Tue, 19 Dec 2023 08:24:19 +0000 http://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290691#Comment_290691 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290691#Comment_290691 Thu, 22 Jul 2021 13:15:52 +0100 minisaurus Posted By: RobLYou can't "gain" more heat from a heatpump than there is available "in" the air.

Thanks for a very helpful reply. So the COP is only useful for comparing the heatpump's consumption with the intake fan's consumption?]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290692#Comment_290692 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290692#Comment_290692 Thu, 22 Jul 2021 13:25:29 +0100 WillInAberdeen
SCOP is actually the ratio of (heat delivered) to (electric power)

But in the spreadsheet it is set up as ratio of (heat delivered) to (heat recovered from air).

As was mentioned previously, the heatpump has an advantage if it cools the outgoing air to much lower than outside ambient temperature, or if there are thermal gains in the building, but that's a bit niche in the UK.]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290693#Comment_290693 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290693#Comment_290693 Thu, 22 Jul 2021 13:52:50 +0100 RobL
Where a heatpump is most useful, is when there is virtually unlimited low grade heat. Then the heatpump can grab whatever it likes, at the higher COP the better.

To answer your question - yes, pretty much. Although in your case earlier they are doing different things - mvhr heating incoming air, HP heating water. If the HP output were heating the building (say it was an A2A hp), then you could compare directly. I would expect very roughly:

MVHR extracts 600W, uses 20We (elec), gives 540W+20W to the building. COP = 28.
Heatpump extracts 600W, uses 100We, gives 700W to the building. COP = 7.

The HP COP is high, but it's now not doing anything the lower power mvhr couldn't do - it still needs all the fans and control the mvhr has anyway.

The other issue I have with exhaust air stuff, is that there's too many variables. That is, variable air exchange is wanted, and completely independently, variable output power(for whatever) is also wanted. I don't see how you can square that without a multi-day heatstore somewhere.]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290695#Comment_290695 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290695#Comment_290695 Thu, 22 Jul 2021 20:25:03 +0100 Ed Davies Posted By: WillInAberdeenAs was mentioned previously, the heatpump has an advantage if it cools the outgoing air to much lower than outside ambient temperature, or if there are thermal gains in the building, but that's a bit niche in the UK.Exactly. Considering that the usual limitation with ASHPs in typical UK humid conditions is the effect of icing if the temperature is dropped too far and that the exhaust air from a house is normally expected to contain more water vapour than the outside air it would appear that “super-cooling” [¹] extract air might be a bit silly.

[¹] i.e., cooling it below the outdoor temperature.]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290696#Comment_290696 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290696#Comment_290696 Thu, 22 Jul 2021 22:27:03 +0100 WillInAberdeen

Incidentally, an ASHP installer told me that icing is a good thing, as the latent heat of sublimation is being recovered by the ASHP. Later, it has to give up the latent heat of melting to defrost itself, but that's very much less. The main issue is the time tied up by the defrost cycle. IDK if he was right.

+1 to what Rob said about ventilation and heating maybe needed at different times. More ventilation in Autumn, heating in Winter.]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290701#Comment_290701 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290701#Comment_290701 Sun, 25 Jul 2021 09:37:42 +0100 WillInAberdeen https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/255762/summary_detailed_monitoring_2_flats_exhaust_air_source_heat_pumps.pdf

A small but detailed study of some social housing with EAHPs to provide both DHW and CH. On rare occasions they performed well (CoP>3) but were mostly a poor fit to the tenants' preferred heating patterns, of heating certain rooms to a high temperature for a few hours of each day. The units also seemed to be undersized for the tenants DHW demands. Possibly a conventional ASHP would also have been a poor fit.

Similar story here
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6696]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290762#Comment_290762 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290762#Comment_290762 Sun, 01 Aug 2021 16:20:44 +0100 minisaurus Posted By: WillInAberdeenhttps://www.gov.uk/government/uploads/system/uploads/attachment_data/file/255762/summary_detailed_monitoring_2_flats_exhaust_air_source_heat_pumps.pdf" rel="nofollow" >https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/255762/summary_detailed_monitoring_2_flats_exhaust_air_source_heat_pumps.pdf

A small but detailed study of some social housing with EAHPs to provide both DHW and CH. On rare occasions they performed well (CoP>3) but were mostly a poor fit to the tenants' preferred heating patterns, of heating certain rooms to a high temperature for a few hours of each day. The units also seemed to be undersized for the tenants DHW demands. Possibly a conventional ASHP would also have been a poor fit.

Similar story here
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6696" rel="nofollow" >http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6696

Interesting links, thanks Will. I'm wondering if issues in the UK with EAHPs are more cultural than technical :) There are hundreds of thousands (maybe over a million) installed in houses over here; they've been in use for at least 20 years, they're dead reliable, but they do require a certain amount of ventilation, which happens to coincide with the ventilation "laws" here.

The ventilation is constant flow 24 hours 365 days; heating is on continuously during the heating season; the immersion starts to kick in (for radiator heating) when the outdoor temp drops below 4-5 degrees.

We heat our house and water for around 80 kWh/m2 - and we've had a few minus 15 nights this winter :)]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290764#Comment_290764 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290764#Comment_290764 Sun, 01 Aug 2021 17:54:22 +0100 RobL
Is that 80kwh/m2 of electricity into the eahp ? Any idea what the resultant heat is out of that? What temperature does the eahp exhaust air actually leave at ?
Most people in the uk have gas boilers, but will eventually have to swap to heat pumps of some sort. I honestly don’t understand how an eahp could be as efficient at central heating as an ashp, but I’m willing to be told!]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290765#Comment_290765 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17129&Focus=290765#Comment_290765 Sun, 01 Aug 2021 18:58:26 +0100 minisaurus
Could part of the problem with rh in the UK is that you bang out 70 degrees in the radiator circuit? Here we have the outdoor temp sensor and "heat curve" that adjusts the rad temp depending on outdoor temp - in my experience this gives a much better indoor climate.

No, 80 kWh/m2 is for all our electricity, including electric oven & hob, but the eahp takes the lion's share.

Our pump is a Bosch Compress 4000 - product literature says max input el 0.54 kW, output 2 kW

Exhaust air is usually around 0 degrees for these pumps.

No, ashps are way more efficient - especially when it gets cold - we had an old eahp when we moved in, so just switched it; but I'm considering switching again to CTCs ashp maybe together with their dirt cheap eahp as we need to run a vent fan anyway.]]>