As someone that’s deeply involved in the design and installation of heating systems, I want to share essential advice for consumers considering an air source heat pump. My business partner, Luke, and I run Optimus Heating, and we’ve achieved recognition in the industry, winning Vaillant’s heat pump challenge last year and being one of 10 chosen projects in Viessmann’s Top of the SCOP’s competition. We specialise in creating highly efficient heating systems, often achieving efficiencies over 500%.
This guide is not a pitch for business but a genuine attempt to help consumers navigate the misinformation surrounding heat pump installations. If you’re looking for a system with long-lasting efficiency, low running costs and minimal noise, here are the six red flags to avoid when considering a heat pump installation.
Red Flag 1: Design Flow Temperature
If you’re being sold a heat pump with a flow temperature of 50C, be cautious. A higher flow temperature can limit the efficiency of your heat pump and shorten its lifespan. Some manufacturers offer a 7-year warranty, which is far shorter than the potential lifespan of a heat pump installed with the correct parameters.
The reason for this limited warranty is that manufacturers have no control over how the heat pump is installed, particularly concerning flow temperature. Many mass-market installations are designed for 50C flow temperatures, which can significantly reduce the lifespan of the compressor and increase noise levels. Additionally, for every degree increase in flow temperature, you may experience a 3% loss in efficiency. A heat pump designed with a 35C flow temperature can be up to 45% more efficient than one designed for 50C.
Lowering the flow temperature naturally enhances efficiency, reduces running costs, extends the lifespan of your heat pump and minimises noise. Always question any company promoting flow temperatures above 45C.
Red Flag 2: Rooms Designed at Different Temperatures
CIBSE design guides recommend different temperatures for different rooms, such as 18C for kitchens and 21C for living rooms. However, designing emitters for varying temperatures in adjacent rooms can lead to inefficiencies.
According to thermodynamics, heat will always move from hot to cold areas. This means that the warmer room will lose heat to the cooler room, causing your system to become unbalanced and your heat pump to work harder than necessary. It’s nearly impossible to maintain different temperatures in different rooms unless they’re vacuum-sealed.
Your home should be designed with a single consistent temperature, with a setback temperature for cooler bedrooms overnight. Always question any company that doesn’t use one consistent design temperature across your property.
Red Flag 3: Third Party Controls, Zoning & TRVs
Imagine a simple two-up, two-down house, where each room has a 2.5kW heat loss/load, totalling 10kW for the entire house. If you install a 10kW heat pump and then zone the house into upstairs and downstairs, shutting off the upstairs zone during the day, you’ve essentially halved the water volume in the system. This reduction in water volume can cause your heat pump to cycle on and off, reducing efficiency and potentially shortening its lifespan.
Heat pumps operate most efficiently when they can modulate their power to maintain a steady temperature. However, third party ‘smart’ heating controls often act like on/off switches, which can cause the compressor to cycle inefficiently. These controls were originally designed for combustion heating (boilers) and are not ideal for vapour compression heating (heat pumps).
The best approach is to use weather compensation controls, which allow your heat pump to adjust its power output based on outdoor temperatures, maintaining a consistent indoor temperature without cycling the compressor. Always question any company that suggests using third party or ambient controls.
Red Flag 4: Buffers, Hydronic Separation & Fixed Speed Circulation Pumps
In 99% of the UK housing stock buffers are not necessary. A buffer is essentially a cylinder of water connected to both the heat pump and the central heating system. The use of a buffer with a fixed speed circulation pump and third party controls leads to huge inefficiencies. The circulation pump on the heating side of the buffer will work at a different speed to the circulation pump inside the heat pump causing distortion across the buffer.
Most heat pumps are designed to operate with a temperature differential (ΔT) of 5C between the flow and return. However, when a buffer is used, the differential on either side of the buffer can become different. This means the heat pump may have to work harder to reheat more than 5C. This can result in huge inefficiencies and reduce the life of your heat pump.
Buffers are often used by installers to simplify the installation process, but this can lead to a lazy and inefficient system. If you really need additional system volume to reduce cycling consider a volumiser. This is essentially the same tin can as a buffer but ideally sits in line on the flow pipe from the heat pump. This adds system volume without the need for hydronic separation.
However, its worth nothing that a well designed system at a low flow temperature with the correct sized heat pump, pipework and emitters will have enough system volume that you most likely do not need to install either a buffer or a volumiser.
If your designer/sales person is specifying either of these things ask they to prove why they are needed. Put simply, buffers massively reduce efficiency and make your system more costly to run.
Red Flag 5: Central Heating Pipe Sizing
Proper pipe sizing is crucial for the efficient operation of your heat pump. The designer should always be able to provide calculations to ensure that your house’s pipework is capable of transporting the full power output of the heat pump. If the pipework isn’t large enough, upsizing radiators will be pointless, as the energy won’t be able to reach them efficiently.
A good designer will consider this and prove their design mathematically, leaving nothing to guesswork.
Red Flag 6: Power Output Size of the Heat Pump
The heat pump you choose should be matched to the specific heat loss of your home. If the heat pump is oversized, it will cycle on and off, leading to inefficiencies and potential damage. On the other hand, if it’s undersized, it will overwork and struggle to maintain the desired temperature, especially in colder weather.
As a crude example, if your heat loss is 3.8kW and you are being sold a 6.9kW heat pump, this heat pump is probably oversized and will potentially cycle.
Proper sizing is essential to ensure that the heat pump operates efficiently and lasts as long as possible.
Closing Thoughts
These are the six red flags to watch out for when purchasing a heat pump. Luke and I are passionate about sharing our knowledge to push the industry in the right direction. We want consumers to be empowered to ask the right questions and choose a system that works efficiently, doesn’t cost a fortune to run and lasts a long time.
Recently, I discussed these points on a Facebook thread about a heat pump installed by a large nationwide company. After someone from that company viewed my LinkedIn profile, the post was removed. I don’t think that’s fair to consumers, so I’m sharing this content here, unaffiliated with anyone.
The marketplace is full of tempting offers, clever marketing and glossy adverts, but many big players are selling you a lie. By asking these red flag questions, you can uncover the truth about what you’re being sold. And if you’re still unsure, leave a comment below or on the Renewable Heating Hub Forums and we’ll try to guide you in the right direction.
Also, please feel free to share this information widely and use it to hold salespeople and designers accountable. Remember, buying a cheap system might cost you more in the long run, with higher energy costs and potential failures from overworked heat pumps.
Nice to see an installer being honest and open as to the potential pitfalls that can befall an installation.
I would certainly recommend you to a friend looking to install a heat pump.
How refreshing to hear someone in the industry telling us what everyone needs to hear. The things you are saying have become pretty clear to the minority who join forums like this to discuss these things, exchange experiences and focus on logical explanations, but seem (by repute) so far to have escaped too much of the industry.
Just a couple of matters for clarification/information.
In your bit about buffer tanks you state (as a red flag) ‘Buffers, Hydronic Separation & Fixed Speed Circulation Pumps’. The first two undoubtedly are red flags. My question is about fixed speed circulation pumps (in systems without hydronic separation). Assuming you have avoided the other red flags, does fixed speed actually matter if the pump is set up for about 5C deltaT at the design temp? At higher OATs the deltaT will drop if the pump speed is fixed, but does this actually affect the efficiency of the heat pump? I genuinely don’t know and haven’t actually seen any data either way despite pouring over manufacturer data books. Obviously it’s a minor point but it would be good to see some data to support this (or not).
Obviously everyone’s interpretation could be different, but my understanding is that a red flag is a warning – an indicator – not a rule; if you see a red flag, you pick up on the need to check further, but the circumstances of the red flag aren’t in themselves an absolute blocker. I have no idea if fixed speed pumps are an issue or not but I’m very happy to accept the advice that if they’re included in a design that I should cross-check that design very carefully. I understand you’re just picking up on a statement to do some technical brain-picking and am not suggesting you’re saying anything more than that, btw.
Personally, I would add some more to the red flag list.
Discussions steering the inquirer to the overall cost after subsidies
If the installer keeps trying to steer the inquirer’s questions about cost back to a “when you take off the Government subsidy (BUS/RHI etc), this is what it will cost" and trys to avoid straight cost discussions that make comparisons easy, the inquirer is likely being “sold to" and the installer may well be trying to hide unpalatable details. For sure, the net cost to change is a valid discussion point but it shouldn’t be the only one.
Installer telling the inquirer what they need
As the old adage goes, God gave us two ears and one mouth so we may hear twice as much as we speak. In order for the potential installer to make recommendations they need to know what the customers needs and wants are, and to find that out the installer has to listen. If they start recommending before asking a lot of fact-finding questions, their advice is not fit for purpose. When they do start making recommendations, they ought to be linking those recommendations back to what the customer has been asking for. It doesn’t, for instance, matter if the rule of thumb is to design a house temperature of 20degC if the customer wants to live in a warmer environment than that and if the installer doesn’t check first then they’re not listening. If they don’t listen to that, what else won’t they be listening to and taking into account?
Plans for the long term
Does the installer have long-term relationships with most of their customers? If the installer is looking to foster a long term relationship with the customer they’re far less likely to be installing something that’s difficult to maintain or costly to run.
How dare you. The customer for a heat pump has absolutely no right at all to have an opinion on what they need because the installer is the expert. I’ve seen this stated quite clearly on this forum on more than one occasion and was recently asked ‘would I tell a surgeon what to do?’ when I had the temerity to suggest that my own view as a customer might have some validity. Surely you have learned that by now!
Well that will rule out many, perhaps most, of the current players in the industry which, just as happened solar panels, will disappear as the grants dry up because their business model is essentially grant harvesting not customer service.
You will of course appreciate that these comments are tongue in cheek, but at the same time have an element or seriousness!
You are correct, I asked the question for that reason alone. Its a statement I have seen before, but I have also seen that statement that 5C delta T isn’t critical. Since both cant be true (or can only be true if they refer to different circumstances) I’m genuinely interested in some fact/technical explanation why heat pump efficiency might degrade if delta T differs materially from 5C, most particularly if its less.
😆 😆
Having had the need many times throughout my life to deal with the medical community on behalf of family members, that’s a very interesting question.
Would I tell a surgeon what to do? No.
Would I tell him or her how to do it? No.
Would I tell him or her what we needed to achieve? Yes, absolutely.
Have I ever done any of the above? No, because every surgeon I have ever dealt with has been guided by the patient’s wishes first and started off by asking questions.
As a result, this hypothetical surgeon’s position is no different from that of a heat pump installer; the customer (patient) decides what should be done, the expert (surgeon or installer) fleshes out the detail and decides how to tackle it, the customer has the plan explained to them and gives the go-ahead (signing the contract or the consent form) and the expert then carries out the agreed plan.
Any “expert" trying to distance the customer from what they intend to do (and charge for) is, in effect, a surgeon playing God with a patient’s life without the patient’s consent. How quickly do you think it would take for the General Medical Council to deal with that high-handed behaviour? On the other hand, any expert accepting a customer telling them how to do their job is absolving themselves of their moral and sometimes legal duty which is equally unethical.
My point precisely, put rather more concisely and probably more clearly.
My answered isn’t the same. And here is an example.
I had a sore ear lobe, and an area that would bleed if I picked it. It appeared in April. Went to the doctor, he sent me to a clinical specialist/surgeon, he said not sure what it is, but what we’ll do is take a section out of your ear and do a biopsy, when it’s healed it’ll look ok. This was in September and the surgery booked for December. Come November the area had healed up, so cancelled the surgery. The next April came along, same thing. My mother said put sun block on your ear, and wear a hat, which I thought was worth a ago. Issue went away a few weeks later and has never returned. Always wear a hat always put sun block on my ear. Now have a similar area on my cheek, so do the same with sun block.
So I don’t do everything the medical professionals say.
The rooms at a consistent temperature is interesting and makes perfect sense because with our existing gas setup the hallway is colder than the living room and every time the door between the two is opened the living room cools and anyone sat near the door gets a chill.
The ASHP design we are going ahead with did as you said and has a consistent temperature but other quotes we had didn’t and rigidly stuck to 21, 18, 22 etc..
We’re looking forward to our installation in the coming weeks.
@Project_Electrify Even though MCS guidelines ‘suggest’ these different temperatures – they are merely that, suggestions and furthermore, they are not living in your house nor are they likely to be paying your bills!
Once the installers have commissioned the system, it is down to you – and this is where the interesting bit starts! Each of the emitters (radiators) is furnished with a lock shield valve which you employ to control the flow thus the heat in each room. Please see:
https://renewableheatinghub.co.uk/how-to-balance-radiators-the-role-of-the-lockshield-valve
for my method. Regards, Toodles.
Interesting that none of my two surveys asked me what room temps I would like/prefer and as for design temp (you get what we offer)! However they were both from who energy suppliers fully expect to get their money back long term. It seems like “we will put in what we want and that is it". Customer? or Victim! One more resurvey and one by an actual installer (I hope). Sadly I expect his price may be out of my budget. Sometimes I wonder why I am bothering.
@Jancold I changed things a bit with 🐙 by insisting rooms had other functions – spare bedroom = office.
I’ve accepted their install won’t be perfect (as in for my needs) but they are so much cheaper it is all I can afford.
Hopefully, over time, it is tweakable one way or another.
I think this is the right way to do things in the current market. Tell them what thet need to hear so you get what you want. It seems to me you are getting a good deal.
I get that. I just did DIY.
It’s almost as if they, the installer, are doing you a favour, somehow they believe the money is theirs, not a tax payer contribution. They are looking for you to pay a bulk of the costs, so really need to minimise what they ‘give’ you otherwise they dilute how much of the £7500 they keep themselves. And somehow they are are getting away with it – criminal
Interesting thoughts Simon. One thing that troubles me about the recommendation to avoid using TRVs is that the user then has little control over individual room temperatures. How would you suggest the occupant makes those kinds of fine tuning adjustments?
@Ian McKay Assuming you don’t require dynamic temperature changes and are happy with a constant temperature in any one room, please carry out the control of temperature using the lock shield valve at the opposite end of the radiator from the TRV (which should be fully open, then the lock shield valve will control the flow.) If you wish to follow my routine, please read:
https://renewableheatinghub.co.uk/how-to-balance-radiators-the-role-of-the-lockshield-valve
Regards, Toodles.
As @Toodles says above. Basically you don’t except as a one off when the system is commissioned (or over time) by balancing the radiators using the lockshield valves.
Low temperature heating, whether by gas, oil, or heat pump, is a different design and control philosophy to the high temperature heating we are used to. The basic idea is to dispense with most controls and run the ‘boiler’ (gas, oil or heat pump) more or less constantly at the lowest possible temperature so that the heat loss from the house is just matched by the heat emitted from the radiators. This gives both the maximum efficiency and the maximum comfort (the latter principally because temperature gradients are minimised). The efficiency gain is more with heat pumps than with fossil fuel boilers, which is why it is emphasised in heat pump installations, but is present even with fossil fuel boilers. The supposed gains from micro-controls, much touted by the manufacturers, are largely wholly illusory and certainly well over stated.
You may need to read a bit more fully to understand this as there are a few interacting factors some of which are counter intuitive. It boils down to keeping most emitters open all of the time gives you a nice constant temperature with minimal temperature gradients in the room (hence maximum comfort) for the least expenditure of energy. The reason that this is the most efficient boils down to the fact that the heat loss from the house is determined by the insulation between the house and the outside walls and insulation between rooms is negligible. The upshot of this is that the amount of heat loss (from the house) saved by zoning a room or an area is quite small, and the reduction is more than offset (in many cases) by the loss in efficiency due to the reduced emitter area which means that the flow temperature must be increased to compensate, resulting in a higher energy consumption (which is what you pay for) even though the loss from the house is less. Heat Geek has some good videos on why not to zone (and when zoning might be a good idea), with some worked examples to prove the point.
Most of us should really be running our gas boilers this way too, but unfortunately the UK (unlike some other countries) didn’t mandate weather compensation, so it wasn’t applied even though most boilers support it because the industry in Europe is centred on Germany which does mandate it. As a consequence most of us are probably paying 10% more for our heating than we need to and have been suckered by the control industry into buying way more ‘bits’ than we really need to.
Hi I am thinking of having a heat pump fitted the installation company is MPC Titan Business Park 1 Aurora Avenue Queens Quay Glasgow.
I currently have oil-fired heating and is a 25 KWH the house is a 3-bed detached bungalow with a solid 9-inch thick wall what size pump would be required and can they use the existing pie work for the radiators they will replace the radiators
I consume 1800 liters per year
1 do you have any experience with MPC
2 would it be cheaper to run a heat pump
3 the house is a constant 20c
any advice would be appreciated thank you, Ian.