Ashp & sahp combined system

Martyn1981

pile-o-stone wrote: »

Hi Mart, would solar provide heatpumps with much assistance in winter? I don't know what your panels are outputting but mine rarely cover the baseload of the house let alone have access for our immersion heaters (or heat pump if we had one).

Hiya, it all helps. My A/C has been running since 9am and most of it from PV, as gen is varying from ~500W to ~1kW. When it's really cold, tends to be sunny, so lots of leccy, and when it's grey and overcast the outside temp is higher, so less (or no PV) but a higher COP and less heat loss (smaller differential with outside).

I'd suggest (and this is just my thoughts) that you think of Nov to Feb as any PV gen is a help, but Mch-May and Sep/Oct could get a lot of assistance. It won't do it all, but will help, and if my sister is able to get something like 10kWp for around £10k, if we build a frame, or a small stable/barn structure, then that could contribute more, in fact much, much more than my off south system does at this time of the year.

The other thing to think of, as packages go, is that the solar will minimse annual daytime leccy purchases, so if on an E7 tariff, you get the benefit of cheaper night leccy for the heatpump, and any other consumption, but minimse the consumption of higher priced daytime units ..... as E7 rates seem to have higher daytime rates than single tariff deals.

JKenH

pile-o-stone wrote: »

If you can source free wood (i.e. you own woodland or have a tree surgeon in the family) then you're OK. If not, then make sure you understand the cost - it's more expensive than gas because of the transport of the wood to your door. We probably buy £2000 worth of wood a year.

Crikey. I have more wood than I know what to do with. Anybody who turns up gets offered it, but usually with the proviso they bring their own chainsaw.

"Chop your own wood, and it will warm you twice." ~Henry Ford

JKenH

pile-o-stone wrote: »

Hi Mart, would solar provide heatpumps with much assistance in winter? I don't know what your panels are outputting but mine rarely cover the baseload of the house let alone have access for our immersion heaters (or heat pump if we had one).

I don’t know if it is just that my ASHPs are super efficient but for a while I was leaving them on 24/7 and finding it very economical. The 2kw one in the kitchen was plugged into a metering plug and usually used less than 200 watts. While I was doing detailed monitoring I had an occasion when it averaged just 120 watts overnight (that night it was 9C outside). When it was close to or just below freezing it used around 400 watts.

On 3 occasions I ran our central heating for 4, 6 and 7.5 hours and leaving the ASHP on continuously produced 24 hour averages of 107, 88 and 74 watts respectively.

We have now settled down to a regime of 2hours C/H in the morning and 2 hours in the evening as we find this keeps the house aired, towels dry and depending on the outside temperature and PV available supplement this with our 2 ASHPS in the kitchen and lounge.

We do have a fairly large house so ASHPS on their own would not be sufficient to heat the whole house.

I feel it is wasteful running them overnight but continuous operation does seem to be the most efficient way.

In spring and Autumn we have just 1hours central heating in the morning when it is cold (operated on a manual boost). Both ASHPs can warm a room up very fast so we only need them on for a short period but ideal at meal times or for an hours TV. On colder spring/autumn days when the sun is shining (basically free to operate ignoring opportunity cost) we leave them running during daylight hours irrespective of whether we are in the rooms.

Apologies if that was a bit disjointed but I was just getting thoughts down as they came into my head.

michaels

Martyn1981 wrote: »

This is a weird suggestion, but one that's grown from an idea/chat with my sister who is looking to replace two oil boilers on a large property with an ASHP.

As ~30% of the outside skin is from the original 140yr old property, and is a solid wall, she needs to consider those rooms as the ones most at risk during the extreme end of weather/calculations.

A potential solution I suggested to her is to back up the heating in those rooms, which are actually rooms that will form part of a business side to the property, with small air con units.

This might sound like duplication, which I suppose it is to an extent, but as the RHI does not allow the use of the system for cooling, and those rooms may need cooling (when folk are there) then spending perhaps £2k on a couple of small ASHPs would work well, and provide the top up needed in cold weather if the large air to water system is struggling.

[Luckily, whilst the property is very large, all of the radiators are also large, and double panel, and double finned, plus the piping is 15mm or larger throughout, so the oil system can heat it very fast, suggesting that lower temp air to water heat pumps will manage, but of course run for longer.]

She is also considering a large ground mount solar system, and if possible (and not too expensive) an upgrade to 3phase. Obviously a heat pump and solar combination help each other out a bit on the economics front as the whole becomes greater than the sum of its parts.

Our 5 bed semi has central heating pipes that are all 22 or 15 but I am sure they would need to be bigger if the central heating water was capped at 40 degrees. Our rads are not small and we have a boiler that modulates down to 4kwh and thus runs most of the time but we need to set the water temp to 60+ when it is minus 10 outside and we are energy cert b rated. It would take massive rads for 40 degree water to do the business and would certianly need all pipes to be at least 22.

zeupater

michaels wrote: »

Our 5 bed semi has central heating pipes that are all 22 or 15 but I am sure they would need to be bigger if the central heating water was capped at 40 degrees. Our rads are not small and we have a boiler that modulates down to 4kwh and thus runs most of the time but we need to set the water temp to 60+ when it is minus 10 outside and we are energy cert b rated. It would take massive rads for 40 degree water to do the business and would certianly need all pipes to be at least 22.

Hi

Really? .... If the issue is lower temperatures reduce the heat exchange from radiator to air and the heat pump is sized sufficiently to maintain the operating temperature & DeltaT between feed & return then surely there's no issue, it's simply the amount of fluid that can be pumped around the circuit without significantly increasing the velocity and pressure requirements that you're identifying, however, increasing the 22mm runs with 28mm will have little/no effect on system temperatures other than for the reaction time of radiators which are furthest from the pump/boiler in very large properties by reducing in-tube flow resistance.

We have 28mm feed & return circuits for this very reason ... the furthest radiator from the boiler is at the end of ~150' feed with plenty of rads on the same run.

As long as the radiators have enough surface area to exchange heat to the air at whatever the fluid temperature may be and this exceeds the building's heat-loss at the time, then the internal temperature will increase ... but remember, as there's loads of energy stored in the internal fabric of the building, whenever the heat source is struggling to match heat-loss, the building & everything inside it acts as a thermal storage radiator and slows any resultant temperature drop.

As an aside, if you're running your boiler supply below 60C, I take it that you have a combi, else how would DHW storage ever reach the recommended 60C & the pump stop pumping ??

HTH
Z

Martyn1981

michaels wrote: »

Our 5 bed semi has central heating pipes that are all 22 or 15 but I am sure they would need to be bigger if the central heating water was capped at 40 degrees. Our rads are not small and we have a boiler that modulates down to 4kwh and thus runs most of the time but we need to set the water temp to 60+ when it is minus 10 outside and we are energy cert b rated. It would take massive rads for 40 degree water to do the business and would certianly need all pipes to be at least 22.

Not sure I understand that in context of what I've posted.

To me, the most important thing my sister discovered on moving in in late November, was that the large radiators could heat up the house in a couple of hours (after 3yrs of being empty). It's only an assumption I admit, but if that can be achieved in such a short time, then they must have more than adequate surface area, suggesting that a longer heating period at lower water temps would work.

[@Z - surprisingly, the next morning, after only one days heating on timer, the house was still 'warmish', before the heating had come back on. I wasn't expecting that, especially after being vacant so long. This seems to me(?) to be another tick for potential heatpump viability. They also bought five hot air blowers and oil rads, ready for a tough start, but haven't needed them.]

Obviously when I mentioned 15mm pipes, I was referring to the lengths feeding the rads, not the main pipework. She's already had one inspection and quote, and they were happy with the pipes/rads. When I say big, they vary from about 2m to 3m, and being double panel, double finned, will output approx 4x the heat of a single flat panel rad.

Totally agree that we should work to extremes, but minus 10C seems a tad .... cold ....... Wales average winter temp is about +6C. And as Z explains, it's not an immediate 'fight' against -10C, since in those conditions you'd expect the ASHP to have been running 24/7, and therefore to have warmed/stabilised the thermal store, so even if temps drop as the ASHP is overwhelmed, it won't be an immediate thing, and therefore you'd need that extreme temp to be sustained for some time.