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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 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 at 6:47
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2 Answers 2

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 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 at 6:50
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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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