New Furnace - Hotwater Heater seems to flameout...

Long story short, when we moved in 3 years ago we installed a 95% furnace in place of an old gnarly gas fired monster. Immediately we started having problems with the water heater flaming out. Since we never lived in the house with the old furnace, we can only assume the water heater worked adequately prior. We had the gas company come out and check and our supply was well within spec. The unit is a Kenmore 50 gal 12 year I think which has about 2-3 years left. We spent some time with the water heater making sure nothing was blocking the vents, giving it a good cleaning etc..Still no luck. What we were able to do was to back off the inspection plate (bad idea, I know) just slightly, assumedly admitting more air into the chamber and seeming to aleviate the problem.

The other day we shut the gas off to the furnace (independent branch off the main gas line), did some tweaks to the furnace (adjusted the fan speeds to fix temp rise), and now the heater is flaming out again.

Before I go calling for repair, is this potentially from:

the furnace creating a backdraft (both are installed in a fairly large, open space)
an improperly adjusted gas regulator on the water heater
a burner on its way out
or X?

I've got a baby on the way in a couple of weeks and starting to get that urge to stay up all night making everything perfect with the house...doing plumbing or HVAC reapairs as a DIY'er is tricky at best, and more so at 2am


Thanks for the advice

Depending on how tight the house is, the WH may not be getting enough air to burn properly. A 95% furnace typically is a closed combustion device, so it is not using conditioned (i.e., air from the house) when running, but if there are leaks in the ductwork, it could be affecting the overall air flow in the house.

See if you notice any difference when say the clothes dryer is running, or a stove exhaust vent is on, or a bathroom vent. All of those also throw out a lot of air in the house and could disrupt the drafting of the WH.

If the forced-draft high efficiency furnace is enough to backdraft the atmospheric drafted hot water heater it's practically an emergency. But it's probably an "orhpaned hot water heater" issue more than a backdrafting-only-when-exhaust-venting problem (though they can be related.) The orphaned hot water heater story goes something like this:

If the HW heater had shared a large flue with the old beastie furnace, the heat of the furnace exhaust kept the flue warm enough in winter to keep it drafting. Without that heat input the flue liner runs cold, and the thing never drafts correctly, and can even backdraft from wind currents. They often seem fine all summer long- then develope drafting issues when the weather turns colder. It's even worse if the flue is a masonry chimney on an exterior wall, which runs cold all the time in winter, since it's not surrounded by warmer conditioned space. With only the HW heater on it, the flue needs to be re-sized for the HW heater's BTU output to draft properly- probably a 3" round, maybe 4", depending on the particulars of number of bends, stack height etc. Even if it usually drafts correctly, an oversized flue with only a hot water heater on it will be cold enough to condense the slightly acidic natural gas exhaust, and slowly degrade the terra cotta liner & mortar, slowly rotting the chimney from the inside out.

Installing a properly sized flue liner might fix the problem, but it can be cheaper/better to just buy a power-vented HW heater and side-vent it like the furnace, and just seal up the old flue. If there are state/local subsidies available, a higher efficiency condensing tank type hot water heater might make sense.

Thanks for the replies. I assumed based on the poor performance of the heater when I wrote the original post that my pilot went out again, but when I got home, I realized it was something else. I'm looking at airflow and thermostat now...

I've thought about swtiching to a power vent, but this has several years left, so I'll try to manage for now. What I did realize in all my research is that there are 2 stages of 'filtration' for the incoming airflow to the hotwater heater. One on the outside of the collar the heater sits on, and one on the bottom of the tank where the air is actually drafted into the combustion chamber. I seem to be missing the outer one, which leaves the innerone doing all the work. I was able to get under there with a bottle scrubber and give it a few good scrubs, while watching the flame, and it seemed to make the flame more robust.

I'm also not convinced the thermostat in the tank is working correctly. when measuring the output water at the closest tap, I'm still coming in about 20 degrees lower than what is suggested I should be based on the setting I've selected. I may 'ignore' the termostat and just set it untill I get to the desired temp and call it a day.

IF there is a lot of mineral deposits in the tank, that can affect the overall performance and sensitivity to temperature. Does it make a lot of popping noises when it is heating?

not at all. and I just flushed it with minimal sediment coming out.

OFten, if it's been there for awhile, the sediment ends up being nearly solid, and may not drain. Depends on how hard your water is, how old, and how much it is used.

You cannot combine a forced draft. 95% furnace with a gravity draft water heater in the same flue. Either the fan is forcing exhaust back down into the water heater and putting out the fire, or the room is starving for air. Be sure you have adequate combustion air. Sometimes old buildings have no direct method of supplying air. Sometimes it is a window that must be left open or a door has been added.

A carbon monoxide detector might be a good investment in your case