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We have a high-efficiency gas-fired hot water heater (in our a basement) that is vented to the outside of our house. When the weather gets below -30°C the vent has a tendency to frost-over. When the water tank heater is unable to vent, it shuts off, depriving us of hot water. So when it frosts over we have to go outside and clean off the vent. Can you offer any suggestions to avoid having the exterior vent getting frosted over?

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Can you post a picture of the existing vent? – longneck Dec 19 '13 at 19:36
Is it a single vent or a fresh air intake and an exhaust vent? – dkwiebe Jan 19 '14 at 0:41
It sounds like you have a tankless water heater. Does it have separate intake and exhaust as dkwiebe asks or is it a concentric pipe (pipe inside a pipe)? Or is it a single vent for exhaust and it draws combustion air from inside (in which case it is not really a high efficiency water heater)? – Keith Hoffman Mar 5 '14 at 6:47

If you truly have a high efficiency appliance (a condensing appliance) horizontally vented, which way does your exhaust vent slope? Most manufacturers have requirements that there be a slope on the exhaust vent, either back to the appliance and a condensate pump to discharge the condensate into the domestic waste line or away from the appliance to prevent the accumulation of the condensate. You might try to find an installation manual for your appliance and see if the exhaust vent is sloped as specified by the manufacturer.

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Generally two things (be sure either/both suits the maker of your furnace before trying - you might also ask if they have any other suggestions, since your problem would presumably be common in your area.)

Insulate the vent pipe (primarily the section outside your house) and/or make it shorter, if at all possible.

Increase the diameter of the vent pipe (more cross-sectional area means much more frost has to form to clog it.)

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It's a code question for a plumber whether he can increase the diameter of a naturally drafted water heater. I'd say no, this is determined by the manufacturer and the btu rating of the appliance. – Keith Hoffman Mar 5 '14 at 6:48
Also I have never seen a high efficiency horizontally exhausted appliance with insulation on the outside of the house on the exhaust pipe. I can't imagine that this would help. – Keith Hoffman Mar 5 '14 at 6:50

One possible solution would be to insulate the exhaust. Any exhaust termination that exceeds the envelope of the house more than 2' is required to be insulated by code so prevent freezing from happening. Another problem may be that the pipe is not sloped back to the appliance, and there may be an elbow that holds condensate water and freezes during the off cycle of the water heater.

Now the best way to find out the issue is for you to post pictures of your appliance, the venting and the termination. Most homes use Power vented water heaters in Canada, however insulating and making sure the venting is sloped can help to keep these from happening in very cold climate areas. However it all depends on the way the appliance is vented.

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This is a common problem, is it the suction air pipe that is frosting over or the flue gas exit freezing off? Sometimes the vent is installed with the vent sloped down to the outside of the building which causes the water produced during combustion to freeze in the exhaust. The vent should always be sloped back to the appliance.

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