A waterheater question

I have a 3 year old water heater, gas fired. Just last night I noticed the top of the water heater is starting to get rusty! What’s the deal?

My theory (which is mine) is this is being caused by condensation. The water heater is in the garage, and because of how the garage is built the exhaust flue for the water heater is fairly long. So I think the metal flue gets hot while the burner is lit, and when the burner goes off the flue cools enough water condenses out of the air onto the flue, and then drips down on the top of the water heater.

Any other ideas? Or if this is the problem, any suggestions on what to do about it? I want to keep this water heater a good long time, but if the rust continues at this rate I’ll be replacing it much sooner than I’d like.

Try wrapping some insulation around the exhaust flue (if that really is the problem). Double check with someone who knows if this is a fire hazard or not but I dodn’t think it is. If you look between the water tank and the outer shell of the water heater I think you will find insulation tucked in there. If that isn’t a hazard then I doubt the same stuff wrapped around the flue would be any worse (but still check with someone who really knows).

Hey Jeff.

You ever lived in Scranton, PA? I knew a guy named jeff that had an id Jeff_42 or something close and am wondering if you’re him.

The problem is indeed caused by condensation, but IIRC, most of the condensation comes from the exhaust gas itself, instead of the surrounding air. I think it has something to do with water being a by-product of methane combustion.

I used to work in a plumbing supply house, and we’d see a lot of gas water heaters brought in for scrap which had rusty tops. This wouldn’t have any effect on the operation of the heater itself, as it’s a glass lined tank, which is surrounded by insulation, and contained in a metal jacket. Even if the top of the jacket were completely rusted, it would not reach the tank itself.

Part of your problem may be the flue itself. If it goes straight up, then any condensed water would run back down, then along the sides of the bell hood and drip onto the top of the heater. Most installations I’ve seen have at least one 45 or 90 degree bend. That might have something to do with it.

At any rate, I wouldn’t insulate any flue pipe, since they get quite hot and you’d have a fire hazard, or at the very least, some funky odors coming from cooked fiberglass. The insulation in the heater is only subject to a temperature range of approx. 120 - 150 F, much lower than a flue pipe.

On a side note, you have more of a rust problem to worry about from inside the tank, and with the T/P valve, than you would from the flue. If you’re not doing so already, you should drain off about a bucket of water from the bottom drain valve every 1 or 2 months. You should also open up the T/P valve just for a second, to make sure it’s not sticking. (Your T/P valve will be up near the top of the heater, and has a small lever-type handle on it. Most codes require that the valve be drained to the outside of the house. If you have no pipe attached to it, use a bucket, but be extremely careful, as the water will be very hot and under some pressure. You only have to lift the spring-loaded lever a bit to get water to come out.)

My advice would be to go down to Home Depot, or whatever you have in your area, and ask about the rusting problem, so you can get another opinion. It’s probably nothing to worry about, and it might be solved by changing your flue pipe configuration.

Dire, thanks for the info. It’s nice to know this isn’t really a structural problem. I’ll also take your advice about bugging someone at the local Home Depot.

Just FYI, the flue runs straight up for about 2 feet, then makes a 45 degree bend (and then joins the furnace flue).

I’m somewhat worried about appearances here. We’re getting ready to sell the house, and even if this is structurally fine it doesn’t look that great. What do you think of treating the top of the heater with one of those rust-fixing formulas, then shooting a layer of spray paint over that (after taking adequate precautions to NOT make a torch out of the spray can!)?

Home Depot!?! Good Lord…don’t ask those idiots. Try posting your question here


It’s the discussion board of Fine Homebuilding magazine. You stand a much better chance of getting the correct answer there than from some HD moron.

Dire Wolf, don’t they now require heaters to be a foot above the floor in a garage? I see a lot of them that way now.

Venkman, most of them have 5 year tank guarantees, right? They are pretty cheap & easy to put in these days if you need another one.