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Unread 01-13-2020, 07:51 AM   #1
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ERV Ducting Question


We recently had our attic spray foamed and I performed sealed crawl space myself. We are now dealing with the stale air and need to introduce some ventilation into our home. The spray foam itself has a bit of an odor, more of a sweet smell than a chemical smell that does linger, but appears to be more due to the fact that the air doesn't change out anymore.

I have decided to install an ERV to provide fresh air and to temper the incoming air. The question is about the ducting. I plan to install the ERV in the attic and duct the incoming fresh air into the hallway so that it will get evenly distributed. What I am thinking of for the exhaust air duct is to simply have the ERV pull from the attic. My thinking is that this will put the attic under a negative pressure, then the fresh air will put the living space under positive pressure, which will result in the light spray foam odor being exhausted.

So, is this a violation of any building codes? Has anyone else done anything like this before? Any input is appreciated.
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Unread 01-13-2020, 04:23 PM   #2
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Re: ERV Ducting Question


You'll only make things worse that way. You want outside air... outside of the building.

That odor is partially from the freon in the propellant and it's damaging to a furnace heat exchanger so keeping it out of the living quarters is a good idea.

Perhaps I misunderstood.
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Unread 01-13-2020, 08:21 PM   #3
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Re: ERV Ducting Question


I’m having a hard time understanding the setup.

1. Will the attic be living space?
2. Will the attic be conditioned space (heated/cooled)?
3. Are there vents to outdoors in the attic (soffits, gable walls, roof ridge vent or other vents) or is it completely sealed up with the spray foam installation?
4. Will there be any connection between the attic and the rest of the house to allow airflow?

Chris
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Unread Yesterday, 10:11 AM   #4
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Re: ERV Ducting Question


Sorry if I wasn't clear. Let me see if I can answer the questions.

1) No the attic is not living space
2) The attic is what I would refer to as semi-conditioned / indirect conditioned. There is no supply air going into the attic outside of the typical minor leaks in the hvac duct work.
3) There are no vents to the outside, the attic is completely sealed.
4) Currently there is no intentional airflow connection between the attic and the rest of the house. However, my scuttle hole is not sealed. And if I move forward with this plan, I would install small vents, most likely in the closets, to allow airflow up into the attic.

Fresh air would of course come in from outside the home, with dedicated a dedicated vent through the attic wall. That air will enter the home through a new dedicated vent located in a central hallway.

What I was proposing was to pull the stale air out of the attic, which would put it at a negative pressure. Then the fresh air (coming in from outside the house) would put the living space at an equal positive pressure. The proposed net result should mean that the spray foam odor would not enter the living space but instead be vented outside the home.

I am not proposing to simply pull air from the attic and dump it into the home. I want to achieve exactly the opposite. I fully understand that the fresh air needs to actually be fresh air from outside the house.

Last edited by cchensley; Yesterday at 10:14 AM.
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Unread Yesterday, 11:44 AM   #5
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Re: ERV Ducting Question


I thought so but just wanted to be sure! Yes, that sounds like a good plan. As I mentioned before, that freon being released is bad for the furnace even though it's harmless in that small concentration to humans. The problem can arise when it is burned since that creates chlorides, fluorides, and phosgene and some of that can break down and combine to produce sulphuric and hydrofluoric acid.

I'm not totally sure but I think the EPA has made the use of freon more restrictive for foam propellant and maybe something else is used now.

We found that out the hard way when we had to replace the heat exchangers on the three rooftop mounted HVAC units on the building where we foamed the wall panels for the walk-in boxes we sold. We had our own metal shop make heavy gauge stainless steel replacements and they worked until we moved away from that business. I wondered if they would have lasted any better than the originals.
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Last edited by surferdude2; Yesterday at 11:47 AM.
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Unread Yesterday, 11:25 PM   #6
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Re: ERV Ducting Question


Thanks for the clarifications. Installing and ducting an ERV/HRV can be a fair amount of work. It’s a solution that should provide long-term improved air quality in your house, if installed properly, but your plan is more about solving what will hopefully be a short-term off-gassing problem (which it would effectively do).

Most ERV/HRV manufacturers say not to put their units in unconditioned space. That’s because the ducts will pick up heat (if the attic is hot) or lose heat (if the attic is cold). In a normal installation it would be possible to mitigate that by insulating the duct work and ERV/HRV. In your situation, though, the air from the attic will be going directly into one side of the heat exchanger and will cool down or heat up the incoming fresh air from outside before it is pushed into the living space. So the heat exchanger won’t be doing its job of recovering heat (or “cold”) from the exiting stale air and transferring it to the incoming fresh air. Instead you’ll essentially be heating the attic (in cold weather) or cooling it (in hot weather) with the air coming from the conditioned living space into the attic.

If you go ahead with your plan, I suggest to at least have the same square inch area of vent allowing air from the living space into the attic as the fresh air coming into the living space so that things are balanced. I get the impression that you’d like to actually depressure the attic and pressurize the living space in order to force the movement of air. That isn’t necessary. The air will flow that way naturally. If you restrict the movement of air from the living space to the attic and pressurize the living space the air will leak out other undesired places, like around unsealed electrical outlets where (if you live in a cold place) the moisture in the air can condense on the cold surfaces on the outer wall.

I suggest that you’d be better off dealing with the attic insulation off-gassing issue by installing a fan to pull air out of the attic via a vent installed on the other side of the attic. Then your ERV could be installed in the typical manner.

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Unread Today, 01:18 AM   #7
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Re: ERV Ducting Question


Hi CC, I have some concerns with what i read but at 1:15 am I'm done for the night. I'll check the thread tomorrow, but basic concern is, that attic needs to be sufficiently conditioned to maintain the interior surfaces above the dew point, well above.

Also, where is the foam and other insulation (like the attic floor) and is the foam covered by drywall or other?

Later
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Unread Today, 08:10 AM   #8
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Re: ERV Ducting Question


Chris,

You made an interesting point that I think I may have been overlooking. My primary concern with installing the ERV is that now the living area needs ventilation. The upstairs is what is commonly referred to as "stuffy", particularly on mild days when the heat pump is not actively running to heat or cool the house. Simply put, our home needs a fresh air supply.

My initial thoughts were that perhaps I could kill two birds with one stone. However, the interesting point you made is in regards to the heat exchange of the ERV not being effective if the ERV is installed in the attic. Our attic typically stays within 3-8 degrees of whatever the living area temperature is, and the humidity is typically almost identical. But in the summer, that means that when our upstairs is 74 degrees, the attic is typically about 82 degrees. So if the outside air is 80-90 degrees, then there is not going to be any conditioning of the incoming air. The converse is true during the winter months. However the temperature differential we have seen in the cold months is usually only about 3 degrees cooler than the living area.

So if I am thinking about this logically, an ERV would be best if the actual air being exchanged is taken from the living area. That makes complete sense now. The more I think about it, I think the odor situation will go away with the air being changed out by the ERV.

And thanks for the warning about the furnace situation, but thankfully, we have an all electric heat pump system so I should have to worry about any of the nasty VOC's that would be produced by the combustion of the attic air.

Bud, thanks for the input about the conditioning of the attic. Our spray foam was installed on the underside of the roof deck and extends completely down to the top of the perimeter walls and top plates. Also, end gable walls are foamed. We have a complete dome of foam, if you will. All blown insulation has been removed and just the framing and sheetrock are left exposed.

Last edited by cchensley; Today at 08:13 AM.
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Unread Today, 08:51 AM   #9
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Re: ERV Ducting Question


Have you covered the inside of that foam installation with anything?

Is it just the attic that has the foam or the entire house envelope?

Did you or anyone measure the house leakage to determine how much fresh air is needed, stuffy is not a sufficient measure J.

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