Does anyone else find that if it is not too cold outside them air source pump is (although still expensive) probably no more than a gas boiler and electricity? But as soon as it gets cold like it has again now, the energy bill is astronomical. Does anyone know how to lower the energy usage whilst still having some heating? Thanks 

Roughly double the kWh yesterday than when it’s been milder. 48 kWh, think will be less today though been colder during day as good day for solar. 5 bed house with good insulation. Worst day last winter was 75 kWh so running at low flow temp (compensation curve) has had impact plus installing thermal blinds. Biggest thing for me is it’s warm, nothing worse than expensive but not comfortable temperature.

Posted by: @lucgw

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Does anyone else find that if it is not too cold outside them air source pump is (although still expensive) probably no more than a gas boiler and electricity? But as soon as it gets cold like it has again now, the energy bill is astronomical. Does anyone know how to lower the energy usage whilst still having some heating? Thanks 

Improve the insulation in your home.

One way is to increase the indoor temperature by 1C or 2C during the warmer part of the day, and then lower it by 1C or 2C during the colder period.

Ensure that your heat pump has been optimised for maximum efficiency.

Posted by: @lucgw

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Does anyone else find that if it is not too cold outside them air source pump is (although still expensive) probably no more than a gas boiler and electricity? But as soon as it gets cold like it has again now, the energy bill is astronomical.

This is pretty much a fact of life with ASHPs, a direct consequence of their efficiency declining as outside temperatures fall, this then being further aggravated by defrost cycles, and of course when it is cold outside, in absolute terms, you need more heat to keep your house warm. Put all this together, and you get a double whammy, or compound effect, as temps fall, you need more heat, but it can only be produced less efficiently. Round number example: if you need twice as much heat when it cold, your bill doubles, but at the same time your heat pump efficiency has halved, so you end up using four times as much energy.

It's too early to say yet, because I still haven't had a full heating season using only a ASHP for heating, but early indications are that in colder weather (around zero and below outside for some or all of the day) the ASHP costs a LOT more to run, but in milder weather it might just cost a bit less than more traditional heating systems. Because cold spells are overall only a small proportion of the heating season, there is a possibility that, over a whole heating season, the high cold weather use and low mild weather use will cancel out, and the total season's energy use will not be bedwettingly high. But I'll have to wait until later in the year, around May time, to confirm whether that is the case or not. 

I did a breakdown of the costs during the last cold spell. 

the ASHP was only just a bit more expensive than oil. But cheaper than everything when not icy cold

https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/oil-vs-gas-vs-ashp-costs

We get very cold weeks like this 4-6 weeks of the year. When I first started researching ASHP, I realised it would be more expensive than kerosene, my then heating fuel, when temperatures were below 2degC. 'Eyes on the bigger prize', at a more typical 7degC seasonal average ambient, the ASHP is around 20% cheaper. The saving is lower however, as I'm now heating the house during the working day due to WFH long-term.

Having a variable tariff and Solar PV with 6.5kWh battery storage has clawed a bit back too, charging the battery on cheap rate tariff periods and using it during the standard and high rate periods. Allowing for inverter losses that saves me £1-£1.50 a day. Mot much in the grand scheme of things, but over a year it adds up to a few hundred pounds, and utilises the battery in the winter when otherwise it would mostly sit idle. That's made quite a difference this week as the ASHP was guzzling the leccy in and out of defrost every 45-50 minutes. Yesterday my quite modest 9 panel 3.6kWh solar PV array generated 11kWh. That helped soften the bill.

@cathoderay a normal year on gas my use was 35,000kW in heat. This year (2022) my use was 7,000. So 1/5 of the energy. I've tracked each of the last 3 years so very confident on my numbers. Therefore given gas isn't 5x cheaper it's more cost efficient to use an ASHP and accept a few expensive weeks a year - for me anyway.

I've just returned after a couple of days away, when the air temperature has hovered around Zero most of the time and find that kWh consumption has, predictably, been high. Not as bad as during the very cold spell in November when it was made worse by the high humidity and freezing fog. This is nothing new.

The heat pump is of course frosted up but what I've noticed is that it has been like this all morning. Now, at nearly midday, it has finally defrosted. Is this normal?

I know that defrosting uses more kWh, but presumably running frosted up is not great either. Is the defrost cycle hard wired or are there adjustable settings that control it? Or is there something wrong with the heat pump if it doesn't defrost quickly?

Mike

Mike,

it depends on the ASHP.  In the Ecodan, it's hard wired based on a particular sensor being below -X degrees for Y minutes (or something like that).  Mine defrosts every 1-2 hours in the right (or wrong) conditions. Yours may be different.   

Posted by: @mike-patrick

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@allyfish Good point.

Judging by how hot to the touch it is sometimes I suspect the tank is not insulated. The replacement will be a smaller (30L) insulated one.

Yes and no doubt when the old tank is removed we'll find a lot of loose corroded material in there which has also travelled around the pipework.

Mike

Finally this week, got the replacement expansion tank installed. Unfortunately it didn't fit the space left by the old one so some extra pipework was required to place it in the adjoining cupboard (my heating system is beginning to resemble a car that's been modded by an enthusiastic but incompetent owner). My wife is not happy as she's had to move a pile of shoes to another cupboard.

New tank is stainless steel (old one was just steel) and insulated so should perform better and last.

The system is now maintaining its pressure (for the first time in over a year) so no leaks. And it is operating more quietly than I ever remember - no sounds of water trickling through the pipework.

For now, this is as good as it has ever been. I'm still not convinced the system is as efficient as it could/should be, especially its cold weather performance.  But without knowing the COP it is achieving this is just a feeling and not something I can prove.

Mike

I’m sorry this thread has stalled .

my concern is that having read through the posts the running costs of a proposed system will be bankrupting .

I am converting my mothers house to enable us to look after her , adding space for us all gives about 436 sq metres . Sold our house and pouring that money into the fabric , to include all triple glazes windows , new roof with 150mm insulation in plus a breathable insulating material as the felt gold tx I think it was .

New slab largely insulated better than old , cavity walls already filled , will put internal insulation on gable ends , the front and back largely new walls with block insulation in .

So I hope I have done all we can , going to use new heat geek tank 450 litre and installer has initially specified two cascaded 11 kw ecodans , I think he may be teetering on 9kw , flow temp though 50 .

Reading the experiences on here it would seem the consumption will be vast , previously oil boiler did the job and required 500 litres every two months for six months and 500 for remaining 6 months , of course the house is now bigger so not a direct comparison. 2500 litres current cost approx £1700.

Running two 11s at that flow temp sounds like it will be exceptionally costly .

If my insulation does the job can I reduce flow to 35 and the pumps will be happy operating at that , would I expect to see a dramatic saving or in reality a relatively small amount because the pumps are too big for job ? 

To be fair to the installer my brief was, I need to keep my mother warm she is so old she feels cold in a heat wave, perhaps hence his caution on size.

@holts A few thoughts: 22kW heat pumpery - have you allowed for the COP which if assumed to be 3 suggests a consumption of approx 7 kW whilst running, if you can achieve a higher COP than 3, then a lower consumption might be achieved.

Perhaps heating the rooms your mother is in most of the time to the elevated temperature whilst the remaining areas might be kept at a lower temperature. This might be achieved by having ‘oversized’ radiators in the room(s) your mother uses most.

Were the loft insulation increased to 300 mm., then this would also help reduce heat loss further.

My wife and I are both chilly mortals for medical reasons and maintain a temperature of 22.5 degrees C. throughout the house 24/7 and our 8 kW Daikin ASHP manages a COP of at least 3.5 and frequently as much as 4.5. Regards, Toodles.