pneumatic thermostats

Steve_51

Hi everyone, I have been asked to change a couple of stats while I am on site repairing the boiler. I have never done any work with pneumatis stats prior to this day. I lookrd at some literature which told me very little, does antone have some necessary info on how the whole set up might operate and how to accurately swap? The stats are in a school where they operate zone valves for hydronic closed loop fan coils.

Thank-you!

Tom Hopkins

Are they a current model or are they an older obsolete model? If they are current, you should replace it with the same model number. If they are an older obsolete model you will have to get them cross referenced. You need to make sure the new stat has the same action (direct/reverse) as the old one. Many stats are day/night - dual main type. They are not uncommon in a school.

They should also be calibrated after they are installed. This will require that you have a guage and a hypodermic needle. I think calibration instructions will come with the stat.

Blackoakbob

pneumatic stats are

fairly straight forward when you change like for like, but
if you change brands you may need special tools, pressure gauges, adapters for calibrating, etc. If you can get the exact replacement it will go well. If the right one isn't
available Robertshaw makes a good universal replacement. It
comes with lots of mounting hardware and adapters. The one you have can operate a couple ways. It may be a two pipe and have a MAIN port suppling air to the valve thru the BRANCH by throttling the output ( in direct acting stats no air press is full heat and 10 to 14 # is closed) or it may be a one pipe which bleeds off the air to the operating valve. One thing to check before condeming the stat is the rubber diaphram in the valve top they can get brittle with heat and age or may have a leak in it losing air and not closing off tight. It's an interesting field of control work and with a little study can be fun and you don't need an electrical license. They have reset controls, setback and many, many applications. Go for it and good luck.
Best Regards.

BillW@honeywell

Are they Honeywell?

If they are, they will have a number like TP970(single temp), TP971 (day/night) etc. on them. Replacements are readily available, so are parts. Get me some more info, and we should be able to help you.

Dale

Practice

We have a few hundred of these stats of various vintages/brands ect. It takes quite a bit of practice to calibrate them. Don't be discouraged if you take some time at it. Always good to learn new (old?) technology. With the DDC controls becoming common in new buildings it's somewhat of a dying art. BTW We have found it cost effective to have a factory person come in once a year to calibrate ours, the speed makes up for the cost.

Steve_51

stats

Thanks for the responses guys. The old stats are Barber Coleman 2 pipe (m+b). The replacement stats that they have ordered in are TP970A, which have 3 tappings. According to the wholesaler this is a staight swap out and to only use 2 tappings and leave the 3rd open. Is this info correct?

Thank-you,
Steve

BillW@honeywell

TP970

The TP970 is a 2 pipe, direct acting single temp stat. M is main air, B is branch, which connects to the valve actuator. The 3rd port isn't used on this model. To calibrate, you'll need a 0-30 psi gauge,part # 305965 an adapter, "football pin" CCT729A and fine point screw driver. Since you're doing room temp, set the throttling range to 4 degrees. The setting device is the black & brass colored slider in the vertical scale. The center should line up with the number 4. Check what the room temperature is, and turn the black setpoint knob on the bottom of the stat until the yellow setpoint needle lines up with the red room temp needle. Insert your 0-30 gauge & pin assembly in the Gauge tap, either in the top center or top right side. Turn the calibration screw with a fine point screwdriver ccw until the gauge reads 0 psi. Then turn the calibration screw cw until the gauge reads 8 psi. Remove needle & gauge, replace cover, set to desired setpoint.

Steve K

Stats

Hi Steve,
You will also need back plates(for mounting), by staying with
the B.C. or Robert Shaw/Seibe/Invensys you will not have to
change the mounting plate. But if the stats are ordered already
it's just a little bit more work. To calabrate a pneumatic stat first
you must know the pressure range of the device you are controling. Say the valve for some fin tube is a 3# to 8# N.O.
valve. This has a 5# spring range, so to calabrate have a accurate thermometer place it next to the t-stat and then set
the t-stat to that temp. the midpoint of the spring range would be
5.5# at ambeint(room) temperature. That is the pressure you would
need to set the t-stat at. This is done by means of a small set screw
usally an allen or slot screw depending on the mfg.
Hope this helps,Johnson Controls has a good on line resorce library.

Have Fun,Steve K,