Just trying to help my neighbor. She has a tank-less hot water system (coil in the oil burner). Baseboard heat is working fine! However I believe she had an Adjustable Thermo Mixing Valve installed a few years back that has worked fine. Well as you run the hot water it slowly fades into lukewarm water at best. I have adjusted the valve both directions with no real significant results. Could the coil itself finally be failing? I will try and post a picture. Is there anything else I could try?

 

hi –

I have the coil also. I think you should be able to feel the temperature differential with your hand (being careful), comparing the output at the hot water pipe from the boiler leading to the valve, and the hot water pipe leaving the valve.

If it’s the coil it should be cool leading to the valve. At least it seems that way to me.

 

They don't actually fail but they become plugged with minerals.

Try what zoesdad suggested. The water line going into the coil should be cold as you draw water. The outgoing line should be too hot to touch. If it's not.... the coil has a problem.

 

Makes sense Duh! Yes the water coming from the boiler is super hot up to the TMV. So that leaves the Adjustment on the TMV with thermometer. I mention that cause when we run the hot water Its obvious, visually noticing the temp goes shooting down observed thru the dial thermometer!

 

There are three blue handle valves in a cluster. If the picture was correctly oriented..... it would be the top one. It needs to be closed.
The gauge only shows you the mixed hot water temperature.
You can't actually see what the water temperature is coming out of the coil.

 

That's exactly right. the top knob is closed all others open! Sorry about orientation.
Gage Shown here after running water for s few minutes, just drops instantly?

 

Well I shut the cold water feed (to the TMV) completely off. While I had the kitchen and bathroom hot only running, the result was both faucets ended up just tinkling water drops or almost nothing at all! Meanwhile I was turning the TMV control both directions (max and min.) with no results. So for some reason or another seems like the TMV control is failing! Seems like its not letting any hot water pass thru.

 

The first most common cause for a tankless coil not operating properly is the temping valve or mixing valve is failing. This has a internal cartridge that can easily be changed. If you have a Watts name brand valve which is the most common.You just need to shut the valve off that feeds water Into the tankless coil then the top of the mixing valve unscrews. Not the dial you turn to adjust the temperature right below that you will see ridges so you can grab it with a wrench. Unscrew it and it will slide out and simply screw the new one in and th iui s usually fixes it and is the most common problem.

Now if this don't fix the problem then there is a issue with the aquastat relay which ou s the main control. This control could be no good due to the probe that detects the temp isnt working or a wiring issue.

if the tankless was actually no good the boilers pressure relief valve would be discharging water onto the floor due to the tankless coil leaking water into the boiler causing it to over pressurize. If this isnt happening then the coil is fine and it is one of the other 2 issues I described above.

I'm a master Pipefitter with 3 generation in my family I knpw these boilers inside and out. I'd be happy to answer any questions.

 

That is an extremely rare problem.

A more common problem..... is the coil gets internally restricted by minerals reducing the hot water flow thru it. This is the most common problem here.

 

If thay was the problem you would have a loss in pressure not cool water.