Air Exhaust Heat Pump

[Peter Ashford]

Some friends planning an eco-refurb on a 1930's semi (solid brick walls) have recently been advised to go for an  exhaust air heat pump ventilation system which will also provide space and water heating.

The refurb aims to provide new insulation , partly external and partly internal, to that roughly equating to the  Carbonlite Silver standard. However, cold bridging (at change from internal to external insulation) and a phased replacement of the existing old UPVC double glazed windows means that U values overall will be somewhat less than that prescribed by the silver standard. Coupled with the inevitable problems in achieving the required levels of airtightness with a refurb. there will no doubt be a space heating requirement in excess of that for the silver standard (40KWh/m2yr for 80m2 semi).

The exhaust air heat pump aims to heat the house (Grd Flr - underfloor heating, First Flr - radiators) and provide water heating with the assistance of solar water heating panels. The system also incorporates an electric hot water immersion heating element which will boost temperatures as required.

Given that the carbonlite silver standard anticipates a heating system based on a condensing gas boiler, CHP etc in my ignorance I am skeptical that the exhaust heat pump system will do as it claims without the need for the electric heating element running for a large portion of the heating season. At this point might a condensing gas boiler, or solid fuel stove, not be a better option for providing top up heat to the system which in turn makes you question the role of the exhaust air heat pump in the first place compared with that of an A rated boiler and a MVHR system?

If anyone has any experience or knowledge of such systems I'd appreciate any info. to aid both my understanding and more importantly avoid installing the wrong water and space heating system in this instance?

Many thanks

Pete

[Nick Grant]

Good to be cautious

John Cantor is giving a presentation that covers this at EcoBuild! I'll let him comment. Meanwhile you have probably sussed the problem with the party wall thermal bridge unless both halves of the semi are being externally insulated.

This looses heat to next door (which will almost certainly be at a lower average temperature) plus there are bridges to the outside walls, ground and cold loft space.

Nick

[Peter Ashford]

Nick

Thanks for the reminder on the party wall insulation as you say it is always going to be an issue with external insulation unless you can do both properties at the same time.

Look forward to hearing what John has to say on the exhaust air heat pump.

Many thanks

Pete

[David OLIVIER]

I wouldn't go near an exhaust air heat pump, unless someone can explain how the tiny amount of heat recovered from the stale air stream from a house can heat a house

D

[Nick Grant]

I think David's answer is probably a good rule of thumb! An argument put by some is that at Passivhaus levels of heat demand an optimised air source heat pump following a flat plate air to air heat exchanger might be an acceptable solution allowing a gas connection to be avoided.

Interestingly the 80% efficient heat exchanger has a much higher COP than a 300% 'efficient' heat pump as David has explained elsewhere.

Personally I an sceptical of this approach even in PH and certainly is renovation but that is little more than a partly informed hunch.

Did you make John's talk at EcoBuild?

[Alan Clarke]

Would be interesting to know who advised an exhaust air heat pump in this situation, and why - as David says the basic physics won't add up and you'll be paying a lot for a direct electric heating system.

I was talking to a builder involved in such systems in newbuild low energy social housing in Wales - tenants have horrific energy bills, as the system isn't robust enough for real life. One household tends to leave the doors open to chat - so all the exhaust air goes through the doors and not the heat exchanger, and of course the heating demand goes up. Another didn't like the continuous fan noise, so blocked up the extracts - no exhaust air, no heat recovery.

Of course these are cases with the system not being used as intended, but the Passivhaus institute has also carried out detailed monitoring (reported at last conference and in a long report in German ref Teves Str) and found the overall COP of the system was 1.3 over the year. ie every unit of electricity only gave 1.3 units of heat in a passivhaus with a very expensive Veissmann compact unit (which Veissmann have now withdrawn, saying what they recommend is a gas boiler and radiators!) In this case I would definitely do the same.
 
Alan