Low Chimney Draft and Chimney Caps

Boilerpro

I have an RBI 735,000 input propane,atmospheric,draft hood boiler. 14 inch draft hood outlet reduced immediately to 12 inch Hart and Cooley B-vent. B vent goes up 6 feet then two back to back 45's and then up another 4.5 feet to facotory chimeny cap.... overall height about 15 feet from draft divertor connection . Despite what design charts say(individual 15 foot B-vent with 2 foot offset good to about 980,000), I seem to have inadaquete draft. Enclosed boiler room, with another nearly identical vent system on larger boiler and 30 x 30 open air intake to outdoors with no screen and wide open louvres, no other exhaust fans etc. Beginning to wonder if the factory chimney cap is reducing draft. Any experiences would be appreciated, as would recommendation for caps that would provide better draft. Thanks all!


Boilerpro

rudy_2

Field Controls

makes a b-vent cap called a 'Star Kap' they used to advertise as helping with chimney draft problems. Never used one, but you might check into it.
I was curious how much draft you have in the stack, should be between negative .02 to .04 WC".
If you have that in the stack, and the draft hood is neutralizing it, you might want to consider sealing off the draft hood and installing a double acting barometric control.
I've done this several times when the draft hood is just not working properly and allowing flue gases to exhaust out the burner area.
You do go out on a limb because you are altering an AGA approved design, but it is something to consider.
Field Controls has a study they had the AGA do several years ago - AGA Report #FTC-07-93 - where they went out on about 50 boilers and hot water tanks with draft hoods which were not venting (even though there was sufficient draft in the stack), removed or sealed the draft hoods and installed double acting barometric controls. This retrofit took care of every single one and got them venting properly.
If you want a copy of the report, just give Field a call (800-742-8368) and they will send you a copy.
Below is a photo of one we did at a local school building with the same situation (plenty of draft in the stack, the draft hoods just weren't working and the boilers were backdrafting into the boiler room). I suspected the problem was that with the combustion air intake right next to the draft hoods (6 of them), the cold air getting sucked up through the hoods was preventing them from venting properly.
We sealed off the draft hoods, put on baro's and they immediately started venting properly.

Boilerpro

Thanks Rudy

Another Idea for the bag of tricks!

Boilerpor