Help with this furnace vent pipe

This is a newer installation. They’ve deliberately punched a small hole in the elbow. There is no barometric damper on this oil furnace. Is this some new philosophy or somebody’s dumb idea?

John Kogel



  1. The hole in the elbow appears to be where they inserted probes for setup and testing. It’s a poor location for the test hole…should be in a straight piece of pipe, ideally 18" from the elbow where there’ll be linear/laminar flow rather than in an elbow where flow is distorted and accurate results may be dificult to obtain.
  2. Unless this is something I haven’t seen before, the manufacturer/CSA Standard B139 usually requires a barometric damper in a naturally drafted furnace.
  3. Furnace flue gas temps at the chimney breeching (thimble) are too low for the installation allowing condensation to form and run out the cleanout. In very severe situations, I have seen orange-brown condensation stains of up to approx 20 sq ft on the basement concrete floor!! Due to an outdoor chimney (wrong place to put a chimney!!), the condensation will be worse. The CSA B139 standard has a chart of furnace BTU output, chimney height, cross-sectional area of flue, etc that indicates what the flue gas temperature should be to prevent condensation

Thanks Brian. should they not plug the hole?
There is no chimney cap, so my perception was rain water causing most of that rust. I’ll pass on a recommendation for a checkup regarding the missing damper.

Professionally it looks much better but this fluepipe is under slight negative pressure created by warm buoyant gases/wind over the chimney top so any leakage on a properly set up system would be into the pipe.

Just a few inches downstream of the elbow is where a barometric damper should be installed…think of that…an unsealed 6" hole in the fluepipe but causing no problems if adjusted properly!!

No oil fired furnaces around here but shouldn’t the flue pipe be Type B in an unconditioned crawl space?

Hi to all,

Brian is undoubtedly correct about the hole being for flue testing purposes, as to the lack of damper some newer high efficiency oil fired furnaces don’t require them as they are forced draft fed, on the issue of the connector to the chimney chase I would expect to see only single walled tubing rather than a “B” vent



The “direct vent” or “balanced flue” sealed flue systems require a manufacturer approved insulated flue. These sealed flue systems don’t vent into chimneys but are sidewall vented (with quite a few guidelines/restrictions). This is a naturally drafted (aspirated) chimney vented furnace that would usually require a barometric damper installed in the single wall flue pipe.

Hi Brian, I have never seen a “balanced flue” residential oil furnace, however I have seen a couple of forced vent oil furnaces when I lived in the frozen North myself.

Maybe John could post the model number so we can look at the manufacturers instructions, those are what should be being defered to anyway.

It appears to be a Comfort Aire ODHA125-D5



If I’m correct about the make here is the installation instructions:



The “balanced flue” phrase is the one generally used by industry. Do a search on it and you get 108,000 hits. It’s a system of ducting that supplies combustion air and removes all combustion gases with no interior building air being used. With house systems, the venting is sidewall with no chimney required as in high efficiency condensing/modulating gas heating appliances.

“Direct Vent” was a manufacturer’s trade name ploy to try to differentiate his product from others.

Brian, we appear to be talking at cross purposes hear I’m well aware of what balanced flue means, I grew up in Europe where that has been the norm for 40 odd years. My point was that I’d never seen that on a residential oil furnace have you?

Anyway back to cases, if I’m correct about that furnace it should have a draft damper, looking at picture 3 it should also have a power vent as the gasses are condensing in the flue due to low draft (not uncommon on more modern oil furnaces.



A power vent through the sidewall at rim joist level (if high enough above grade) would be a solution. The other less costly solution would be to sacrafice a % or so efficiency so that flue gases are warmer when entering the flue and condensation will not occur.

As equipment became more efficient with lower flue gas temps, the oil heating industry did not adjust and improve as far back as 1993. During my first year as energy advisor/analyst with the prov. gov., I was called on a few real screw ups by the masonry, heating and ventilation subs on new houses.

One was a new small retirement home for a couple returning to NS. The mason built an (*1) exterior brick chimney with (*2) 8"x12" clay flue liners. The furnace was way (*3) oversized for the house. The HRV system was (*4) piggybacked onto the main supply/return trunk ducts of the furnace and not ducted individualy to rooms from the HRV unit in a dedicated duct system.

*1- This is the worst place to install a chimney, especially in cold climates. A flue liner has to be very warm to hot to create sufficient draft to safely exhaust flue gases. So…It has to be warm…let’s put it outside and see if it still will work! SAD!!! SAD!! SAD!!! Are we still evolving or has that stopped? WETT in Canada would like to see exterior chimneys banned…even the insulated ones as they still can get cold between firing cycles of the appliance.
Look at most old farmhouses and such…the chimney is usually up through the center of the house! The chimney would always be warm, providing the needed draft upon appliance startup…and any parasitic heat loss would be to the house…a bonus.

*2- The furnace would have vented properly with a 5" round flue!! This oversized exterior cold flue immediately caused condensation of flue gases. These flue gases ran down and seeped through the liners and brick through the exterior insulated wall, showing up at the floor baseboard joint of the main floor within 4 months of use.

*3- The furnace would only run in short cycles, never fully warming the exterior chimney flue and contributing to the condensation.

*4- Since the HRV fresh air is distributed only during the short heating
cycles, it caused air quality problems especially at night in the master bedroom with 2 people sleeping there. An auto setback t’stat would set the temp back at about 11 PM and since it was a very efficient house, heat was not called back on til 3-4-5 AM, depending on the outdoor temps. This caused the room air to get stale…the owners would open a window…and they had a whole house ventilation system!!

Thanks, I’ve called for a repair based on the above evidence of flue problems.

John Kogel