I asked 3 plumbers the same question and they all gave me the same answer which did not answer my question. My thermostat on my hot water heater seems to be bad. No matter hwere I set the temp it comes out at exactly 120 degrees. I like it to come out at about 102 degrees as it has for the past 10 years or so. It is in my shop and I use the hot water to take showers.
My question was. Is their a secondary safety thermostat built into the thermostat that keeps the water from getting over 120 degrees or is it just that my thermostat won’t adjust anymore? My concern was that it might get too hot, I can live with the 120 degrees as long as I feel it won’t keep creeping up.
They all had the same answer about the blow off safety valve if it gets too much pressure. I allready knew about this and it had nothing to do with my question. How could I have rephrased my question, I was getting frustrated.
The main problem with the question you posted here was that it gave no indication of what it was about. I have edited it to indicate the subject. Please use descriptive thread titles.
Do you have a downstream tempering valve that limits water temp? Or temperature-controlled faucets that either control or limit hot water temps?
Tempering valve: You’re looking for a tee valve on the water heater outflow with a knob and a connection to cold water. It thermostatically blends hot and cold to a preset outflow temp. They are more common on “boiler” type water systems than the more precisely limited gas or electric water heater, but code or a prior owner may have installed one.
I did the original installation and all the plumbing. Just a gas hot water heater with no attachments. I am a little puzzled why it stays at exactly 120 degrees. It makes me think it has some kind of mechanical default built into it in case of failure of the thermostat.
As far as I know, water heaters have one thermostat. The pressure relief valve prevents the heater from spectacularly exploding and destroying your house if the thermostat fails (yes, they can do this – watch the Mythbusters test if you haven’t seen it – the heater rocketed five hundred feet into the air). Your question was “if the thermostat fails, is there another thermostat that will prevent it from getting too hot”. The plumbers understood it to mean “if the thermostat fails, is there a second thermostat (or something else) that will prevent it from getting too hot (and exploding)”, which is by far the more serious concern.
Given the horrendous consequences of failure, and given that one of the two safety systems in your heater seems to be operating incorrectly, I’d be moderately concerned about continuing to use it without determining exactly what’s wrong.
My hot water heater has two thermostats-one at eye level, and one closer to the ground. Check to see if there is a small panel closer to the floor. If so, and if there is another thermostat, make sure that both thermostats are set to the same temperature to get the desired effect.
This brings up another question. I of course do have the proper blow off valves installed but if I didn’t what would keep the excess pressure from just pushing back into the water system once it exceeded inlet pressure of about 80#? Unless maybe the 3/4" pipe could not relieve the pressure fast enough?
It will do that. You would need to close off the water supply to the tank and then have the relief valve fail. The relief valve opens from heat or pressure.
Depending on where you live, there may be a check valve installed that would prevent water from backfeeding. In the OP’s case, since there’s no check valve then it would feed back into the water system if the pressure got too high. The pressure can build up faster than it can be pushed out through the water inlet, though. In any event, chances are the pressure relief valve is functional, so no worries.
As for dual thermostats, I’ve seen that on a lot of electric heaters but not on gas. Maybe some have it and I’ve just never seen it. I dunno.
As for any other safety device, most gas water heaters (at least newer ones) have a thermal cutoff, but it doesn’t function like a thermostat. If it gets triggered, it shuts off the gas and you have to reset it and re-light the heater to get it to work again. It wouldn’t cause the symptoms that the OP describes.
The symptoms of the OP make me think that the thermostat has a problem.
Enough force to take a water heater through a roof and 500 feet in the air is MORE than enough to shear either
A) two 1/2" 90 degree solder joints or the copper line itself, and a 3/4" flex or rigid gas line.
or
B) two 1/2" galvanized couplings/dielectric unions, and a 3/4" gas line.
Fittings are galvanized steel and plenty brittle under those kind of forces. Hell, even the nipples would probably split and fail.
MikeG, I think HoneyBadgerDC’s point was that if the pressure starts to rise to dangerous levels in the tank, it will force water back out the supply line. Which is probably true, but not something I’d care to rely on as a safety feature.
Where I live a check valve on the incoming water line has been required for decades, and is required to be installed in older houses when the house is sold. In fact, the rules recently changed to require a dual check valve instead of the old single flapper valve - I recently needed to upgrade the check valve since I am in the process of selling my house.
The check valve is installed immediately after the water meter, which is far from the water heater in my house. Even if the OP installed and plumbed the water heater, there very well could be a check valve back at the water inlet.