# Negative pressure in house after winterizing

OmCheeto
Gold Member
...
Not sure what you were after with R-Value, but Greg is asking about direct air leakage....
Ratios!

What's the ratio of heat loss from Greg's house via:
the lack of insulation
vs
direct air leakage
?​

We know that Greg has a 3 story house, with one of the stories being at least partially underground, and from a previous post, the upstairs is unheated, so, I'll interpolate his above ground surface area as being 189 m^2. (1000 ft^2 floor area with 8 ft tall walls)

Using your temperature numbers, I calculate that with the fan on, he would be losing 635 watts.
With a nominal R-value of 13, his walls and ceiling will be losing 1815 watts.

Since we don't know his actual air leakage rate, nor his aggregate R-value, it's difficult for me to recommend a course of action.

Also, since we now know his bathroom fans have "back draft dampers" and he has exterior flaps, I would not recommend adding more flaps.

ps. obligatory graph:

This tells me, that even at 10% of the fans flow rate, it's still losing 1/27th as much energy as the house, itself.

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OmCheeto
Gold Member
Coincidentally, the wind is also quite brisk at my house, and the temperature is near freezing.
Not knowing how to measure air flow leakage through my bathroom fan, I decided to do an experiment.

I used 4 pieces of masking tape, and secured one of those ultra-light weight produce bags over the inside vent opening.
It just kind of pulsated like a plastic lung, so it appeared that I had no net leakage.

Still curious, I replaced the produce bag with a dry cleaning bag, and inflated it with a hair dryer.
It took ≈3 minutes, to evacuate ≈1/2 of the 1.1 m long x 1.24 m circumference tube, yielding a flow rate of 0.000374 m^3/sec. (0.8 cfm)
Which from my calculations gives me a net energy loss from the vent of ≈10 watts.
Which is not quite as high as my 1700 watts lost through my walls and ceilings. (R-13, 1000 ft^2, 8 ft tall walls, diff temp 40°F)

My guess is that there is some type of Bernoulli effect going on at the roof vent, which is where my fan exhausts to.
As far as the costs go, my fan leak costs me ≈90¢/month, and my poor insulation is costing me \$150/month.

The conclusion of the following is that a proposed alternative effect of why the bag shrank is negligible, so you can ignore it:
Another possible explanation for the bag shrinkage may be the back and forth flow of air through the vent. The vent tube runs through the attic.
So I took some measurements:
attic RTD reading: 5.46 kΩ (My house is wired, for science. There's also an RTD in the crawl space.)
attic temp: 43.1 °F, 6.2 °C, 279 K

Ambient bathroom temp: 70 °F, 21.1 °C, 294 K​
From the ideal gas law, we know that PV = nRT
Given that P, n, and R are all constant, Volume must be proportional to Temperature: V = kT
Volume = 0.135 m^3 = k * 294
therefore k = 0.135/294 = 0.00046
Volume @ 279 K = 0.00046 * 279 = 0.128 m^3
0.128/0.135 = 0.95
Which tells me that the bag would only shrink 5% if the bags air was being chilled by 15 K. (27 °F)​

OmCheeto
Gold Member
Today I calculated that Greg's bathroom fan is theoretically capable of a 500 kw heat loss. (@ a 40°F temperature differential)
I found that, most humorous.
So I did another calculation, throwing in another variable, and came up with a number of 236 watts.
That still seems a bit high.

This may require some "empirical" data, to capture something I'm missing.

Evo
Mentor
So pretty much I should stop wasting my time and just deal with the energy bill?
I have new thermal windows from Home depot in all of my windows, not a whiff of a breeze. I also have storm doors at every door, and NO wind coming through the doors. Also, I can remove the thermal glass in the spring and just have screen doors in cooler weather.

Keith_McClary
Gold Member
In houses with the old leaky storm windows the stack effect was noticeable because the moist air leaking out the upstairs windows would frost up the inside of the storm windows. The lower windows would be clear because of the relatively dry air leaking in.

I think I’ve nailed the culprit to being the fire place. The damper is pinned open due to the gas logs and are only have a moveable metal mesh grate over the opening. If I install fixed glass doors would that help? They wouldn’t be air tight.

jim hardy
I think I’ve nailed the culprit to being the fire place. The damper is pinned open due to the gas logs and are only have a moveable metal mesh grate over the opening. If I install fixed glass doors would that help? They wouldn’t be air tight.

Why pinned open - what does that have to do with "gas logs"? Is there a constant pilot or something?

The glass doors I've seen don't seal that great, and if you do have a pilot, it would need some air anyway.

OmCheeto
Gold Member
I think I’ve nailed the culprit to being the fire place. The damper is pinned open due to the gas logs and are only have a moveable metal mesh grate over the opening. If I install fixed glass doors would that help? They wouldn’t be air tight.
Tough call. You might want to check with the manufacturer.

Why pinned open - what does that have to do with "gas logs"? Is there a constant pilot or something?
Exactly, by design the damper needs to be pinned open for pilot exhaust.

Exactly, by design the damper needs to be pinned open for pilot exhaust.
How often do you use it? I'd consider turning the pilot off, and lighting when I use it, and removing the pin.

Tom.G
Why pinned open - what does that have to do with "gas logs"? Is there a constant pilot or something?
Pilot lights on those are extremely rare. More likely a safety consideration for when someone doesn't turn the gas valve completely closed. As an alternative safety feature you could install an automatic gas shutoff valve that uses a thermocouple to keep the gas on only when it is hot. They are used on almost all gas appliances with a pilot light. The drawback is you have to hold their Reset button depressed until the thermocouple heats up on startup.

Some areas require a fireplace to have an inlet for outside combustion air. That and fairly tight fitting glass doors would greatly improve the efficiency. I don't recall the exact number, but without outside combustion air, fireplace heating efficiency is negative below 40° to 50°F.

If you do install a duct for outside combustion air, construct it as you would a chimney; i.e. good clearance from combustable materials like wall studs and siding. The reason being that when the wind blows and the air intake is on the downwind (low pressure) side of the house, you can get a backdraft with the chimney acting as air intake and the smoke going out what you thought was the intake. Just something to keep in mind.

OCR and jim hardy
jim hardy
Gold Member
Dearly Missed
I think I’ve nailed the culprit to being the fire place. The damper is pinned open due to the gas logs and are only have a moveable metal mesh grate over the opening. If I install fixed glass doors would that help? They wouldn’t be air tight.

They'd have to help. Plus they're a barrier against soot & water vapor from combustion.
Just make sure the unit still gets plenty of combustion air.
I would contact whoever manufactured your gas log and get a copy of their installation guide. There should be guidelines as to size of chimney opening required .

I strongly urge you to have someone skilled in the trade examine your installation . Sealing up your household air leaks has changed how your fireplace 'breathes' .
Combustion air that used to leak in through the walls will now have to come down the chimney. Has the flame become less blue and more yellow since sealing the leaks ?
In all probability everything is fine but a CO meter reading would be reassuring. As would a chimney inspection to look for bird nests and lost frisbees. You don't want combustion products in your living room .

old jim.

OCR