Water Heater Vent Freezing up...

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SpudMuffin

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I have a tankless water heater. We had to run the vents 35' to the side of the house where there was no porch. The rise is better than 1/4" per 10'... And we used 3" PVC... However, because it has been below zero the PVC is freezing up on the inside of the pipe... Last night we had no hot water so I took a gallon of hot water and slowly poured it down the exhaust vent and that did the trick...

I figure as long as its -30 I might have to do that every other day? Any other ideas?
 
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Jadnashua

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It that entire run through unheated space? Is the effective distance (this includes the additions for elbows, etc) within specs for the device? How is it terminated outside? Could it be from snow being blown in, and melting, then refreezing? Is it exiting on the side of the house with the prevailing wind?

When my boiler was installed, they installed the termination upside down...the condensate dripped down and sealed off the inlet when we had our first really cold weather. The installation manual showed it correctly, but the installers either couldn't read, or didn't look, or didn't care until they had to redo it!

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SpudMuffin

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Everything is to specs and it is installed with good slope. The run is in a crawlspace that is about 45 degrees...
 

LLigetfa

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The rise is better than 1/4" per 10'
How much better? When I had my new furnace installed, they put such a minimum slope on it that the velocity of the exhaust kept pushing the condensate up hill. When the condensate accumulated, it got heavy and sagged the pipe so it no longer had a proper slope to drain. A few more cycles and it made gurgling noises and failed.
 

SpudMuffin

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Well I used the larger option of pipe (3") so that isn't a problem. I directed one of my heater ducts into the crawl space and that took care of it until we get back above zero...
 

Terry

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tankless-frozen-vent.jpg


-6 degrees somewhere. Not where I'm at though. I did ski in 10 degrees this week though.

This vent is on a second story roof.

tankless-frozen-vent-2.jpg
 
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LLigetfa

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I drive a truck for a company that delivers water to portable washcars. The washcars have atmospheric tanks that they pump from. Sometimes the tank vent builds up with ice until it is completely sealed. We get calls complaining that they are out of water but when we get there, it is just that the pump cannot suck because the vent is blocked. We have a delivery schedule so they should not run out of water unless something goes wrong. Sometimes when I get on site, I find long icicles hanging under the washcar from a water leak. Sometimes the power goes out and the inside of the washcar is a skating rink.

I had hooked up to one tank and started to fill it when I noticed the vent was blocked with ice. My truck has a boiler on board for hot water pressure jetting so I started putting hot water to the vent. Meanwhile the fill pipe was building up pressure in the atmospheric tank. When the hot water let the ice loose it shot out like a rocket.
 

Dana

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A heat tape used for freeze protecting potable plumbing applied to the section of pipe that is outside of the insulation boundary of the house should work.

If the crawlspace walls aren't (yet) insulated, in a location as cool as yours (IECC climate zone 7) insulating them to the current IRC code minimum R15 continuous insulation would be worthwhile, even if there is some insulation in between the joists under the subfloor. That would mitigate against the vent condensing/ frost-up problem, but would also have a measurable heating season fuel use benefit (unless you're already at R50 between the first floor subfloor & crawlspace.)
 
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