Hot water capability with an air to air heat pump

dingoxh

Hi there,

I am looking for a bit of advice with regard to heating and hot water for a renovation project.

We are planning a full reno of our family home and are looking at options for heating as part of this. The grants (either individual or OSS) for a heat pump are of course tempting. But my concern with an air-to-air heat pump is to do with hot water. Am I correct in thinking that a HP cannot produce enough hot water to run baths and showers? Not really a fan of electric showers to be honest as backup. Is there another way, perhaps a combi boiler to provide hot water on demand? 

Perhaps I am overthinking this or am misinformed about the potential of a HP. Grateful for any advice you may be able to provide. We are a young family taking on a lifetime type of project, so it is critical we get it right.

John.G

The largest single phase ASHP output I think is around 15/16 Kw, ASHP outputs are based on a air temperature of 7C and hot water output of 35C, a fall in air temperature and a required rise in HW temperature results in a very significant reduction in power output. For example 7C/35C gives a 14.5 Kw output, 0C/45C gives 9.8 Kw and 0C/55C only gives 9.23 Kw. So even if thse power outputs are sufficient to heat your house, would struggle to supply sufficient HW for a growing family excet you install a 300L cylinder or such like?.

You cheapest option might be dedicated PV for hot water. Combined with a large buffer tank you should get year round 24-7 hot water. PV is cheap if you don't need to have it grid tied or need batteries. You could probably loop your heat pump circuit into the buffer tank to preheat the water to 35c.

You are correct in saying heat pumps and hot water are not happy bedfellows.

Water John

If you are able to, location and finance wise, you could opt for ground source HP. a large storage tank and only use night rate electricity. The large highly insulated tank is for both room heating and hot water.

dingoxh

https://www.boards.ie/discussion/comment/121132598#Comment_121132598

Very helpful insight. to save on space, is putting in a combi-boiler a viable alternative for hot water or is that overkill? We have natural gas in the area and also used a combi in the past and really was impressed by it. We are keen on HP for the long term savings but if HW is a problem, then we need a plan for that. I think a lot of the promotion about Heat Pumps is disingenuous as it doesn't fully call out the hot water issue so i realyl appreciate you honsesty.

https://www.boards.ie/discussion/comment/121132607#Comment_121132607

we are in an urban mid terrace. Not sure we can do a ground HP. How much more expensive is a ground versus air?

https://www.boards.ie/discussion/comment/121132563#Comment_121132563

Thank John, again helpful stats and experience. Heat pump on its own seems not a runner.

Beginning to wonder if it is all really worth it or should we just stick with gas for heat and water.

Water John

Urban mid terrace rules out GSHP. Use A/W for space heating. Most units nowadays have a small hot water tank also. That water is usually around 40c I think. It goes over 55c on Saturday for Legionaires protection. Ours in a similar location is Daikin, but we don't have young people in the house. Neither did we bother changing the already installed electric shower, but we could have. The heating of the HW is of a lower efficiency than the space heating.