Help with house venting

[nate379]

My house is sealed well enough that there is a makeup air setup. I imagine this is kind of common on newer houses.

What that is is a 3" PVC pipe that runs through the top plate of an interior wall (in my hallway) and goes up over the insulation into the attic. At the bottom of the wall there is a vent.

I noticed when I had first bought the place if the dryer was running it would make a bit of neg pressure in the house (like a vacuum). Open a door and it would pop open, and it would be hard to close.

If the bath vents are on or the range vent it's even worse.

I put a woodstove in the house this fall and without a window being cracked open, it will pull smoke in the house. Now the stove has an outdoor air kit, but even still there is enough vacuum in the house it pulls through any little crack. It doesn't smoke out the house, but enough to smell it.


I talked to the builder and he wants to add another pipe, but doesn't know what size to go with.

Want to keep it as small as possible for less heat loss, but fix the problem. I suggested doing another 3" but putting a reducer to 2". If the 2" isn't big enough, just would need to pull reducer off and 3" the whole way.

#1 I was wondering is there any time of baffle/flap that would allow air to come in but not go out of that pipe?

#2 any ideas how I could figure CFM of devices and base that on pipe size?

1 window open about 1" is enough to make up what the pipe can't do. Window is 2ft wide.

Bath fans are around 100cfm, but I have no idea on the range hood or the dryer.

 

[jklingel]

It sounds like you have a very tight house, which is very good... if you can ventilate it properly. Why no HRV system? FYI: "ASHRAE’s residential ventilation standard (Standard 62.2) sets the minimum ventilation rate at 7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupiable floor area. ASHRAE Standard 62.2 calls for baths to attain 50 cfm intermittent or 20 cfm continuous." A very knowledgeable individual on the greenbuildingadvisor site likes the American Aldes Airlet 100s for passive air intake. This may help: "The Home Ventilating Institute recommends one square foot of open air inlet per 300 CFM of ventilation fan capacity." Once you figure the cfm you should have for your house, use this to calc your vent size. Hope that helps, and I would encourage constant ventilation. Any way to get your wood stove its own air supply, or does it have that already? j

 

[Hlidskjalf]

An HRV would definitely make the most sense.

Having a negative pressure in your house can be dangerous. Exhaust gasses from appliances can be pulled into the house.

 

[nate379]

HRV won't make the "most sense" because I don't have that kind of $$ and the way the house is built there would be no easy way to install and have it accessible. Yes it's possible, but I got quote from $3-5k for unit and install.

There is NO way I could recoup that kind of $$ in even 20 years for "lost" heat from having the attic vent.

 

[jklingel]

HRV won't make the "most sense"

There is NO way I could recoup that kind of $$ in even 20 years for "lost" heat from having the attic vent.

Then ventilate with fans. There was a study reported on another site that basic fan ventilation (Panasonic makes very quiet, programmable fans) cost $188/yr, while HRV cost $88. It's hard to justify HRV, esp retrofit, IF you can get good ventilation otherwise. If I were building new, I think I'd install HRV, mainly for the air quality.

 

[nate379]

That is what I have. BUT, like I said, if you pull air from a home you need to replace it. The replace part is the reason I started this thread.

Then ventilate with fans. (Panasonic makes very quiet, programmable fans)

 

[wbrian63]

Want to keep it as small as possible for less heat loss, but fix the problem. I suggested doing another 3" but putting a reducer to 2". If the 2" isn't big enough, just would need to pull reducer off and 3" the whole way.

Good idea with the reducer. You could also restrict the flow thru the grille - removing it and putting a cardboard cutout behind it or partially obstructing the connection to the pipe.

1 window open about 1" is enough to make up what the pipe can't do. Window is 2ft wide.

A 1" wide x 24" long window gap is 24 square inches.

The ID of a 3" Schedule 40 PVC pipe is approx 3", which is 7.065 square inches (PI X Radius Squared for area of a circle, so 1.5 x 1.5 x 3.14).

So, if you're needing 24sq inches of makeup air, a single 3" pipe isn't likely going to cut it. That being said, how much makeup air you need depends on how far away from the exhaust source the intake source is.

Air acts like a fluid. Think of your house as a body of water with various obstructions in place which are the walls, furniture, etc. Now in that body of water somewhere is a 3" drain. Open the drain. Does the water drop evenly? Probably not, because the obstructions prevent the even flow of water towards the drain. Same thing in your house. Install a big-*** vent hood in the kitchen with a makeup pipe somewhere on the other side of the house and you'll still have makeup shortfall, even if the makeup pipe can provide enough flow to compensate.

You need more makeup air, and the best way to provide it is at a slow velocity, that way the winter chill air has time to disperse into the conditioned space.

I'd try to locate some more makeup systems closer to the draw points. For the bathrooms, don't do it in those rooms, nobody's gonna want the Alaska artic chill coming across the floor when the vent fan is on - maybe just outside the room would be better.

I don't think you need a 1:1 exhaust to makeup ratio. There are already other sources of return into the house thru existing gaps in the structure, although obviously few with the symptoms you've noted.

#1 I was wondering is there any time of baffle/flap that would allow air to come in but not go out of that pipe?

You might be able to use an AAV (http://www.toolbase.org/Technology-Inventory/Plumbing/air-admittance-valves) - but these are designed to respond to the heavy draw created when water rushes down a drain pipe. I don't know if they'd open properly for your situation.

#2 any ideas how I could figure CFM of devices and base that on pipe size?

Well, you know your bathroom fans are 100cfm, and google is your friend for the venting capacity of your vent hood - the trick there may be finding the model # (try looking behind the screen next to the fan - if it's a standard-issue builder-grade vent hood).

That being said, it's not as simple as just adding it up. CFM is a volume measurement, and that can be obtained with larger openings and lower velocities, or smaller openings and higher velocities. Your vent fans, at 100CFM probably use a 4" duct. The vent hood - 5 or 6" is typical. Higher volume units can use 7 or 8" or even larger. That doesn't mean you need the equivalent in cross-section of makeup piping. As noted previously, you don't need 1:1, just enough to lessen the "not enough makeup air" symptoms you're facing.

Try and locate another makeup pipe closer to highest draw point and see what happens.

Just my 2cents.

 

[Dragster Racer]

How about running the fart fan vent over to the 3" pipe in the attic? LOL.
This is a problem we have having a bit of in our new house. ***** to seal up a house nice and tight, only to have to open a window! So you choke down the air in the wood burner, and now the house pulls from the chiminey. How about some high temp sealer for the stack? Not sure what to use though. Funny that more thought hasn't gone into this with most appliance makers so that they provide fresh air for themselves.

 

[tdkkart]

If your stove has an "outdoor air kit" I'd be talking with the manufacturer and find out why it's not working. If it's installed correctly to their specifications for your stove and it is still not working then they have a design issue that they need to fix.

 

[Dragster Racer]

I think what probably happens is that he chokes it down for long burn, which cuts down on the supplied air from outdoor. Plus, even if it is wide open, and the house wants air, it will **** from any part of that system.

 

[stealthbob]

I think what probably happens is that he chokes it down for long burn, which cuts down on the supplied air from outdoor. Plus, even if it is wide open, and the house wants air, it will **** from any part of that system.

All my vents are vented out through an air exchanger, no exhaust fans but I also have no wood stove (wish I did )

Modern houses are so tight, I loose water in my toilets when it gets windy as the pressure pushes in and out the vent stack.

 

[nate379]

It is working, just the stove isn't designed to operate in a vacuum either. It is not sealed to that extent. When the stove is burning for a long burn it pretty much is a smoldering fire, so the stove has quite a bit of smoke in it.

If your stove has an "outdoor air kit" I'd be talking with the manufacturer and find out why it's not working. If it's installed correctly to their specifications for your stove and it is still not working then they have a design issue that they need to fix.