Is there an economical electronic replacement for a pneumatic thermostat?

[Cagey57]

Let's have a show of hands. Mine is raised at this point but you asked more than 1 question.

Will Spy Guy take up the small task of learning some basic pneumatic control logic and application? {Hopefully} Or that of enlisting the help of someone who already knows it? {Preferably }

Or will he continue searching for someone who will help him justify selling the customer on some electronic thing they don't need and which won't resolve their issue while having an infinite payback period? {Not the best solution, IMO}

Did he want an answer to a question? {I think so...}

Or just an agreement with an assumption? {Possibly, I'm not sure }

@SpyGuy,
PECMSG answered your question in post #2, 2sac added a (expensive, maybe $1k worth) retrofit suggestion in post #9.

Unless something is terribly wrong with the existing pneumatic valve actuators the basic "Problem" can be corrected for less than 300 bucks (plus some learning time) with a new Pneumatic Thermostat.

Pneumatic Control Theory is the basis for almost all the current DDC controls. A little study will yield massive results.

Good Luck on this one !

[pecmsg]

@SpyGuy,
PECMSG answered your question in post #2, 2sac added a (expensive, maybe $1k worth) retrofit suggestion in post #9.

Unless something is terribly wrong with the existing pneumatic valve actuators the basic "Problem" can be corrected for less than 300 bucks (plus some learning time) with a new Pneumatic Thermostat.

Pneumatic Control Theory is the basis for almost all the current DDC controls. A little study will yield massive results.

Good Luck on this one !

Let me guess..............Something about 3 - 8#'s, 8 - 13#'s, 0 - 10 VDC?

[R123]

I'm doing Tenant Improvements on a commercial space in a mall that was built in the early 1980's. The existing HVAC consists of a set of boilers and chillers on the roof, with heated and chilled water piped throughout the building. In each of the occupied spaces, there's a fan unit with heating and cooling coils. The thermostat is a Honeywell TP970 or TP972 Series (I can't remember the specific model). There are two pneumatic lines connected to the thermostat: one from the building source of compressed air (supply), and the other to the valves that allow the heated and chilled water to flow into their respective heating and cooling coils in the fan unit (control). Both the heated water valve and the chilled water valve are controlled by the single pneumatic line from the thermostat. When the thermostat allows pressurized air to the water valves, one valve opens and the other closes. When the thermostat bleeds off the pressurized air to the water valves, the open valve closes and the closed valve opens. The fan runs constantly.

This is a VERY inefficient system. Because there are only two states to the thermostat (on/off), the system is either running full hot or full cold. There is no time when there is not full heated water running through the heating coil or full chilled water running through the cooling coil because the thermostat is simultaneously opening/closing both hot/cold water valves with an "on/off signal" via its single pneumatic control line.

For example, if the thermostat is set to 70°F, then when the room is even slightly below 70°F the fan unit is blowing full hot. As soon as the room temp creeps above 70°F, the fan begins blowing full cold. Once the room temp crosses back down below 70°F, the fan starts blowing full hot again. In other words, the HVAC is constantly fighting itself to stay on temperature. Needless to say, the tenants are not happy that the fan will be alternately blowing full hot and then full cold air on them whether it's during the highs of summer or the lows of winter.

So I was hoping to offer them an electronic thermostat that can control the two pneumatic valves (hot and cold) independently. Obviously, this would mean separating the two valves from the single pneumatic line from the thermostat and running a dedicated pneumatic line to each of the valves.

Does such a thermostat exist that is also relatively economical? I've only worked with fairly modern commercial heat pumps and furnaces, so this is my first experience with an old pneumatic system.

I know this thread is a couple weeks old and others have chimed in… The way your fan coils are SUPPOSED to work is like this:

Thermostat is supplied with 20# air from building air compressor and drier. Operating range of thermostat is 3-15# with 9# calibrated to setpoint at room temp. IE: room temp is 72*, set stat dial at 72* and calibrate output pressure to 9# after adjusting throttling range. For direct acting stat, spring pressure on cooling valve is 10-15# and spring pressure for heating valve is 3-8#. That gives a “dead zone” from 8-10# with neither valve open. Stat is modulating so when room temp goes above setpoint, cooling valve starts opening and when temp goes below setpoint, heating valve starts opening. NEVER should they both be opening at the same time. Reversing acting stat operates reverse of direct acting where cooling valve is 3-8# and heating valve is 10-15#. Northern climates with predominantly heating tend to have direct acting so unit fails in heating upon loss of air pressure and Southern climates with predominantly cooling tend to have reverse acting for cooling on fail.

You don’t appear to understand pneumatics which is ok because it’s a dying breed. If it “truly” works like you say, then someone working on it before you jacked it up but I guarantee you it wasn’t designed that way.

[gbfromsd]

To modulate, or not to modulate. That is the question.

[pecmsg]

To modulate, or not to modulate. That is the question.

Hell be back after everything is changes to electronic and still doesn't work!