I have an analog t-stat which controls baseboard heating and an Aone HVAC unit on the roof. When in heating mode, it sends a signal to two modulating control valves in the hot water feed to the baseboards.

These two valves are leaking by in the closed mode. They are jammed into a ceiling cavity and difficult to replace.

Is there a way of taking the t-stat signal and converting that to a digital signal (which would not effect the existing operation) and which would shut-off the supply pump? I do not want to replace the t-stat.

We need a lot more info to even determine if this is possible. The valves may still leak because the pressure isn't removed.

What also might be possible, is to never allow the valves to close fully.

Is it possible to replace the valve in a different accessiable location and force the bad valves fully open?

We need to know how these valves are controlled. 0-10V, proportianal control? Is a feedback signal available.

Agreed valves leak because pressure is on from supply pump.
Location of piping and valves is completely jammed in.
Valves are on 4 - 20 ma signal. Feedback signal not available.

Roger

You can use a 4-20 mA isolator and set the zero and span of the output so the valves don't close all the way. e.g. make 4-20 mA (0-100%) be x-20 mA or say 5-100%.

e.g. http://www.api-usa.com/pdf/api/api_dpi-2.pdf

This does have an issue because it's loop powered, that the output must start at 8 mA minimum. You can convert that into %.

These would not be subject to the 8 mA minimum, but would require an additional supply.

http://www.api-usa.com/signalconditioners_signal_splitters.php

Lots of companies make these modules.

The system may actually turn off the pump anyway when both valves are satisfied, so just being able to set the zero should be all that you need.

You can also get a 4-20 Alarm module and use it to look at the input value and then turn off the pump. You would have to invent some logic so that when both inputs were off, the pump would turn off.

These modules don't come cheap. Some actually require software to configure. Some have displays. Some do not.
Some have span and zero screwdriver adjustments. Some are loop powered. Some are not. Some are isolated. Some are not.