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Class B vent

Posted by bus_driver (My Page) on
Sun, Nov 21, 10 at 18:23

Encountered something new. On a renovation, the natural gas to the water heater was turned off some months ago and the heater removed last week. The vent has dripped water, apparently in the morning, each day even in the absence of rain. Yes, I know that rain is not supposed to enter the vent. I suspect that moisture condenses in the cold vent and forms frost during the night. When the Sun hits and temperatures rise above freezing by 9am, the frost melts. This my speculation. Anyone seen this before? And what is the effect on a heater that is not operating during a vacancy in cold weather?

Follow-Up Postings:

RE: Class B vent

BD, I've seen that on both hot water tanks as well as furnaces! Typically, the water makes it to the basement, hits the elbow at the wall, and flows down the vent pipe which is at angle and then drips into the heater / furnace and evaporates.

In the past, this wasn't a problem, as both devices had pilot lights and the vent pipe would be warm and the water would just evaporate!

It isn't uncommon to look at the vent pipe and it looks OK. Yet if you were to look inside the pipe and it's very rusty inside and sometimes rusted over 50% through!

In your case, when you turn off the gas to the WH, you could remove the vent pipe from the WH! Just don't forget to reinstall it!!!

RE: Class B vent

I suspect the condensation is from the air inside the house, and removing the hw heater vent pipe wouldn't help.

When one device is removed from a b-vent (eg when a furnace is replaced with a HE model the inside of the b-vent is supposed to be reduced or sleeved (so I am told) to reduce the volume, and also to deal with the potential corrosive effects of more condensation. I also gather it's theoretically possible to still vent a condensing HE furnace via the b-vent but you need the sleeving.

I watched HVAC people replace my neighbour's furnace, and they fed flexible aluminium ducting up the old b-vent. I guess it won't corrode the same as steel, but I do wonder about galvanic action between the old steel and the aluminium. I guess the latter's supposed to act as a sacrificial piece.

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