Power Burner with Sidewall Power Venter Problems

Chuck_21

We have a steam boiler with a modulating power burner 1260mbh gas input.The flue is 12in with a total length of 12 feet, 1-90deg ell, and a tee with barometric damper. It reduces at the wall to a 10" Power Venter rated at 1300mbh. At that rating 75 equivalent feet of flue is allowed.

On low fire we are exhausting all flue gases No Problem.

When modulating at high fire we start getting flue gases back through the barometric damper.

The manufacturer for the power venter feels everything should be operating okay because it falls within their ratings. There is a damper in this power venter and we were told not open it all the way because a swirling action could occur at the baraometric and cause the gas to come out. We haven't tried that yet.

Another possibility would be that the power burner is drawing in too much combustion air.

Any suggestions would be helpful.

Thanks
Chuck

Rudy

Some more info would be helpful

Some draft, O2, stack temp, etc readings would be helpful in figuring out what's going on. Pretty tough to figure out otherwise.

[Deleted User]

Push me...

pull you.

I've never had any good luck with this scenario. I know it HAS to be done in certain situations, but I hate doing it because you have so many parameters, all of which are extremely critical, to control in order to maintain precise operation.

Probably the hardest single parameter to control would be the requirement of -02 to -05 negative pressure at the breaching. Too much and you've got problems. Not enough, and you've got problems. Set it up for worst case scenario, and you'll have efficiency problems for the 98% of the time that you are NOT at design conditions.

Typically, the exhauster comes from the factory with ZERO reduction in suction potential. If it has been closed, I'd say it is safe to open it to get rid of the spillage, but you need to get someone qualified to make sure the appliance isn't overfired in the first place.

Simply overcoming the spillage by increasing the ventors capacity may be masking another issue.

If you're not comfortable dealing with things that can kill or maim, call in a professional.

ME

JackFre

While the problem

can certainly be combustion air related, and I think I'd start there, another thing that can compromise capacity is the piping to the PV. A lot of times when you are at or near capacity the vent connector lay-out becomes more critical for the system to "reach" its high end capacity. Are you feeding the PV with a 90? Getting fittings to close to the PV inlet creates turbulence in the flow. If the fan is not seeing a good clean laminar flow on its inlet that can reduce its capacity. I think you want at least 5 pipe diameters of straight pipe on the inlet to let the PV. Also, try to layout for 45's rather than 90's. I represent Tjernlund in New England. Is it one of ours?

Chuck_20

It's a Field's SWG-10. We reduce at the wall to the 10" fan inlet and have approximately 10 feet of straight 12" prior to that reduction.

Mark are you from the MA, CT area?

Unknown

Chuck

Do you have the damper on the inlet side of the venter adjusted to move the proper amount of byproducts you will be getting on high fire? Also, is this boiler a pressure-fire boiler with a breech damper at the canopy outlet? If so this needs to be adjusted properly to maintain the positive over-fire condition you are looking for. I may also add that you should be using a dual-action type barometric and that you should have manual reset spill switch installed in the piping before the barometric if you don't have one already. Hope this helps.

Glenn Stanton

Manager of Training

Burnham Hydronics

U.S. Boiler Co., Inc.