Pneumatic Thermostat Question for Experienced People
First off, thank you for taking the time to check this out.
On our campus we are running primarily pneumatic thermostats from Johnson, Seimens, and Honeywell. (Plus some older version of the same i.e. Powers, etc.) Before coming to my current position, I had not worked with pneumatics just DDC. I have been here for about 8 months and have gotten to know pneumatics pretty well. Shortly after I started (about a month or so) my department decided to desend on an air handler and map the system and calibrate everything. I got advice for proper calibration for dampers and valves from an experienced friend. Things were going well as no one before me had ever calibrated stats correctly. Now I am having to recalibrate these same stats a few months later.
Do pneumatic thermostats need periodic calibration?
Does a change in the weather/seasons somehow affect calibration?
For reassurance purposes, a satisfied damper should be at 50% for a hot deck/cold deck application, right? a satisfied radiant valve should be closed, right?
Double Check Senario
I have a multi-zone air handler running a hot deck and a cold deck. Zone 1 is an interior room with no radiant heat and a damper actucator with a spring range of 5 - 10 psi. Calibrating the thermostat to a satisfied temperature should have a 7 1/2 psi branch pressure, correct? Zone 2 is an external room with radiant heat valve of 3 - 8 psi and a damper actuator with a spring range of 5 - 10 psi. Calibrating this thermostat to a satisfied termperature should have an 8 psi branch pressure, correct?
This is what I have been doing per the advice from my experienced friend, seems logical and correct to me but want to double check...
Thanks for reading this far. If you have any thoughts or suggestions they would be greatly appreciated. And thank you in advance for any replies.
>Do pneumatic thermostats need periodic calibration?
Not monthly. System should be checked during the controlled season. During heating season the radiation should be checked to verify it is operating properly while the load is within the control span of the stat/valve. If the room is over set point the valve should be shut 100%. If the room is below setpoint by more than the span of the stat the valve is 100% closed. This is not the best time to be inspecting the calibration. Try to schedule the walk through while the valves are modulating within the span of the actuator. Then a quick check of the temperature vs setpoint will verify the system operation. Check the baseline without handling the stat. The bimetals in these stats are very sensitive and can pickup heat from you hands very quickly. Then it seems to take a long time for the system to dissipate the heat added. I usually use a handful of digital thermometers and set one on top of the stat. do this 5 times. Go back to stat #1 check the reading now the temperature should have settled. Now move that thermometer to stat 6. go back check stat 2... get the picture.
If you find you need to repeatedly calibrate a given stat, replace the stat and put a guard over it. (You might try the Guard first as hands on the stat can throw off the calibration of a good stat.)
>Does a change in the weather/seasons somehow affect calibration?
Not normally. Though the load on the multizone damper may make it look like a calibration shift has occurred. Remember the stat is a P only controller. Remember the supply air temperature from the unit may vary the load balance point.
>For reassurance purposes, a satisfied damper should be at 50% for a hot >deck/cold deck application, right? a satisfied radiant valve should be closed, >right?
Yes, in general. Usually it is easier to just pick the middle of the thermostat's span for setpoint. Just calibrate all your stats to the same spec like 8# at setpoint. (there really is not much difference in 1/2# of branch pressure. JCI T-4002 factory spec setting is 2.5 psi/F°. So it is 0.2 F°. That is approaching the precision limits to measure the bi-metal temperature in the field. )
For your exterior zone 2, I would consider changing the damper actuator range to 8-13#. Even if the heating valve closes at 8#, by then you may be partially opening the cold deck. If the damper was 8-13#, the valve would close first, then the hot deck would close and then cold deck open.
This can be particularly important depending on your hot water pressure. A 3-8# valve range really means that the valve might need 9, or 10 or even 15 psi to close fully against the pressure. In that case it's open while the cold deck is open, really wasting energy. You can look at a closeoff chart in the valve literature to see just what air pressure is needed to close off against the water pressure. With small Honeywelll valves we used 2-5# ranges on heating valves where sequencing with another actuator. to help prevent the overlap.
You can install a ratio relay and change your output to your heating valve to ensure it closes all the way!!!
Originally Posted by tlp261
Law Of The Thermostat: He who has the thermostat wins!!!!!
Thanks everyone for the insight. It was/is very helpful.
Before you calibrate again please look at the "condition" of the air coming from the compressor/dryer/prv. Most pneumatic systems become "hydraulic" systems if not properly cared for. Check the blowdown valve at the bottom of the tank for water. Also check for rust. My guess is that you have oil and water in the pneumatic lines. This can be blown out using nitrogen on a line with all devices removed.
Just my $.02 worth.
Ah yes, we have preventive maintenance personnel that check the air dryer and compressor on a monthly basis and I know he does his job well. So that does get done. But I didn't know you could blow out the lines with nitrogen. Good thing to know. Thanks.
Originally Posted by oldhweller
Another product is CALGON PNEU FLUSH , its made to clean Pneumatic lines , works great.
I did not know this product existed. Thank you very much for bringing this to our attention. Thanks. Mike
Originally Posted by Control Man
Glad to be of assistance. used it on quite a few jobs , 1 was turning into a HYDRAULIC control system from all the oil , cleaned it up nice.