Yes, the same amount of air must be replacing what is exhausting, which is why the fans are programmed to run about 1/3 time.
Can one fan make the entire house negative? Yes, but only if the house is tight enough. Apparently, as I and John Straube have been saying, 2ACH50 is "tight enough".
The day we did the blower door test was 24° outside, and the baseline delta-P was -2.2 Pa at the front door (1st floor), so the 2nd storey ceiling pressure should have been approximately +2.2 Pa or 4.4 Pa total stack effect pressure.
With one bathroom fan running, the manometer delta-P increased to -4 Pa, which should have brought the ceiling pressure to perhaps 0.4 Pa. With both bath fans running, the downstairs delta-P was -8.6.
on the day of the test...what was the temperature inside the house?
Was the woodstove operating?
also... can you explain in more detail the woodstove and...
"a 4" inlet in the first floor for direct-coupled woodstove combustion air"?
I don't know much/anything about woodstoves...
Is your stove and chimney an airtight "system" with respect to the interior of the house?
does the combustion air inlet connect directly into the stove or is there an opening near the stove?
Is there a damper to close off the combustion air when the stove is not being used?
I assume the inside temperature was close to the thermostat setpoint of 68°.
I don't think you can do a blower test with a woodstove running, as -50 Pa is likely to overwhelm the chimney draft (about 42 Pa at 300° flue temperature) and cause backdrafting, even with a supposedly "air-tight" stove.
The new EPA-certified woodstove had an option for direct-coupling of combustion air supply to the firebox of the stove (as a number of them do now), but I don't believe it was connected to the 4" PVC intake pipe under the slab (that terminates in the bottom block of the chimney as an air plenum). The intake duct and cast-iron cleanout door were sealed for the test.
There is no damper in the combustion air system, since normally it would lead directly into the "air-tight" woodstove and be, effectively, a sealed combustion unit, isolated from the interior airspace. There is, however, one of my "patented" inverted U-traps:
The one weakness in this system, with a concrete block chimney (which I believe is far more durable and safe than Metalbestos) is that the cast iron cleanout door is not well sealed and should have a gasket. But it's at the lowest point of the house (most negative stack effect pressure) and the normal chimney draft pressure (at room temperature) would be 8-10 Pa, which would partially balance the house negative pressure.
There's no "reply" button for me on Robert's post with the inverted U-trap photo, but here's a question. I can't tell if you're saying you connected the woodstove to the outside air duct, or not. Also, your post refers to a block chimney but I don't see it. Curious about these details for future reference.
The woodstove was not direct-coupled at the time of the blower door test but was later connected to the air intake in this house.
Here's the air plenum and combustion air duct termination in the base of the block chimney:
In homes which were entirely wood heated (plus passive solar) with no central heating system, and in which the wood stove would be running much of the winter, I have close-coupled the stoves to a nearby floor or wall register which provided outdoor make-up air. This had the advantage of avoiding cold air being drawn in from elsewhere across the floor to the wood stove, thereby creating drafts, and also allowing the woodstove to depressurize the house to act as a non-electric whole house exhaust "fan" and draw fresh air in through the remainder of the air inlets.
Otherwise, fresh air was provided by the intermittent use of bath and kitchen exhaust fans. In one house, I connected a dehumidistat to the bath exhaust fan, and in another there was an indoor composting toilet which had a low-speed fan that drew indoor air over the compost pile and exhausted through the roof, which provided continuous ventilation.
They communicate by Tweet, of course ;-)
where does the other pipe below the chimney go?
Ah! The other mystery pipe?
You can just see it at the bottom plate below the U-trap. I ran two 4" PVC pipes under the slab to the chimney pad, since two 4" pipes are roughly the same cross-sectional area as the 6" flue pipe (equal intake and exhaust).
But, when we got the wood stove, the direct-couple kit had just a 4" round intake connector, so I capped off one of the two pipes at both ends. I guess it makes sense that 4" of cold air can supply 6" of hot, expanded air.
When you create a thermal air trap as shown, how do you calculate the pressure benefits of the trap?
Bud, I'm not sure exactly what you mean by "pressure benefits", but I did discuss this somewhat on the previous page.
It's a common practice among HVAC professionals in cold climates, and sometimes called a Saskatoon Loop.
It's similar to the common rule of thumb in roof ventilation that a minimum 3' head between intake and exhaust vents is necessary in order to get any appreciable air flow - but in reverse.
Any negative chimney effect will inhibit cold air back flow into the conditioned space unless there is so much envelope leakage that a strong negative pressure is created at the point of combustion air entry.
This thread has me lost. Dianne is going to have to work on a better way to string these posts.
Anyway, if a vent goes directly through the envelope there is (with temperature and location) a pressure between the inside and outside to move the air. If either inside or outside vent pipe goes up or down, that pressure is altered by the difference in the weight of the air in the pipe vs the air around the pipe. Once I understand where the openings on your inverted loop are, I will be able to estimate what its benefits are.
PS, I have my doubts about the saskatoon loop??