Brian Paul

I made an initial enquiry to the DNO (Western Power) They came back very quickly ...and said - where available - all new connections are now 3 phase - and would be in this case. I was advised that they would allow 11kW export over 3 phases with no additional restrictions. It does change the thinking a bit but welcome news anyway. Many thanks to NickfromWales for this advice

Thanks to PeterW, ProDave and SteamyTea for these comments. Perhaps a more conventional arrangement would be better - now looking at 2 tanks again. ProDave : 4kW or 8kW? I agree that it would be difficult to use 8kW easily in the summer (possible production up to 80kWH per day) but I do plan to provide at least 1 charger for EV - 2 if it seems justified. If the network supplier limits output to 3.6kW I would use a limiting inverter to ensure that happens. The proposal for 8kW is more to do with getting best use in the darker months where a doubling of input can make a much bigger difference and provide enough for ashp even at reduced sunlight levels. Batteries are possible - but not until they are more cost effective.

Sorry - illustration in first post got caught out by file size. Correct one to follow

I am in the process of designing (with architects!) and commissioning a small low-energy house which will rely mainly on solar pv for energy. There is no gas at the property. I have over 15 years experience of solar thermal and solar pv. My current house uses solar pv exclusively for hot water for more than 7 months per year, uses gas for ch & backup hw for colder months. My current 3 immersion design cylinder has proved very effective. I have trawled through various heating and dhw designs including thermal stores and multi-tank solutions but cannot reconcile the different requirements (eg thermal stores need a good temperature gradient to provide dhw - heat pumps provide 55º as a working maximum). I have also noted comments on this forum. (“Hot water and heating are different – keep them separate!”) However I’m still looking for an efficient & effective solution. I am considering using a large unvented cylinder (say 500 litres) which will also act as a buffer for a heat pump and a thermal store for underfloor heating. For at least 8 months of the year the total input would be from solar pv through immersion heaters. For the colder months there would be the possibility of input from a heat pump. Primary heating would be through 3 immersion heaters. I have included a diagram. I see the following as advantages : 1. A single tank with straight-forward plumbing. No complex valves or diversions. 2. Separation of circuits for input/output. Antifreeze needed only in the heat pump circuit. - Better heat transfer to UFH with water than with antifreeze mixture. (and much less of it) 3. Ability to maximise use of solar pv (8kW) throughout the year. 4. Possibility of running as a fully electric system without heat pump - under consideration for at least one year to assess relative costs. Also provides alternative heating if heat pump fails. 5. Typically : we boost heat in the mornings (before 9.00) and in the late afternoon/early evening. Background temperature maintained at other times (although with a good store in the slab this will become less pronounced). Best efficiency from an ASHP will be when the outside temperature is highest and the best use of solar pv will be between 9.00 and 17.00. So having a facility to store heat at these times will maximise both efficiencies. I recognise one potential problem: In our current location, with excessive water hardness, cylinders frequently deposit more than 0.5kg calcium salts per year. We have completely avoided this by quality water softening and would do so in the new location. Although the physical design is simple the efficiency will depend on the control circuits. I propose to design & program these myself rather than rely on the current commercial provisions which seem to be over-complicated but still limit options to those provided by manufacturers. All of the plumbing would be installed by G3 qualified tradespeople. So I would welcome constructive guidance … 1. Is there any commercial system which already follows these lines which I’ve missed? 2. Has anyone on this forum taken a similar line – with what results? 3. Are there any genuine down-sides which I’ve missed?

After many years of renovations & extensions we thought our building days were over. But now we are Northerners looking to build our very last house in the SW (after we have negotiated planning). I've followed discussions on Buildhub for around 18 months since our circumstances changed. We are looking to build a near passive house of around 120 m2. My initial questions are about heating requirements for such a building. Passive house figures suggest around 1800 kWH p/a and allowing for 'near passive' I'm looking at 2500 kWH p/a. Historically about 25% will be in January - ~600 kWH - giving around 20 kWH per day with a possible peak demand of 2kW. So: 1. Are these figures realistic? 2. Our proposed solar PV would provide a net surplus over the year with an import requirement of around 2500 kWH during October to March. (Which near enough equates to heating demand.) Using an ASHP with COP 2.5 (not seasonally adjusted & I'm not an optimist) this could be reduced to 1000 kWH. (or 600 kWH if very optimistic) So is it worth specifying an ASHP? How long do they last? Cost of servicing? Any advice gratefully received.