Reducing negative house pressure?

[Vinmassaro]

I purchased a home from 1958 a few years ago and it underwent a major renovation (all new windows and doors, main level ceiling torn down and reinsulated). The house is a split level with two fireplaces, one upstairs and one downstairs, with separate flues. The first time I lit the upstairs fireplace (it is an open fireplace and we only light it for decorative purposes), I started getting smoke coming out of the downstairs fireplace. Opening a window upstairs did not fix the issue. After having it looked at by a chimney guy, he said the house has a negative pressure problem after he got on the roof and put a lighter over the downstairs flue, and could see that the house was sucking the flame in. I have a door in the basement to my garage, and he told me to crack it open when using the fireplace. This did solve the negative pressure smoke issue when I light a fire upstairs now, but lets in a lot of cold air into the basement. It's also likely that my oil furnace is usually running at the same time, sucking more air.

With it being very cold in CT these past few weeks, I've been reading on how to reduce energy costs. The house already has new windows and doors so there is a lot less air to be pulled in through those. It seems one of the easiest DIY jobs is to insulate the basement rim joists with rigid foam. My unfinished basement is about 1500sq ft. There were a few foundation vents in the basement, but the previous homeowner covered them up. I was able to uncover one of them when I found out about the negative pressure problem, but I have still been cracking the door just in case. I just know that if I insulate the rim joists, it will only further exacerbate the negative pressure problem.

I'm hoping someone can steer me in the right direction to a product I would need to introduce fresh air into the basement, when it is needed, to reduce the negative house pressure. The chimney guy mentioned a "Fan in a can" and googling that has come back with a few Field Controls products but I can't find much detail on how or where they should be installed, or how they should be wired. I am fairly comfortable working around my furnace if given pictures or diagrams, having installed new Honeywell Truezone dampers and Truezone control panel to upgrade the original Trol-A-Temp system (with some help from another forum).

Sorry for the long post, but I thought the backstory was useful. Thanks!

 

[LLigetfa]

There are probably two factors at play here. One is air leakage out the top half of the house. It is more difficult to find than where air leaks in so often homeowners only seal up where they feel cold air coming in and raise the neutral pressure plane (NPP) in the house. You need to lower the NPP. Opening a window upstairs did not help as you found out since that window was above the NPP. The basement door is below the NPP so that worked.

So then, step one is to seal up as much as possible in the upper half to lower the NPP. A smoke pencil can be used to find where air is leaking out. Thermal imaging cameras too can help if you know how to interpret their results. Keep in mind that anywhere air leaks out, it will carry moisture with it that then condenses in the walls/ceilings which promotes rot, insect infestation, and diminishes insulation values.

The second factor is the flues drawing air out. That was why the upstairs fireplace raised the NPP. Limiting how much air can escape up the flue will improve the efficiency of the fireplace. This can be done with a well sealed insert or doors.

After all that, there is still a need to provide make-up air. Often there will be make-up air ducted into the cold return of the furnace where the furnace then warms it so that it is not so drafty. Those however usually end up spilling cold air out the nearest cold return register. Another means to get make-up air into the home is to have dedicated make-up air ducts to the fireplace so that the fire heats the air coming in from outside. Some wood burning appliances also use dedicated combustion air from outside so they don't steal from room that would air raise the NPP.

Some other appliances such as furnaces and water heaters use dedicated outside air for combustion and those help to not influence the NPP. They also reduce the risk of flue draft reversal causing CO to enter the home.

 

[Sponsor]

 

[WorthFlorida]

You should add a few carbon monoxide alarms. It appears your furnace is not drafting and the exhaust its being pulled into the home as LLigetfa stated. Even when you get the problem solved and having a few battery operated CO alarms in the home is a good thing. They're only in the $20-$30 dollar range now even for plugin models.

One cause of NPP is the prevailing wind if your home is exposed. As the air flows around the home and the position of the roof sloops, the downwind side of the house will have lower pressure. This will suck air out of the home (attic space,etc) and air from other areas (such as your flue) will be pulled in. When homes were first being made airtight when better windows came to the market one way to provide combustion air for the furnace room was to put a 3" pipe and a few elbows throught the outside wall to provide air. Hence direct vent water heaters and furnaces then came onto the market.

Don't forget the clothes dryer. From a Whirlpool dryer safety specification:
https://outlet.whirlpool.com/digitalassets/WED95HEDW/Ventilation Specification_EN.pdf
The capacity to handle up to 230 CFM (cubic feet per minute) of airflow for each dryer in the system.

You have a perfect example when old homes are buttoned up too tight. Just keep this in mine that as you seal up leaks, you're building yourself a big plastic bag around the home. THIS OLD HOUSE addressed this a few times in the last 30 years and generally an air exchanger was added to provide fresh air intake and exhaust stale air.

 

[VAWellDriller]

I had the exact problem in my new construction house.....my simple solution, which instantly fixed the problem was to extend the flue of the upstairs chimney. Originally the 2 flues were finished at same height and smoke would roll out of one and down the other. I now have about 1' of difference and everything works great.

 

[Vinmassaro]

Thanks for the replies. I forgot to mention that entire house was rewired during the renovation since it had a low-voltage electrical system. We have hardwired smoke/carbon monoxide detectors on each level of the house and they have never gone off.

As far as sealing the upper level of the house goes, the main things I can think of are the old, large (38" x 44") whole house fan at the top of the stairs, and the ceiling hatch to the attic. The attic itself has a combination of older fiberglass insulation and cellulose sitting on top of the ceiling joists. I wish I didn't have the whole house fan because it doesn't seem too useful in the summer as we prefer to use the central air when needed. It is old and noisy and we never use it.

The furnace is a Williamson Low Boy. I figured I would mention this in case it impacted anything.

Some questions:

1. Would it not be advisable to insulate the rim joists without first somehow sealing and insulating the whole house fan and ceiling hatch to the attic? How should these be sealed?

2. We don't use the downstairs fireplace at all. Would adding a chimney cap damper to the downstairs fireplace flue and a flue stretcher on the upstairs fireplace flue be a good additional strategy? What about sealing the downstairs fireplace from inside the house?

 

[Dana]

If the air is being sucked IN to the top of the other flue (which is presumably at the top of the house), that wouldn't necessarily be due to stack effect air leakage at the top of the house into the attic, but it might be if the air leak from the attic is really huge, and it might be. Normally the warm air leaking into the other flue would have a higher stack effect draw than at the attic floor plane though, even without a fire in the other fireplace. If it's not going to be used, installing an inflatable chimney-balloon plug would stop it from being able to draw air down the unused flue. A gasketed top-sealing flue damper would also take care of it.

When the chimney is cold, and the heavier cold air is dropping down the flue via convection. This is pretty common for chimneys built on the exterior of the house than those that run up through the interior to the attic. It's also much more likely to happen when the outdoor temps are under +10F than when it's +30F.

Further down the list, open hearth fireplaces are abominably inefficient, (often drawing more cold air in than the heat they're putting out, but best-case ~15% thermal efficiency), a local air pollution problem, and a PITA. Installing an air tight wood burning insert with a big viewing window fixes those issues , turns it into a space heating appliance (at 65%+ efficiency.)

Whether or not you fix the attic plane air leaks first, insulating and air sealing the basement rim joist, foundation sill, and foundation walls will not make the problem worse, and is highly recommended for both energy efficiency and indoor air quality reasons. Clearly the convection path is down the cold unused flue, pulling air in from an optimally bad location. The tighter you make the house, the fewer issues you'll have. It's almost impossible in a retrofit (witout fully gutting the house) to make the house so tight that it can't support single fireplace draw without backdrafting, and harder still to make it so tight that an air tight EPA rated wood burning insert wouldn't work without ducted combustion air. Some inserts have provisions for ducted combustino air, but it's only a 2.5" or 3" duct, not larger.

 

[LLigetfa]

When the chimney is cold, and the heavier cold air is dropping down the flue via convection.

I presume the and is a typo. Everything is a balancing act so stack effect, NPP and cold down-drafting interact. Lowering the NPP might not be enough to counter the cold down-draft but might possibly prevent it from starting in the first place. Granted, sending a constant stream of warm air up the flue to keep it up-drafting is not energy conserving. If that fireplace is never used then a top damper should help. An inflated bladder can also be placed inside the flue.

I don't agree with you that only sealing the rim joist won't make the problem worse. Anything that raises the NPP can make it worse. It was evidenced that opening a basement door which lowered the NPP, made it better.

 

[Jadnashua]

An air-air heat exchanger can be made to allow makeup air that doesn't exhibit huge losses. I think I read that they are now required in new construction in at least some places in Canada (and probably aren't a bad idea in lots of other places, too). Ideally, all combustion devices would be closed, i.e., drawing their own combustion air from outside rather than the dwelling. Then, it's easier to manage getting fresh air into the dwelling for best health and extracting the stale air.

 

[Dana]

I presume the and is a typo. Everything is a balancing act so stack effect, NPP and cold down-drafting interact. Lowering the NPP might not be enough to counter the cold down-draft but might possibly prevent it from starting in the first place. Granted, sending a constant stream of warm air up the flue to keep it up-drafting is not energy conserving. If that fireplace is never used then a top damper should help. An inflated bladder can also be placed inside the flue.

I don't agree with you that only sealing the rim joist won't make the problem worse. Anything that raises the NPP can make it worse. It was evidenced that opening a basement door which lowered the NPP, made it better.

The warm air leaking out the attic fan lowers the pressure, the cold air above the leaky unused flue is cold and falls down the flue a bit faster, sucking in smoke from the adjascent flue in close proximity. Interaction with other air leaks are minimal, and those other leaks are (in most cases) smaller than a poorly sealed flue damper on an open hearth fireplace. Sealing up the leaky flue is the beginning of the solution, and until it's sealed up air leaks within a few vertical feet of the lower fireplace aren't going to affect much. It's not going to make it better, but it's not going to make it worse.

The band joist leak may be as big as all of the window & door crackage combined, but it's nowhere near as big as opening up a window to relieve the pressure difference. Only when a really BIG leak is sealed up (such as the leaky unused flue or the attic fan) will there be a significant enough adjustment to the depressurization problem to make a difference. There are a couple of very simple convection loops in progress here- the fireplace being used is pulling gobs of air up the flue, depressurizing the house, the second being the attic fan hole too. The cold air in the top of the unused flue is sinking, and mixing with the warm air rising from the house, but it's density is still WAY heavier than the fire-heated air going up the active flue. Any additional depressurization from the attic fan hole only makes that happen quicker.