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I have a duct that comes directly from outside into my supply duct of my furnace. When the furnace begins to heat the house it draws in fresh air. If I put in an HRV where does the fresh air hook to. I was told it hooks to your main return duct just upstream of the furnace’s air handler. If so do I have to plug the hole I have now?. If the fresh air has lots or humidity in it, won’t it rust out my heat exchanger.
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Are you asking about the heat exchanger (HX) in your existing furnace? Or in the proposed HRV? My own HRV HX is plastic. Can't rust. I run it through the dishwasher once a year to clean it out. I haven't seen an HRV with a metal HX.
The HX in your furnace is (should be) fairly tough. Because the exhaust gases on the hot side are somewhat acidic.
Fresh outside air can have up to 100% relative humidity. But the %RH will be lower after the HRV HX during the heating season. But some moisture in the indoor air that is being exhausted can be condensed as it is cooled by cold incoming air.
If you have a cooling season, then some moisture in the outdoor air may condense in the HRV HX as it gets cooled by outgoing, stale, air-conditioned indoor air.
Both of these sources of condensed moisture from indoor or outdoor are are addressed by the HRV. The HRV has an internal drain pain and one or two condensate drains. These should plumbed through a P-trap to your DWV piping.
If the existing furnace draws outside air in for distribution through the house, where does that extra air go, eventually? I say "extra" because it would seem to create an imbalance between return air from the house and heated air from the furnace. Does the extra air leave through leaks in the house?
If the fresh air drawn in by the furnace is for combustion (as in a direct-vent combustion device), you don't want to connect your HRV to that. The HRV should only connect to the distribution ductwork - one connection to each of the two air handling ductwork - return from the house and out from the furnace. Plus the appropriate check valves to keep the HRV exchanging air in the house and to prevent the HRV from blowing air through the furnace.
Why do you need an HRV? My house is so tight (0.07 air changes per hour w/o the HRV) that the HRV is clearly needed. Your house is almost certainly not as tight. If your house is above 0.33 to 0.5 air changes, then it would be of minimal benefit and I would recommend against it. Check with your code authority what the required air changes are for you. You may want somewhat more than the minimum if you have someone with a very sensitive nose. An HRV can also help reduce mildew if your house is pretty tight. -David
*The fresh air duct coming into your house SHOULD be connected to the return duct going from the house to the furnace. Most current furnaces have stainless steel heat exchanger/copper & aluminum AC coil and humidity from outside air will not harm them.An HRV will have 4 connections. One set (in/out of the HRV) will exhaust "old" air from your house. This can be used to exhaust bathrooms, can take air from any place in the house or can take air from the main return plenum. The air is passed through the unit and discharged outside. One connection will take the place of the existing fresh air supply and take air from outside, the last id the supply from the HRV to the furnace. This should be connected to the furnace return, closer to the furnace than the exhaust, if that's how it is attached. Check out renewaire.com as an example.Depending on the type, an energy recovery ventilator will exchange both heat and humidity between the two air streams. And although it is not a common practice, providing fresh air into your house, independent of leakage, is essential for air quality.
*Tim gave a clearer description of HRV connections than I did. Where I said "one connection", I meant to say one coneection in and one out of each set of distibution ductwork. There are of course four air connections (two in and two out) on an HRV. -David
*Dave, you asked "If the existing furnace draws outside air in for distribution through the house, where does that extra air go, eventually? I say "extra" because it would seem to create an imbalance between return air from the house and heated air from the furnace. Does the extra air leave through leaks in the house?"What I see in most residences (and a lot of other types of buildings)is that the house is usually negative because the dryer, the water heater, in some cases the boiler/furnace, the range hood and bathroom exhaust fans all expel air and/or products of combustion with no makeup source and rely on inleakge. The "extra" air will serve to balance this effect and, unless force fed, will only tend to make the house less negative or possible neutral.