# Negative Pressure in House, esp. Basement



## velvetfoot (Dec 14, 2012)

I'm trying to address a smokey odor in basement, likely smoke coming down oil burner flue, since oil burner is not used hardly at all anymore.  There is a low flow radon fan in the basement, and an insert on the first floor.

I opened the basement window a little this morning, and there was a big flow of air coming in.  I left it open a crack and will see how that works out by when I get home in the evening.

When the stove on the first floor gets totally cold, there is some cool air coming down the chimney, but a little burning newspaper gets the draft flowing in the right direction and there is no problem with smoke.

The house is about 7 years old now, and an energy audit this past summer with a blower door test showed that it was quite tight.

I've read about the stack effect, make up air, etc, but still wonder where all that air is going to.  Perhaps I will see if air is escaping through the bathroom fan dampers upstairs, maybe block them off temporarily.  I've sealed the ceiling penetrations on the second floor the best I can, but I can revisit them as well.

Is negative pressure in the basement a normal thing?  If the open basement window works, should I just put in a cheap air supply vent, like the Condar ASV-90, http://www.condar.com/asv.html ?  In the basement?  On the first floor?  Both?

For other background, the oil chimney terminates several feet ABOVE the wood chimney, which is SL300 interlocking galavnized pipe in a chase.  It'd be hard to extend the wood flue and/or shorten the oil flue.  Prevailing winds tend to blow smoke in direction of oil chimney.  I've been told that the reason oil chimeny was higher was because the oil smoke would eat up the other chimney.

I don't think there is an outside air kit available for my Hampton, but would consider an asv there.

I'd like to spend a little more time down in the basement and the smokey odor could be annoying.

Punching holes in the wall to let the cold air in just grates me the wrong way.

Thanks for reading.


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## jharkin (Dec 14, 2012)

I thought I had a negative pressure problem earlier in the year due to smoky smell around stove. Turns out I  just needed to redo the griddle and door gaskets.

Negative pressure in the basement is completely normal. Due to the stack effect the house draws air in at the basement level and pushes it out upstairs. This is why we often hear of people having to kick start their draft on basement installs.

In your case, with a very tight house you might indeed need an outside air kit for the stove. I'll defer to others with more expertise in this are  who are sure to chime in soon.


.


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## velvetfoot (Dec 14, 2012)

I have a feeling that the stove doesn't really take that much air.  Not sure about the radon fan which takes it suction from a pipe under the slab.  It might be able to quantify the flows, I don't know.  Maybe measure draft (magneholic (sp?)).

I can see the smoke kind of drift up and into the general direction of the oil chimney cap.

The root cause maybe people expect too much of basements.


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## MaintenanceMan (Dec 14, 2012)

The problem is most likely from your radon fan. Are you running a furnace blower to circulate air/heat or anything?


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## Corey (Dec 14, 2012)

It seems to be fairly easy to get neg pressure in a basement, especially if all the hot air is coming up the steps to the rest of the house and cool air can't return to the basement.  I suppose there are several things you could try as a fix: (some may or may not apply)

Change height of wood burning flue and or oil burning flue.  Having the wood burner higher may help smoke 'overshoot' the oil burner.
Damper the oil burner flue to help prevent back draft when not in use.
If possible, close the door between downstairs and upstairs.  This will stop the hot air rising and pulling neg pressure in the basement.
Use a 'stuffer' fan to blow cool air down the stairs and give the basement positive pressure
Open any shared HVAC ducts between down/upstairs.  These can allow a path for airflow as well.

Basically, if it's just a problem with the basement, you're looking for anything to stop hot air rising out, or help cold air get back in there.  If it's a whole house issue, then you would need to look more on the make-up air side.


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## velvetfoot (Dec 14, 2012)

MaintenanceMan said:


> The problem is most likely from your radon fan. Are you running a furnace blower to circulate air/heat or anything?



No furnace, a boiler that doesn't get used.


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## velvetfoot (Dec 14, 2012)

There already is a field controls power damper in front of the barometric damper, but I guess the seal isn't perfect - as I reacall there were stainless steel fingers, or something, on its edges.

There's no source of hot air in the basement, and I keep the basement door closed.


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## TradEddie (Dec 14, 2012)

A basement will always tend to be at negative pressure to the outside air, this is part of the stack effect.  If your house is tight, then it may even make the smoke smell worse, because air has fewer other paths to enter. The radon fan may be making it worse too, ideally it should only be drawing from under the slab, but its likely that there are some air paths through or around the slab.  If you can't improve the damper seal on the flue, keeping a window cracked down there will give the air an easier path to enter.  Don't pressurize from outside, because that will be forcing cold air in the winter, and damp warm air in the summer.  The idea to pressurize from upstairs is interesting. 

TE


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## FyreBug (Dec 14, 2012)

How about an Heat Recovery System in the furnace room (HRV). They go for about $800 but they will probably fix a chronic problem.


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## begreen (Dec 14, 2012)

Is this the house that has the boiler flue above the insert flue. If yes, I would tackle that first, reversing their heights. The passive HRV is a good idea too.Is there one already in the home?

http://www.viking-house.ie/fine-wire-hrv.html
http://www.ventyourhome.com/the-hrv-system


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## DanCorcoran (Dec 14, 2012)

I don't think the radon fan is involved.  We had one in our previous house.  In order to work, they need to suck air from beneath the slab.  During installation, one of the tests is to take a test measurement, from beneath the slab, at the most distant point from the fan.  If there is good suction measured there, there couldn't be any significant leakage through the slab elsewhere, causing any significant negative pressure in the house.  If there were, you wouldn't get an adequate suction reading at the test site.


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## velvetfoot (Dec 14, 2012)

Reversing the heights is interesting, and something I might eventually do, but it won't be cheap, and the building will still be under negative pressure, for better or worse.  So, there's a chance that, in some future operational scenario, oil smoke fumes are sucked down the wood stove chimney and into the house.


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## velvetfoot (Dec 14, 2012)

Do the HRV include a makeup air function or do they just exchange air from inside to outside?  This thing is about makup air, ie, incoming only, with only sketchy info at present where all that air goes!


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## begreen (Dec 14, 2012)

The expense may be less than imagined. As for the oil fumes concern, I'm not sure about that. Is there negative pressure on the first floor as well? How is the house ventilated?


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## velvetfoot (Dec 14, 2012)

The house is ventilated with windows. Which have been closed for the winter, until this AM when I cracked open a basement window.   There is a boiler, but it is not used.
Yes, there is some negative pressure on the first floor, as noted in the first post about starting the fire when the stove gets cold. Opening a window on the first floor also results in incoming air flow.


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## begreen (Dec 14, 2012)

Sounds like investing in a good HRV is the next order of business.


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## TradEddie (Dec 14, 2012)

DanCorcoran said:


> I don't think the radon fan is involved. We had one in our previous house. In order to work, they need to suck air from beneath the slab. During installation, one of the tests is to take a test measurement, from beneath the slab, at the most distant point from the fan. If there is good suction measured there, there couldn't be any significant leakage through the slab elsewhere, causing any significant negative pressure in the house. If there were, you wouldn't get an adequate suction reading at the test site.


 
Unless the Radon fan has reached its blank-off pressure, there is air entering the space below the slab, drawn through that space and out through the fan.  It is irrelevant to the operation of the radon system whether that air comes from outside (through the soil/backfill), or from inside the house. All the test you described will do is ensure that the "leakage" from whatever source is not so great that it affects the operation of the system. You can still suck soda up a straw with a leak, its just doesn't work as well.
Unless intentionally designed that way, basement slabs are rarely sealed airtight to the space below, so a radon fan will more than likely draw air out of a basement, adding to the effect of the stack effect.

TE


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## TradEddie (Dec 14, 2012)

HRV and ventilation solutions are not addressing the problem.  The problem is not lower pressure in the basement, this is normal. The problem is a smoke smell, coming directly from the oil chimney, or coming from the wood chimney down the oil chimney. If from the wood, what are your burning habits, is your wood well seasoned?  Can you safely seal the oil chimney? Changing stack heights might work, but then you might get the same problem with smoke smell though stove when running the oil furnace.  Has the furnace been cleaned? 

TE


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## DanCorcoran (Dec 14, 2012)

TradEddie said:


> Unless the Radon fan has reached its blank-off pressure, there is air entering the space below the slab, drawn through that space and out through the fan. It is irrelevant to the operation of the radon system whether that air comes from outside (through the soil/backfill), or from inside the house. All the test you described will do is ensure that the "leakage" from whatever source is not so great that it affects the operation of the system. You can still suck soda up a straw with a leak, its just doesn't work as well.
> Unless intentionally designed that way, basement slabs are rarely sealed airtight to the space below, so a radon fan will more than likely draw air out of a basement, adding to the effect of the stack effect.
> 
> TE


 
I agree completely with your assessment.  I just believe that the operative word is "significant", i.e., there may be some leakage, but I don't believe the OP's problem is a low-volume radon fan and cracks in the slab.  It may be contributing in some minor way, but I'd bet that turning off the radon fan wouldn't produce any noticeable improvement.


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## velvetfoot (Dec 14, 2012)

I believe I tried turning off the radon fan once and it made no difference, except that the radon levels shot up.

The wood is well seasoned and the smell is wood smoke.  The boiler was cleaned this summer, but it hasn't been in operation for several months after I put in an electric water heater.  I might try to temporarily seal off the oil chimney as a test at some point.  Who know, maybe it's getting sucked down the chase somehow.

If I were to leave the basement window totally open, wouldn't the negative pressure gradually increase to equal outside pressure?  Should that be what I'm striving for?


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## DanCorcoran (Dec 14, 2012)

Yes, but wouldn't you then be sucking in cold air? An outside air kit (OAK) lets the stove suck outside air in directly, from the outside to the stove, without bringing that cold air through the house.

As to the oil chimney, I've read elsewhere on this site that smoke can exit one chimney and then be drawn down another unused chimney nearby.  This happens because the cold outside air is denser than the warm air in the basement, so it forces its way down the second chimney into the basement.


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## TradEddie (Dec 14, 2012)

DanCorcoran said:


> I agree completely with your assessment. I just believe that the operative word is "significant", i.e., there may be some leakage, but I don't believe the OP's problem is a low-volume radon fan and cracks in the slab. It may be contributing in some minor way, but I'd bet that turning off the radon fan wouldn't produce any noticeable improvement.


 
Agreed, negative pressure is totally normal even without a radon fan, the problem appears to be woodsmoke being drawn down the furnace flue.



velvetfoot said:


> I believe I tried turning off the radon fan once and it made no difference, except that the radon levels shot up.
> 
> The wood is well seasoned and the smell is wood smoke. The boiler was cleaned this summer, but it hasn't been in operation for several months after I put in an electric water heater. I might try to temporarily seal off the oil chimney as a test at some point. Who know, maybe it's getting sucked down the chase somehow.
> 
> If I were to leave the basement window totally open, wouldn't the negative pressure gradually increase to equal outside pressure? Should that be what I'm striving for?


 
Opening a window will likely solve the problem, by providing an easier path for air to enter, but this draws cold air into your house. It will not equalize over time, the air will warm (by cooling your house) and rise, to escape somewhere in your upper floors, maintaining negative pressure in the basement. An OAK might help if you have a very airtight house, but even with an OAK, your basement will still be at negative pressure and will still want to draw air in through any available path. Blocking the oil flue to test is a good start, just don't forget you've done it and try to restart the furnace.

TE


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## velvetfoot (Dec 14, 2012)

Opening the basement window did seem to help-nothing like 30 degree air streaming in.   I went upstairs with an incense stick, like I've done before, and found nothing (like I said, the blower door test checked out well).  Turning off the radon fan had no effect.  Cracking open a window on the first floor where the stove is had no effect, though air is coming in.  Time seems to have no effect.  I can't understand why it wouldn't equalize.  Where does the air go?


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## jharkin (Dec 15, 2012)

The negative pressure exists because the air has places to escape on the upper floor. Lots of very very small cracks add up. Opening the window doesn't change that, the stack effect will only stop if the entire inside equalizes to the outdoor temperature.


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## suprz (Dec 15, 2012)

Just a uneducated idea, but do you have a attic ?   And if so, is there a way to get up there?  If so maybe there is warm air escaping the attic?  Maybe not enough to show on the smoke from the incense stick test but alot of little leaks add up to a bigger effect?  I believe the wood smoke smell in the basement could be caused by the boiler flue being taller than your wood burning flue and being cold an slightly sucking colder air laced with chimney flue smoke into the basement because of the negative pressure.  So the hot smoke from your lower chimney rises and the taller boiler flue catches some smoke from time to time and sucks it down even a small amount would give you a slight smell. As far as the negative pressure, hot air rises and cold air drops as anyone can attest to. Even if your house is very tight, i am thinking that there is always enough air escaping any house to create some type of convection effect. Like i said i am no expert at all, but i have worked in the HVAC area throughout my life, and these things are what i have observed.  That being said,  i would not want to install something in my basement letting in cold air either,  i know that there is no way my house is airtight at all, but if i open my basement window right now, i can guarantee there would be a rush of cold air streaming in because i have observed it first hand.  I contribute it to the counter weighted damper on the furnace flue though. It is always in operation even with the furnace off.   Oh well, i think i am rambling... I hope you can find the problem and find a solution.


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## metalsped (Dec 15, 2012)

Hook up an OAK and be done with it. It will basically bypass your pressure issues, without having to spend an arm and a leg.


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## woodgeek (Dec 15, 2012)

I'll stir the pot.

--Seems like you can disable the radon fan temporarily and see if it changes the backdraft rate. Flip the breaker if hardwired.
--Cracking a basement window should decrease the backdraft (as a test). While the stack effect pressure is still present (your upstairs are a few Pascals higher pressure than the basement) opening the window lowers the point which is in balance with the outdoors (the neutral pressure plane, NPP) to be close to the window.
--Solution: figure out a way to heat the oil chimney 24-7 with a small heater (i.e. a draft inducer). Try an incandescent light bulb on a dimmer, tied to the metal flue, and wrapped with non-combustible insulation, so that the wires and dimmer are outside the wrapping. Or you could get a heat tape or coil that can take the heat when the boiler does operate.....

likely the 10-20W required would cost less than the open window, and prevent cold air intrusion...potentially saving energy net.


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## velvetfoot (Dec 15, 2012)

suprz, you're not rambling, that's exactly what seems to be happening., plus I had a surprise this morning!
There used to be more air coming through that weighted damper but I put a motor operated Field Controls flue damper in there;  still some smokey smell though, when window's closed. 

Overnight I left the basement window open about and, for good meeasure, the window next to the stove a half inch or so.  Air is streaming through both windows this morning.  There doesn't seem to be much of a smokey odor in the basement at all.

And....here's the surprise:  the second floor was almost the same temperature as the first floor!  I don't know which window opening is contributing most to this effect, but I guess it makes sense.  Of course, the basement is cooler.

Or I could just be losing my mind.

I still can't get over the fact of that much air leaving the house, again, with my looking for leaks as well as that blower door test.


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## velvetfoot (Dec 15, 2012)

Yeah, I was also thinking of stuff like blowing the radon exhaust out the oil stack; if I tie it in before the burner, maybe it would supercharge my oil heat?  

Of course, if I changed my backup to propane I would get an efficient condensing boiler with no barometric stuff to fool with.  A little expensive for just replacing one backup source with another.


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## TradEddie (Dec 15, 2012)

I have found that while its very easy to find inlet leaks on the lower half of the house, using an incense stick, or just by feeling for drafts, but  much more difficult to find the air leaks upstairs.  There is no draft of cold air, and the incense stick doesn't show it either.  If anyone knows of an effective leak detection method for upstairs (apart from IR camera) I'd love to know.
I suspect your blower door test was rated "good" in the sense that you didn't have serious leaks, or that it was typical for the construction type, but unless they recommended an HRV at that time, you shouldn't assume your house is tight.

Strange things can happen when you open windows,when my stove is going good, sometimes it gets too hot in the stove room, so I open a window, obviously it cools the stove room, but it *increases* the upstairs temperature, presumably because there is a greater volume of air moving upstairs.

TE


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## jatoxico (Dec 15, 2012)

Have you tried turning the boiler on to warm the flue and see if that helps? I leave my boiler on in the summer even though we don't need it for hot water but I set the temps low (100-120). Hardly uses any oil but runs a little. If you did the same now maybe the warm flue would not suck air down so easy and I doubt you would use much oil at all. I always think it good to exercise the system anyway.


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## velvetfoot (Dec 15, 2012)

Yes TE, I'd also like to know a way of finding outgoing air leaks.  The Hampton manual says to crack open a nearby window when burning, so I guess it's done a lot.  This whole thing of opening windows, cutting holes in walls, running the oil burner, etc, just cuts across my grain.


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## velvetfoot (Dec 15, 2012)

Here is a picture of the chimney tops.
The smoke kinda wafts upwards and in the general direction, at times, of the oil chimney.
Maybe the wood chimney could be extended and the oil chimney shortened.  I don't know.


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## begreen (Dec 15, 2012)

velvetfoot said:


> Here is a picture of the chimney tops.
> The smoke kinda wafts upwards and in the general direction, at times, of the oil chimney.
> Maybe the wood chimney could be extended and the oil chimney shortened. I don't know.


 
I don't see anything too technically challenging there, especially if the boiler stack can be reduced by one length and still be above the cap.

Back to negative pressure. Does the house have a lot of recessed lighting fixtures? If yes, are they sealed?


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## velvetfoot (Dec 15, 2012)

That could give me hope. I imagine, however, that the height difference was intentional and for some reason. However, if the oil chimney could be easily reduced so that it is below the wood chimney, then maybe it will do the trick. I guess I should consult a chimney sweep.

There is 1 total recessed fixture that I sealed. I also caulked the electrical boxes on the ceiling as well.I will also try to see if there is any leakage thru the walls that terminate at the ceiling, without going into the attic.


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## nate379 (Dec 15, 2012)

I had to create an air leak into my house because it was too tight. If the dryer was running, or bath fans, opening the front door would cause it to about hit you in the face and it was hard to close it.

Wasn't so much a problem before I had the stove, but after I had to leave a window cracked open or it would pull smoke from the stove into the house. Stove and chimney pipes aren't exactly "air tight" This was even with an OAK.

Anyhow what I did was drill holes in the top plate of one of the interior walls and ran 3 2" pipes through and into the attic.  Put a "U" and bit of pipe going down to make an air damn.
In the stud bay at the bottom I have a grille/vent.

HRV would be a better solution, but I wasn't going to drop $4-5000 for one installed.  I'll just use a bit more firewood.


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## Treacherous (Dec 16, 2012)

velvetfoot said:


> Yes TE, I'd also like to know a way of finding outgoing air leaks.


 
Would one of these thermal leak detectors help you?


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## velvetfoot (Dec 16, 2012)

Wow that's negative pressure in the extreme.   
But still, where does all that air go?


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## velvetfoot (Dec 16, 2012)

Treacherous said:


> Would one of these thermal leak detectors help you?


I don't think so, unless you were on the downwind side of the leak.


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## velvetfoot (Dec 16, 2012)

Here is another picture of the chimneys:


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## TradEddie (Dec 17, 2012)

Treacherous said:


> Would one of these thermal leak detectors help you?


 
That may be the very model I have!  Upstairs leaks do not let cold air in, they let warm air out, so there is no cold spot to find.  Those detectors are great downstairs, and good at finding areas where insulation is a problem too.  Luckily there are not so many potential places for air leaks upstairs.

TE


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