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Negative Pressure in House, esp. Basement

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by velvetfoot, Dec 14, 2012.

  1. velvetfoot

    velvetfoot Minister of Fire

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    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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  2. jharkin

    jharkin Minister of Fire

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    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.


    .
  3. velvetfoot

    velvetfoot Minister of Fire

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    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. :)
  4. MaintenanceMan

    MaintenanceMan Burning Hunk

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

    Corey Minister of Fire

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    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.
  6. velvetfoot

    velvetfoot Minister of Fire

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

    velvetfoot Minister of Fire

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    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.
  8. TradEddie

    TradEddie Minister of Fire

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    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
  9. FyreBug

    FyreBug Minister of Fire

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

    begreen Mooderator Staff Member

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  11. DanCorcoran

    DanCorcoran Minister of Fire

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    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.
  12. velvetfoot

    velvetfoot Minister of Fire

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    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.
  13. velvetfoot

    velvetfoot Minister of Fire

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    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!
  14. begreen

    begreen Mooderator Staff Member

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    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?
  15. velvetfoot

    velvetfoot Minister of Fire

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    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.
  16. begreen

    begreen Mooderator Staff Member

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

    TradEddie Minister of Fire

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    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
  18. TradEddie

    TradEddie Minister of Fire

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    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
  19. DanCorcoran

    DanCorcoran Minister of Fire

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    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.
  20. velvetfoot

    velvetfoot Minister of Fire

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    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?
  21. DanCorcoran

    DanCorcoran Minister of Fire

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    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.
  22. TradEddie

    TradEddie Minister of Fire

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

    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
  23. velvetfoot

    velvetfoot Minister of Fire

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    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?
  24. jharkin

    jharkin Minister of Fire

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    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.
  25. suprz

    suprz Member

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    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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