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Thread: "ganging" pneumatic t-stats
07-25-2012, 02:26 PM #1
"ganging" pneumatic t-stats
I am about to open up a bunch of office space into one large area, as opposed to smaller rooms and offices. This 3000 sf area is served by 8 or 9 VAV's...and I want to be sure that the t-stats dont compete with each other.
If I were to connect all of the "signal" outputs together, would this then "average" the signal and make all of the VAV's act identically? (assuming calibration and air valve size are the same).
No budget for DDC, even tho I pitched it as hard as I could....
07-25-2012, 02:42 PM #2
No it wouldn't .
Your'e assuming quite a bit here and I dont understand your reasoning. With one or 8 stats you should reach a sustained average temp. with specific areas responding to accute heat loads.
Chances are the calibration from one stat to the next is off a little or a bunch not too mention you most likely have velocity reset controlers that take an input from a single stat to provide minimum and maximum CFM per VAV.
It would be a mess even with one stat serving all the VAVs which you wouldn't have enough of a signal. ( volume ) to accomplish.
Competing VAVs shouldn't be an issue if the Stats are located in their respective areas.
There is nothing wrong with a pnuematics set up as long as it is maintained correctly.
07-25-2012, 06:49 PM #3
I did it with two vav's years ago. Two small offices now turned into one. As far as I know it's still like that and it worked well at the time. There was only one hole in the marble for one stat so that's what they got."If you can't explain it simply, you don't understand it well enough
"Don't you just love when the flames burn off the wiring schematic?" hvacvegas-HTALK member
07-25-2012, 07:05 PM #4
MAYBE with two VAVs you coud get away with it.
One of the problems you run into is you lose signal volume with that many actators AND your reset controlers will act up. Its one of the reasons why you see pilot positioners on large actuators.
Each velocity reset is set up for that VAV when commisioned. A signal on the other aide of the room isn't going to control that VAV accurately.
I would rather toss the thermostats up into the grid ro sense plenum temp rather than jerry rig them.
07-26-2012, 05:21 PM #5Regular Guest
- Join Date
- Jul 2001
The signal to the actuator is from the velocity controller not the tstat so your loss of signal volume is a bit ?able.
Have setup plenty of large sq footage areas in this manner for TEMPORARY control after the areas were gutted and real estate company was trying to lease it and only wanted to worry about 1 tstat for them to fiddle with.
Had a few they wanted to keep like this after the space was leased and occupied , but too my problems and rezoned to suit tenants.
07-26-2012, 05:33 PM #6Professional Member
- Join Date
- Jun 2009
- StL, MO
If you connect multiple themostat branch lines together the result is a low signal select. The 'stat with the lowest pressure output will bleed off the other thermostats.
You can connect about as many volume controllers as you want to a single thermostat. I've seen as many as twelve. The reason this works is that the volume control "T" connection is a dead end; typically a small diaphragm inside the controller. You can prove this to yourself with a squeeze bulb.UA LU 562
07-26-2012, 05:43 PM #7
Without a pilot positioner your'e dead in the water. I had a building with old McQuays with variable inlet guide vanes and big actuators to drive them.
Having velocity controllers means that velocity reset between min and max settings is possible regardless of setpoint.
Either way I dont see it working and imagine cold and hot spots throughout the space.
I've come behind guys who did stuff like that, usually building " engineers " and wish they would have just left it alone. It's hack service work in my book.
07-26-2012, 06:47 PM #8
I've controlled about 8 vav boxes with reheat with one tsta and a capacity relay.
07-26-2012, 08:08 PM #9Regular Guest
- Join Date
- Jul 2001
Well 6 seems your hung up on pilot positioners , so I will bow out on this thread , my 38 years and thousands of VAV box installs , repairs , service and I can count the # of piot positioners on a VAV on 1 hand.
No need for a long reply , sorry I posted in the 1st place.
07-26-2012, 10:07 PM #10
I brought them up to more or less illustrate my point about losing branch volume when T-ing off to too many actuators.
If youv'e done it and it's worked then thats great, I just dont think its a effective way to control 9 VAVs over a 3000 sq foot open space.
I've got a building thats 3 floors of open space with cubicles and I'm not running into the problem the OP was concerned with in his first post.
( t-stats ) fighting each other. As long as they remain calibrated it the floors space temp seems to maintain a even and confortable temp.
Also one stat for nine Vavs when you have exterior pocket heaters just doesnt sound like it would work effectivley.
08-09-2012, 03:03 PM #11
thanks for all the input
this might be a good time to begin the painful conversion to DDC, eh?
Buildings are 60 years old, and I think a reducting with new VAV's is the proper way to renovate this space. Hope I can convince the bean counters!
08-09-2012, 03:19 PM #12
If I understand the sequence right it's best, IMO, to limit the number of TStat's in the common area and then run the signals from the used TStats into pilot one to one relays where the strength of the branch signal only signal the pilot realay, which uses main air for power and the TStat signal for a signal only. So the power comes from the pilot relay and not form the TStat.
Not saying this right but this will allow contol of many devices by the use of only one TStat. Hope this makes sense. Control man can clarify what I'm saying if it does not make sense."The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
- Alexis de Toqueville, 1835