Heating, Air Conditioning, Fridge, HVAC/Pneumatic Thermostat


QUESTION: I live in a 50 year old hi-rise and my thermostat recently started to leak/hiss. The hiss is constant and will only quit when I crank the dial to maximum heat. It is a Honeywell TP970A2020.
Should I just replace or am I able to repair the existing unit?
Thanks in advance,

ANSWER: Hi Brent,

This one takes me way back to when when I started in this business. What you have is a direct acting stat. Rise in temperature, rise in pressure. So when you turn the stat way up the pressure drops to 0. That's why the hissing stops.

I don't think you can fix the stat, as it would take special tools to do it. Your best bet is to replace the stat.

Expect to pay about $ 100.
Let me know if you need help ordering.

---------- FOLLOW-UP ----------

QUESTION: Thank you Jim.
When I remove the thermostat from the wall, there are 3 air lines that it mounts to. 1 of the 3 lines is blowing air with constant pressure. If I block it off, it triggers the heat. My fear is there is just a leak between the lines and the thermostat and if I replace the thermostat, the leak will remain. Or is this just a common symptom of a thermostat failure?

Hey Brent,

The tube that has the constant pressure is the "main air". It has pressure on it all the time, about 20psi. The 3 pipe stat is called Day/Night stat. It automatically sets the stat down at night.

I would change the stat out. It sounds like the stat is bleeding off all the time. This can be a bad O ring or gasket.

Let me know what happens.

The cost on the 3 pipe stat is about $170.

Heating, Air Conditioning, Fridge, HVAC

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Jim S.


I can answer questions on gas or electric furnaces, ac units and heat pumps. I don't work on boilers,chillers,or home appliances.


I have been in the HVAC trade for over 35 years. For the past 15 years I have owned my own business. I am semi-retired so I can answer questions in a timely manner.

I attended trade school for 5 years on weekends and evenings. This was very hard to do, but well worth it!

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