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Discussion Starter · #1 ·
Hi guys,

I'm noticing that my house may be at a negative pressure. I've noticed backdraft from the fireplace, and seemingly colder than normal drafts from doors, etc.

I'm wondering if this can be alleviated by increasing the air-intake from outside to the furnace. I have Lennox G51MP series furnace with air vented & drawn from outside. My understanding is that the air intake is primarily to replace combustion air. I'm wondering if the furnace can be adjusted to intake more air to not only replace combustion air, but also to add dilution air and create a positive air pressure in the house.

Thanks, any help is appreciated.
 

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There is a way to add in outside air to the furnace but will take some new equipment like a ERV or HRV.

Any other gas burning appliances consuming oxygen?

Another possibility is a poorly air sealed ceiling. Stack affect from warm air exhausting out of the roof can cause neg pressure in the house by pulling air through holes and gaps. Things like plumbing and wire penetrations that were never sealed are typical sources.


Much has been written on the subject of air sealing and reducing stack effect.

Some info on the topic
http://www.myhomescience.com/understanding-stack-effect/
 

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Discussion Starter · #3 ·
There most certainly is air being lost through the ceiling/stack effect. It's an older house and may need improved attic insulation. I was hoping to be able to correct the negative air pressure problem with an easier fix like dilution air to furnace.

I'll look into ERV or HRV.

Other gas burning appliances are dryer and hot water tank. Both are vented without combustion air feed. The backdraft problem does seem to come and go (perhaps time with when dryer is on, or other ventilation), and I was hoping this air could be replaced in better ways than through penetration, or pulling air from attic/backdraft from fireplace.
 

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HRVS and ERVs provide balanced ventilation, they don't change the pressure in the house.

Your attic may need air sealing, not just more insulation.

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It's possible to measure the pressure in the house with a manometer - one tube goes to the outside to reference normal atmospheric pressure the manometer measures the difference.

You can also get a blower door test done to measure air tightness and see if the house is tight in general or it's caused by a leaky ceiling. Part of the test is checking pressure difference with the blower door fan off with all exhaust fans/combustion appliances on.

After that they depressurize the house and see how much airflow is needed to hit a certain pressure differential.

Costs quite a bit of money though.

probably not worth the trouble and money.
 

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Backdrafting can be a serious problem and something we (energy auditors) test for under worst case conditions. Those combustion exhaust gasses can include CO when the burning process fails to be complete and that can occur without warning.

As mentioned, when you have excess high leakage it creates a more negative pressure in the low areas. Check for those leaks, here is a handy guide for air sealing.
https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/TBC_Guide_062507.pdf

Note, sealing any leak, high or low helps to reduce the total leakage, so if you have access to the ceiling in the basement that can work as well.


The fireplace, that is another issue.


Bud
 

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You should have combustion air in the same room as the water heater. What I usually see is a 6" insulated flex duct going from the outside wall near the ceiling, to the floor near the water heater. Then either bend the end up or stick it in a bucket to make a "cold air trap" so that the cold air doesn't just constantly spill onto the floor and make the entire room cold. That's the easiest way to do it.


The first pic is of the intake hood that goes on the outside wall. If it has a flapper inside of it, remove that before installing. The next 2 are examples of "cold air traps".....








 

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Only really tight houses need that - it's very uncommon where I am.

Since negative pressure can be caused by excessive ceiling leakage, it's better to make sure that's not a problem first.
 
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