Are you measuring the flue's output with that thermometer? Nothing would normally get that hot except combustion leftovers and things possibly heated by that. Check the flue.

It's one of those thermometers that just sits on a little stand (digital). I placed it on top of the water heater, and that's where we're getting the high temps.

Do you think that the flue is blocked?

At 160F, you're almost certainly measuring combustion heat that should be going up the flue instead of popping off your fire alarm. Especially since you say your alarm sensor is of the rate-of-heat-rise type, my first guess would be obstructed flue or flue installed in such a way that it's not functioning as it should. If either one, it's likely easy fix.

While your there next to the unit, turn the heat setting up so the burner lights up and leave it there for a bit while you look/feel/examine the flue and area around it on top of the heater. I'll bet you'll find the problem quickly. Don't forget to turn the control back down to its normal range when you're done.

Well, I did what you're suggesting, and got the same results -- I turned the control back to normal once I hit 150 degrees.

I'm going to do a little test now. We have an exhaust fan in the ceiling of the garage. There are three air intake vents in the garage door, and the idea is that the exhaust fan brings the cooler air from outside inside the garage, and then exhausts the hot air into the attic, where it gets pushed through a gable vent back outside. The exhaust fan is 7-8 feet from the water heater, so I'm wondering if perhaps it's strong enough (1350 CFM) to be pulling the exhaust from the hot water heater away from the flue and into the garage proper.

I'm going to try turning off the exhaust fan once the water heater cools down, and then turning the heater back up to high with the exhaust fan off to see if the problem corrects itself.

Negative pressure in an enclosed structure (by using an exhaust fan for example) can definitely draw the products of combustion back into a building, which is probably what you're seeing unless the flue is faulty.

If the fan draws more air than the door vents can supply, then the hw heater will be having the hot air drawn back in. be careful.

Not knowing what the average bathroom fan draws, I'd say 1350 cfm is a pretty good airflow.

Well, I ran the second test, with the fan off, and nothing was triggered. So I'm pretty certain now that we're getting a negative pressure issue.

If I got a power vent or a power direct vent, I'm thinking/hoping that this would solve this problem, since the exhaust gases would be sealed off from the garage, and would be forcibly expelled outside. What do you think?

This entire discussion is asinine.

Some half-witted fool installed a "rate of heat rise sensor" two feet above the water heater and now you are considering tossing out a perfectly good water heater because the alarm went off?

That shinny galvanized pipe extending upwards from the top center of your water heater is the flue pipe, and the flue pipe is intended to convey the exhaust gases from the burner outdoors where it vents to atmosphere. Do you suppose it might be possible that as the exhaust gases are conveyed on the inside of that pipe that perhaps the pipe wall might get hot and radiate some of that heat energy into the surrounding space?

Anybody with even a modicum of understanding about water heaters and how that alarm sensor functions would have known that a sensor of that type should not have been installed that close to the heater.

Don't know how big your garage is but I think you need a source of make-up air for the water heater's burner if your garage is that tight. Even WH in closests have well-vented doors. Also, 1350 cfm is pretty hefty exhaust vent.

Disagree with lazypup about the sensor. May or may not be positioned in the best possible place but the top of your water heater shouldn't be getting that hot anyway. If it was exhausting normally through the flue, it wouldn't. That sensor just called your attention to an unsafe situation.

Fix the flue thing/negative-pressure thing/make-up air thing and this problem will go away. Any/all of these need to be made right for a safe installation anyway.

Would a power vented water heater fix the problem? I'd rather not get rid of the exhaust fan, as it does cool down the garage nicely. Right now I've turned it off.

lazypup, I disagree with your conclusions. The problem only occurs when the exhaust fan is on -- it's not the heating of the flue pipe that is causing the problem, as if the exhaust fan is off, the rate of heat rise sensor does not trip. Also, your tone is not appreciated. Further, you conjecture incorrectly, the sensor is not installed "two feet above the heater," as you suggest: perhaps another "half-witted fool" should read the original post more clearly before posting?

There is no mention of an exhaust fan in your original post however, in your original post you made the following statement:

Quote"I placed a little digital thermometer on top of the water heater (the sensor was a couple of feet away from that)"

Is not "a couple feet" approximately 2 feet from the heater?

A power vent will make this situation worse, 2 exhaust fans running at the same time. All you have to do is increase the size of the air intakes or add a few more.

You say you have three intake vents, what size are they?

If the water heater is next to a outside wall, put a vent hole in the wall the size spelled out in your local codes. that will take care of the problem. if it isn't next to a wall run the proper size duct to supply outside combustion air. See your local AHJ for the proper size air intake requirements. this is assuming you live in a warm climate.

Or you could change the water heater to electric and that would solve all your problems. Then you can run the exhaust fan in your garage all you want.

Good deal. I've taken a look at all the options, and I think think I'm just going to get a power vent model. There is sufficient combustion air, really the issue is the exhaust being sucked into the fan. With a power vent model, the exhaust would be "sealed" and pushed directly into the flue. (We have a dedicated, short-line exhaust flue for the hot water heater, so this should be an easy install.)

Bigger air inlet vents would help but when ventilating in such a situation there is that risk of sucking back those combustion products, and on the upside you will use less gas, if you end up with an high efficiency model.

I don't suppose you've considered some sort of alternate hw system, eg solar? You would still need some sort of backup but that could be electric or gas demand.

I think that the negative pressure issue is correct, although that's probably not the best place for a heat detector.

That said, you need to solve the negative pressure situation or install a water heater that gets its combustion air from the outside and (obviously)vents to the outside. It's not enough to simply use a powervent to vent the unit to the outside. Depending on the degree of pressure differential, you could still have problems just using a powervent.

Units that use outside combustion air are made for places where there isn't sufficient make-up air for combustion such as a closet or a very tight house.
Most of the time when the heater can pull combustion air from the outside it's called a "direct vent" option. However, you have to be careful with the terminology because a direct vent water heater can be something that side vents without a blower (and also can pull combustion air from the outside)

So, if you can't solve the negative pressure problem, I would look at powervent heaters with an outside air intake option, a side vent direct vent unit (no AC power needed) or a tankless unit with a direct vent option.

Argh -- I was hoping that a power vent of the exhaust gas would be sufficient to solve the problem, and that I wouldn't necessarily need a direct vent of outside air for combustion. Just having a power vent of the exhaust gas would be easy to retrofit into our system, and as a big added bonus, the power vent systems seem to be incredibly energy efficient. Having a direct vent for the combustion from outside air would be more difficult to retrofit in, and such systems for whatever reason don't seem to have as high energy factors as the power vent-only designs do.

I'm thinking that perhaps I may just permanently disable the exhaust fan. The exhaust fan helps in late spring early fall when the outside temp in Phoenix swings from high to low greatly overnight. In the summer time, it doesn't help that much, since the outside temp stays hot all the time. I already have radiant insulation on the inside of the garage door (we're south facing), which has greatly assisted in rejecting heat from being soaked into the garage.

Alternatively, I may look into solving the negative pressure situation. Presumably, I could cut more holes into the walls of the garage, and then verify that the exhaust gases are not being sucked out of the flue. I'm thinking I may just be on the threshold of the negative pressure situation, insofar as the problem does not occur all the time. On the flip side, I don't really want to start cutting holes into the walls of the garage willy nilly unless I know it's going to solve the problem.

Your negative pressure issue with that 1350cfm fan will be solved ONLY by having enough air inlets of sufficient size. Or you could downsize the fan. How large are the present inlets?

It's summer now. Why not just turn the exhaust fan off for a while and see if the problem recurs. I'll bet it won't. And you'll have your answer and have spent nothing.

Yeah, the problem goes away when I turn off the exhaust fan. It's not a problem now, as the garage cannot be cooled any lower than ambient temp anyway, and the ambient temp is quite hot (106 right now, for example), and doesn't go down much at night. The radiant insulation on the garage doors prevents the garage from getting more than 5 degrees or so above ambient.

I like the exhaust fan for the spring and the fall, where temps may increase to 90+ during the day, but quickly drop into the 70's at night. At that time, it's nice have the exhaust fan decrease the garage temp to the 70's in the evening so I can work in the garage comfortaby.

I did some research, and apparently an 8"-round duct provides about 350 cfm of air inlet. At about 50 sq inches for such a duct, that means about 7 cfm per sq inch in general. I have to measure my air inlets (basically, three rectangular grates cut into the garage doors) to see if I'm getting 1350 cfm worth of air inlet. If adding just 1-2-3 air inlets would work, that's going to be my first choice.

In your calculations don't neglect to add dimensions to make up for losses from louvers, screen coverings, and eventual obstructions from twigs, leaves, spider-webs, etc. Considering cut-out size alone won't quite cover it. I'm sure there are rules of thumb for such considerations.

hello,

regarding negative pressure issues: my water heater was flashing error code 3 blinks=pressure switch OR TCO, resulting in the burner turning off until the exhaust cooled down. turned out it the temperature of the blower exhaust was above the temperature limit. ALSO, the burner flame had too much orange in it indicating lack of oxygen.

problem was that i had installed storm doors over all the exterior doors which made the house much more air tight, so the power vent blower had trouble supplying combustion air.

my solution was to partially close the gas supply valve to the water heater . the supply air was then able to meet the reduced requirement of the burner, and only sporadic orange flame tips.

This is a very bad (half) solution!

Fix your air intake issue. If you need to create an outside air intake, do that. Better yet move to something that isn't a gas fired appliance OR one that has an outside air inlet.