Negative pressure question?

Patrick Powers

I recently installed a Delta Elite with a Beckett AFII. My problem is after shut down house draws some oder down through system and out burner combustion air intake. I installed a clean air intake, but problem still persists. The vacuum tee shows possitive air coming into the house. The house seems to be trying to get air from where ever it can. There is a dryer in the same room which is adding to the problem. Is anyone familiar with the Field Controls MAC metering air control? Open for suggestions.
P.S. The burner does have pre-purge post-purge controls.

Jim Davis

Negative Pressure

Houses fighting for air is more common than most can imagine, but when it is on oil you can smell the problem. Had an oil furnace once that actually rusted out in the summer because the chimney was bringing in humid air because of duct leakage when running the A/C, resaturating the residue in the flue. A Field MAC was installed and this cured most of the problem. Also added a combustion air register to the plenum to compensate for any duct leakage that was not sealable. With boilers a Fan In the Can is recommended.

Patrick Powers

negative pressure

It is a boiler. Do you think a fan in the can would be the way to go as opposed to a Field MAC?

Jim Davis

Fan in the Can

Yes this is your best option. The MAC needs ductwork and a fan to pull in air. Make sure chimney has cap so it can't get wet. Also there is an inexpensive way to create a post purge on the flue, rather than the burner

Patrick Powers

exhausto-fan

One other time I had this problem was in a resturant. They install a super duper fryolator hood and smoke eaters. Things got real ugly there on shut down it was an old american standard double door steam with an oil fired water heat. We ended up installing an exhaustofan on the top of the chimney and wired it to a Field Controls power venter controller, so the fan did not run the whole time. What is the inexpensive alternative for post purge that you mentioned. I certain would not want to go through what we did in the resturant. BTW all I had to do was crack the back door and everthing went up rather than down the chimney. Remember this is only on the off cycle (shut down) that the oder is there.

tombig

Cooktop hoods in a commercial application should have their own built in makeup air.

Jack, CVMS

Makeup air

Totally agree, but what do you do when the makeup air is -50°F and freezes the soup on top of the range?