Pneumatic 2 pipe Honeywell T-stat

[rnoel]

Gentleman,

It's been a while since last post, but I have tuned in from time to time to keep sharp.

I am new to this building and gathering data day by day. What I have gathered thus far is following
- Meeting area complaints of to warm, hard to get in this room for much more info, may have to schedule after hours follow up. Found thermostat not responsive, this is a simple two pipe single temp control thermostat. Controls vav Titus velocity controller on lower scale for vav cooling and on higher scale controls reheat valve. Replaced t-stat with known working t-stat and was able to get around 6 psi max in cooling, allowing some cooling for staff, shot about 60 degrees at register with temp gun, but not full velocity. I noticed I was not able to see 15 psi when lowered stat to lowest setting, like in some other zones. (Noted some slight air bleed by stat also) Could this be a faulty velocity conroller, loose tubing in the wall or the like? (Doubt tubing issue).
Thanks..

[catalina_mike]

The branch or controlling pressure is not capable of much volume. So if you have a leak in the hot water valve or velocity reset controller it will not be able to keep up and will not control properly. Stats do bleed air when the bi-metal responds to temperature changes but should not be doing it continually unless it is a single pipe thermostat they bleed almost all the time. Now lets be sure of the action if in cooling you had 6 psi (sounds funny) by lowering the thermostat set-point you should see the 15... Normally DA thermostats were used for most VAV applications (unless changeover stats) temp up pressure up with 3-6# reheat valves for heating. So any leakage of air on the branch will not allow the cooling to increase. You would calibrate the room thermostat at 7-9#. You can pinch off the brance line and verify the thermostat does reach the 15 as a starting point then look for leaks. Either the HWV or Velocity reset controller.

[rnoel]

Thanks Catalina...
This is a D/A T-stat, on re-visit I will check devices for leakeage

[ckartson]

I use hemostat to pinch off branch line and see if pressure builds at the stat but that just proves the stat is good you probably have a bad diaphragm in reheat or some other bad valve. keep pinching off lines and you will find it. Finally a use for old roachclips.

[simux]

A lot of volume controllers reversed the signal. For example a R/A thermostat with a reverse volume controller and a normally closed vav box. On a call for cooling the pressure goes down at the thermostat and is reversed at the volume controller for cooling within its max and minimum settings.

[catalina_mike]

Yes and also in the Midwest and eastern engineers specify "NO" hot water reheat valves.

[simux]

I really hate vav boxes without reheat valves. I also hate the vav boxes that recurculate the return air through a filter using a fan. ARGGH