Thursday, April 22, 2010

PassiveHouse ERV

Part of the beauty of a PassiveHouse is that we won’t need to have a furnace.

In order to pull this off we need our rockstar envelope (see previous post) but we'll also need to use the highest-efficiency heat-recovery ventilators available  - and will probably have to import it from Europe. The best systems use counter-flow air-to-air heat exchangers, while most HRVs in North America rely on parallel-flow heat exchangers. With counter-flow, the HRV can achieve heat-recovery efficiency of over 90% .  If we go for certification with the Passive House Institute – then we need an HRV that they have certified (the requirement to use Passive House certified equipment seems like a bit of a money-making racket to me but that is grist for another posting).

HRVs bring in fresh air, exchanging heat with the outgoing interior air; any moisture in either air stream is retained. ERVs exchange both heat and moisture, so that moisture levels in the house neither increase nor decrease appreciably as a result of the ventilation; this is important for applications where you want to either keep unwanted moisture out of a building or keep it in.   We will study the need/desire for an ERV because of summer humidity – but in general, I think we will be going with an HRV.

Ducting will be designed to be as straight as possible and has very few trunks.  Long 6 or 8 inch runs will come from a main vertical duct run in the house.  Ducting has a smooth interior to minimize friction and a corrugated exterior for strength.    Simple balancing components are provided in the outlet registers.

We will need -

  • Silencers that acoustically isolate the HRV unit from the ducting to limit noise;
  • Control modules;
  • Bathroom and kitchen hood switches that can boost the air volume temporarily.
  • An antifreeze-filled ground-loop to temper inlet air and prevent frost in winter in the HRV.  If the HRV is running in defrost it is not as energy efficient.  We are also studying the idea of earth tubes – but not sure it is right in Ottawa’s summer situation.
  • Small, in-line "post-heaters"  to condition the ventilation air

The wood stove will provide supplementary heat  (this will require a long post to make the argument I am sure), and it looks like the banks and the insurance company will want baseboard heating just in case. 

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