ReedRichards

50oC means that you stand more chance of fitting the new radiators where the old ones had been. You might be able to pay a bit extra to "oversize" the radiators to allow a lower operating temperature. It depends how flexible the Octopus fitters are with regard to paid-for deviations from their standard spec.

The first thing my heat pump did, about a day after it was installed, was to come to a complete stop with an error message. It turned out this was due to a "blocked" filter causing insufficient flow and was easily rectified by the installer. But given the sensitivity to flow, I worry that an auto-bypass valve might cause the flow rate error before it was able to operate. Also, does the flow reverse for defrosting, or is that just my fanciful imagination? The central heating pump causes enough pipe and radiator noise that I would not want it running when it was not necessary to do so. I'm not aware that I can program a run-on but I'll look into that. But is it desirable? My heat pump is limited to 20 minute cycles and if it goes off after 15 minutes, as it often does, do I really benefit from circulating water round the heating system rather than leaving it to stand?

One thing that my present setup does not achieve is circulation through the heating system when the heat pump is off. There is a short period of a few minutes when both the water pumps are on whilst the heat pump is not active. But as soon as the heat pump stops pumping heat then the water pump for the central heating goes off. I would have to strip off some insulation to measure the pipe temperatures to and from the buffer; I haven't got around to doing that so I don't know if I get mixing inside the buffer or not. Even then, I have only the one thermometer and measuring a temperature differential is quite difficult because temperatures very rarely hold constant. Also, I could not use the existing tank alone as a volumiser because the water pump inside the ASHP would have to pump water through the stopped central heating water pump in order to defrost.

My buffer tank has an immersion heater fitted, although it has never been connected. It's also integrated with (underneath) the hot water cylinder which makes it short and squat and probably the opposite of the optimal configuration. I wonder if I could add a tall thin buffer, bypass two ports on my existing buffer and just use it as a volumiser?

Do you have room for a second cylinder in series with the one you already have? The one cylinder would act to "warm up" the cold feed for the second cylinder. I don't see that Octopus could object to that and that way you could keep what you've got.

Yes, I understand that bit. But me giving to next door means that the wind turbines a mile away don't need to do that, so they end up sending electricity further away than would otherwise be necessary. So although my electricity goes nest door, the consequence is to send the quantity of electricity that I generated off somewhere further away.

Is it really as simple as that on a local level? From my house I can glimpse the tops of two wind turbines, part of a set of 10 (I think) nearby. So when the wind blows,which is most of the time, then my locale as a whole (which is not densely populated) will be exporting electricity so the main issue will be the transmission losses before that electricity gets somewhere big enough to use it all. If I export my solar PV electricity I'm just adding to the electricity that loses power in transit. Wouldn't it be better using it locally, in my house?

I'm paid on the assumption that I export half of what I generate. I don't know how much I actually do export but it's still quite a fair percentage of my summer generation. I don't see that this is "morally bankrupt"; what's the morally profitable course of action in my circumstances? I have a 12 kW heat pump that draws up to 6 kW when heating the hot water, that's half power. That would seem much the same as a 7 kW heat pump heating the water on half power, 3.6 kW except that it will take twice as long.

I have only radiators and I run them with Weather Compensation; it works fine. Since radiators generally require hotter water in cold outdoor temperatures than you use for UFH, then you probably derive more cost benefit from using WC with radiators than you do with UFH.

I disagree too. My immersion draws 3 kW and my solar PVs are well capable of providing that (except in winter). My heat pump draws up to 6 kW when heating the hot water and that would require some import. Since I am paid for deemed rather than actual export then I can either heat my water for free using the immersion heater or at some cost using the heat pump.

55 degrees is scalding hot, you would get third degree burns in 10 seconds. Do you perhaps have a blending valve on the outlet from your hot water cylinder?

@JohnMo always says this and I always respond that neither 24/7 operation nor limited use of thermostats is a prerequisite of Weather Compensation. If you want to take WC to the limit of peak efficiency then, probably, yes. But I don't have 24/7 operation and I use two zones with two thermostats and I'm still sure that WC saves me money over using a fixed water temperature.

Weather Compensation makes the Leaving Water Temperature less warm when it is warmer outside. With your settings the LWT will be 42oC if the outside temperature is -4oC or less. Given that we have a cold spell at the moment Weather Compensation will set your LWT to about 42oC. If that is what you use without Weather Compensation then it will make no difference at all for the present. I find it hard to see how it could ever change the balance of temperatures around your house. Can you measure the flow and return temperatures across your radiators? Given that you have a buffer tank with an unusually large volume of water I would have thought the buffer tank temperature must approach the LWT in which case I would expect that the temperature of the water entering your radiators should be close to the LWT and the water leaving your radiators should be a maximum of 5o less than the input water. If your house is of timber-framed construction then there will be a small cavity between the insulated timber frame and the exterior cladding. This cavity is, I believe, open to the outside air. If so, then adding exterior wall insulation will make little if any difference to the effective insulation of your house.

So that would mean one degree down in LWT for every degree up above -4oC. So 15oC is 19 degrees above -4, 42-19 = 23oC so yes, I agree.

Wouldn't the best thing to do be to start with a standard tariff then experiment with different patterns of usage to see if they keep you warm enough? Monitor your usage and if it looks like you could make use of an Economy 7 tariff then you can switch.

Doesn't "a thin layer of screed" indicate that @Rupe30is not trying to heat a giant concrete slab but actually quite a small one?

They wouldn't @JohnMo. Weeks from now my comment would have become unintelligible and potentially misleading had you not responded. But just at the moment the two most recent threads in "Air Source Hat Pumps (ASHP)" both mention Midea, which is an usual state of affairs. Anyone considering purchasing a Midea heat pump is surely going to read both of them, not just one in isolation. So much for topicality.

...which wasn't meant to be taken seriously.

But it makes the lights flicker.

I don't find Sunamp to be very forthcoming with a lot of the technical details of their product. One is forced to make assumptions. 😞

So I wasn't completely wrong; it's just that Sunamp (apparently) did not pursue the development of the 43oC PCM unit that would have been much more suitable for use with a heat pump.

I can't see any reason why a Sunamp wouldn't work equally well. Sunamps are possibly a bit more temperamental in their operation than a tank of water; just an impression I have formed without ever having actually used one. E.g. I think it may be possible to get a Sunamp with a different lower-temperature phase change material that is better suited to a heat pump. If you operate your ASHP at higher temperatures it just increases your running costs so it's really not a good option. If you have to run your radiators at higher temperatures then you lose all the economic benefits of having UFH for as long as they are on, possibly for all the time if your heat pump is not smart enough. Yes, that's just paranoia. The comparison must have been between the Sunamp and a tank of water used as a thermal store. Even then I don't see why it would be true [Edit Maybe I can. A hot water thermal store cools as it supplies heat, a PCM used to store the same amount of heat supplies most of it at the phase change temperature]. But I [really] don't see that you can go wrong with a hot water cylinder that supplies hot water directly provided you can find the space to fit one. You'll want to store your hot water at 50 C or less so you need a larger volume of storage than if you were using a higher storage temperature. Or you could even combine a Sunamp with a smaller volume storage cylinder using the Sunamp to preheat the cold water feed to the cylinder.

If you have radiators it's simplest if you can run them at the same low water temperature as your UFH. This means they either need to have a really large surface area or instead you use UFH on both/all floors of your house. I don't understand why you think a heat battery gives you "better on demand" hot water than a hot water tank. Can you explain your "fears"?

Switch off the heat pump and turn on the immersion heater. Once the tank is heated with the immersion heater then see if the hot water still takes a long time to come out of the taps. If it does then it's manifestly not a problem with the heat pump. But do get an independent assessor who can explain to you how things have been done. Most people on this forum would have taken more of an interest when things were being installed.

Hi @Marvin I fixed my problem; it's @Wozewho has the issues now. They seem to have been badly let-down by their plumber who has installed or perpetuated some very odd pipework.