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Pneumatic Stat troubleshooting—night & day stats

At our church built in 1968, we are troubleshooting our pneumatic thermostat system that controls our hot-water heating throughout the building. It works well, but we haven’t used the night/day functionality in 20 years and wish to use it again to save energy.

Question: Can someone explain to me how separate day & night thermostats are typically connected?

Background: We have about 35+ Powers (Siemens) pneumatic thermostats. In four different areas of the building, we have an air handler with 3 separate zones. (The other rooms have radiators.) For each of these air handlers, two of the zones seem to be secondary with their own stats to control their own hot water valves for the heating coil in their own ducts. The primary zone (in each set of 3) is the only one that can call for the air handler to turn on in addition to its own hot water valve. This zone has side by side day and night stats for different set points. There are manual controls in the boiler room to switch each of these air handlers and their related zones to day or night mode, presumably changing the pressure from 20 to 15 to activate the other stats. (During day mode, the fan runs continuously for ventilation). What we find unusual is that we have 2 separate stats instead of a single stat with day & night settings (which we know powers made and every online example uses). What we can’t figure out is how they are connected.

From each day/night switch in the boiler room, there is a single main line heading to that part of the building. When we remove both stats, they appear to have their own (blue) main lines since there is air coming from both. So we assume they’re not connected in series. So would the branch lines be connected then before the pipe heads back to the air handler and hot water valve?

We do have a local HVAC service company we love, and they are great at replacing individual parts that break and keeping the system running, but they don’t seem to understand how the air lines are all connected. No church members know how the system works, so the building engineer and I are trying to figure it all out.

Thanks for your help!

Pastor in Minnesota


  • ratio
    ratio Member Posts: 3,675
    With two separate stats for night & day, I'd be looking for a manual switch or a time clock that operates an air valve to switch between them.

    Follow the air lines!

  • ScottSecor
    ScottSecor Member Posts: 863
    In the buildings we worked in with pneumatic systems there was a timer that changed the pressure going to the building.

    In basic terms, if the pneumatic tubing to the building or the air compressor failed, the entire building would overheat (fail safe). Based on this logic, if you send pressure to the system (around 7psi if memory serves me) during the daytime you would allow each zone to have maximum heat. During the night you would intentionally raise the pressure slightly (15psi if memory serves me). This would in theory allow say 72 degrees max building temperature during the day and approximately 62 at night.

    The way this was done was with two solenoid valves and two pressure reducing valves. Day solenoid and prv would be open with seven day timer (typically 6am to 6pm at a school, M-F). Night solenoid and prv would take over the rest of the time. On Saturdays for a basketball game the custodian would bypass the night setback and put the system into "day mode."

  • ScottSecor
    ScottSecor Member Posts: 863
    Just checked one of my books, day pressure should be around 15 and night around 22. Drawing is a little different than I tried to explain, but I think you'll get the idea.
    EBEBRATT-Ed Member Posts: 15,721

    Day and night pressures can vary slightly depending on the manufacturer. I think Seimens/Powers is 18 & 25 I would start at the air compressor and follow the lines. You will probably find two pressure regulating valves. 1 would be day and the other one would be night. and they would be controlled by two air solenoid valves (ep switches)

    Pneumatic thermostats can be bought to do day and night with 1 stat or heating and cooling with 1 stat but can't do both functions (day/night & heat/cool) with 1 thermostat. That could be the reason you have multiple stats. Don't know if your doing cooling as well.

    To many unknown things for us to troubleshoot without more information but......don't give up!

    Pneumatic controls are fairly simple, reliable and trouble free. They do require a little maintenance and calibrating of thermostats, servicing of the air compressor but none of this is rocket science. Problems is none of the young guys even know what pneumatic controls are.

    Most control people in this day and age will tell you to rip it all out....I wouldn't.

    There are videos on you tube a lot of good information there. Also if you need controls, parts and technical help "National Energy Controls" in PA is a good supplier.

    Have you looked for any original building drawings?? A control print existed at one time, that would be a big help.

    If not, is most of the tubing visible so a dwg. could be made? (probably wishful thinking)

    I guess I would try and make a sketch of how the air lines are piped in 1 section of the building. If you can get most of it we can probably fill in the blanks.

    Good luck
    JUGHNE Member Posts: 11,086
    edited March 2018
    The system I am somewhat familiar with was installed in 1961.
    It is a day/night. You change over in the boiler room with a simple manual knob that switches from one regulator psi to the other.

    They do have day/night tstats with the manual over ride lever on the bottom. D/N tstats have 2 adjustable wheels inside.

    The outside air dampers were disabled and blocked off. The building has enough air infiltration for its present use.

    One thing of caution is if you have outside air blowing over a heating coil, then the function of those controls are critical to prevent freeze up for that unit.
    JUGHNE Member Posts: 11,086
    Where in Minnesota is this located?