Pneumatic Controls Hot Deck Cold Deck

[joedeck1]

We are working on a hot deck cold deck system with pnumatic zone dampers. The cold deck damper is closed with no air preasure and the hot deck is open. The rooms have Johnson T-4002 201 single purpose high velocity stats. The problem we are having is that there are two actuators on one stat and the calibration is not going too well. We can't get the dampers to
modulate properly. I have tried to find info on the stats and all I found is how to calibrate them. Does anyone know if they can work a mixing box properly with two actuators? And would changing the spring preasure in one actuator help in any way?


Thanx in advance for any advice :)

[Edited by joedeck1 on 08-15-2005 at 11:49 PM]

[old fitter]

Just guessing, sounds like these two zones have been grouped together perhaps due to a TI. If that stat is a single pipe feeding both actuators, you may have a fitting,tube,or diaphram leak.If single pipe, it is fed with a five thou[.005]restrictor then that branch signal is teed to the actuators.Any leak wont let the pressure build to stroke the dampers. If the stat is two pipe,test with a gauge and a full call for cooling,you will soon find your leak.Dont forget to re-calibrate,to mid point if hot and cold water, or re adjust strke if steam or dx.

[DeltaT]

Are you sure that the thermostat controls the hot/cold deck dampers actutators directly? Some have a hot/ cold deck discharge controller. Some have a variation.

The hot deck is normally open, called "fail safe",should a simple thing like the pneumatic compressor stop then all the building would go into heating to prevent freezine.

The cold deck goes to a normally closed position to help prevent the chiller(s) into going into full cooling.

If the T4002 is calibrated correctly it's quite possible that you have pneumatic pilot relays which stage the dampers to sequence so as one opens the other closes. The relays should be on the side of the damper actuators and they need to be set properly or you never will get any control.

More info if you please.

[joedeck1]

I guess I didnt explain very well. I have air preasure and no leaks. My failsafe position is heat. The problem is getting the stat to modulate so the air can be mixed in the mixing box. The dead band is so close you get full heat or full cool with no mixing. I was wondering if a T-4002 was capable of mixing. It says in the specs it is a single purpose stat. Have you guys used this stat in a mixing box situation? I think a T-4054 is made for a mixing box but I'm still trying to find a solution as of now.

Oh and the stat is the only thing operating the dampers.

The crazy thing about this deal is the hot deck cold deck and actuators are all new. Four penthouses full of it. And they did nothing down in the building, didn't replace a single stat.

Like having a new car with no steering wheel :)

[Edited by joedeck1 on 08-16-2005 at 09:04 PM]

[okemechtech]

What you need to know is the hot and cold deck actuator spring pressures. You can add a positive positioner to shift the spring range on one actuator if the deadband is too small.

[old fitter]

I guess I misunderstood, I assumed this was a Multizone air handler.Sounds like what you have is a hot and cold mixing box. First check your actuators,the spring ranges cant overlap.If this is a typical JC job,you might have a 4-8 N.O. htg actuator and a 9-13 N.C. clg actuator.Out of the box stat sensitivity is 2-3 #s per degree F.,so you can see that a 3 degree temp change will send you from full heat to full cool.Check to see if: someone changed[increased] your sensitivity,your air balance is correct,and the stat is in a good location,near the "mixing" portion of the space, not in the path of the supply air. You can always change the sensitivity,even down to 1# per degree.

[DeltaT]

What old fitter said!

There is a big different to us old guys that know what a real hot and cold deck application is compared to mixing boxes.

The mixing boxes come out of the factory either set up with one damper actuator to modulate between the hot/cold supply. Or they can have two which are factory installed with the proper spring ranges.

If the thermostat is acting as if it is a two position thermostat then most likely someone has messed with the sensitivity as stated above. Check the sensitivity then recheck the calibration and you should be OK if there are no other problems such as linkage hangup or some other odd control sequence.

Also, the easiest way to troubleshoot the dampers is with a squeeze bulb or better yet, make your own variable supply pressure using a minimum position switch which you probably have laying around.

[Edited by DeltaT on 08-16-2005 at 10:27 PM]

[joedeck1]

This is an old style hot deck cold deck setup. There are four penthouses each has a hot deck cold deck that operates close to a quarter of the buildings space. Each hot deck cold deck has around 25 zones. Each zone has what I call a mixing box, where your hot deck comes off of the top and the cold deck on the bottom and then the air mixes,goes into one duct and out in the room. Each zone represents a room. The specs called for one actuator with a single shaft to control both dampers. For some reason they could not use a single shaft and actuator to work both dampers so they used two. One on top for hot deck and one on bottom for cold deck. Hot deck N.O. cold N.C. So I have two actuators and two dampers with one stat for each room. I can't get the dampers to modulate. For example I had a room temp of 74 I set stat for 65 and nothing happens. I turn stat to 55 and get full cooling. Turn to 85 and get full heating. Can't get anything in between. I have adjusted everything I know to adjust without anything close to a solution. I just don't think the stat is capable of modulating two actuators. :(

Hot decks are supplied by steam boiler in another building. I have only worked on a few hot deck cold deck systems. They are not very efficient IMHO. always heating and cooling at the same time and relying on dampers to mix the air to a good discharge temp does not even sound logical to me. Running a steam boiler in the summer to pull humidity. What were they thinking?





[Edited by joedeck1 on 08-17-2005 at 12:40 AM]

[joedeck1]

Originally posted by okemechtech
What you need to know is the hot and cold deck actuator spring pressures. You can add a positive positioner to shift the spring range on one actuator if the deadband is too small.

I have looked at the spring tension. with two actuators on one stat, let me know if I'm wrong here :). Both actuators are 8-13 psi range, 8 closed 13 open. If I change the tension on one say like the cold side, since the hot is N.O. It will still close the hot side because it is set at a lower tension.

Now if I had a different stat that was capable of operating in two different preasure ranges it could be possible. Keep in mind if I go that route I would need to change tension on close to 100 actuators. Man that would be a bad week....

[okemechtech]

So you at least see how the dampers are operating. There is an overlap, where both dampers are open at the same time. This condition blends the air so that there is always some airflow. Think of it this way; The normally closed 8-13# cooling damper starts to open @ 8# and is fully open @ 13#, whereas the normally open 8-13# heating damper starts to close @ 8# and is not fully closed until it reaches 13#. So between an 8-13# stat signal there is some blending going on. The crossover point is at midspan or 10.5#. Of course this is theoretical operation. If there is any mechanical resistance, binding, or position deviation the spring ranges are thrown off. Make sure that you don't have any severe binding problems.

Which is why systems like this so energy inefficient that they are no longer being installed.

You may want to pay a little more attention to the hot deck and cold deck settings at each main coil, Resetting the deck temperatures would help reduce energy consumption.
It is most likely already being done, but look into recalbration. Some systems were reset based on outdoor temperature or zone high/low signal selection.

[DeltaT]

Originally posted by DeltaT
Also, the easiest way to troubleshoot the dampers is with a squeeze bulb or better yet, make your own variable supply pressure using a minimum position switch which you probably have laying around.
[Edited by DeltaT on 08-16-2005 at 10:27 PM]

Once again, if you can modulate the dampers with a squeeze bulb or minimum position switch which will gradually increase/decrease the branch line pressure to the dampers, then you problem is the thermostat and its sensitivity.

[old fitter]

If the whole building has had these actuators replaced incorrectly,why not just buy a new actuator for the hot deck,4-8 spring. What a hero youll be. I just ask that you act like you know exactly what your doing; anything can be fixed with pneumatics,you could even see if they sell just a new 4-8 spring,and your in business.

[okemechtech]

My recommendation is to stick as close to the original concept of the hot deck cold deck mixing box system. You want to be careful in selecting a modification, especially when you have that many zones.
The idea of changing one spring to a 4-8# spring range may create a ventilation problem at an 8# stat signal. The original mixing box theme would never be able to have both dampers closed at the same time if you were using one actuator.

[old fitter]

Sure, and at 3lbs per degree,how long do you think the branch line will stay at 8psi? Assuming normal deck temp at about 55 and 90 degrees,your overlap wont cool or heat very well until the stat calls for the extreme.If you are concerned about a closed box at 8psi,adjust your linkages and set screws to provide a minimum flow at closed pressure.

[hvacbear]

Originally posted by joedeck1
This is an old style hot deck cold deck setup. There are four penthouses each has a hot deck cold deck that operates close to a quarter of the buildings space. Each hot deck cold deck has around 25 zones. Each zone has what I call a mixing box, where your hot deck comes off of the top and the cold deck on the bottom and then the air mixes,goes into one duct and out in the room. Each zone represents a room. The specs called for one actuator with a single shaft to control both dampers. For some reason they could not use a single shaft and actuator to work both dampers so they used two. One on top for hot deck and one on bottom for cold deck. Hot deck N.O. cold N.C. So I have two actuators and two dampers with one stat for each room. I can't get the dampers to modulate. For example I had a room temp of 74 I set stat for 65 and nothing happens. I turn stat to 55 and get full cooling. Turn to 85 and get full heating. Can't get anything in between. I have adjusted everything I know to adjust without anything close to a solution. I just don't think the stat is capable of modulating two actuators. :(

Hot decks are supplied by steam boiler in another building. I have only worked on a few hot deck cold deck systems. They are not very efficient IMHO. always heating and cooling at the same time and relying on dampers to mix the air to a good discharge temp does not even sound logical to me. Running a steam boiler in the summer to pull humidity. What were they thinking?





[Edited by joedeck1 on 08-17-2005 at 12:40 AM]



Sounds like you need to lower your sensitivity/gain (depending on what school of thought you follow).

They system you describe sound ineffiecient what is the application?

[core]

...is exactly what you will be doing if you keep that current set up.

This is what I hear that is wrong.


#1 Wrong dampers...The company that installed those dampers hasn't a clue about pneumatic. Clearly the thermostat can't suppply enough air to operate both dampers at the same time. We know this because the dampers work at the extreme ends of the scale and not mid-point. What you need is a pilot driven damper. This is a damper that has TWO tubes going to it (a) a signal tube from the t-stat and (b)Main air tube to drive the damper. There is no way on Gods green earth you will drive two damper bellows with a t-stat. You can verify this by calculating the amount of air volume needed to drive both dampers.

#2 You need OVERLAP....All Hot / Cold deck systems need overlap. Just "What will happen to the Fan if there is no overlap?". If you are using a Centrifical forward curve fan you fan will start "Riding the curve" this is bad because as dampers start to modulate, with no overlap, the fans Brake Horsepower will increase with Static pressure rise. You will or may reach a point where your fan starts to ride in the "Unstable area" of its curve. At this point many things can happen like fan surging and breakers or fuses tripping.

What you need too do is simple. You must present logical facts to the people who approve repairs. You need to track down via the prints the original damper or equivalent including t stat.

Like I said there is no way your current system will work with that set-up.

Hope this helps

Core

[hvac3901]

first what is the velocity control used? do you have one, and have you checked? are you refering to the ahu level or zone level stuff? sounds like a double duct box, is it CV?

i hope you did'nt turn too many knobs. 4002, is just the thermostat, it sends a signal to the velocity controls, if you have velocity controls they are controling velocity regardless of what your damper motor spring range is, PERIOD!

maybe i'm a bit confused, but it sounds to me like somethings missing.

[Edited by hvac3901 on 08-21-2005 at 02:13 PM]

[Control Man]

Have worked on Loads of MULTI ZONE AHUs in the past 28 years and they are not complicated and the older installations have no velocity controls.

* BASIC * sequence of operation is Thermostat sends 0 - 15 psi range signal 0 - 8PSI modulates the HOT DECK DAMPER from OPEN to CLOSED 9 - 15 PSI modulates the COLD DECK DAMPER from CLOSED to OPEN . Many use 1 actuator with a dual face damper , some have dual actuators with seperate dampers for each deck as well as some have PILOT POSITIONERS on the actuators.

[hvac3901]

controlman

agreed the most simplistic pnuematic in operation, but find it hard to believe that someone retrofited to new boxes and maitained the mixing boxes as they were. i wonder if our freind is somewhat stuck on the old terminology and trying to relate something new to us. please note my post did pose a few questions. i wonder also if maybe i jumped on this thread a little late? hard to believe that it just won't work until at the extreme setting of the t-stat.

sincerely,
ron

also would lke to know if there are two fans involved here, i.e. hot deck fan and cold deck fan?

[Edited by hvac3901 on 08-21-2005 at 07:35 PM]

[Control Man]

Rare to see 2 supply fans on older HOT / COLD deck systems, most of the problems I have run across on this type of system is incorrect diagnostics and many times it is as simple as a miscalibrated thermostat or a slight air leak.

[westsidemech]

The t-4002-201 is a High volume output stat. Calibrate to mid spring range.
Determine ambient temp value.
Turn setpoint dial to ambient temp.
Adjust output to mid spring range.of the controlled device.
How johnston likes to do it.
So at a satifyed temp is were it is cailbrated. same air flow
Both dampers open to mix.at mid spring range. Don't know if you have different spring ranges on the dampers or pilot positioners.

[westsidemech]

What do the design specs call for on this system? The factory sensitivity set up on the stats is 2.5 psi per degree change. We are energy pigs here. heating and cooling at the same time. Any of them work correctly.

[propmanage]

If you have a pilot positioner, adjust and calibrate it?

Sounds like you have a heating coil with valve, a cooling coil with valve. Each controlled by sensor set point. Then you have the dampers for the Hot/cold deck controlled at the T stat. Place a gage on the damper line and warm up the thermostat to see a change at the damper. If no change you may not be talking about the same controls to the same damper or you may need to calibrate and possibly adjust the band at the T stat. You may also have a bad one and need to replace it.

Trouble shoot trouble shoot

[propmanage]

If you have a pilot positioner, adjust and calibrate it?

Sounds like you have a heating coil with valve, a cooling coil with valve. Each controlled by sensor set point. Then you have the dampers for the Hot/cold deck controlled at the T stat. Place a gage on the damper line and warm up the thermostat to see a change at the damper. If no change you may not be talking about the same controls to the same damper or you may need to calibrate and possibly adjust the band at the T stat. You may also have a bad one and need to replace it.

Trouble shoot trouble shoot

[dageek]

good to see the old guys sharing with the new guys to the pneumtic field . . . the old fitter has his stuff together and knows his stuff . . Johnson has some good application notes which might help a lot.

as far as the squeeze bulb, use that instead of the grad relay or regulator . . . with the squeeze bulb, you can check for leaks at the same time you drive the actuators....

keep up the good work 'Older Fitter', you da man