Barometric damper on power vented furnace

[Reis Pearson]

Is there any reason why or why not someone would install or retain the barometric damper on a new furnace installation?

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[Brandon Whitmore]

My guess is that someone was not qualified to install that furnace, but installed it anyway's. You may want to have the client's check to ensure permits were pulled for the change- out. I am not aware of any gas appliance that requires a barometric damper.

Here's some info: Draft Regulators - Barometric Dampers: A Guide to Barometric Dampers on Oil Fired Boilers, Furnaces, Water Heaters: inspection, adjustment, cleaning, troubleshooting

[Bob Harper]

Reis, your tag said "power vented furnace'--do you mean fan assisted draft induced? Most anything "power vented" with the blower at the appliance will be a Category III or IV using smaller diameter pipe, usually AL29-4C or PVC respectively. This appears to be a natural draft appliance vented into a masonry chimney using single walled galvanized steel vent connector with a bullhead tee and a double acting MG-1 Field Control barometric damper. The main problems I see here are:
Is this chimney suitable? need Level II- Is that a liner?
No cleanout visible
tin tape on connector
baro. oversized--should be same size and vent connector

Actually guys, the National Comfort Institute teachs removal of draft hoods and replacement with double acting baros. with spill switches( see at top of baro. damper). It is an improved design believe it or not. The baro. still provides dilution air whereas Cat. I fan assisted units do not so it reduces condensation. It also provides vent relief for standing pilots being double acting.It also of course allows you to limit the draft.

I'll ping DavidR to chimne in on this one, too.

HTH,
Bob

[Darren Miller]

Is that a return opening within inches of the damper????

[Darren Miller]

Bob;

Wouldn't the 'cooler' air from a damper cool the exhaust too quickly, causing condensation problems?

[Reis Pearson]

Reis, your tag said "power vented furnace'--do you mean fan assisted draft induced? The main problems I see here are:
Is this chimney suitable? need Level II- Is that a liner?
No cleanout visible
tin tape on connector
baro. oversized--should be same size and vent connector

Thank you Bob... yes, you are correct and I misspoke. It is a fan assisted draft induced furnace. I had already written up the lining issues and the cleanout was on the other side (altough blocked by some crappy framing job). I wrote it as "Improperly vented and needs to be repaired." Thanks for the info on the barometric damper.

Cheers,
Reis

[Bob Harper]

Darren wrote:
Wouldn't the 'cooler' air from a damper cool the exhaust too quickly, causing condensation problems?

Darren, that's a good explanation of just one of the problems with draft hoods. At 80% nominal efficiency (it's actually much lower), you will still be making water vapor that needs dilution air to lower the dewpoint and prevent condensation. Draft hoods are fixed so once you have enough dilution air, they continue to cool the chimney and, as you pointed out, reduce draft. A barometric damper, on the other hand, opens only when there is excessive draft. Sure, there is some leakage around the valve plate but that's usually enough to provide the dilution air. As it cools the stack, it closes so the draft stabilizes for proper combustion.

A lot of these furnaces don't have an adequate post-purge cycle to flush out the last moist flue gases and prevent condensation. This is an inexpensive modification any tech should be able to do but they really should take the training first.
HTH,
Bob

[DavidR]

Darren wrote:
Wouldn't the 'cooler' air from a damper cool the exhaust too quickly, causing condensation problems?

Darren, that's a good explanation of just one of the problems with draft hoods. At 80% nominal efficiency (it's actually much lower), you will still be making water vapor that needs dilution air to lower the dewpoint and prevent condensation. Draft hoods are fixed so once you have enough dilution air, they continue to cool the chimney and, as you pointed out, reduce draft. A barometric damper, on the other hand, opens only when there is excessive draft. Sure, there is some leakage around the valve plate but that's usually enough to provide the dilution air. As it cools the stack, it closes so the draft stabilizes for proper combustion.

A lot of these furnaces don't have an adequate post-purge cycle to flush out the last moist flue gases and prevent condensation. This is an inexpensive modification any tech should be able to do but they really should take the training first.
HTH,
Bob

Right as usual Bob.

That double acting barometric will lower the dewpoint of the flue gases by allowing minimal dilution air and flow to still happen in the flue where without the barometric it would only occur when the inducer is running.

A barometric by definition does the exact opposite of a draft hood, a barometric attaches the appliance to the flue where a draft hood is a disconnect.

Depending on the variables here post purge may have fixed the problem instead if it was a moisture related problem.

Judging by the position of the gate on that barometric there is either a really good positive pressure on that room or a really good negative draw on the flue.

That barometric should also be retrofitted with a auto reset spill switch tied in with the pressure switch.