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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
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I have experienced the same problem. My intake frosts over and the furnace shuts down with a differential pressure fault. The cause is; when the intake air temperature falls below the dew point the relative humidity of the intake air becomes greater than 100% which means the moisture in the air will condense or precipitate out of the air onto anything that is colder than the dew point. When the furnace fires it draws a huge amount of air in through the cold air intake port. If the air temperature is less than the dew point, the excess moisture in the air immediately condenses on the cold intake screen (if you have one) and the inside of the intake pipe. As long as you draw this moisture saturated air across the cold surface of the intake port and the subsequent frost layer the moisture will continue precipitating out and make the layer(s) of frost thicker and thicker until the pipe is completely closed off or the screen is completely plugged. The fix; First item of business was to remove the bird/bug screen. This eliminated about 80% of my issues. But every once in a while when it would get really cold (-20 to -50F) and the dew point was above 0, the intake pipe would completely fill with frost. So, I reconfigured my intake pipe to be as close to the building as possible and constructed a foam insulated hat that covered the top, sides and outboard surfaces of the intake, leaving the space between the building and the intake uninsulated. This allowed the intake to be "heated" by the escaping heat from the side of the building. I was only looking to get the intake pipe temperature above the dew point and I believe this has worked.

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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.

  • Even with proper slope there is a lot of moisture exiting the pipe at extreme cold temps this freezes and causes problems. – Ed Beal Feb 7 at 15:10
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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.)

  • 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
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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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I have a Navien 210 hot water heater, in cold weather I take the cover off the unit inside the house and use the combustion air that is fed from another fresh air pipe in my boiler room that also feeds my boiler for house heat

  • Are you sure the heater is not backdrafting under these circumstances? – ThreePhaseEel Jan 3 '18 at 23:16
  • Hello, and welcome to Stack Exchange. @ThreePhaseEel is right; there's a risk that you could get exhaust gases pulled back into the house (e.g. if the house is otherwise well-sealed and you turn on the exhaust fan over your stove). – Daniel Griscom Jan 7 '18 at 23:52
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Tom’s response on Nov 7 17 is spot on with the exception of why the moisture drops out in the intake. When moisture saturated air is drawn into the air intake it increases in velocity and creates a slight pressure drop. The pressure drop creates what is referred to as the JT effect which is a cooling effect. The cooling of the saturated air causes what ever moisture is in the air to drop out and cause frosting. The frosting if left unchecked will eventually block the intake to a point where the differential pressure permissive on the hot water inhibits the hot water heater from firing. The solution resides with creating a design to cause the JT effect to occur in a warm region of the intake piping. Reducing air velocity in the piping sections that are exposed to the outside temperatures will move the JT effect to a warm region of the piping. If the JT effect occurs in the piping within the house the inside temperature will offset the frosting risk by compensating the temperature drop due to the JT effect. If the intake is insulated as Tom describes you are effectively moving the temperature differential further out of the house to the external piping where the JT effect is causing frosting. They balance here is very slight but just enough to freeze off the intake piping. So the trick is to design the system so the JT effect occurs in a warm region of your piping. If you increase the intake diameter and force the JT effect further into the piping inside the home you will get the same result of no frosting. If you heat the external piping to compensate for the JT effect outside you will get the same result of no frosting. One other consideration is to reduce air flow to a level just above the threashold of the pressure differential switch by throttling the air exhaust. The reduced air volume also reduces the air velocity and moves the JT effect back into the house piping. This might explain why some houses have problems and some don’t. Large heaters from some manufacturers have over sized blowers and move more air then needed.

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    Hello, and welcome to Home Improvement. This seems interesting, but it's hard to read without any paragraph breaks (the "Wall o' Text" effect). Would you spend a little time, edit it down to the core ideas, and add a few paragraphs? Thanks. – Daniel Griscom Feb 7 at 17:23
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With all the answers talking about the slope there must be a lot of bad installs out there. I have seen this happen with a properly installed intake / vent line. The extreme cold outside and the humid air of the exhaust causes the frost to form. I have seen this below -10. I first tried insulating the pipe where it exited the roof, I then tried some self regulating heat tape on the pipe, this fixed the problem I probably waste some power but the heat tape is only 10' long and it has kept the pipe from freezing up again. If you use this method make sure to get self regulating tape that is for use with plastic pipe. The self regulating can be wrapped over itself and won't over heat like some heat tape will.

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My problem is with the intake pipe frosting over due to the high amount of moisture being blown out the exhaust pipe. I've tried separating the pipes more and directing the exhaust up and lowering the intake. I've been told that the moisture is coming from the gas itself, and the only way to cure it will be to move the intake pipe to the other side of the house.enter image description here

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Dec 4 at 22:39

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