ReedRichards

Possibly not, but if you have been running cold water into a bucket for long enough to fill the bucket and start on a second one then, in order not to freeze your hand, you only test the water temperature every few seconds. Whatever happened, the amount of time in which the tap may have run warm rather than hot was insignificantly short compared to the amount of time it ran cold. I was so traumatised with my old bad plumbing that I installed a secondary return loop for my hot water, even thought the pipe runs were shorter from the new hot water cylinder.

I used to have a Combi boiler and a very long pipe run. There used to be a long wait to get hot water with enough flow to fill at least one bucket of water but when the hot water came it was immediately hot, not tepid and warming up slowly. So I don't think there was significant mixing in the pipes nor time for the cold pipes to significantly cool the hot water. Therefore I don't think @JohnMois correct in his account of the sequence of events, I don't think cold pipes and possible lack of insulation will kick-in significantly until you turn the tap off, they will determine how rapidly the water that is then trapped in the pipes will lose heat. If you had a thermal imaging camera you could look at the pipes inside a wall and see how rapidly they heat-up when hot water flows through them. The faster you see them get hot, the less insulation they have.

In which case you should be able to achieve hot and cold water at similar pressures.

Do you actually have an unvented hot water cylinder (in which case there should be a pressure vessel nearby) or is it vented (in which case there will be a header tank in you loft and not so much hot water pressure)?

That's absolutely correct. The ASHP heats water in a cylinder and has no influence whatsoever on the water pressure (the same would be true however your cylinder is heated). If your cylinder is unvented then your hot water pressure should be set to something approaching the pressure of your mains cold water. However if you have a narrow (15 mm) pipe from your rising main to your hot water cylinder that could restrict the hot water pressure.

I had an oil boiler before I had a heat pump. But I don't know how efficient it was at converting oil to heat and the actual amount of heat required to keep my house at temperature will vary from one year to the next. But making some estimates, I appear to achieve an SCOP of approximately 3, which is approximately what I was promised. That's for heating and hot water.

22 mm copper pipe has an internal diameter of 20.2 mm. Therefore a 1 m length of pipe holds 1.28 litres of water (if I have done my maths correctly). Suppose your shower has a low flow rate of 10 l per minute of hot water. If it takes 2 minutes to get hot water then your pipes must hold 20 l so the length of your pipe run is 15.6 mm. That's very long but it's not impossible if your pipes don't follow a direct route. If your hot water flow rate is higher then the length of the pipe run would be lower. This is a plumbing issue and nothing to do with your heat pump. If by "return pipe" you mean a hot water secondary return then that is by no means a standard plumbing feature unless you're a hotel. If your hot water pipes are really 22 mm in diameter that is good for filling a bath quickly and bad for any other requirement. I think 28 mm pipes are recommended between the heat pump and the cylinder (or is it the buffer tank) but not elsewhere. If your hot water pipes were 28 mm then your problem would be even worse.

I know short cycling is held to be bad, but 35 minutes on 20 minutes off seems like a particularly long cycle where the cooling of your rooms during the 20 minutes off might be noticeable. Do you actually feel a difference after the adjustment in terms of a more even room temperature?

But that's not the way a TRV operates, at least not a "dumb" one. My understanding is that they are much more analogue than digital so they reduce the flow as they reach their set temperature and open up a bit if they drop below their set temperature. If they keep open just enough to maintain their set temperature then they are doing exactly what a manually throttled radiator would do, maintaining just enough flow to give you the room temperature you want.

What's the minimum bend radius?

Yes, most certainly it's modulating but the modulations appear to be quantised. They go up in steps rather than across the full range of minimum to maximum, although these steps seem to be reached by slowly increasing the power over a few minutes.

My OH would love to find a local swimming pool where the water is 27 C. They are supposed to be at 28.9 C but often warmer, which my OH finds too hot for swimming (although fine for those who just like to stand in the pol and chat).

That's excellent but a lot of heat pumps, mine included, cannot do "temperature ranges"; I'm not even sure what that means. My heat pump has a single programmed weather compensation "curve" (actually a straight line). There is a control feature that allows me to shift this up or down by + or - 5 degrees in one degree steps but this is a manual control feature which cannot be programmed. So if you go with @JohnMo's recommendation, make sure you have a heat pump that is sophisticated enough to do give you the control you actually want. I have two zones, one for the bathrooms and one for the rest of the house. I did this so I can heat the towel rails in summer in order to dry the towels. Each zone has a thermostat; I have only radiators. All the radiators have TRVs but only a few rooms, like the bedroom, are set to less than the max setting. I use a night time set back and gradually increase the main zone temperature during the day; something I have been doing since I got my first programmable room thermostat in 1998. I keep reading that having more than one zone is "bad" for efficiency but I have yet to convince myself that this is true for my particular heat pump. As best as I can tell it has a series of stepped target output powers. The step it aims for is higher the colder it is outside. It ramps itself up to that step and holds the power to see what happens. When the house is at temperature, or near to, it holds that power until (I imagine) the return water gets too hot and which point it stops and starts again 20 minutes after the start of the previous cycle. So it runs in 20 minute cycles nearly all the time except when it is working hard and the cycles coalesce. Given this actual mode of operation I find it hard to convince myself that having two zones is bad for efficiency.

That's if you want your rooms at a constant temperature 24/7 and you are prepared to spend as much time as it takes adjusting flow rates to achieve this.

But worth it, in my opinion. For a while I had the discharge pipe from my cylinder running along the skirting board to an external wall. Then we revamped the bathroom and I could run it under the floor to an internal drain pipe. The tundish incorporates a non-return valve so you don't get drain smells.

Why did you opt for a thermal store? It seems like a good way to minimise your heat pump efficiency. Normally you improve your efficiency by running your space heating at lower water temperatures when the outside temperatures are warmer but it seems to me that having a thermal store will prevent you from doing that.

Not necessarily a bad decision. At current fuel prices, a properly installed heating system using a heat pump should be marginally cheaper to run than a gas boiler (assuming you achieve an SCOP of 3). And it's very green. But if you thought your solar panels would generate enough electricity in winter to run the heat pump then you made a bad miscalculation. And if you tried to install a heat pump as a drop-in replacement for a gas boiler and you did not change your radiators that most likely explains why the radiators no longer do their job.

Surely there may or may not be an efficiency penalty? If a smaller unit can modulate down to lower output powers than a larger unit then yes, there will be an efficiency penalty with the larger unit. But if minimum power is the same either way then I don't see anything to cause an efficiency penalty.

In my old house there was a flat roof above the living room extension which we made accessible by substituting a door for a long window. We then sought permission to put a balustrade around it but this was refused, essentially on safety grounds. The rationale was that with a balustrade in place we might then be tempted to throw a party (or some such) and fill the flat roof with a large quantity of people. The extension had not been constructed to bear this amount of weight. The absence of a balustrade would limit the numbers ever likely to be on the roof and therefore be, on balance, safer. So we put plants in pots around most of the edges and just took care not to fall off.

How will you get it up there?

You really ought to be able to control when the unit comes on and therefore to make sure that it does not come on at night during summer. You can certainly do that with my LG heat pump and I would not consider a unit that "does it's own thing" and cannot be controlled in this way. If you can find a copy of the manual it should tell you how to program the timings.

Unfortunately this means that your tanks are outside the heated fabric of the building. So they are exposed to cold air and the heat they lose is wasted. You can buy high tog duvets fairly cheaply, I wonder if it would be worth sewing one or two together to make an insulated cover for your tanks? I have this issue with my heat pump system, although it's not nearly as bad as in your case. Heat pumps typically target a small temperature differential around the heating system so that requires much higher flow rates than you would use with a gas or an oil boiler. If you reduce the flow rate it will make your heat emitters less efficient but It would not necessarily reduce the overall efficiency (I think). In my case I'm not sure if the noise originates from the pump and is carried along the pipes or if it really is flow noise. Could you try filling your wall with sound-proofing insulation, if you have not already done so?

My buffer tank has no temperature probes nor any other form of electrical connection. I'm not aware of any setting for the heat pump that can tell it that it has a buffer tank, let alone report the temperature if a probe was present. Occasionally I see a power blip in the middle of the night which could be the heat pump circulating water to check that the return water is not too cold; that's the only means it has of getting some idea of the buffer temperature. I suppose the buffer tank guarantees that I don't void the warranty by operating the heat pump with less than the specified minimum volume of water (which is 50 l). And it provides a reservoir of hot water with which the heat pump can defrost itself as necessary. My main problem with the buffer tank is that it adds an audible internal pump whereas the external oil boiler I replaced kept the central heating completely silent.

Just on a point of information, that's not how my heating system works. When there is a call for heat the heating pump turns on. About a minute later the ASHP internal pump turns on (or there is a delay in this being reported on the controller). After another couple of minutes the compressor is reported as turning on. As soon as the compressor turns off, all pumps cease operation. This does sometimes happen if there is a room thermostat calling for heat but because (I presume) the return water temperature gets too high. I have a third party controller which restricts the number of cycles to 3 per hour. It doesn't know I have a buffer tank so doesn't know that it would be okay to leave the heating pump running. So it doesn't. If a thermostat is still calling for heat the sequence repeats 20 minutes after the start of the previous cycle.

Well you think the Sunamp is being reheated by the ASHP but the Sunamp thinks it has been waiting for heat to the point where it give up and takes matters into its own hands. So perhaps there has been a valve failure so heat from the ASHP doesn't reach the Sunamp or a blockage so heat doesn't get there fast enough? Or there's a problem with the Sunamp. Could a temperature sensor have failed? I heard a rumour that some units have an internal pump; could that (if it really exists) have failed?