The Use of Air Source Heat Pumps in Non-Domestic Historic Buildings

00:00:00:09 - 00:00:30:13

Speaker 1

Thank you, Matt. And good afternoon, everyone, and welcome to this Technical Tuesday webinar, which will share the findings of our latest work, The Air Source Heat Pumps in Historic Buildings. I'm Dan McNaughton, a senior building services engineer at Historic England. And it's nice to see some familiar names in the audience and some new ones, too. From my work, I've encountered a lot of misconceptions about heat pumps, and in particular, there have been a lot of questions raised about how viable this technology is in historic buildings.

00:00:30:20 - 00:00:56:07

Speaker 1

So we saw the need for more advice about this topic to be available. And I'm delighted to be joined today by Andrew McQuatt of Max Fordham, who has worked closely with us on this work and led the investigations. We've been really pleased with his work and to give you an idea of his expertise in this subject area, Andrew peer reviewed CIBSE AM17, which was published last year and this is the industry publication

00:00:56:07 - 00:01:12:00

Speaker 1

if you if you've not heard of that one and that covers heat pump installations for large non-domestic domestic buildings. So that's enough for me. I'll be available with Andrew for any questions that you may have at the end. Just pop them in the chat and I'll have a look through those. And for now, I'll hand over to Andrew.

00:01:12:00 - 00:01:12:13

Speaker 1

Thank you.

00:01:14:04 - 00:01:43:00

Speaker 2

Thank you very much, Dan. Okay, so as Dan has said today, today my talk is, is really going to be based on the work we've been carrying out on behalf of Historic England, looking at issues, heat pumps in historic buildings. And this the work that I'm going to talk about today is very much a continuation of the work that we carried out last year, looking at the viability of air source, heat pumps in historic buildings. But that report focused very much on domestic and small commercial buildings.

00:01:43:11 - 00:02:03:12

Speaker 2

And if you want to have a look at that report, I think that Dan is going to drop that a link to that chat, a link to that in the in the chat and a that you do. So between November 2022 and February 2023, we carried out phase two of this of these and of this study.

00:02:03:19 - 00:02:31:11

Speaker 2

And we did this by looking at four additional case studies. Now, the report is still in in draft form, and it hasn't had the final go ahead from the key study participants. So in order to respect their confidentiality, I'm not going to be talking about these projects by name today. Rather, I'm going to focus on some of the most interesting things that came out, that came out of looking at these studies.

00:02:31:11 - 00:02:54:23

Speaker 2

And these things are going to be looking at how to design air source heat pumps for efficiency. I'm going to think about the system design temperatures in the context of heat pumps, thinking about activity and comfort, looking at some difficult to heat species, understanding heat loss, and specifically how to measure heat loss in order to improve your heat pump design.

00:02:55:14 - 00:03:22:19

Speaker 2

And then lastly, we'll think about refrigerant choice. So I'm just going to get straight, straight on. So designing for heat pumps. So the performance of a heat pump is really well understood and is entirely predictable. If you want to know how efficient a heat pump is going to be, a given flow temperature and a given outside air temperature, then all we have to do is look up at the technical guides, look at some tables or some graphs.

00:03:22:19 - 00:03:49:03

Speaker 2

And the information from the manufacturers is there. So why then do we see performance variations across sites with some sites reporting that the technology saves them money while others that actually costs some cost them a fortune? Well, for me, it's because we must consider a heat pump and the heating system acting as one harmonious system.

00:03:50:03 - 00:04:12:23

Speaker 2

And it is often the design of the heating system that's going to come to dominate the overall system performance. And that's and that's over and above the selection of any particular make or model of heat pump. So why is that? Well, with the exception of CO2, heat pumps, the critical factor determining heat pump efficiency is the temperature of the water

00:04:13:03 - 00:04:44:22

Speaker 2

that leaves the heat pump. So sound system design aims to satisfy the heat loss of the building at the lowest possible flow temperature. So the heat emitter, be that the radiator or the underfloor heating, the fan coil unit, all of these things must, must be designed to deliver the right amount of heat to make those spaces comfortable. And of course, the larger the heat emitter, so the larger the radiator, the cooler that need to be in order to deliver the right amount of heat.

00:04:44:22 - 00:05:08:03

Speaker 2

So too, it's the size of the heat emitters that that so critically impact the whole system performance. And as I've said the cooler the water that's leaving the heat pump, the more efficient the overall design is going to be. In some cases, if the radiators are small, that may be a choice that you've made because you just can't make them any bigger or they're historical in nature.

00:05:08:10 - 00:05:15:15

Speaker 2

And then we may have to run these radiators at a higher temperature. And that is, of course, going to impact overall and system efficiency.

00:05:17:16 - 00:05:38:03

Speaker 2

And the second point I want to make and so we've spoken there about the decisions you make in terms of the size of the radiators. The next point is about how we choose to operate our heat pumps. And this can have a surprising influence on the overall performance. So it's it's burned into our collective psyche to switch things off to save energy.

00:05:38:12 - 00:05:58:15

Speaker 2

So smart thermostats, for example, have been switching our heating off the moment we leave our front door for years in order to save energy. And like most things these days, there is a there's a lively on that online debate whether you should leave your heating on or switch it off to save to save energy. But it's also often the case.

00:05:58:20 - 00:06:21:01

Speaker 2

And even a question that can sound relatively simple is, in fact quite complicated. And it's complicated because it depends on a range of factors. And what I will say, you know, at this point again and again, but maintaining your building at a constant temperature and not allowing it to cool down too much when you leave or overnight will use more heat energy.

00:06:21:01 - 00:06:42:13

Speaker 2

The question we have to ask yourself is, can that increase in heat energy be offset by the efficiency improvements of running a heat pump? Well, so bear with me on this analogy so we can think about heat pump efficiency a little bit like a car's fuel economy. So a drag racer is going to cover the set distance very, very quickly.

00:06:42:18 - 00:07:02:01

Speaker 2

I think we can all agree it's going to use a lot of fuel. And in the process, firstly, because it's running on a relatively inefficient combustion engine, and secondly, because air resistance increases by the square of off your velocity or your speed. So every time you double the speed, the air resistance goes up by a factor of four.

00:07:02:18 - 00:07:25:08

Speaker 2

So if you cover that same distance in a small electric car driven slowly, then, then I think we can agree that the small electric car driving slowly is going to get to the finish line after the drag racer. Essentially the small electric car is going to have spent more time switched on, but because of its improved efficiency and its slower speed, it's going to use less energy to travel.

00:07:25:11 - 00:07:48:06

Speaker 2

And that that distance, of course, you could say you could decide to pick away at these performance improvements by gunning your electric car as quickly as you as you can. But that will result in increased wear and tear as well. So as we've made clear, maintaining your building more or less constant temperature and not allowing it don't kill them too much will require more heat energy.

00:07:48:17 - 00:08:05:21

Speaker 2

And I think this is this is a lot like asking that small electric car to keep driving a little bit further down the track. So we know that small car is so much more efficient and that despite that extra distance, the car is still going to have consumed less fuel. And this is similar to how a heat pump works.

00:08:06:04 - 00:08:35:17

Speaker 2

But rather than speed, they get their efficiency from operating at lower and lower temperatures. So these are little example, a heat pump friendly, 45 degrees, low temperature, 76% of the heat that the heat pump is delivering is simply being moved from outside to inside. And the remaining 24% is coming from the electricity that's driving the compressor. So at this point, a heat pump, it's going to have it's going to be about 32% cheaper to run than a gas boiler.

00:08:36:18 - 00:08:56:11

Speaker 2

So then if we think about the traditional approach of keeping everything turned off to the last minute and then only turning things on just before we need them, well we're going to have to run that same heat pump at a higher temperature in order to produce hotter water to get the building to heat up quickly. So for argument's sake, let's see, we ask that heat pump to produce 70 degree water.

00:08:57:06 - 00:09:24:15

Speaker 2

Well, at that point, the heat from the environment, free heat, that drops down to 60% with 40% of the energy coming from the electricity that's driving a compressor, which then makes the heat pump at this point 16% more expensive to run than the gas boiler. So the improved and heat pump efficiency at 45 could compensate for the overall increase in extra heat required to keep the building on for longer.

00:09:25:23 - 00:09:47:16

Speaker 2

And of course, yeah, this is going to require heating to be left on for longer. And it's like asking that little electric car to drive slightly further down the road. Often we find the optimal solution is to set a desired internal temperature and then let your building only cool down by, say, two or three degrees at night. And we let the logic of the control pump and do all the work.

00:09:47:16 - 00:10:09:19

Speaker 2

We give you the autonomy to run the heat pump at its maximum efficiency and you achieve the internal temperatures. But we do need to be careful because we need to balance a range of factors here before we come to the conclusion about what is right for your building. And these factors will be things like what is the thermal inertia if you building, what are the occupancy levels, what are the occupancy profiles?

00:10:10:06 - 00:10:29:10

Speaker 2

And so a building like a church is a really good example of this because it's going to have a high thermal area and it may only be occupied once a week. And so the balance may be kept in favour of only heating up that building. And very rapidly, when it's needed and over keeping it keeping it warm all the time.

00:10:29:17 - 00:10:47:04

Speaker 2

So we have to be careful. We have to make sure that we judge each project on its own particular merits. Now, I don't know if any of you have come across a geek, but I'm a I'm a big fan. I think they explain things very clearly and they've put together a really nice video on explaining the nuances around this topic.

00:10:47:04 - 00:11:10:23

Speaker 2

So if you if you're interested, I strongly encourage you to have a look at he and should you leave your heating left on all the time or not? And they go into in much more detail than I have there and explain all the nuances. It's a very, very useful video. Okay. So I often talk about low, low temperatures.

00:11:11:02 - 00:11:33:07

Speaker 2

And when we're talking about heat pumps and the benefits of running them at low temperatures. So some of it's the one we've already covered is that if you run your heat pump at a low temperature increases the heat pump efficiency. And we've also got reduced wear and tear because if you run a low temperature, it's likely that your heat pump will be starting and stopping less and which is kinder to the system components.

00:11:33:07 - 00:11:56:22

Speaker 2

We can we can enjoy increased occupant comfort by maintaining at more stable temperatures. And we can also helped improve at building fabric protection. And because again that due to those stable temperatures is helping to drive moisture out. And when I see low temperatures and I want to quickly cover what do I mean by low and how do they compare historically to fossil fuel heating systems?

00:11:57:24 - 00:12:22:09

Speaker 2

So CIBSE defines air temperature regimes and as low, medium and high temperature for the commercial heating industry. And low temperature is anything below 90 degrees. And then you can see medium 90 to 120 high temperature above 120. Now, we can forget all that because when I'm talking about low temperature, I'm usually talking about anything less than 55 degrees.

00:12:22:09 - 00:12:47:06

Speaker 2

And anything less than 55 is going to keep on friendly. Greater than 55 is typically not as efficient for heat pump. So before the introduction of condensing gas boilers, heating systems were typically designed to operate at 82 to 71. That's 82 degrees, leaving the gas boiler or whatever boiler it was, and 71 coming back from the heating system.

00:12:47:24 - 00:13:14:16

Speaker 2

And the advent of condensing gas boilers led to new systems being designed to achieve a maximum return temperature of 50, with a typical at design temperatures being 70 from the boiler, 50 coming back from heating system. And the reason the reason we do that is because a condensing gas boiler gets its efficiency over a non condensing gas boiler by allowing the energy trapped in the water vapor and flue gases to be condensed and recovered.

00:13:14:19 - 00:13:36:15

Speaker 2

And if the return temperature comes back too hot, we can't condense the flue gas and we can't make those energy savings. So the advent of condensing gas boiler should have really ushered in an era of heating system upgrades to ensure that every, every premises was benefiting from from the the condensing part of these new gas boilers.

00:13:37:05 - 00:14:06:00

Speaker 2

And this I don't think this was always done. I think in many cases and a lot of domestic cases, no one condensing gas boilers were removed and a new condensing gas by others was simply added. And and the energy savings may not have been realized. And I think I think this is because of the energy savings being only around a maximum of 20% and gas being so cheap, the fact that the boiler failed, perhaps failed to deliver and that these savings went unnoticed.

00:14:06:13 - 00:14:32:11

Speaker 2

However, that's not a luxury we have anymore with heat pumps, because heat pumps run on a fuel because it costs a lot of money. You know, electricity is not cheap. And the second part is the system design has such a big impact on overall heat pump performance. So if we get this wrong and we did a poorly designed heating system, we may end up with a system that costs twice as much to run and as the optimal system.

00:14:33:00 - 00:15:00:06

Speaker 2

So we're not going to get away with simply switching out heat pumps for boilers without considering heating system. And the good news is building regulations now require all new heating systems to be to be designed with a maximum temperature of 55 degrees. And there are, of course, exceptions made for historic buildings. For some historic buildings, and upgrading the heating system to operate at low flow temperatures just might not be possible.

00:15:00:20 - 00:15:24:01

Speaker 2

And in these situations, it may be may be totally appropriate to run heat pumps at higher temperatures. And except there was increased running costs because everything in life is, of course, a balance. And what might be a priority for one project might not be the priority on your project. I'd like to have a quick think about activity and comfort, so don't worry about the equation.

00:15:24:10 - 00:15:50:21

Speaker 2

I think that essentially to achieve to achieve comfort in any space, your body's heat losses and heat gains have to be balanced with your body's metabolic rate. And our metabolic rate is driven by our level of activity. The heat loss is dependent on things like air temperature, the humidity of the air, the temperature of all the surfaces that surround you, the speed of the air that's moving past you.

00:15:50:22 - 00:16:35:04

Speaker 2

And, of course, what clothes that you're that you're wearing. So, as designers, once we once we decide what comfortable space temperature is going to be and we go about making trade offs and trade offs between the fabric upgrades and the size of the heat emitters in the space. So I want to make it clear that a well-maintained, historic fabric, fabric that eliminates drafts is perfectly capable of providing comfort for almost any activity and using a low temperature, heat pump friendly and design. But as we've already discussed at the low running costs, heat pump compatible heating system requires larger radiators.

00:16:35:09 - 00:16:58:17

Speaker 2

And so these can cost more money and they can be space consuming. And there's always going to be a point where you just can't keep adding space emitters. You're eventually going to run out of space. So at this point, the designers and the clients, I think, should take a step back and consider whether improvements could be made to the fabric performance of the building.

00:16:59:02 - 00:17:21:02

Speaker 2

And this doesn't need to be deep retrofit and it's maybe focusing on controlling drafts or perhaps is increasing the flow temperature of the heating system and accepting those high running costs. I want to make the point that this this can only work up until up until a point. Cold drafts are always going to be an issue.

00:17:21:02 - 00:17:41:10

Speaker 2

And in any building old only if you're if you're sitting there and you can't escape them. And no matter how much heat you throw into the space, you're unlikely to ever achieve comfort. So at this point, perhaps we have to reconsider how we use species. So a way of thinking about this is, is an example of a person that's perhaps just come in from the outside.

00:17:41:17 - 00:18:06:02

Speaker 2

They're walking through an exhibition space. They still got the coat on and perhaps 16 degrees will provide an adequate comfort in that space. But 22 degrees will be more appropriate for someone in a restaurant, perhaps they're sitting there and they're sitting and they're dressed and dressed to impress. So every building that I've ever designed has as one thing in common. Old or new,

00:18:06:06 - 00:18:38:19

Speaker 2

I've had to pay very particular attention to entrance spaces because these are spaces that have really high infiltration and are always challenging to heat and because of that large amount of airflow. So in the worst cases, building owners, operators want to keep the doors open all the way through the winter and encourage people to enter and of course, to provide unhindered access to the sites and research does show that that many people are deterred from entering if a door is closed.

00:18:39:03 - 00:19:10:14

Speaker 2

And however, you can overcome this simple signage outside saying things like, we're open, please come in, or we're keeping the door closed to save energy. And of course we can add electric push buttons and activate doors and to address any accessibility issues. So step one should always be to consider some sort of physical intervention and to reduce drafts, something like, you know, and a draft lobby, for example, because only from a solid foundation can you make an entrance space both comfortable and energy efficient.

00:19:11:12 - 00:19:37:16

Speaker 2

But even with the best of steps based, often, you know, draft lobbies in place is always going to be difficult. And that's because every time the doors open, it's quite likely that the reception staff are going to be deluged by a big slug of a freezing cold, freezing cold air. And this is a problem because reception staff are often are often sitting at desks and they're sitting there for long periods of time, and they're going to require similar comfort conditions to people working in an office.

00:19:37:20 - 00:20:10:03

Speaker 2

And that may be 21 degrees with relatively low air movement. So it becomes very, very wasteful and to try and heat the air inside these leaky spaces and because every time the door open, all that air or a large majority of it is lost outside. So if we try to overcome this issue just by increasing the size of radiators, firstly where eventually could run of space, but also just by making hot air with radiators.

00:20:10:03 - 00:20:45:15

Speaker 2

And that hot air may be lost, we may require a different approach. So a more economical approach might be to try and heat the occupants directly using things like infrared heaters. So an electric infrared heater operates an efficiency of about 100%, and that's compared to a heat pump at about 400%. But the difference is with infrared heating, the heat is delivered directly and from the source to the surface or indeed the person is therefore less susceptible to that air change in the air leaving the room.

00:20:45:15 - 00:21:06:12

Speaker 2

So it might be might be a good option there. So it becomes, with heat pump design, it becomes very important to understand the heat loss of our building. So if we if we underestimate the heat loss of our building, we're going to end up with a heating system with heat pumps and radiators that are too small and won't heat our buildings correctly.

00:21:07:10 - 00:21:38:12

Speaker 2

If you go the opposite way and you overestimate the heat loss, we risk installing vastly oversized, very expensive to buy heat pumps and heating systems. From the world of fossil fuel boilers, over sizing has had a limited impact. Firstly, because boilers are pretty cheap and secondly, cycling doesn't have much of an impact on their efficiency. So cycling is common to all heating systems, especially in spring and autumn.

00:21:39:01 - 00:22:03:13

Speaker 2

And it happens when the minimum output of the heat emitters and heat generators sorry. So the boiler or the heat pump and exceeds the heat demand for the building. And at this point, we have to simply switch off the boiler of a heat pump in order to avoid the building overheating. And as I said, it's not much of a problem for gas boilers as when you switch them on, they quickly reach their peak operating efficiency.

00:22:03:18 - 00:22:32:09

Speaker 2

So starting and stopping isn't is a huge, huge impact on efficiency. And it's but obviously it's not great for wear and tear. And the difference with a heat pump is that heat pumps go through a slower and less efficient heat pump cycle, sorry, start up cycle before they reach their optimum steady state efficiency. So our larger oversize heat pump is going to be switching on and off more often and it's going to spend more time in that inefficient start up period.

00:22:33:09 - 00:23:01:20

Speaker 2

So research on the impact of cycling on heat pumps and says that if a cycle lasts anywhere between 10, 15 minutes it's probably not going to have a huge impact on overall efficiency. But certainly don't want to see cycle lengths less, less than 10 minutes. Otherwise, you can have an overall lower efficiency. And also, we have to consider the fact that heat pumps are expensive.

00:23:02:06 - 00:23:35:02

Speaker 2

So at the moment, an air source heat pump is about ten times more expensive than the gas equivalent. And of course, they take up significantly more space. So it's really important to understand heat loss. And you are at a really accurate design. So what can we do to help us do that? So to understand the heat loss of a building we typically, typically consider and for tanks, we've got the building size, we've got the ventilation system, how airtight that building is and what are the existing insulation levels.

00:23:35:14 - 00:24:07:20

Speaker 2

Now building size and what the ventilation system flow rates are should be relatively easy to measure and to an acceptable accuracy. But things like insulation levels and air tightness, they must be they must be estimated based on visual inspection, which can lead to some pretty big inaccuracies in the final calculations. But luckily, there are there are options to improve accuracy of heat loss calculations, and these include measurement techniques such as and airtightness testing.

00:24:08:22 - 00:24:36:17

Speaker 2

So accurately measuring the air tightness is probably one of the most significant factors in any heat loss calculation. So what we do is we put a fan in the external doorway and we either pressurize or depressurize the building and we use that and the resulting data to work out how leaky the building is. For large buildings it may not be possible to do this for the whole building, but you could do this certainly on sample areas of the building and get some really useful data there to improve your designs.

00:24:38:01 - 00:25:06:02

Speaker 2

And the nice thing is the air tightness testing is mandatory for newbuild properties in England therefore, and there should be an established industry capable of carrying this testing. The next one is in-situ u-value testing. So the u-value of an element is basically just a measure of say for a wall and how quickly heat passes from one side to the other and accurately measuring u-values and or estimating and can be really difficult in historic buildings.

00:25:06:02 - 00:25:24:09

Speaker 2

You know, you may not really know what the wall building is and you certainly don't have any as built drawings. So in situ u-value testing can be can be a particularly useful technique. And this involves measuring the temperature on both sides of the wall and then measuring how much heat flows through that wall over a 72 hour period with a heat flow meter.

00:25:24:18 - 00:25:52:00

Speaker 2

And that that allows you to just do spot checks around the building, again, helping you to increase the accuracy of any sort of calculations that need to be done. And the third example here is called a core heating test. And this is probably the most the most accurate test you can do to assess a building's heat loss. So this test involves measuring the internal and external temperature over a period of approximately three weeks while the building is unoccupied.

00:25:52:11 - 00:26:09:18

Speaker 2

And what happens is you plug in a load of electric heaters and you measure how much energy is required to maintain the building at a certain temperature. And that's going to give you a really accurate figure. Of course, the catch there is possibly that the building has to be unoccupied for three weeks. So that's not going to be possible for everyone.

00:26:10:21 - 00:26:33:10

Speaker 2

There are no and proprietary systems that are available that can measure heat loss parameters in in occupied buildings. But of course, with anything, the accuracy will decrease. But it may be worth it, given that you don't have to you don't have to empty building for three weeks. And the last thing I just want to make a quick note about is don't underestimate the usefulness of historic energy data.

00:26:33:24 - 00:26:55:02

Speaker 2

Now, of course, if you're going to make a lot of changes to your building, the historic data might not be may not be that useful. But if you're not going to be making that many fabric changes, having a look at how much your building was using and is a really good way of just maybe double checking how someone has sized your heat pump in the future.

00:26:55:02 - 00:27:20:10

Speaker 2

So, you know, that data can be really useful. And of course, it has its name, it has its limitations. And lastly, before we I stop for four questions, I'd like to have a little chat about the refrigeration choice. So of course, you know, why are we doing this? We're deploying heat pumps because it's the form of heating that's going to release the lowest amount of CO2 into the atmosphere.

00:27:21:13 - 00:27:39:15

Speaker 2

However, what we to what we need to do is we need to consider the broader environmental impact of actually of everything we do. Otherwise, we may end up damaging the environment in other ways. And the type of refrigerant used in the heat pump is one of those things that can definitely undo the positive work we're trying to do.

00:27:40:02 - 00:28:04:11

Speaker 2

So refrigerants could damage the environment in the following ways. First one is ozone depletion. The second one is global warming potential, and the third one is PFAS chemicals. So if we take those in turn. So well, firstly, a heat pump salesperson is going to tell you that this just isn't an issue. Heat pumps don't leak there and heat and the refrigerant is carefully collected at the end of their life.

00:28:05:12 - 00:28:25:03

Speaker 2

Well, heat refrigerants I'm afraid you know do leak and some of it will make its way into the atmosphere when we carry out whole building carbon assessments CIBSE recommend accounting for at least between 2 and 6% of refrigerant leaks per year and a further 3% lost at the end of the recovery. And of course that's an average figure.

00:28:25:13 - 00:28:53:08

Speaker 2

So some heat pumps will suffer a catastrophic failure and lose all their refrigerant and other heat pumps will remain and hermetically sealed and release and almost none of their none of the refrigerant. But depending on your point of view, it may not sound like a lot. And but for some systems, this can account for as much as 13% of the whole life carbon emissions, and that includes the manufacture of the heat pump and the carbon associated with running that heat pump over its entire lifetime.

00:28:53:13 - 00:29:18:04

Speaker 2

So it's not it's not an insignificant amount of carbon there. So for many years now, manufacturers have only used refrigerants with ozone depletion potential of zero. So in reality, this isn't something that we need to worry about these days. So it would be like walking around the supermarket, checking to see if your kind of deodorant still contains CFCs, is there banned, it’s not going to happen.

00:29:19:01 - 00:29:48:00

Speaker 2

But once we move our attention, we move their attention from ozone depletion. We started to think about the global warming potential of refrigerants. So global warming potential or GWP is a measure of how potent a refrigerant is as a greenhouse gas and we compare it to how potent CO2 is. So many older systems and unfortunately, some new system still being installed today.

00:29:48:04 - 00:30:18:08

Speaker 2

We use we use refrigeration like R410a. So for every one kilogram of R410a that escapes into the atmosphere that will have the same global warming potential as releasing 2088 kilograms or basically two tonnes of CO2 into the atmosphere. And it's because this that R410a is currently being phased out. Its common replacement is R32 and that has a much lower global warming potential of 675.

00:30:18:08 - 00:30:39:02

Speaker 2

But it's still far from insignificant. And while we don't have any specific dates for the phase out of R32, we know there's likely that R32 will be will be phased out. So on the face of it, we can look at a modern refrigerant such as R1234ze with a global warming potential of only seven.

00:30:39:02 - 00:31:06:21

Speaker 2

It appears that it solved all of our problems. But as is often the case, if we only focus on one measure of environmental impact, we may end up and inadvertently causing another problem. And I think this is exactly what's happened here because refrigerants like R1234ze introduce PFAS chemicals into their mix. Now PFAS chemicals are in everything from clothes to cookware and have been linked to various health problems.

00:31:06:21 - 00:31:34:08

Speaker 2

And we're really only just starting to understand what their long term effects are. But they are forever chemicals, meaning that once they're released into the environment, they'll take up to a thousand years to break down. But luckily there are alternatives that we can use right now. So natural refrigerants and so R290 has a global warming potential of only three and R744 has a global warming potential of one.

00:31:34:21 - 00:32:00:20

Speaker 2

But it's often the case and everything in life comes with the catch. So R290 is actually camping gas or to be more precise, it's propane and is therefore very flammable. So of course extra care is required around siting of an R290 unit. But put in perspective, if every Waitrose supermarket in the country can safely use propane chillers, the challenges are clearly manageable.

00:32:01:13 - 00:32:35:08

Speaker 2

And don't forget, we're currently piping methane under pressure directly into our homes and businesses for heating now R744 is simply CO2 and the global warming potential of one. So CO2 heat pumps can produce high temperature water. And by that I mean and temperatures from 70 opposite 90 degrees. But in order for those types of heat pumps to be efficient, the return temperature from the heating system must be coming back at about 30 degrees or lower.

00:32:36:00 - 00:33:18:15

Speaker 2

So understanding why 30 degrees is important is basically a lecture in itself, but it is it does represent a really critical point on the CO2 refrigeration cycle. To achieve sub 30 returns you're going to require some modifications to your existing heating system. But again, this has been done successfully many times and as long as we learn from the projects of the past, we can we can make CO2 heat pumps work very well. So, and just to sum up and go through to my conclusions. Heating system design is crucial to crucial to optimize performance of heat pumps and it's often the temperature of water leaving the heat pump that the

00:33:18:15 - 00:33:46:07

Speaker 2

critical factor in determining the resulting heat pump efficiency and heat pumps are more than capable of heating historic spaces but in any building who you can't control, the drafts you might have to think about and how that space is used. And some species, such as entry spaces are just hard to heat regardless of the heat source. And it is essential to accurately understand the heat loss of your building in order to improve the accuracy of your system

00:33:46:07 - 00:33:56:09

Speaker 2

design and natural refrigerants such as propane or CO2 should be the first port of call when it comes to selecting a refrigerant. Thank you.

Do you recommend the use of Glycol in a domestic system? Or trace heating instead?

Manufacturers' installation instructions must be followed. Some manufacturers permit anti-freeze protection valves instead of glycol, but instructions must be followed to avoid voiding the warranty.

Protection against freezing is necessary in case of a prolonged power outage. The use of trace heating may require a battery backup system, adding expense and complexity.

Further information on Glycol is given in "Heat pumps in Historic Buildings, page 10."

Thought there was heat loss calculations possible through using the Purrmetrix system in occupied buildings?

This appears to be a commercial system. However, I have no personal experience to assess its effectiveness.

Is there any data on lifetime costs of ground versus air source heat pumps - comparing capital, running, replacement cost, longevity and so on - and which do you prefer if both are technically viable and cost is no issue?

In many cases, the cost of installing a ground source heat pump is high and may take a long time to recover through the cost savings associated with improved efficiency. However, a ground source heat pump may be the only option in situations where an air source heat pump cannot be accommodated due to site noise constraints or for visual reasons.

Does the heat pump equipment need to be externally mounted?

Air source heat pumps will be usually installed externally although they can be installed in plant rooms if adequate ventilation is designed. Ground and water source heat pumps are designed to be installed internally.

Are heat pumps steal-able? Gas boilers are sometimes stolen from vacant buildings.  Heat pumps by their nature are located outside. Do we need to consider security? Fencing? Barriers?

We haven’t heard any reports of a problem with this but anything of value is at risk of theft. Air source heat pumps typically weigh over 100kg so you will need at least 2 people to be able to move this manually. If you are in an area at high risk of theft or vandalism then CCTV or other security measures could be taken.

For spaces such as churches, when doing heat loss calculations to help size ASHP using underfloor heating, do you consider the whole volume or occupied zone?

I’ve tended to calculate the heat loss using the space volume. It may be possible to rationalise this but the convective component of the heating output will still stratify and this can be around 50% of the heat output. The radiant component of the heating output will have the greatest contribution to comfort within the occupied zone. I’ve often had to supplement underfloor heating with additional emitters either due to the limited heat output available from underfloor systems or from not being able to install an underfloor system in all areas of the church.

It is essential to consider the heat loss from the entire space while ensuring that the occupied area remains comfortable. This can be achieved through radiant methods like underfloor heating or by ensuring proper mixing of the air in the space. Careful attention must be given to maintain a comfortable environment.

Are we seeing an increase in the number of people who can maintain heat pumps, particularly propane/CO2 types?

The government has recently announced heat pump apprenticeships with the aim of increasing the number of installers but it is not certain if this will cover these 2 refrigerants.

This question applies to the selection of any equipment. It is always important to question the presence of the manufacturer or approved installers in your area.

Are local conservation planners open to ASHP being installed in historic buildings and gardens?

There seems to be more case studies relating to planning proposals involving air source heat pumps in historic buildings and we are aware of many examples of this technology being used.

To increase the thermal performance of underfloor heating with a heat pump would you reduce the distance between the coils/pipes so a low flow temperature works better?

Yes, by spacing the pipes closer together, you can achieve higher output or reach required output at a lower flow temperature.

How efficient is it to use ASHP with cascade units that then bring up the water temperature of the water to still use old emitters that require higher temperatures? And how cost effective is this?

Propane units can reach 70-degree flow temperatures, so a cascade may be unnecessary. Cascading multiple ASHPs or operating a single heat pump to achieve high temperatures results in similar overall efficiencies.

If you decide to combine a water-to-water heat pump with an air-source heat pump, it can significantly reduce the overall efficiency. This is because the amount of free heat delivered by the air-source heat pump is fixed, but you continue to add energy via the compressor in the water-to-water heat pump.

Is there much advantage of locating external ASHP on south elevations rather than colder north elevations?

Finding a sunny location can enhance performance compared to a north-facing location, but I don't have numbers for the degree of efficiency improvement. Given that in the winter, when the heat pump will be doing the bulk of its work, the sun isn't intense and is often overcast. Therefore, I wouldn't recommend making significant compromises, like sacrificing an amenity space, for a marginal performance improvement.