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Rick, HVAC Supervisor
Category: HVAC
Satisfied Customers: 22016
Experience:  40+ yrs. experience as a licensed oil & gas technician.
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Have a 5 yr old Viessmann heating system with weil McLain

Customer Question

Hi have a 5 yr old Viessmann heating system with weil McLain auxiliary water tank. Heat for the house seems OK but hot water for shower / bathing no longer hot enough. Turned up thermostat on tank helps but not the answer. any ideas
JA: Did you double-check the thermostat's settings? And what about the filters?
Customer: Did check thermostat setting. turned down and up responds. Have it set now for very hot but when running it lasts a short time. Don't see any filters?
JA: Do you plan on doing the work yourself?
Customer: I would like to.
JA: Anything else we should know to help you best?
Customer: Has not been serviced since installation. Heating system is propane. Radiant heat. Keeps house comfortable in Maine winter.
Submitted: 5 months ago.
Category: HVAC
Expert:  Rick replied 5 months ago.

Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.

Based on your initial description it sounds like you have a bad thermostat on the Weil tank. We could do some troubleshotting if you like.

Customer: replied 5 months ago.
sure what steps would I take to test thermostat
Expert:  Rick replied 5 months ago.

Do you have a dedicated pump for the tank or does a zone valve control boiler water flow?

Customer: replied 5 months ago.
Expert:  Rick replied 5 months ago.

It's one or the other. The answer can't be no

Customer: replied 5 months ago.
Separate pump for tank
Expert:  Rick replied 5 months ago.

Ok, does your temp dial on the tank have numbers or just warm, hot, very hot etc

Customer: replied 5 months ago.
No numbers just the words
Expert:  Rick replied 5 months ago.

Ok, where is it set now and do you have a thermometer that you can use to check the water temp?

Customer: replied 5 months ago.
set at very hot. Not sure where to test for water temp.
Expert:  Rick replied 5 months ago.

You'd just put the thermometer under running hot (only) water but if it's set for very hot and the water isn't scalding then you likely have a bad thermostat. One more test to confirm it's not something else. Trace where the thermostat wires connect to either the separate pump relay (for the tank's pump) or the boiler controller and jump those terminals. You can also connect the 2 wires together at some point if that's easier. When you do does the pump run? It can be difficult to tell so you can put the handle of a screwdriver to your ear and the tip on the pump. You'll hear the motor spin if it's running. Does the piping from the boiler to the tank get hot? If it does then replace the thermostat. If not you have a circulation problem.

Expert:  Rick replied 5 months ago.

If the piping does get hot then replace the thermostat

Customer: replied 5 months ago.
OK did a quick test of the water it is at 100-102. The pipe from the tank to domestic hot water seems very hot to the touch. After mixing valve ( think that is what it is) the pipe is warm. Could it be this?
Expert:  Rick replied 5 months ago.

You didn't mention a mixer (3 pipes connected to it) until now. If the hot line into the mixer is very hot and the mixed water out to the house is only 100 then the mixer is the problem and it needs to be rebuilt.

Customer: replied 5 months ago.
Doesn't look like something you rebuild. Hot water comes out of tank. T connection meets with cold water input. Knob on connection but can't turn it.
Expert:  Rick replied 5 months ago.

All mixer/tempering valves can be rebuilt. Sometimes (with aggressive water) the valve mechanism can fail internally and the entire valve must be replaced.

Customer: replied 5 months ago.
Ok looks like I have a direction. Should I be able to turn that knob on mixer. Has plastic knob that seems like it should turn but didn't want to force.
Expert:  Rick replied 5 months ago.

The knob should turn (easily) but that won't solve this problem even if you could turn it. The fact that it's seized up is another indicator of the need to repair or replace it.

Expert:  Rick replied 5 months ago.

I have been called away. If you have more questions I'll respond when I get back (should be less tahn 1 hour). If not I'd appreciate a favorable rating. The thread won't go away and I'll answer related follow up questions even after you rate me favorably

Customer: replied 5 months ago.
Ok thanks for your help.
Expert:  Rick replied 5 months ago.

You're welcome