When you’re planning a heat pump installation, most of the conversation tends to revolve around one thing: the heat loss calculation. It tells you how much heat your home loses and, in turn, how powerful your heat pump needs to be. But here’s where many installations go wrong… knowing the right size of heat pump is only half the story. If your pipework can’t physically carry the amount of heat that the pump is trying to deliver, the whole system will underperform, no matter how good the heat pump itself is.
Heat pumps in the UK typically work by moving warmth around your home through water circulating in pipes that feed radiators or underfloor heating. The amount of heat that can be transferred through those pipes depends on the temperature difference between the flow and return (known as ΔT) and the rate at which the water is moving. Boilers typically operate with a 20C temperature difference, which allows a lot of heat to be carried with relatively low water flow. Heat pumps, however, are designed to work more efficiently at lower flow temperatures, usually with a ΔT of around 5C. To move the same amount of heat with a ΔT of 5C rather than 20C, you need roughly four times more water flow. And when the water has to move that much faster, you need larger pipes to carry it quietly and efficiently.
To put this into real terms, we can look at the kind of heat that different pipe diameters can handle at that 5C temperature difference, based on data from the CIBSE Domestic Heating Design Guide. A 15 mm copper pipe can carry roughly 3 kW of heat. A 22 mm pipe can carry about 7 kW. A 28 mm pipe moves around 12 kW and a 35 mm pipe can handle roughly 18 kW at sensible flow velocities of about one metre per second.
These are not exact figures (every system has different pressure drops depending on layout and pipe length) but they make one thing clear: pipe size matters, and it matters a lot. If you’re installing a 16 kW heat pump, for example, you’ll likely need at least 35 mm primary pipework to deliver that heat effectively. Try to squeeze all that through 22 mm pipes and the system will struggle. You’ll end up with noisy, turbulent water rushing through the pipes, a stressed circulator pump and a system that can’t deliver its designed output or efficiency.
This is why pipe sizing isn’t some minor technical detail – it’s fundamental to system design. Every section of pipe must be able to carry the combined heat load of everything downstream of it. The further you move away from the heat pump, the smaller the load becomes, so the pipework can gradually reduce in size. But if your installer simply reuses existing boiler pipework (often 22 mm copper) for a large heat pump, you can almost guarantee performance issues.
Smaller pipes and longer runs mean higher resistance, known as pressure loss, and the pump has to work harder to maintain flow. Over time, that extra effort means more wear, more noise and higher running costs. It’s a false economy to save a few hundred pounds on pipework when it compromises the entire efficiency of a system that costs thousands.
It’s also important to remember that at the lower operating temperatures of heat pumps, water becomes more viscous, which increases resistance by up to 50%. So while a low ΔT can make the system more efficient in theory, it also demands higher flow rates and larger pipe diameters to keep performance stable. That’s the delicate balance in good design: move the water fast enough to carry the heat, but not so fast that you create turbulence, noise and wasted energy at the pump.
If upgrading your pipework isn’t practical, there are limited workarounds, but they need to be properly engineered, not guessed. Some designers might increase the ΔT slightly to reduce flow rates, though this can dent efficiency. Others may try to spread the load across larger or additional radiators, but that still depends on the pipework’s ability to supply enough energy to each circuit. The truth is, there’s no way around physics: if the pipe is too small, it will throttle your system. You can’t cheat the flow rate, and you can’t expect great performance if the bottleneck is built into the system.
For homeowners, the lesson here is simple: don’t let your installer brush past pipework. Ask them to explain what pipe sizes are needed to deliver your system’s output and how those compare with your existing setup. If you’re told that your old 22 mm boiler pipes will “do fine” for a new heat pump, ask for the design data that supports that. Numbers don’t lie, and in most cases, the maths will show that they won’t. Good installers know this and factor it into the design stage, not as an afterthought once problems arise.
At the end of the day, a heat pump is only as good as the pipes that feed it. You can have the best machine on the market, a flawless heat loss calculation and even the best intentions, but if your pipework is undersized, you’ll never see the comfort, efficiency or performance you were promised. When planning your installation, make sure pipe sizing is part of the conversation. It might not be the most glamorous topic, but it’s the backbone of an efficient system. The right pipework will give you a quieter, smoother and more reliable heat pump that performs exactly as designed.
And if you’re not sure whether your system’s pipes are up to the job, don’t guess. Ask your installer to show you their flow rate and velocity calculations, or better yet, share your details on the Renewable Heating Hub forums and get advice from people who’ve been through it before. It could save you a lot of frustration (and a lot of money) down the line.
I’ll be sharing our journey because we’ve got undersized primary pipework running from the heat pump to the house, and we’ll share the maths and methodology with you in the weeks and months to come on how we plan to overcome this.
@mars Im going to put a slightly different angle on this post because, whilst I accept its importance in principle and for some properties, Im not so sure it should figure quite so significantly in consumer minds for the vast majority of properties.
Whilst its certainly the case that pipework size really does matter, its not necessary (or even practical) in many, perhaps most, cases to do extensive calculations or even to be 100% certain, and an attempt to do so would be way over the top. There is a very real role here for ‘engineering judgement’ as a first filter, with a contingency plan if the engineering judgement turns out to be wrong.
Take my house as an example, 200sqm and 7kW loss, fairly typical of a suburban detached house I imagine. All that was necessary (and the only thing practical without taking up flooring and floorboards) was to note that there appears to be a 22mm primary for upstairs, a 22mm primary for downstairs, that the rads are on 15mm tails and that there is a 22mm flow/return pair for the DHW. Now of course its possible that the 22mm primaries are sized down to 15mm ‘too early’, but there is no practical way to discover that without taking up carpet and floorboards upstairs and solid flooring and floorboards downstairs. Far better to proceed on the most probable assumption ie that it is OK, and then solve a problem if it occurs. I would suggest that the same logic applies to most houses where both the primaries and the feeds to the rads (ie the bits that you can actually see) are OK, which is likely to be a majority.
A similar approach was taken with the cold water feed to my DHW cylinder which is 15mm only. The installer checked the garden tap (which is on the end of a long run), noted that the flow was >20l/min, and made a reasonable assumption that the flow to the DHW cylinder would therefore be OK, because the mains pressure was sufficient to overcome loss in the pipe. There was a fallback option if not, which in practice wasn’t needed (and yes the installer did check before leaving!). That’s exactly what I would want any installer to do. What I definitely didn’t want (which some who I firmly rejected proposed) was to run a new pipe without first proving that the old was sufficient, which is realistically only possible by experiment.
So whilst I agree that pipe sizing should be part of the conversation, there is, in many cases, no reason to expect it to be a big deal, provided that the installer has made some simple observations and those ‘check out’. If we don’t recognise this there is a risk of going hard the other way, ie forcing a large amount of unnecessary disruption and attendant cost just to be 100% certain that the system will work the first time it is switched on.
@JamesPa, that’s a fair and well-argued perspective, and you’re right that in many cases, particularly with ‘typical’ detached or semi-detached homes, a bit of pragmatic engineering judgement goes a long way. I also agree that no one wants to see homeowners tearing up floors or spending thousands chasing theoretical perfection when, in reality, most systems will run just fine if the primary pipework is adequate and properly balanced.
Where I think things have gone wrong (and why I wanted to highlight this in the article) is that engineering judgement has too often been replaced by guesswork or wishful thinking. A lot of poor-performing systems I’ve seen (and heard from homeowners about) including our own stem from installers assuming the existing boiler pipework would “probably be fine” for a heat pump, without checking lengths, pressure drops or even the basics like ΔT or pump head. When it isn’t fine, the result can be noisy systems, low flow rates and underperforming emitters, all of which could have been avoided with a few quick flow calculations or at least an informed check. All of this is present in our own system.
So I think we’re saying broadly the same thing: pipework doesn’t automatically need to be ripped out, but it does need to be understood. A decent installer with good judgement can usually make that call without turning it into a forensic exercise, but the key word there is decent. Too many don’t have that level of understanding yet, and that’s where the trouble starts.
Your approach (verify what you can see, make reasonable assumptions where you can’t, have a fallback plan if things don’t behave as expected) is exactly how it should be done. The problem is, not enough installers are doing that middle step at all. They just assume and walk away.
So yes, for most homes it’s not a big deal, but for some, especially larger or older properties, it’s absolutely critical. My aim with the piece was to get homeowners asking the question, not demanding that installers rip everything out. Because as you rightly point out, the balance lies somewhere between blind faith and unnecessary disruption.
@Mars
Agreed. There is another point to be made here, namely that you cant make the judgement/calculation on pipe size unless you have correctly determined the loss and, as we know, surveys dont necessarily accurately determine the loss. Since the tendency appears to be to overestimate loss, there is a risk that pipework is unnecessarily replaced based on this. Personally i would put more effort into loss determination before even thinking about pipework, as this is the foundation of everything else. I would possibly go so far as to say that a surveyed loss that has not been sense checked by some means shouldn’t be used on its own as a reason to replace any pipework.
I’ve in the process of rectifying my system. My installer couldn’t get the flow rate through the primary pipe work they put in, so I now have 2 pumps just on the primary circuit to get the required flow rate. I asked them to re-pipe the primary, but no, they won’t even admit that wider pipes would help. We did build a decking structure over the primaries so swapping them would be a pain, but I’ve now got a 6 pump heating system for an open loop system…… I think the installer thinks I’m being over dramatic but I’m not happy about it.
I can understand 2 but what are 6 doing. Do you have microbore or very long runs? How big is your house?
@JamesPa
No, the house was back to brick and the installer put in every single heating pipe in my house. I have 2 on the primary run to the low loss header. One off the header on a rad run, one off the header that then splits and runs to the 2 UFH manifolds, and a pump on each UFH manifold.
@davidnolan22 Oh my, thats a lot of pumps and is certainly wrong. You should change the primaries, remove the buffer and insist on a better design.
@ASHP-BOBBA
Hi, I think there is a chance that the system “works" with the new 6 pump set up. We won’t really know until it gets much closer to design temp. If it does “work" and the pumps and the header can get fluid around the house to heat it with a COP that’s above the MCS minimum (which I think it will be), and the energy consumption is ball park what they quoted: where do I stand?
Can I go to NAPIT and say: “it works, but it could be better". From reading these forums, the formal complaints procedures seems a difficult and often fruitless process.
@davidnolan22 I genuinely do not know the answer, unless your home is 500m2 I doubt you need that many pumps and there would still be a better way to do it even if it was.
I think the main issue can be zoning, so many installs the engineers zone in smaller houses, what we know is they work better with less restrictions but I your point, if it performs like they advised then what can you do?
Er…no comment!
It’s a complete and utter waste of time and energy, especially with the certification bodies like NAPIT…
@Mars
So, I’d be left with: asking the installers to do what I want despite me not having zero heating engineering experience or qualifications; leaving it as it is if it just about works, or, paying a competent person to sort it out. Several thousand I expect and I bet they don’t want to get involved within the warranty period.
It’s the complete lack of design, planning and execution in so many installs that really gets to me.
Our original system was bodged from day one. Every time we raised concerns, the stock response from the installers was, “We’ll add another pump.” By the end of it, we had four pumps and no real improvement.
Fast forward to our recent so-called “retrofix,” and the pattern repeated itself in reverse. The installer scoffed and mocked at the four pumps, ripped them out and replaced them with one massive pump. Same logic, just a bigger plaster on the same wound. Still no design, no evaluation of pipe sizing, no system thinking. Just “add more force.”
That approach doesn’t work. It never has in heat pump installations. It’s a cowboy mindset… the idea that you can blast your way to success instead of taking the time to plan, calculate and understand what you’re working with. I know it takes time. I know it takes effort. But that’s the job of a heat engineer. That’s essentially what you’re paying for as a customer.
In spring, we’ll be tackling a proper retrofix (built on design, maths and strategy) to show how thoughtful planning delivers uniform, efficient heat across a property, even a complex-ish one like ours.
Heat pumps demand respect and understanding, not guesswork and brute force. Especially with multiple zones, you can’t wing it. These systems need thought and it’s long overdue that the industry started showing some.
@davidnolan22
So far as I can tell from the ‘other place’ (buildhub) there are customers that absolutely insist on microzoning however much their peers on the forum tell them otherwise. It’s perhaps therefore understandable that some installers zone more than is ideal!
Whilst it’s true that, if it works to an adequate spec,* there may not be much you can do legally, you can still do a lot practically. Obviously you can’t fix the fact that 6 pumps consume roughly 6 times the energy as 1 pump, but other than at the ends of the season that is hopefully a small proportion of the total. However you can:
Attempt to minimise the buffer penalty by ensuring that the volume flow rate on the ashp side is larger (ideally only just larger) than the volume flow rate on the emitter side, so that the thermocline is at the bottom not top of the tank, and thus there is negligible dilution of the flow temperature by return water.
Operate as a single zone by setting any zone thermostats a couple of degrees above target so they act as limiters not controllers
Adjust your WC curve down as far as possible so that the house when operated 24*7 single zone, is just at the right temperature and no more
If necessary balance the zones for equal temperature (or desired temperature difference) by adjusting pump speed, flow regulating valves or lsvs
Obviously keep a note of anything you change and do it methodically, slowly (leaving it to stabilise for a day or more between changes) and make notes so you can retreat if you need to.
I suspect you know most of this so forgive me if I’m stating the obvious.
If you do the above well you should end up with something quite close to single zone open loop operation at the optimum flow temp with hopefully a relatively small buffer penalty.
Feel free to ask for guidance or explanation if you do decide to take this approach.
* building regs require that heating systems are designed to be as efficient as reasonably practical which could form the basis of an argument for scop>2.8, the min requirement for BUS. If you do invoke this check precise wording!
@JamesPa
Hi,
I’ve done most of the above, and I think could well “work" this winter.
The thing that bothers me most is just the lack of any care on the design. The 6 pumps could have easily been avoided with some care and attention to detail; my house was ripped back to brick: they put in every single pipe and still made a mess of it. I’ve dialled back most of the pumps where I can, and I think when all 6 are working I’m about the 200 to 250W mark, still seems a lot when for much of the years the ASHP itself only draws 600-800W. And lastly, pumps break. New pump and call out fee I imagine will be 300 to 500 quid a time. I imagine I’ll send a few thousand pounds over the next 10-15 years swapping failed pumps.
If my installer leaves it as it is, I imagine over the coming years I’ll just bring someone else in to put it right. Another few thousand pounds. I could involve NAPIT, but I would worry they would make the situation worse not better. My installer has told me their only other NAPIT case made they put zoning in due to building regs. I don’t want that.
I still go back to the day their salesman was sat in my kitchen and promised me “the easiest and most forgettable heating system you’ll ever own". What a load of bollocks that turned out to be. If I had my time again, I’d have put in one more gas boiler and come back to it in 15 years. What I’ve been through in the last 2 years was not worth it.
I completely agree, really I do.
However I also know that the whole construction industry has a fair number of chancers, and like it or not they mostly get away with it.
For that reason I pretty much assume that if I get any construction work done I may have to clear up the mess myself and certainly I need to supervise it very carefully. Obviously I try to avoid this scenario and mostly, but certainly not always, I am successful (only a couple of years ago I was scammed for a £5000 faulty roof). I guess I’m pretty much resigned to life as it is not as I would like it to be.
Sad I know, but not special to heat pumps or to the UK so far as I can tell. Don’t forget that we have almost all been paying 10% more for our gas heating than we needed to, and enjoying lower levels of comfort, because condensing boilers are almost always set up in the UK so they don’t condense. The industry has had since 2005 to adapt, but it hasn’t. When my gas boiler was last serviced the guy even reset the flow temp, which I had turned down, back up to max before he left! That says it all.
@JamesPa
And then there was the window company I had to deal with…… Horrific……
I’m not sure how I would go about finding a trades person again.
The more we learn about about an ASHP potentially replacing a standard or condensing boiler heating system, fuelled by oil or gas, and while being enlightened by this insightful RHH resource, the more we might be resigned to upgrade our existing 20+yr conventional oil boiler like-for-like. Even an ‘upgrade’ to a condensing boiler has drawbacks.
It’s early days, currently determining our Heat Loss and also raising our game regards insulation (basically loft), as DG was upgrade 7yrs ago. But, the deal-breaker might likely be the microbore pipework feeding every rad throughout the property (186m²) and the cost to put this right by someone who knows what they are doing; rather leaving it to ‘sticking plaster’ applied by installers with a borderline skill-set. (… blimey, 6 pumps = 6 problems baked in for the future !). We will not be compromising with pumps, buffers, a partial re-plumb, larger rads, etc.
We still plan on installing PV & Battery as part of an ‘energy mix’, including a diverter (I know, a moot point) for DHW for the c. 6-7 mths per year the CH will not be needed (deduced from decades of usage).
We are also at a point in our vehicles’ life-span where we might well move to one EV; given they now offer a reasonable endurance between charges. At this point in the .gov and energy retailer “bribe-cycle" I see little merit in seriously considering feed-in tarrifs. There will be changes over a timeframe yet to be determined; with AUS and CAN being cases in point. However, the EV industry is maturing: now more ‘leading edge’ than ‘bleeding edge’; that’s only my opinion and likely only of value to me.
Regarding having an ASHP, I sincerely hope we can make it work for our house for several reasons: some selfish, some altruistic. However, having been involved in many aspects of engineering: industrial, civil, and military grade; from Apprentice to C.Eng, and admitting that the most enjoyable times were spent holding tools rather than holding meetings, I have developed a ‘sensitive nose` when it comes to passing the (my) “sniff test".
That said, I might still be dupable with some of the current cohort sadly being “snake oil" peddlers who can be odourless; akin to carbon monoxide and equally toxic. One example being the way that some ASHP manufacturers ‘spec’ their wares.
As ever, caveat empor whilst trying to be ebullient.
Hi @colinc. Your concerns are well-founded. At least you can make a well-informed decision either way. The laws of physics and thermodynamics can’t be bypassed. High mass flow low delta T design is best for ASHP, using a low design temperature. It can work with microbore, at higher design and higher deltaT emitter temperature, but that largely depends on where the microbore manifolds are, what size pipework feeds them, and how much mircobore there is. You may not want the upheaval and expense of changing it out along with a fair few radiators if you want to design for a lower system temperature.
Also to consider is your lifestyle and how you want to heat the property. If you prefer the heating for a few hours in the morning and evening, and unheated during the day, that doesn’t optimise the ‘low and slow’ heating efficiency of a heat pump. Frankly, you’re better off with an over-sized fired boiler to chuck a load of heat into the building quickly. ASHPs can’t do that, and are not sized to do that. They have a small % margin of duty-in-hand over the property design heat loss, vs the typical 200-300% oversizing of most domestic boilers. No surprise when people try to run ASHPs like a boiler the house never properly warms up. Most of the low grade heat provided is lost into the cool internal fabric of the building, which has to warm through along with the internal air volume. The interior fabric of the house becomes a big thermal heat sink. This pre-heat load isn’t calculated in any ASHP sizing computer programme – they assume a steady-state heat mass flow balance. Again, the laws of physics – we ignore them out our peril.
Our conversion from dirty old oil boiler to ASHP coincided with a shift in work-pattern to WFH, so we’ve really appreciated the low and slow all-day winter heating. It’s affordable for us, and it’s great having the whole interior fabric of the building warm. Risk of condensation and mould in wet rooms is eliminated. The old non-condensing oil boiler would have been prohibitively expensive to run all day, and at 26kW with limited modulation, incredibly inefficient for a property with a 9.5kW heat loss. We have leveraged the advantage of also installing solar PV & BESS + a dynamic electricity tariff, which has shifted 90% of our import to low rate and given us a significant energy cost saving compared to oil fired CH and DHW.
It was a leap of faith at the time, and we were ignorant of how to optimise ASHP efficiency. In the early days when all the ASHP commissioning parameters were wrong we questioned our decision. We kept faith, got advice, not least from here, (and now we reciprocate that), and it has been a good investment for us.
I wholeheartedly agree with everything @AllyFish has written, and our journey was similar.
We had microbore piping in our old system. Until we started ripping it all out as part of a full re-plumb, I didn’t know exactly how much. Turned out we had 22mm copper for most of the heating system, branching down to 10mm microbore to feed individual radiators, so in theory this may not have been too bad if the microbore pipes could carry sufficient heat for each individual radiator. I measured the internal diameter of some of the 10mm microbore piping ripped out, and it’s internal diameter was only ~6mm.
The other concern for me was the state of the existing piping and radiators. It had been in place for at least 20 years, maybe longer, on an old oil fired boiler system and had never been flushed or topped up with inhibitor. Flow rates of 5-6L/min for the old system was obviously OK, but using that same heating pipework for a low flow temp system requiring 25L/min flow rates was a complete unknown. I would not want to assume that those high flow rates would be achievable bolting an ASHP onto an existing old dirty system. So we replaced all radiators (no doubt full of sludge) and pipework. I think this is where retrofits are a real risk as large parts of the system design are simply unknown, and I’ve never heard of retrofit installers undertaking extensive system flushes before bolting a new ASHP onto an old emitter system.
Much of our new copper pipework is now on show, fixed to walls just above the skirting board. It looks a little industrial, and I don’t dislike it but I appreciate that may not suit every property or tastes… but I’d much rather see my piping and know it’s good than have issues hidden out of sight. The upshot is we have a modern ASHP based system, with 24L/min flow rates and we run flow temps of 30-35C all year. As @AllyFish says this makes the house far more comfortable and issues of condensation and mould which had affected us for years are hopefully a thing of the past.
@Old_Scientist Just to say that during our retrofit, we had 7 out of 10 radiators replaced and the system was (if I remember correctly), given a full flush before the old radiators were removed and a power flush was performed before the system was commissioned with the 7 new radiators and a dual fuel towel rail added to the ‘armoury’. When the system was ‘serviced’ by Daikin (long story related elsewhere), on all 3 occasions, the engineer remarked on the low resistance of the whole system. Regards, Toodles.