back draft

[henk2845]

I have a problem with downdraft through the vent pipe of my water heater. I recently installed a continous flow gas water heater (replaced the standard 40 gal gas water heater). I now have a problem with down draft through the vent pipe. I never noticed it on the 40 gal heater, but the draft is so strong that it prevented the gas from firing. The gas nozzles on the new heater are directly underneath the vent pipe. I put in a damper and it helped somewhat, but I am afraid it is not venting enough, causing some carbon monoxide to get back into the utility room, where the heater is located. Also, the utility room is getting real cold in the winter and I can feel cold air coming down the vent pipe into the heater and into the room, despite the damper I installed. Someone mentioned that I should extend the vent pipe on the roof. How high should it be extended, if any?

Thanks for any suggestions

henk2845

This is NOT RE.

[southpaw]

Hi Henk

  The reason your vent is down drafting is because cold air is heavier than warm.

When your storage type water heater was hooked up the vent goes directly up the center of the tank, this keeps the warm air constantly flowing up the chimney.

when the burner was off the hot water supplied the heat to keep the draft going.

This is part of the efficiency advantage of the tankless water heater, no constant flow of hot air up the vent.

Now, if your tankless heater is not running the vent cools to outside temperature and any negative pressure in the building, bathroom or kitchen fan or wood stove draft will start a cold  downdraft in your vent pipe which will only stop if you can close it completely or warm the vent and the air in it to start the draft.

I have seen cold downdrafts in tankless water heaters freeze and split the heatexchanger.

Extending your vent would only help if you could keep the entire length warm to eliminate the column of cold air from falling out on the floor.

A power vent may be the only cure.

Southpaw

[dnix71]

I live in south Florida, but have gas hot water and a gas stove. The code here requires a 3 foot high stack on the roof. The last roofer just hooked the water heater vent to the attic. When the plumber saw that on a service visit, he had a fit and walked off the job.

The city fixed it for us, for a fee. There was a proper stack on the roof nearby, but the roofer never connected it properly and the roof inspection missed it.

I would look at the cap on the vent on the roof. If the cap and flame arrester are intact, then it should be hard to get a downdraft. The maker of the new heater must also specify the diameter of the vent. It may be different than the original heater.

Where I last worked in Pompano Beach we had gas fired thermographers on our presses. If the wind blew from the south we would get sick if we didn't open the doors, because that wind forced exhuast back down the stack into the shop. That setup supposedly met the code.

Installing gas devices is best left to a pro, but that's no guarantee. I had to install a replacement stove in my apt because the city worker ran away after seeing the current hook up. I live in a standard wood/block duplex, but someone used x-pipe to add a meter in the back for my stove and copper water line to carry the gas from the meter to the stove. X-pipe is for mobile homes, but it has a city meter on it and no one there wants to deal with how it got that way and stayed that way for 20 years.

[dnix71]

If a power vent is required, then the unit has to be set up not to run if the power is out, otherwise you risk killing yourself and family from CO.

[dnix71]

But a power vent doesn't have to use inside air. It could be setup to pull outside air for combustion, so you don't waste warm house air.