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?

Hi Minisaurus

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.

Icing maybe not such a problem here: if indoor is 20C and outdoor 10C, the MHRV can cool the outgoing air to say 11-12C, but an EAHP could cool the air to say 2degC. As well as twice the sensible heat, that would condense most of the moisture vapour so recover lots of latent heat, like a dehumidifier.


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.

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 :)

Rh is not a problem in our house - today we have 50 % rh - rh generally follows the outdoor rh, presumably because our input air is via 7 no. 75 mm wall vents :) It can get a bit dry in feb-march, but that's because it's so dry outside - we do have those clay things that you fill with water hanging on the radiators, they're pretty good.

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.