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Sweating Fuse Box

Ive been building my dream garage for about a year now. A few months ago we installed the heater. It is a natural gas Procomp 30000btu heater, and it works well. A few days ago I was working in the garage and I noticed a small puddle of water on the concrete floor, right under the fuse box. You could see a water mark on the sheetrock under the box, where a stream of water ran down to the floor. I removed the cover to find a lot of water in my fuse box. There was condensation on the back of the box, on the busses and there was water dripping off of the wires that comes out of the standoff. This dripping water was pooling up on top of the breakers.

The box is mounted on an exterior wall, with no insulation between the box and the exterior plywood. The garage is well insulated, and finished with drywall throughout. The heater is a natural gas (blue flame) ventless heater and the owners manual does say it can produce a little bit of water, but that it should just form a puddle under the heater. The outside air temperature was 10f inside 63f. The stand off goes up through the roof and does include a weather head.

Not sure what other details you guys would need to know. Feel free to ask.

Thanks in advance!

Re: Sweating Fuse Box

Condensation happens when warm moist air comes into contact with a cold surface. Ventless gas heaters generate high humidity, the box being on an exterior wall with no insulation provides a cold surface. Either install a dehumidifier or move the breaker box.


Re: Sweating Fuse Box

The box should be mounted on the inside face of the insulated wall. At a 10 degree outside temp, I bet the back of the box against the sheathing was close to 35 degrees or lower. Well below the dew point in the garage. If the conduit to the roof is against the exterior sheathing that is another place for condensation to form and run back into the box.

Re: Sweating Fuse Box

Outdoor panels make sense in warm locations. But in your area, it should be inside. Get a licensed electrician to move it in.

Re: Sweating Fuse Box

After reading the owners manual paying special attention to the water vapor produced by this unit, I find that ventless heaters such as this, produce appx 1 ounce of water per 1000btu. On the lowest setting, it is 16500 btu/hr so that would be nearly a pint of water per hour. on the lowest setting.

All things being relative, that sounds like a lot of water to me. Guess I am shopping for a dehumidifier!


Re: Sweating Fuse Box

Here's an idea: build a door or removable panel to cover the electrical panel. It should have at least 1" of rigid foam insulation, and a gasket or weatherstripping to seal the edges. Use closed-cell foam weatherstripping.

That will effectively put the panel outside of the heated area. The weatherstripping will prevent moist air from reaching the cold electrical panel; the insulation will prevent condensation on the door or cover panel that you build.

Re: Sweating Fuse Box

A dehumidifier may help with moisture in the air a bit and with stored wood, but it likely won't help with condensation at the box when it is essentially against an uninsulated wall at 10 degrees. An electrician should know better than to install it like that.

Re: Sweating Fuse Box

Wade, the NRC now requires any conduit from a cold area or underground feed to a warm area be sealed. They must be sealed at least at one end.

There are two methods I use: Expanding foam and duct seal both are available at an electrical supply house and are listed by UL.

As the mfr. literature stated "products of combustion" of your appliance is water vapor and it will condense on any cold surface such as a cold window, water glass or in your case a power panel.

All of the methods mentioned are fine to solve your problem.

Re: Sweating Fuse Box

I'd seal the conduit at the weatherhead, then I'd loosen the panel and slide a sheet of 1/2" styro (like R-Max) behind it then reinstall. That isolates it from the exterior surface doing most of the 'chilling'. It will stick out 1/2" from the wall afterward but that shouldn't matter in a garage. "Picture frame" with trim if it bothers you. And seal things at the weatherhead, not at the bottom, the reason being that if it's sealed at the bottom, a build-up of water could occur in the conduit, leaking or dumping quickly into the panel when the sealant fails.

Unvented gas appliances need outside air feed and ventilation, which sort of offsets using them for heating purposes. You'll need a dehumidifier and it would be wise to install a carbon monoxide detector as well. With all this, it's usually better to go with vented gas appliances wherever you can. Save the unvented stuff for only places where proper venting is highly impractical or impossible ;)


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