Main burner comes on without water pressure

We live off-grid and collect/store rainwater (under our house) which is pumped (via 1/2HP pump) to a pressure tank and then to our Paloma PH-24M-DP propane fired (indoor) water heater. This setup has not changed in 20+ years.

However, a couple days ago the heater started acting strangely. Normally, after we light the pilot & turn the dial to the ON position, the main burner does not fire until after we turn on a hot water faucet. For some reason, the main burner is now igniting as soon as I begin turning the dial from Pilot to ON even though there is no request for hot water. If I do nothing (i.e., do not turn on a hot water tap), the heater will just heat the water to almost boiling temps.

Any idea why this is happening?

I have uploaded a couple pics for reference (direct shared links to Google Photos fail to embed):

 

It sounds like your flow sensor is either jammed or defective. When was the last time you demineralized it? Depending on the water hardness, that may need to be done at least annually. If not done, mineral deposits will start to reduce the efficiency, and eventually, can cause it to fail to operate properly.

 

Hi @jadnashua , thanks for helping. Here's an exploded view of the parts. Do you know where the flow sensor would be?

https://www.dropbox.com/s/2tl0prmazeooem3/paloma-ph24-water-heater.pdf?dl=0

 

Again, I"m guessing, but it looks like you have a problem with the water control assembly. The troubleshooting says some dirt in there could be the source of the problem. It might be that the diaphragm has failed. A call to the manufacturer may help resolve it.

This is one of the hassles with tankless systems...they're more complex than a simple tank type, and not everyone has the experience and training to fix them efficiently. That being said, the documentation isn't all that generous, either, at least from what I could find.

 

Wouldn't a flow sensor require a computer or at least some type of electronics to read the sensor? Our unit doesn't seem to have any PCB's on it whatsoever.

 

The installation manual I could find didn't have much information in it!

There are ways to sense water flow that only require a switch, like a little flapper in the water stream. That could go to the interlock system that enables the burner. WIthout more information from the manufacturer, it's hard to say. They do indicate in their troubleshooting section the possibility of the diaphragm being clogged or broken. That could move based on the lowering of pressure when you open a valve to draw water. Hard to say for sure, but it does point towards that.

This is where a good tech, schooled in that particular brand and model comes in, and where the utility of the things breaks down...lots of people may install them, but not as many really understand how to fix them and may just throw parts at it until it works. WIthout a better manual (than I've found, anyways), it's all guess work. Call the company. Some will talk to an owner, some will only talk to a certified tech. Hopefully, they'll talk to you or may be able to direct you to someone that should be able to fix it for you.

 

Mechanical feedback systems for monitoring flow & modulating the flame on tankless water heaters have for about a century now. Mechanical things wear out, stick, break, get jammed up by crud & gunk- it might be fixable with a cleaning if you can figure out what you're dealing with.