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I have a combined radiant heating system/domestic hot water system. This unit is a middle efficiency unit rated at 87% The boiler is connected to a B-vent chimney with single wall vent connector. The boiler is also equipped with an electric damper to reduce air going up the chimney when not operating.

The B-vent was replaced 15 years ago and is in good shape. The vent connector is so corroded that last night it collapsed. It was replaced at the same time.

I have shutdown the unit until I can get it vented properly again.

A: Why is my vent connector pipe corroding so badly?

B: How can I prevent this from happening again?


My suspicion is that the boiler is oversized (90,000 BTU) and is short cycling. Even on cold winter days it rarely runs more than 2-3 minutes at a time with 10-20 minute off times. This results in hot humid air in the vent and chimney which is kept there when the electric damper closes. Water condenses in the exhaust system, and never really dries out.

My thoughts:

  • Disable the damper in the open position.

  • Punch a hole in the damper so that it doesn't shut all the air off in the off position.

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  • If your suspicion is correct, the right solution is to adjust the controller settings so that it runs at least 5-10 minutes with longer off times. That will fix not only the problem but also prevent premature wear and tear all around. Don't know about gas, but my central heat-pump had a huge warning about this.
    – Jeffrey
    Jun 24 '20 at 16:48
  • Can you get a smaller boiler, or a smaller burner package (fewer BTUs) for your existing boiler for that matter? Jun 25 '20 at 0:14
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I would start with a couple of small 1/2” holes if you disable the damper totally you loose efficiency quickly where several smaller holes will allow a draft and the combustion moisture to dry 15 years is a fair amount of time but I have had systems last much longer in some cases when doing annual inspections I have recoated single wall with zinc rich paint it could have the last piece dipped in a batch but excess moisture is probably the cause so start small and see if that works without totally disabling the energy saving feature of your damper.

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On replacing the pipe, I found that the damper had a 3/4" knockout apparently for this purpose. I've removed it, and we'll see what happens in the coming winter. This amounts of a tiny air leak, but with a 20 foot stack effect should help to keep the exhaust system dry. Since the house is an older one, built in the mid 80's, and we heat with wood a lot, this is an inconsequential air leak.

The pipe was inexpensive -- four 3 foot sections, two adjustable corners, and a T ran me $80. Replacement was straight forward, with the usual minor problems of working with sheet metal. An extra pair of hands holding one end is useful. I used 26 ga pipe this time. This is heavier, I think, than the last time I replaced it.

Guys at the Heating and gas company say that this is a common problem in our climate especially with medium efficiency units vented through older stacks. The units with their higher efficiency produce some NOx gasses which combine with water vapour to produce acid that chews on the pipe. Anyway the guy claimed that the vent pipes are good for only about 15-20 years.

One comment suggested swapping the unit for a smaller one. While this is doable, it would cost thousands of dollars. We do a lot of heating with wood, and keep unused parts of the house cold. The house is well designed for winter solar gain. On a sunny day even at temps of -20 C (roughly 0F) we build a morning fire, let it go out, and build another when the sun goes over the hill in late afternoon. The unit is properly sized for worst weather scenarios, and no auxiliary heat.

At present the indirect hot water tank is used as a buffer. This increased the cycle time from 2 minutes on/15 off to a cycle about 2.5 times as long during cold weather. This decreased but did not eliminate condensation in the stack.

A better solution to this would be a variable fire water heater where the blower and gas mixture are adjusted according to need. W. O. Stone makes a series of units that have a 4:1 fire range. E.g. It fires at 25% of full when the water temp falls below 140, increases to 50% if after two minutes the temp falls below 137, increases to 75% if it falls below 135, and increases to 100% if it falls below 132

Other solutions:

  • adding an additional buffer tank. Space makes this difficult.
  • adding an on-demand 20,000 BTU water heater. These are usually not designed as water reheaters, and a supply of not quite hot enough water may result in boiling in the heater.
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  • Yeah, given your load swings, a modulating burner for your existing boiler might be a wise investment Jun 28 '20 at 15:24
  • As far as I can tell, there is no replacement burner available. I would have to replace the entire unit. I have also considered trying to find out if the BTU consumption of the unit can be modified -- Replace the nozzles with smaller ones. This would be simple to reverse when we sell the house. The burners are simple air entrainment ones -- there is no fan. Can a valve be installed like a gas stove valve to permit easy adustment of the unit? Jun 30 '20 at 13:19
  • The issue with choking the gas to the burner down through an orifice or preset valve is you'd have to find a way to adjust the combustion air as well, and that'd require a full-on combustion analysis setup (a good HVAC tech should have a combustion analyzer on their truck, but not all do) Jun 30 '20 at 22:51

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