Bosch tankless water heater problem


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Old 02-20-16, 01:37 PM
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Bosch tankless water heater problem

My Bosch Aquasar 125B has been great until it flooded my house last month while I was on vacation. The burner didn't shut down when the water was off and it heated the water until the pressure relief valve blew. At that point, it should have shut off the gas, but it didn't, so boiling water continued running out of the pressure relief valve (which should have been plumbed to the outside, but wasn't). So hundreds of gallons of water seeped through the walls. I'm dealing with that now, but needed to fix the water heater.

So I read through the manual and learned about suggested maintenance (which I had not done).

I installed three way valves, to easily flush the unit and then pumped 5 gallons of pure vinegar through the heater for an hour. Then flushed with water. I took apart the water valve and found the brass orriface the plunger goes through to be corroded. So I ordered a rebuild kit and replaced it. Just to be sure, I pumped vinegar through the whole thing a second time. I hooked it up and lit the pilot, But the burners also lit, even though water wasn't being used. This time, when the water inside the heater boiled, it made a squealing racket and boiling water came out the relief valve and the unit shut down, as it's supposed to in this event. So I took the water valve apart again, cleaned it and reinstalled it. Same problem.

All the service bulletins on the Bosch say this problem is the water valve. So I assumed I did something wrong with the rebuild. Therefore, I bought a completely new valve and installed it...same problem. Is it possible I did something wrong in the rebuild TWICE and also installed a defective water valve? Or could there be some other reason that the burners are lighting, even though no water is in use?

I know this unit is ten years old. But it's very simple. NO microprocessors, just a thermocouple, a pilot, burners, heat exchanger and water valve. I hate to just throw it away. I called a service company and they wanted $85 for the service call, but then wouldn't diagnose it at all. They just suggested I buy a new one from them.

Bosch will only talk to licensed contractors, so they won't make suggestions. Can anybody help? I know the water valve regulates the flames, but how exactly does that work? Does the valve trigger something on the burner, something that could be replaced?

I read about a homeowner who cut the spring inside the diaphragm to make it trigger, but a brand new valve shouldn't need to be modified, should it?

Thanks in advance for any advice!!!
 
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Old 02-20-16, 05:43 PM
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Welcome to the forums.

Not sure if there are any Bosch techs here. You definitely have a water valve problem as that's what activates the gas valve. It sounds like there isn't enough water pressure to turn the gas off.

I read about a homeowner who cut the spring inside the diaphragm to make it trigger, but a brand new valve shouldn't need to be modified, should it?
I'd replace the entire unit before I'd consider attempting that.

Possibly someone that can help you directly will stop thru.
 
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Old 02-28-16, 02:06 PM
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Fixed it!

I can't remember my login, so I had to join up again with a different screen name. But I wanted to let you all know I fixed it, and despite the advice to the contrary, it had NOTHING TO DO WITH THE WATER VALVE.

The water valve has a pushrod that moves up and down with water flow. Since I replaced the entire valve and it didn't fix the problem, I looked to see what that rod pushes against.

It pushes against a similar rod in the gas valve. I sprayed it with WD40 and grabbed the rod with a pair of needle nose pliers and gave it tug. It was stuck solid. After pulling a pushing a few times with the pliers, it suddenly loosened! Put it all back together and now it works!
 
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Old 02-28-16, 03:27 PM
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I was under the impression from Bosch tech service that the water valve and gas valve get changed as a complete unit.

I had thought you changed the entire valve assembly.

If it was stuck.... I'd want to know why so that it doesn't happen again. WD-40 is not a lubricant and won't really do much in your application as far as a lasting repair.
 
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Old 03-02-16, 04:11 PM
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fixed it!

I don't know what Bosch recommends, other than the owner's manual and the tech bulletins (of which I read every single one for ALL their tankless heaters). Remember, they won't talk to us lowly home owners. You're right, WD40 isn't much of a lubricant. However, I believe the reason it was stuck, was because in 12 years, I never flushed the unit. Really, once i pulled it with a pliers, it easily moved in and out and since WD40 is also a solvent (as are all oils) I used it just for added security. I could always lube it with something good like white grease or silicone or the stuff they provided when I swapped out the water valve. But it was so easy to fix, I know I can do it again anyway.

Thanks,
 
 

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