Positive over fire draft help needed
What could cause a positive over fire draft? Unit is a Spirit VHF - ABT oil fired furnace that was installed in 2019. I’ve paid 3 techs to look at it, plus a couple buddies, but nobody can figure out why I have positive over fire draft that is emitting smell into basement. Of course, none of them have used a manometer or draft gauge, but instead they only do a combustion analysis and smoke test. I piped the flue out my basement door and up the side of house and still had positive draft. So I know it’s not chimney or flue pipe related. I had the heat exchanger in my driveway for testing/cleaning, so I know it’s not dirty or blocked. The Beckett AFG burner is set up correctly with z gauge and new nozzle/electrodes. Combustion analysis is always perfect. No smoke in smoke test. The oil pump is set to 120 psi per the sticker on burner and the nozzle is correct per tag on burner. The burner runs like a top. This furnace has a soft combustion chamber that has some cracks on the inside, but nothing that goes through. All gaskets have been replaced. The techs seem to want to down fire it, but I don’t believe that is the proper solution. We’ve adjusted the barometric damper every which way and still can’t get a negative draft. This happens on calm days, windy days, warm days, cold days, garage door and windows open, everything closed and even when the chimney is up to temp and has a nice draft. What could it be?
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Find someone with a manometer or draught gauge for starters, and check and make sure that the problem is real…
Is there a barometric damper?
Then, assuming you have excess positive pressure at the start of the breaching, make sure the barometric damper is free, if there is one. Check the draught after the barometric. it should be at least slightly negative, and the barometric should be free swinging to control it. If you have positive pressure after the barometric, or there is no barometric, somewhere there is a blockage in the rest of the breaching or in the chimney.
Consider: the only thing that can create a positive draught number is the blower on the burner plus an obstruction somewhere…
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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I believe the boiler is designed to run with a positive over-fire combustion chamber pressure.
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Sorry, furnace
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In the I/O manual on page 3 it clearly states the following:
B. DRAFT REGULATORS:
A draft regulator is supplied with the furnace and should be installed according to the regulator manufacturers recommendations. With the burner operating, use a draft gauge to adjust the regulator to the proper setting. (refer to the instructions enclosed with draft regulator to adjust to the proper setting). When the burner air supply and draft are properly adjusted, the overfire draft should be a negative (-).01" to (-).02" WC 1, as measured at the 5/16"overfire air tap (See Fig. 4). This tap is provided in the upper burner mounting plate. To measure the flue draft, punch a small hole in the vent connector pipe as close to the furnace as possible and always before the draft regulator.
Note: 1. Draft overfire may be positive for high fire applications but not to exceed (+).02" WC.
So this furnace can clearly be operated with a positive overfire draft, but that is on High Fire applications. If your Heat Loss calculation allows for a smaller firing rate on your Spirit furnace, then you may be able to get a negative overfire draft by using a smaller firing rate nozzle.
I also agree with @Jamie Hall in reference to measuring the actual Draft with a Draft gauge or manometer. that heat exchanger (HX) has a pressure drop of about .02" WC if I recall correctly. (you can call Thermopride to confirm.) that means if you have a -.03" WC draft at the breach (between the furnace exhaust outlet and the draft control) then you should have a -.01"WC draft over the fire. If you have a greater pressure drop thru the heat exchanger, then there is a blockage in the heat exchanger. You will need to open up the cleanout ports and remove the vent connector (SmokePipe) and run a brush thru those passage ways and vacuum out any debris or build up that may have accumulated over time. HX Cleaning instructions start on page 8 of the manual.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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What kind of flue pipe runs up the side of the house and for how many feet.??
You understand if you're getting fumes in the basement you are at risk for C0 poisoning??
I would make sure the chimney is adequate and that you have enough combustion air, the flue passages clean.
If you still have fumes, I would suspect an undersized chimney or a blockage
I would have the tech down fire it until the problem can be resolved for sure.
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Installed in 2019. Was it manufactured in 2019 or 2018?
"Combustion analysis is always perfect." Can we see, or is it a secret?
You ran a temp flue out a door an up the side of the structure to 3 ft above the peak within 10 ft? That must've been a party. While you were up on the roof, did you happen to peer down the chimney with a light?
Did you have the furnace running (burner and blower) in the driveway? Did you put it up on blocks first?
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"I piped the flue out of my basement door and up the side of my house.."
Well there's your problem.
Pipe the flue to an actual chimney. Or install a power venter.
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I believe the OP ran a new temporary chimney to see of it solved the problem.
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Exactly…
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A temporary flue made of single wall sheet metal will not offer you the same draft as a properly installed masonry or manufactured chimney. Depending on the outdoor temperature that is surrounding the single watt “Chimney” the temperature drop will be much greater on the single wall “Chimney” Compared to a proper Chimney. A draft gauge will tell you what you need to know about your problem.
Since you can not actually see inside the heat exchanger, you need to use a brush with a long handle to go thru that heat exchanger and move any debris out of the round outer section
Once tyou have brushed the heat exchanger, (you can do that without moving to the driveway) then you get a vacuum hose and run it in the outer radiator section to remove anything that has fallen to the bottom of the radiator. This accessory for a SootMaster vacuum is perfect for getting into the cleanout openings and snaking around the bottom of the Outer Radiator section.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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