Air to Air Heat Exchange Ventilator?


The existing home I purchased has an Air to Air Heat Exchange Ventilator. What is the theory of operation?
Each bathroom has a switch to manually turn it on, plus, it has a humidistat mounted on the wall in the main area of the house. Is this just some overgrown ventilator fan to take care of humidity in winter? We're in Wisconsin.
Last winter we used it when we showered but it never seemed to run on its own via the humidistat which was set around 40 percent humidity. Never had moisture on the interior of the windows and my wife has around 30 house plants.
Any thoughts on correct operation?
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Sounds like you have a Heat Recovery Ventilator (HRV), in which outgoing warm moist stale house air heats incoming cold dry fresh outdoor air. They may condense water from outgoing air, which helps transfer heat. Unlike more expensive Energy Recovery Ventilators (ERVs), they don't re-evaporate water into incoming air. HRVs and ERVs are useless in most US houses, since they naturally leak lots of air in wintertime. On mild days, when they don't leak at all, exhaust fans and humidistats can help.

It sounds like your house naturally leaks so much that the heat exchanger never needs to turn on to reduce indoor humidity, despite all those plants. Andersen says an average family of 4 evaporates 2 gallons per day of water. Each green plant might evaporate 1 pound per day.
If you evaporate 48 lb/day (2 lb/h) of water at 40% RH and 70 F with an indoor humidity ratio wi = 0.0063 pounds of water per pound of dry air and wo = 0.0017 outdoors (January in Madison, WI) with C cfm of air leaks, 2 = 60C0.075(wi-wo) makes C = 97 cfm, enough for 6 full-time occupants, based on ASHRAE's 15 cfm per occupant standard.
You might raise the RH and/or lower the fuel bill by airsealing the house, with lots of caulking and a blower door test. With no heat exchanger, reducing leaks to 15 cfm would save about 24hx7673(97-15) = 15.1 million Btu of sensible heat per year in Madison's 7673 degree-day climate, eg 150 gallons per year of oil. With your heat exchanger, the savings would be larger. Raising the indoor RH would raise wi and the latent heat loss, ie the energy needed to evaporate water, at 1000 Btu per pound, but it would likely make the house more comfortable and lower the plant water consumption, for a further heating fuel savings.
Nick
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wrote:

Actually it seems to be just the opposite. Eight year-old house and it appears to be heavily insulated, attic crawlspace has 24" of batt insulation, Anderson dual-pane, low-E windows thru out and basement ledge also fully insulated. I've done a visual on the outside and couldn't find anything to caulk. House has a sealed fireplace, can't think of the brand, I'm at work at the moment, but the type that the doors get locked into place and it has a blower to circulate the hot air with an outside air draw. Propane use for the forced air heat is quite good. 800 gals for the heating season, including water heater, clothes dryer and stove. House is approx 3000 sq ft ranch. Full unfinished basement. Now I've never had it tested, so its not scientific, just basing this on past experiences with my 5 prior homes.
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JJ wrote:

As Nick points out, an ERV transfers moisture as well as heat. An ERV will work to control Humidity, while an HRV does not.
HRV only transfers heat. You get fresh air injected without creating a big burden on the furnace. The theory is that with a new house, the caulking and sealing has become so efficient,we aren't getting enough fresh air injected into houses. Under 4 ACH50 (4 air changes per hour at 50 pascals of over pressure -blower door test) and we need a ventilator as natural ventilation has been cut to under 100 CFM.
Canada's new building standard for Energy Efficient housing (not mandated, just a goal for builders and consumers) is even tighter than this hence most houses built in Canada since 2000 using this spec as a guideline will have one or more HRVs in them
HRVs see the most use in heating dominated climates. But even there, an ERV to transfer humidity as well as heat makes sense. ERV's shine in areas that either have high external humidity or high internal humidity.
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An HRV will work to control Humidity, while an ERV does not, in the sense that an HRV can reduce excess humidity in a perfectly airtight house.

That depends on the size of the house, no? A 2400 ft^2 1-story house might naturally leak 4/20 = 0.2 ACH (using "the rule of 20"), 0.2x2400ft^2x8'/60 = 64 cfm on a winter day, enough for 4 full-time occupants, but on a mild day, it may not leak any fresh air, so even air-leaky houses can benefit from a mechanical ventilation system, eg an exhaust fan with a humidistat.

Not for a typical air-leaky US house. A mythical US house in Madison Wisconsin that only leaks 2 cfm (built to the Canadian IDEAS standard, with 2x60/2400/8 = 0.006ACH, ie 0.125 ACH50) might waste (70-16)(15-2) = 702 Btu/h in January, with an exhaust fan to supply the rest of its 15 cfm fresh air requirement, like a 200 W bulb running 24 hours per day, about $200 per year, with electric heat at 10 cents/kWh, or $70, with a heat pump with a COP of 3, or $0.00, with high-performance solar heat.
An HRV with 90% efficiency might reduce this to 70 Btu/h if condensation and freezing of moisture from outgoing air were not a problem, which it might not be if the occupants tried hard to avoid making moisture with latex suits and ERV mufflers for breathing and covered cooking pots and no floor mopping or plants, but if the outdoor humidity ratio wo = 0.0017 pounds of water per pound of dry air with Pa = 0.08 "Hg, warming it to 70 F would make the RH 100Pa/0.748 = 11%. Most people would find that uncomfortable.
How would we make an ERV breathing muffler? Cloth mufflers seem to work well when it's freezing outdoors, condensing water when we breathe out and re-evaporating when we breathe in. Steve Baer does an interesting experiment: breathe out into a 1/4" ID x 3' long glass tube and watch condensation travel halfway down the tube, then breathe in and watch it disappear in the other direction. I can't breathe out fast enough to make more than 2' of condensation. The air that comes out of the end of the tube might be close to 70 F and 100% RH.
With indoor air at 70 F and 50% RH and wi = 0.00778, maybe the best we can do with an HRV or an ERV is to keep the outgoing air temp above 32 F, which limits the sensible heat recovery to (70-32)(15-2) = 494 Btu/h. With an HRV, we lose an additional (15-2)x60x0.075(wi-wo)1000 = 356 Btu/h of latent heat. We might lose less with an ERV, but not much less, since it's hard to evaporate cold water into cold air. But we seem to require the cold air to condense the water.
For 100% energy recovery in this mythical house, maybe we need something more like an ERV breathing muffler, with evaporation and condensation or adsorption of water at higher temps, something like two Scandinavian breathing walls with a periodically-reversing fan in a room partition between them? With Typar membranes vs vapor barriers? The in-breathing wall might be part of a solar air heater with an infinite R-value :-) How can we avoid lots of condensation and frost in the insulation?
Nick
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