You shouldn't have to change the setting at all to get a constant temperature year round. Typically the gas valve has a temperature probe built in, so I would try changing that first. Nothing else comes to mind that would impact the temperatures. When are you taking these readings? Obviously, the temps will go down the more water you use until it heats up again. I will say most of the gas water heaters I worked on were manual controls. I did change maybe one with the newer type digital valve. When I get more time, I will look over my manuals, and see if I have one on this model. I wanted to give you some sort of an answer for now.
Hi, BryanS - I always take the readings throughout the day, usually across multiple days. I also try to take the temperature when the flame is off, since the temp should be near the current set point. For this current situation, the water temperatures were really low in the morning when I got up, but I didn't make any temperature measurements until later in the day. I then tested the temperatures over a 4+ hour period, constantly raising the thermostat when the flame was off but the temperature was still low. I moved the dial almost 1/4 of the full dial circumference before the temps were around 120 DegF again.
If you are in Phoenix metro especially (or anywhere else in our state for that matter), I highly recommend draining and flushing your water heater at least once per year due to the water hardness and high mineral content. You will be surprised by how much sediment (from every water company) is in the tank. Especially after 18 months.
Hook up a garden hose to the drain near the bottom. Run hose out to driveway.
Open drain valve.
Examine drain water. You will see ALOT of very fine tan sediment.
Let it flush until sediment clears. Close drain valve.
This sediment build up will affect the holding tank's thermal conductance properties, which in turn affects the thermostat's performance.
Also due to the nature of the typical gas water heater, the temperature in certain situations may vary by up to 30 degrees F higher or lower at the bathtub, shower, sink, etc. A flakey, really out of spec thermostat will allow more variation.
This model should have a draft test port at the top of the water heater. This could affect the temp swings. Check the manual on how to perform this test if you still have it. Check your flame too. A properly operating burner should produce a soft blue flame. Blue tips with yellow inner cones are satisfactory. Anything else is bad. Also verify nothing is obstructing the rooftop flue pipe vent cap.
Unfortunately, AO Smith does not rate very good when in comes to water heaters imo. If after completing the above, and you still have issues, you will probably need a new thermostat and/or manifold assembly. Most likely thermostat. Check which one you have, Robert Shaw or WhiteRogers.
Hi, beam current - I drained my tank about 6 months ago, and there was absolutely no sediment in the tank (I was pleasantly surprised!).
I'll try draining again in the next few days. I'll also get on the roof to see if the vent cap is blocked. I will also do a draft test this weekend.
I noticed this problem immediately with this WH. I had a similar problem with my previous WH, too - but not as frequent. I was hoping that the more modern tank/thernostat/thermopile designs would have helped, but it doesn't look like it now. Maybe something common - like a blocked vent cap - is causing this issue.