One word of advice. All heat pumps pause to run defrost cycles at outdoor temperatures below about 3C which reduces their overall output more than the -2C specification might indicate, so make sure that your heat pump is rated to more than your house requires. For instance my house demand is 5.5kW but I find that my 7kW heat pump just covers it at sub-zero temperatures.

I had the sense that, in the SE, defrost would not run often. But if defrost starts around 3C, this may be more of a factor?

That word of advice isn't entirely accurate.

The heat pump will run a defrost cycle whenever it starts icing up. That is certainly likely at outside temperatures approaching or going below zero but it's also perfectly possible at higher outside temperatures too. More relevant is how moist the outside air is, since there'll be relatively little icing up at (for instance) -5 if there's almost no humidity but probably quite a lot of icing up on a misty day at higher temperatures with lots of water in the air.

The South East is, as you know, comparatively warm and comparatively moist, so heat pumps will definitely go through their defrost cycles quite regularly in the cold months. I have certainly seen my heat pump go through defrost cycles at OAT greater than 3degC and seen plenty of times where there's been almost no defrosting at subzero temperatures. In other words, it's nuanced.

Many manufacturers sell only one or two models of heat pump, then add software limits to make intermediate power models. The result is, if you buy a "5kW" heat pump which is actually a 7kW machine limited to 5kW, then the lower end of the modulation range will be exactly the same. Only the maximum power is different. When we say "don't oversize the heat pump" we're mostly worried about it cycling a lot because the lowest possible power at the low end of the modulation range is too high. But if the two machines you're hesitating between are actually the same, then... why not pick the bigger one? That will extend the modulation range upwards, but the minimum power will be the same. That may still cause problems if there's not enough flow, though.

On the other hand, if the manufacturer makes 3-4-5kW machines from the same chassis, and 7-9-12kW from another chassis, then the 5 and 12kW models have full modulation range allowed by the hardware, but the limited models have the low end of the modulation range from the bigger heat pump!

I'm not sure how Vaillant splits their models, you should check.

I doubt it, but would be really interested to find out.

For some actual data I recommend reading the NESTA survey from 2023, this is the largest and most comprehensive independent survey I know of about 2000 heat pump owners and about 1000 boiler owners 

The broad brush answer to the question is about 80% in both cases, but the heat pump owners are more polarised than the boiler owners.  Of course dissatisfied heat pump owners are more likely to blame it on 'new' technology whereas dissatisfied boiler owners will more likely blame it on the plumber.

For me this tells us more about the heating industry that it tells us about either boilers or heat pumps.

Also for me 80% is too low whatever the heating technology, although that itself begs the question whether expectations are realistic, a question to which I doubt we know the answer!  My suspicion, for which I have only very limited evidence principally from this forum, is that the dissatisfied are a mixture of people whose installer has done a poor job, and people who are unrealistic in their expectations. Principally the former, but definitely a sprinkling of the latter.  Heating seems simple but is actually complex and individual to people and houses.  Perhaps it's unrealistic, with current technology, to expect it 'just to work' in a way that suits _you_ without learning a bit about it, whether it's gas, oil, resistance electric heat pump, infra red or something else.

All in all, I think anecdotal evidence and interpreting it to form a "gut feel" is about as good as you're going to get.

Absolutely agree.

We are looking to have one installed so have gone beyond what we read on in forums, talking to people living nearby that had them installed. 

Whether "someone is happy or not" is a question that leads to essentially blending different sets of people. Some are happy as it is financially working, others because their fossil fuel system was not working well and it is now fixed, others becase the doomviews did not materialise and their heatpump is keeping the house warm, others because the new system did not cost them much more than fitting a new boiler.. I even found some that loved that this pushed them into installing better looking radiators! And of course, most like the fact that keeping their house warm is causing a lot less emissions. 

I tend to see it as: what is valuable in the data for a specific prospective heatpump user? Assuming they really know what would make them happy.. And how much is their happiness dependent on the few days the work will take part of it vs the years while it will be working..

My particular concern is to make sure we are well equipped to enable it to work financially long term, despite electricity being 2-4 times higher than gas. My view is our policy makers are not sufficiently bold in that respect and this makes people trying to make a rational decision hesitate. 

The water pump is the circulation pump for ASHP, just located inside not outside.

We pushed for a system without a buffer as per advice of many in this forum. The Arotherm fits this and with a heatloss of 5.1, the 5KW version was tempting. But I am hearing that some leeway is needed, like this nearby heatpump user shared: 

One word of advice. All heat pumps pause to run defrost cycles at outdoor temperatures below about 3C which reduces their overall output more than the -2C specification might indicate, so make sure that your heat pump is rated to more than your house requires. For instance my house demand is 5.5kW but I find that my 7kW heat pump just covers it at sub-zero temperatures.

I had the sense that, in the SE, defrost would not run often. But if defrost starts around 3C, this may be more of a factor?

Social media definitely distorts perception. The happiest owners become advocates, the angriest become campaigners and the huge middle just gets on with life. But as I’ve been working through a very large amount of homeowner data recently, one thing that keeps jumping out at me is how many systems appear to sit in this strange average category.

The house is warm enough. The bills aren’t outrageous. Nothing is catastrophically wrong. So the homeowner assumes the system is good.

But average isn’t necessarily good.

I’ve seen plenty of systems where the owner believes everything is working properly because they’ve adapted to it. Then you look underneath and find high flow temperatures, weather compensation disabled, excessive cycling, poor controls strategy, badly configured pumps, hydraulic separation issues, immersion heaters doing heavy lifting or systems running far less efficiently than they should be.

The homeowner often has no frame of reference because they’ve never lived with a properly commissioned heat pump before. So functional early becomes interpreted as good/excellent.

And to be fair, excellent systems absolutely do exist. When they’re designed properly, commissioned properly and controlled properly, they can be superb. But heat pumps are far less forgiving than boilers as we always hammer on about. A mediocre boiler install can still broadly work. A mediocre heat pump install often leaves performance on the table without the homeowner necessarily realising it.

The survey point is interesting too. Timing matters massively. Ask somebody in April, a few weeks after an initial install, and you’ll often get a very different answer compared to after two winters, multiple defrost cycles, servicing costs and a proper cold snap.

One thing I can say from looking through years of homeowner discussion is that the volume of troubleshooting and complaint discussions has risen enormously alongside deployment growth. Even normalising against forum growth, installer and performance-related complaints increased roughly 24-fold between 2021 and 2025. That doesn’t automatically mean "most systems are bad", but it does strongly suggest the industry still has a consistency problem around design, commissioning and aftercare.

Personally, I think the better question isn’t "Are most owners happy?" it’s: "How many systems are actually operating close to their potential?" I suspect that number is considerably lower than the broad satisfaction surveys imply.

Firstly, @deltona, you're right about the power of general discussion - social media in particular - to introduce assumption in a way that they are then accepted as fact. A warped portrayal of reality either way. I should point out, though, that in order to try to get past those assumptions and subjective comments, one needs to be pretty scrupulous in not introducing one's own assumptions. A case in point is

...

I'm not interested in your opinion of your system, you form an extreme minority.

...

That's also an assumption. I strongly suspect the assumption is correct, but it's an assumption nonetheless. I mention this to highlight the pitfalls, especially since quite a bit of your language in the post implies your own personal assumptions still being given pretty free rein. There's no law against it, but it could make it more difficult for you to get a solid answer to your question rather than the answer you want to hear (or at least the answer others think you want to hear).

Now to your underlying question. When we got our heat pump, we didn't know anyone else with one. However, several friends and neighbours were very interested and so once ours was installed we found quite a few people following how things progressed and, since the install, we've had two who've gone on to get heat pumps themselves. They've listened to our comments about picking a capable installer and, although neither ended up using the company we used, they have both had successful installs at sensible prices (after the grant, approximately the cost of an expensive boiler swap). Both are happy enough to be starting to evangelise themselves. One, I might add, was a first time house buyer and subsequently new parent who can most certainly not afford the high running costs of a botched install, so that gives some valuable context.

We also found, after our installation, that one of our neighbours was running an air to air system that's now about 15 years old. He was happy enough with it that he recommended the concept to another neighbour who has since moved away. That latter was considerably less than happy with their A2A installation, and now we've dug in a bit we find the design in that latter installation was not good and so doomed from the start. The 15-year-old first A2A installation, however, is still eminently practical, reliable and economical, so it's not the fault of the technology.

We've had several other friends and neighbours asking quite regularly about our experiences, so there's plenty of interest generally. The question of "can it still manage to heat the house in freezing weather" is one that keeps coming up, as is the question about noise, so the misconceptions you were talking about on social media are pretty persistent. However, the skepticism around our estimates of savings has evaporated with the independent experiences of those who've followed our lead and picked good installers, since their savings are mirroring ours.

In terms of the wider public, I think finding out a true representation of satisfaction will be hugely challenging because of the different groups of people involved. There will be plenty who simply don't know enough to judge; some will be happy with a poorly performing system because they don't know better, some will have unrealistic expectations, some will have a perfectly good system that they're running inappropriately etc. The survey you mentioned will also only have been filled out by the subset of the population who are predisposed to answer surveys, and in the widest sense there's also the truism that we only hear about the extremes - those who're in enough trouble to start complaining and those who're so satisfied as to want to tell the world.

All in all, I think anecdotal evidence and interpreting it to form a "gut feel" is about as good as you're going to get.

is it one of those at the bottom ? Sorry I edited the pic but it went dark 

RH (Relative humidity) is the % of moisture in the air relative to the maximum possible moisture content, which depends on temperature.

Here's a psychrometric chart, it's about absolute/relative humidity and temperature.

Click on the point 5°C, 90%RH ("humid" winter air), you get about 5 g/kg of moisture. Now bring that air into your house and heat it to 20°C. That doesn't change the absolute humidity, so follow the horizontal arrow. You get 20°C, 35%RH air, in other words pretty dry. 

So humid outside air (ie, high RH) in winter is only a problem if the outside temperature is above 15°C.

Now how to size for cooling...

First take your heat loss, suppose you got 10kW at 20°C deltaT (indoors 20°C, outdoors 0°C) and express it in kW/°C of indoors/outdoors deltaT. However, the outdoors temperature is not necessarily the air temperature: if a the outside surface of a sun baked south facing wall reaches 50°C, then heat leaking in through this wall must be calculated with a deltaT from an outdoors temperature of 50°C. Likewise roof tile temperature will probably reach 60°C so heat gain through the loft insulation must be calculated according to temperatrue inside the loft, which may (or not) be much higher than outside air temp.

Add solar heat gain for each window. This depends on the angle between the plane of the window and the direction of sun rays. It can add up very quicky to kilowatts especially for south/west facing windows so you need to consider louvred shutters or other means of keeping heat from the sun outside. Stuff like curtains etc, being inside, catch the sun rays and turn them into heat inside the house so do nothing to keep the heat out.

Now in summer, take 35°C, 60%RH outside air, bring it into the house and cool it to 25°C: follow the back arrow.

You get 25°C, 100%RH air, so sweat doesn't evaporate, even with a fan blowing it's uncomfortable, it feels like a swamp. That's what you get with a cooling system that can't dehumidify.

Now if you turn on the fan coils and drop RH to 50-60% by condensing water out of the air (red arrow), even if the temperature is exactly the same 25°C, it'll be much more comfortable.

However to achieve this each cubic meter of air (1.3 kg) needs have about 12 grams of water removed.

Water latent heat of vaporization is 2260 kJ/kg or 2.26 kJ/g so the heat pump needs to suck 27kJ of heat for each m3 of outside air that needs to be dehumidified.

For 200m3/h of air, this adds 1.5kW to the cooling load. So unintentional air leaks and air lost through ventilation and replaced by warm humid outside air must be considered.

Then you get your total cooling load. Depending on variables it may be similar or even higher to the heating load, in this case you may need a heat pump that is oversized for heating but adequate for cooling, or maybe the opposite.

Another consequence is that the flow temperature must be below the dew point to allow your fan coils to dehumidify the air, which means as said above all the pipes will be cold enough to attract condensation and must be vapor tight insulated, which is a meticulous finicky job with zero tolerance for error.

Multisplits don't have this issue because heat is transported as latent heat. If your main load is cooling, consider multisplit instead of hydronic. In fact I am surprised you're planning hydronic with fan coils only. Usually the reason to go with hydronic is an existing radiator system or radiant floors, then a few fan coils added for cooling. However if you design the system from scratch with fan coils only it would make more sense to use a multisplit...