A New Way to Duct HRVs revisited
Hey everybody,
I find Martin’s article A New Way to Duct HRVs pretty fascinating. It seems pretty sensible for a well sealed/insulated home, however the postscript from Dr. Straube says that 10cfm of conditioned air wouldn’t help with regulating the temperature in a closed off bedroom. I guess that makes sense at that rate.
My question is: what rate WOULD air have to be introduced to a closed bedroom to even out the temperature? 50cfm? 100cfm? I’m sure it depends on a few factors, but what’s a ballpark rate for a 150 sq ft bedroom in pretty good house?
I like what Martin envisioned: conditioned air from a single mini-split and fresh air from a cheap Panasonic HRV dumped into a living room, and then supplied to the bedrooms via short ducts. I would think a quiet Panasonic fan per room could push enough conditioned air into the bedrooms (return air accomplished with jump ducts) to keep the temperatures comfortable and the air fresh. These fans could even be operated with a bedroom door switch, ensuring that no extra power is used when the bedroom doors are open. Thoughts?
As always, thanks to GBA for all the quality information.
Michael
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Replies
Michael,
Before calculating how much air would be needed to raise the temperature of a room with a closed door, it's important to distinguish between ventilation air (outside air introduced into a building to improve IAQ) and air that is transferred by fan from one room to another.
An HRV or ERV is used for ventilation. It distributes 100% outdoor air to various locations inside a building. Ventilation always entails an energy penalty, since outdoor air needs to be conditioned to bring it to indoor conditions, and because an equal volume of interior air always needs to be exhausted whenever outdoor air is introduced into a house.
The more you ventilate, the greater your energy penalty.
Overventilation is therefore undesireable. For years, the ASHRAE 62.2 formula suggested that most single-family homes ventilate at a rate between about 45 cfm and about 80 cfm. The new (controversial) formula pushes recommended rates for many homes above 100 cfm.
In any case, you can see how a house with 5 or 6 rooms might ventilate a bedroom at a rate of about 10 cfm or at most 15 cfm. This low airflow rate limits how much the ventilation air can contribute to raising or lowering the temperature of the room.
The other limitation that constrains this method is the specific heat of air, which is 0.0182 Btu/cf/°F.
If you want to transfer air from one room in your house to another using a through-the-wall fan, you can. Again, your limitations are going to be the low delta-T (while a furnace might be blowing 140° air, you will only be blowing 72° air) and the specific heat of air (0.0182 Btu/cf/°F). The other constraint is whether or not the occupants find the fan noise acceptable.
As some point, you begin to realize that you are trying to jury-rig a forced-air heating system; it can be argued that if you want to install a forced-air heating system, that's what you should purchase in the first place.
What he said: It all depends on what the heat load of that room was, and the delta-T you're willing to tolerate.
Say you have a bedroom that pencils out at 2000 BTU/hr at 60F indoor temps at the 99% outside design temp of when unoccupied, adding two sleeping humans and no lights on it will have a heat load of about 1500 BTU/hr. If you stipulate that a 10F delta is OK (70F in the common space, 60F in the bedroom), every cfm is worth 10F x 0.018= 0.18 BTU/minute, or (60minutes/hr x 0.18=) 10.8BTU/hr. So to support a 1500 BTU/hr heat load takes 1500/10.8= 139cfm.
If you can get the unoccupied heat load down to 1000 BTU/hr, with 2-humans you'd have a heat load of 500 BTU/hr, and be OK at 45cfm.
Of course this ignores the substantial amount of conducted heat through the partition walls at a delta-T of 10F, but no matter what it takes a pretty low-loss room to get there on ventilation air alone- it's easier to just let the Great Dane share your bed when it's cold out. :-)
For a rough-cut of how much heat you'd get through the partition wall via conduction, assume a U-factor of about0.25- 0.4 BTU per square foot per degree delta. (About R2.5-R4, roughly where you end up when you add the air films to the ~R0.5 of gypsum per side.) If you have 100 square feet of partition wall, that's potentially as much as 100 x 0.4 x 10F= 400 BTU/hr (which presumes some air movement breaking up the air films on the walls in both rooms, but not those inside the wall cavity) or 250 BTU /hr in a calm-air scenario, which can a significant fraction of the total for a low-loss room, enough so that 10cfm at a 10F delta-T makes a difference. (About as much difference as the Great Dane from a room temp point of view..)