Positive pressure?

Something a little odd happened today.

I have my manometer hooked to the "Low" side so my draft reads positive all the time.

I was working in the attic so I had the door off the access hole upstairs and at some point during my million trips to the shed for tools, I noticed the manometer had gone to negative .02 wc which should indicate pressure.

I put the cover back on and it returned to it's normal reading.

Any guesses on what would have caused this?

Sounds like having that access way open let warm air rise up thru your attic creating negative pressure where your stove is and it cannibalize your draft.

I'm wondering if it was strong enough to pull air DOWN the chimney.

That's the only way I can see positive pressure in the stack.

Yes it can be strong enough. The house itself is also a chimney.

franco b wrote:Yes it can be strong enough. The house itself is also a chimney.

Yep, the "stack effect" of the house combined with the open attic access allowed lots of warmer to escape creating a vacuum affect in the lower portions of the house. When air escapes like that, make up air has to come from somewhere and it looks like it was powerful enough to overcome the chimney draft. Was the stove running at the time? Is the stove in the basement?

rockwood wrote: Was the stove running at the time? Is the stove in the basement?

Yes and yes.

There you go
Opening a window low in the house at the same time you open the attic access should let the chimney keep functioning properly.

rockwood wrote:There you go
Opening a window low in the house at the same time you open the attic access should let the chimney keep functioning properly.

I have a window next to the stove cracked a bit and I'm in process of installing a fresh air intake for my oil burner so I have the intake hood installed but the pipe just hanging there.

There is a fair amount of air coming into the basement.

The family was heading out and I had a feeling I knew what was going to happen. Sure enough, the CO detector was going off when we got home. Had to open up all the doors to air things out.

I had to reinstall that attic panel and everything is working normally now.

my guess is that your stack isn't taller than the highest portion of your home, at least not by much.
This is why stack height in relation to the house (including unused spaces such as the attic) is important in determining whether or not a chimney will perform under just about any condition. A stack that terminates at a height less than 2-4' above the highest portion of the home is subject to dramatically different performance in different weather and other (attic door open etc.) conditions.

I went to the extreme of installing vents to the outside in the basement on two sides of the house. Whenever it gets warm out like in the 50's OR we need to open windowstats, I'll open these vents to neutralize any pressure that may be compromised. For me, these vents make a huge difference in maintaining a reliable draft in the chimney.

Berlin wrote:my guess is that your stack isn't taller than the highest portion of your home, at least not by much.
This is why stack height in relation to the house (including unused spaces such as the attic) is important in determining whether or not a chimney will perform under just about any condition. A stack that terminates at a height less than 2-4' above the highest portion of the home is subject to dramatically different performance in different weather and other (attic door open etc.) conditions.

I think by todays standards, my chimney is a little short. It is in the middle of the side of the house so that the peak is perpendicular to it but it's no more than 2 feet MAX taller than the peak.

It's a two flue, terra cotta lined brick chimney that runs from basement to the roof.