Replies

1. 
   rdesigns | Dec 14, 2009 01:27pm | #1

   It could take decades, depending on several unknown factors.

   Bath fans are typically used for just a few minutes at a time intermitently thru the day. For an HRV system to re-coup its cost for that application would take a long, long time.

   If the bath fans were run continuously as a means of controlling indoor air quality for the entire house, then the HRV would pay back more quickly--just how quickly would depend on more unknowns, like how many CFM are we talking about, what is the efficiency of the particular HRV system, what do you pay for power or gas, just to name a few that readily come to mind.

   One thing I can say, is that makers and salesmen of HRV's have been known to exagerrate the efficincies of their equipment.

1. 
   Gene_Davis | Dec 14, 2009 04:15pm | #2

   Then what is the value of the HRV at all?

   The house is being built by the owner from plans by me. He is extremely capable and is doing everything. It is a hillside walkout, 31x31 footprint, three-layer-cake. One full bath on each floor.

   Very well built from a thermal-envelope standpoint, HVAC being handled by the latest and greatest by WaterFurnace, open loop geothermal, well-sourced, and our electric rates are down around 4 cents. Nice and low.

   All the baths are ducted from ceiling inlets to the HRV unit (Fantech) in the mechanical room adjacent the WFI unit.

   Basement is ICF, walls all built with special thermally broken 2x6s, insulation in walls is medium density spray in place foam, ceiling up top under the cold roof has about 24 inches of blown cellulose on top.

1. 
   rdesigns | Dec 14, 2009 06:12pm | #3

   The HRV is valuable as a means of controlling indoor air quality throughout the house.

   Your OP seemed to ask if the HRV would be a practical substitute for bath exhaust fans.

   With your latest info, I would say that the house needs both bath exhaust fans and the HRV. If the house is really tight, and it sounds like it is, the bath fans will have insufficient makeup air to replace what they need to exhaust (about 50-60 CFM each, probably). There is also the need for outside ventilation air for maintaining IAQ, something like 20 CFM/person at least. If this air is introduced apart from an HRV, there is considerable loss.

   For example, with a 76-degree temperature difference from inside to outside (70 inside, -6 outside), you need 70 BTU's to heat each CFM of outside air, on an hourly basis. An HRV will recover about half of the lost energy.

   The value of bath fans (and kitchen exhaust) is as a means of spot ventilation; capturing humidity and odors at their source.

2. 
   Clewless1 | Dec 14, 2009 10:00pm | #4

   Pretty much what the other poster said. Given tight construction, ventilation becomes important and with 10,000 heating degree days (where is this located, in your refrigerator? ;) ), you are tossing away a lot of heat energy ventilating (again, depending on your projected frequency and level of ventilation). Expect an effectiveness (read efficiency) of about 50% from an HRV. If I remember, I'll give you a quick formula to use to calc the cost (and then the savings) of your HRV so you can judge for yourself the economics of it.