AO Smith FCG 75 water heater does not maintain temperature

allanfan

My water heater would not kick in to reheat the water automatically. See attached picture.

I have to manually set the temperature knob to "A" or "B" to trigger the water heater to heating again. I've been manually "turn on" the water heater every night for the past 2 weeks. Any idea how to fix? Thank you.

Big Ed_4

What do you normally have it set on ? What happens if you leave it on B ?

allanfan

I should have added more context — I moved to this house about 3 weeks ago, and the water heater has this issue ever since. I think the house has been vacant for a while, so possible there is some sediment deposits.

I usually leave the temperature knob at the normal level, i.e. the level before "A".

I'll leave it at "B" overnight and see what happens tomorrow. I suspect even by leaving it at "B", it would drop to 80 degree and not reheat. But I'll report back tomorrow.

hot_rod

what does the temperature drop to before it re-fires?

Lets say A setting is 115 degrees F. It may need to drop to 95 F before it fires up again. The control has a delta that it works around.

No harm in going to B setting.

Ideally you don't want to supply over 120F to the faucets. Check the temperature at the faucet right after the tank turns off.

allanfan

My experience has been that this tank never automatically re-fires. So the thermostat on the tank generally bottoms out at 80 degrees.

Jamie Hall

Can you send a picture of the gadget with A and B settings? Is there an aquastat other than that invollved?

allanfan

See attached picture. I don't know what is an aquastat, and I don't think there is such a device here.

Larry Weingarten

Hi, Modern controls can have a thirty degree deadband, or space between on and off points. I'd just turn the knob slowly hotter until you hear and feel it click on. Then, live with it a bit and see if it reheats by itself.

Yours, Larry

allanfan

I left the knob at "B" last night, it reached 125 degree last night. To my surprise, it was still at 125 degree this morning. It went down to 115 degree this evening. I'll continue to leave it at "B" and see if it comes down more tomorrow.

allanfan

After leaving the knob at "B" for 2 days, it was back to 95 degrees last night. I'd assume it should have kicked in below 100 degree?

I manually "kick in" the heater by turning the temperature back to "normal" and then turn to "B".

Larry Weingarten

Hi, You could leave it halfway between B and C, and see how it behaves. 🧐

Yours, Larry

hot_rod

Where are you measuring temperatures? At a running faucet?

Gas fired WH control read temperature near the bottom of the tank. So if it shuts off at 125, the top of the tank will be a bit hotter due to stratification. Unless it is constantly circulated with a recirc.

Cold should enter the tank at the bottom, when the control sees the cut in temperature it will fire. But you may still have warm water being pushed out.

If it runs hot for a few seconds and turns cold it could be a failed dip tube.

Since there are no temperature numbers, you will have to test with a thermometer.

Although controls used to have temperature numbers, I think they got removed and replaced with letters for liability reasons?.

Bottom line, if you are measuring at the faucet, the initial water from the tank may be 125 and the bottom of the tank is lower, maybe 105. As @ Larry mentioned there is at least a 30° difference from where it turns off, then back on.

A small possibility the control is bad.

Certainly no harm in turning it up a bit more. If it comes our near 130°consider a mixing valve. Plumbing Above that scald potential rises, which can be an issue with small children or the elderly..

allanfan

I didn't measure the temperature, i just read the thermostat at the hot water out:

So far, my experience has been the thermostat at the hot water out feels pretty accurate.

allanfan

Shall I try draining the water heater? (I've never done that before, hopefully not too hard)

This water heater has been sitting idle for an unknown period of time (maybe over a year), so I think there is a possibility of buildup around the thermostat at the bottom/inside the WH.

Larry Weingarten

Hi, The thermostat probe behind the control would have to be buried in sediment to create this sort of problem, and I've only very rarely seen that much sediment. So, I doubt draining or flushing will make any real change.

Another thing I've found on modern heaters is that as the room they live in warms up, so does the delivered water temperature. So, if the room your tank is in is colder than normal, it could be a big part of the problem.

Yours, Larry