Pressure loss

[RVKC]

I’ve got a pneumatically controlled PIU in a VAV system that will not energize. The pneumatic thermostat builds enough pressure to close the VAV when calling for heat but will not provide enough pressure to activate the fan switch and heat strip switch. I’ve replaced both switches with known good devices but the results are the same. All of the other PIU’s in the system are still functioning properly. Any suggestions?

 

[tecspm]

Are you sure the stat branch line builds pressure on a call for heat. Normally the opposite is true. The branch line exhausts to drive the VAV controller to close to minimum for reheat airflow(The VAV controller will have a seperate supply main).
To verify the stat:
Start at the stat-is supply pressure sufficient to close the switches, if not problem is in the pneumatic supply not the stat.
Next check the stat output-disconnect the branch and connect an accurate gage. Turn setpoint to full cooling(on DA stat temp up pressure up)- gage should read supply pressure.
Next,if not, depends on the type of stat but most brands have replaceable arms. Old school actually... today most would just try a new stat.
If yes then verify tubing to VAV controller. Disconnect branch line at controller and repeat above steps....
Enough steps for now, Reply with how you make out.

24 Years Under My Belt,
Too Many Cheeseburgers Over My Belt,
But I Love To Talk Controls.

 

[imok2]

Also you can use soapy water in a spray bottle at all pneumatic connections but not on stat.

 

[RVKC]

Strangely enough, I swapped the stat with 2 other stats and still had the same problem but when I jumped the stat completely, I would get heat. After soaping all the fittings and finding no leaks, I rigged a test squeeze bulb to pressurize the actuator to find that it was leaking enough pressure off to prevent the fan / heat PS to be energize. A new actuator fixed the problem.

My question now is should a properly functioning pneumatic stat (TK-1101-404-2) always bleed off a little air pressure???

Thanks for everyone’s help.