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jweiner 01-16-2009 07:50 AM

HRV Exhaust Duct
We have an HRV unit. The insulated exhaust duct exits our house through a box sill at the back of our home. This box sill area is not accessible as we have finished our basement. Is there any risk of condensation/moisture issues in this box sill given that we live in Wisconsin and it gets pretty cold here? For example, when the unit is not running, can cold air enter this exhaust duct and cause condensation around the box sill area. I know that it shouldn't since the duct is insulated, but there are often small gaps in this insulation.

Also can a bathroom exhaust vent also have outside cold air enter back into it and cause condensation on the outside of this exhaust pipe. I also have a bathroom vent that exhausts through a box sill in a finished part of my basement and want to avoid any moisture/water issues down the road. Thanks in advance.

jweiner 01-17-2009 05:37 AM


yuri 01-17-2009 09:22 AM

What is a box sill? Post a pic of it and we can help.

jweiner 01-17-2009 11:34 AM

A box sill is where the joists rest on the foundation and abutt the skirt board of the house, generally in the basement.

yuri 01-17-2009 12:45 PM

I have a HRV with similar installation and no I would not worry about it. The bathroom fan will have a flapper on the hood outside to prevent backdrafts and I would not worry about it. Check it to make sure it is not broken.

jweiner 01-19-2009 02:22 AM

Oh yeah - thanks for reminding me. I forgot all about the little fllapper on the (white plastic) bathroom exhaust vent. I sort of wish there was a similar flapper on the metal exhaust vent from my HRV unit. Why don't those metal vents have some sort of flapper built into them too?

kennzz05 01-19-2009 02:26 AM

perhaps a question better posed to the manufacturer

yuri 01-19-2009 06:11 PM

The exhaust should have a flapper and the intake no flapper. At least that is the way Lifebreath does their units.

dgbehrends 01-20-2009 01:51 PM

I have an 8inch duct vented for my Venmar HE 2.6 HRV between my I-beam joists above my block foundation (in my unfinished basement) in what you call a sill box. It has a flapper on it. However the installers did a terrible job of sealing up the hole around the duct and I could see daylight and feel tons of cold air coming in, as it was -26 below a week ago. I took some great stuff expanding foam and sealed it up so no more daylight but I still feel a draft. The duct plus insulation wrapping takes up a good portion of the sill box so it was very hard to get around it and seal all sides. This spring I'm going to take off the vent on the outside and seal it up much better. My intake can't have a flapper so it is wide open with only a grid to keep the critters and bees out.

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