Ruud thermostat won't kick on burner properly

Michael O'Neill

I have a standard seven or eight year old 40 gallon Ruud "Achiever Professional" propane fired DHW tank. Over time the old thermostat had to be set higher and higher to heat the water warm enough to take a comfortable shower or bath. Finally it could be set no higher. Thinking the thermostat was the issue I replaced it with a brand new one, exactly matching the old one, and properly designated for use with propane.

The problem is persisting. The water in the top of the tank measures at 110 degrees, measured by a temp gauge installed at the top of the tank (same opening where the PRV is located), which makes for a tepid bath. However if I turn the hot water tap on in the bathtub for a very few minutes, the burner in the DHW tank will kick right on and the temperature in the tank rapidly increases to 125 degrees at temp gauge. But you shouldn't have to do that. That's the same way it had gotten to working with the old thermostat.

Since I've already installed a new thermostat, I'm stumped how to solve this issue or what the problem may be. Any ideas? There didn't appear to be any sediment in the tank when it was drained while the new thermostat was installed. I don't know what any other factors may be that could be causing this, or how to correct the issue. Thanks for your consideration and advice.

Mike

EdTheHeaterMan

@Michael O'Neill That sounds like a problem that a new thermostat should fix. I am assuming the thermostat is a combination thermostat, gas valve like most gas fired water heaters use. Thes only other serviceable parts are the burner and the tubes that connect them to the gas valve. Unless there is another over-temperature cut off that is special to that tank.

I am wondering if there is a dip tube that is not working, The dip tube is inside the tank and conducts the cold to the bottom of the tank. If the dip tube is broken off, then the cold water can take the shortcut to the hot outlet while the bottom of the tank stays hot and the burner wont come on.

Michael O'Neill

Ed, how would you go about checking if the dip tube had failed? I'll shut off the cold water supply, unhook the flex hose and see if I can look down the pipe using a flashlight. There's really no other parts, so maybe that's it.

The burner does come on quickly when the hot water is turned on in the bathtub so you'd think that cold water is being released at the bottom of the dip tube. And besides that the tank should cool from the bottom up at night setting off the thermostat with no cold water entering the tank top or bottom. This is such a weird problem. Thanks for your suggestion and nice visual aid by the way. The tank pictured is even a RUUD one.

Mike

JUGHNE

You can usually pull the dip tube out of the cold inlet.
I had to replace mine and I used soft 3/4" outside diameter ACR copper tubing.
I formed a flare on the top end of the tubing to keep the tube from falling into the tank.

Larry Weingarten

Hi, As others have said, it could be a dip tube problem. Two other possibilities that might behave in a similar manner are a recirculation line that has a failed check valve and a crossover in a fixture that allows cold water into the hot side. When you check hot water temperature, are you feeling the hot outlet line on top of the tank? That's the best way to know if it's the heater or something else.

Yours, Larry

Michael O'Neill

I'll pull the dip tube out tomorrow, check it out, and get back to you all. I watched a youtube video on DHW dip tube related issues that described exactly the symptoms the DHW tank has been exhibiting, We'll see tomorrow if that's it. At least that's a fairly inexpensive fix if that's the problem. Thanks Larry and JUGHNE. I do very much appreciate your suggestions.

Mike

Michael O'Neill

I ordered a new dip tube and will pull out the old one and replace it Thursday if the new tube arrives then as promised by Supply House. Only cost $8.00. Hopefully this will do the trick. If not, back to the drawing board. Shame to have wasted $150.00 on the replacement thermostat though.

Michael O'Neill

A new dip tube was installed and didn't correct the problem. The old dip tube looked a little worse for wear but no major breaks. Next up was to look at the possible problems that Larry suggested, particularly a possible crossover in a fixture that might be letting cold water into the tanks hot outlet.

There are two tempering valves on the hot water line, not to include the one on the shower valve, that were the next suspects. One, A Honeywell Series 1-MA, is set to provide 75 degree water as needed to a 125 gallon aquarium upstairs. The other, an inexpensive Watts Series 70A tempering valve, is right over the DHW tank and keeps the hot water flow from the tank regulated at 120 degrees. That is useful during the heating season when I heat the DHW tank via a flat plate heat exchanger connected to my wood boilers output, rather than by propane. That tempering valve is a fail safe in case one of the aquastats that kick on and off the circulator to the flat plate hx fails and the DHW tank overheats.

My new question to you is, can one of these tempering valves allow their cold water input to bypass them and flow backwards into the top on the DHW tank without there being a malfunction within them? Or do they have to be malfunctioning for this to happen? I would think they'd be designed so this couldn't happen without there being a malfunction, I don't really know? I did end up plugging the cold water input into both of them and the DHW tank is now functioning as it should, So thank you Larry for solving this issue at last.

Now I've got to figure out how to get the tempering valves plumbed back in and operating as they should again. Thank you all for your help.

Mike

EBEBRATT-Ed

@Michael O'Neill

If the tempering valves are in otherwise good shape you might be able to get rebuild kits for them.

Larry Weingarten

Hi, The 70-A was common back in solar thermal days, and with hard water, would lime up in the open position. I had people wanting to rip out their solar because they could only get warm water. There are kits for that valve, but often just removing and cleaning up the internals works nicely.

Yours, Larry

Michael O'Neill

Larry Weingarten said:

Hi, The 70-A was common back in solar thermal days, and with hard water, would lime up in the open position. There are kits for that valve, but often just removing and cleaning up the internals works nicely.

Yours, Larry

You know, I do have one of those rebuild kits for the 70A, Larry and Ed. I got it when the 70A's valve froze and wouldn't turn. Like you say Larry, probably gummed up with hard water deposits, even though the water is softened. I'd yet to rebuild it though, when this current problem cropped up, and instead shut off the cold water intake to it as a stop-gap.

The other tempering valve may be the culprit. or maybe the cold water cut off valve to the 70A wasn't working. I pulled that out too just in case. I'll go ahead clean and and rebuild the 70A, and reinstall it with a new cutoff ball valve on the cold water input line and see what happens.

Larry, I do owe you one for the solution to the overall problem. Thanks again.

Anyone, can any cold water bypass these tempering valves if they are functioning properly?

Mike

Larry Weingarten

Hi, I don't have enough experience with the range of valves to be able to answer "Anyone, can any cold water bypass these tempering valves if they are functioning properly?", but there is a good chance that @hot_rod does. A question... do any of the tempering valves "see" hot water all the time? If so, that could encourage then to get stuck open.

Yours, Larry

ps, I'm glad to help! Maybe when Wetstock can safely return, I'll collect that "one"