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Is there an economical electronic replacement for a pneumatic thermostat?
I'm doing Tenant Improvements on a commercial space in a mall that was built in the early 1980's. The existing HVAC consists of a set of boilers and chillers on the roof, with heated and chilled water piped throughout the building. In each of the occupied spaces, there's a fan unit with heating and cooling coils. The thermostat is a Honeywell TP970 or TP972 Series (I can't remember the specific model). There are two pneumatic lines connected to the thermostat: one from the building source of compressed air (supply), and the other to the valves that allow the heated and chilled water to flow into their respective heating and cooling coils in the fan unit (control). Both the heated water valve and the chilled water valve are controlled by the single pneumatic line from the thermostat. When the thermostat allows pressurized air to the water valves, one valve opens and the other closes. When the thermostat bleeds off the pressurized air to the water valves, the open valve closes and the closed valve opens. The fan runs constantly.
This is a VERY inefficient system. Because there are only two states to the thermostat (on/off), the system is either running full hot or full cold. There is no time when there is not full heated water running through the heating coil or full chilled water running through the cooling coil because the thermostat is simultaneously opening/closing both hot/cold water valves with an "on/off signal" via its single pneumatic control line.
For example, if the thermostat is set to 70°F, then when the room is even slightly below 70°F the fan unit is blowing full hot. As soon as the room temp creeps above 70°F, the fan begins blowing full cold. Once the room temp crosses back down below 70°F, the fan starts blowing full hot again. In other words, the HVAC is constantly fighting itself to stay on temperature. Needless to say, the tenants are not happy that the fan will be alternately blowing full hot and then full cold air on them whether it's during the highs of summer or the lows of winter.
So I was hoping to offer them an electronic thermostat that can control the two pneumatic valves (hot and cold) independently. Obviously, this would mean separating the two valves from the single pneumatic line from the thermostat and running a dedicated pneumatic line to each of the valves.
Does such a thermostat exist that is also relatively economical? I've only worked with fairly modern commercial heat pumps and furnaces, so this is my first experience with an old pneumatic system.
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Get someone that knows pneumatics to fix it.
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It's not broken. Its working as designed. The problem is that the design is crap.
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Plus, there are many advantages to modern electronic thermostats. Such as programmable schedules, remote monitoring, etc.
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Never seen a system that alternates between the 2 valves. slowly open and close yes! That's why I recommended getting a pneumatics controls contractor before ripping out and replacing a lot of components.
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Economical?
Put modulating actuators on the valves and add modulating stats
Officially, Down for the count
YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET
I know enough to know, I don't know enough
Why is it that those who complain the most contribute the least?
MONEY CAN'T BUY HAPPINESS. POVERTY CAN'T BUY ANYTHING
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Slowly? Does it matter if the valves open/close in 1 second or 10 seconds? It's still alternating between 100% hot and 100% cold as the room fluctuates around the set temperature.
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Originally Posted by
2sac
Economical?
I don't see why an electronic thermostat with a couple of solenoid valves to control the pneumatics would cost more than a few hundred dollars.
Originally Posted by
2sac
Put modulating actuators on the valves and add modulating stats
If I'm correctly understanding your recommendation, that would allow both valves to be partially open at the same time. That would mean the air is being simultaneously heated and cooled. That may improve comfort, but does nothing to improve efficiency. If the room temp is a couple of degrees within the set temp, why does the air need to be heated or cooled at all?
But the way the system is currently designed, there is ALWAYS water flowing through one of the coils, either heated or chilled. Because when one valve is closed, the other must be open. There is no other possible condition the way the valves are set-up.
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Originally Posted by
SpyGuy
I don't see why an electronic thermostat with a couple of solenoid valves to control the pneumatics would cost more than a few hundred dollars.
If I'm correctly understanding your recommendation, that would allow both valves to be partially open at the same time. That would mean the air is being simultaneously heated and cooled. That may improve comfort, but does nothing to improve efficiency. If the room temp is a couple of degrees within the set temp, why does the air need to be heated or cooled at all?
But the way the system is currently designed, there is ALWAYS water flowing through one of the coils, either heated or chilled. Because when one valve is closed, the other must be open. There is no other possible condition the way the valves are set-up.
EP switches aren't cheap. Anything pneumatic isn't cheap. They won't do anything for efficiency.
Why are they running boilers and chillers simultaneously year round?
You can modulate the valves with a 0-10vdc or 2-10vdc actuator using a signal off a stat reading discharge temperature. A Honeywell T775M, 2 discharge air sensors, and 2 modulating actuators w/linkages for whatever valves you have in the system. Set your heat parameters, set your cooling parameters, and the control will modulate whichever valve it needs at the time, not both.
Officially, Down for the count
YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET
I know enough to know, I don't know enough
Why is it that those who complain the most contribute the least?
MONEY CAN'T BUY HAPPINESS. POVERTY CAN'T BUY ANYTHING
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Yes, there is an electronic thermostat that will replace your pneumatic stats, but that won't solve your problem.
You are not understanding pneumatics. The beauty of pneumatics is the 'infinite' control.
For example, you'd want one valve to operate 3-8 lbs, and the other to operate 8-13 lbs.
Likely you are following other techs who also had no clue what they were doing.
The wireless pneumatic thermostat is made by Cypress. Problem is, it will give the same output as a straight pneumatic stat. But it will interface with a computer system that can even be remotely accessed.
I do a triple evac with nitro to remove non condensables.
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Post Likes - 3 Likes, 0 Dislikes
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Originally Posted by
BBeerme
Yes, there is an electronic thermostat that will replace your pneumatic stats, but that won't solve your problem.
You are not understanding pneumatics. The beauty of pneumatics is the 'infinite' control.
For example, you'd want one valve to operate 3-8 lbs, and the other to operate 8-13 lbs.
Likely you are following other techs who also had no clue what they were doing.
The wireless pneumatic thermostat is made by Cypress. Problem is, it will give the same output as a straight pneumatic stat. But it will interface with a computer system that can even be remotely accessed.
That's probably the same stat JCI used to sell and has since discontinued, and it was twice the cost of a standard DA/RA stat
Officially, Down for the count
YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET
I know enough to know, I don't know enough
Why is it that those who complain the most contribute the least?
MONEY CAN'T BUY HAPPINESS. POVERTY CAN'T BUY ANYTHING
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What you describe is not how it was ever supposed to work.
It's not either/or - not an on/off thermostat - and if it is the people who last worked on it created that obviously incorrect situation.
There is a dead-spot in the middle of the actuation pressure range. One valve slowly closes - then: dead spot at set point with both valves closed. If the space temperature overshoots past the set point far enough to require the other valve to begin to open - sumting wong boss.
You don't need more control - you need less. <g>
PHM
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Originally Posted by
SpyGuy
I'm doing Tenant Improvements on a commercial space in a mall that was built in the early 1980's. The existing HVAC consists of a set of boilers and chillers on the roof, with heated and chilled water piped throughout the building. In each of the occupied spaces, there's a fan unit with heating and cooling coils. The thermostat is a Honeywell TP970 or TP972 Series (I can't remember the specific model). There are two pneumatic lines connected to the thermostat: one from the building source of compressed air (supply), and the other to the valves that allow the heated and chilled water to flow into their respective heating and cooling coils in the fan unit (control). Both the heated water valve and the chilled water valve are controlled by the single pneumatic line from the thermostat. When the thermostat allows pressurized air to the water valves, one valve opens and the other closes. When the thermostat bleeds off the pressurized air to the water valves, the open valve closes and the closed valve opens. The fan runs constantly.
This is a VERY inefficient system. Because there are only two states to the thermostat (on/off), the system is either running full hot or full cold. There is no time when there is not full heated water running through the heating coil or full chilled water running through the cooling coil because the thermostat is simultaneously opening/closing both hot/cold water valves with an "on/off signal" via its single pneumatic control line.
For example, if the thermostat is set to 70°F, then when the room is even slightly below 70°F the fan unit is blowing full hot. As soon as the room temp creeps above 70°F, the fan begins blowing full cold. Once the room temp crosses back down below 70°F, the fan starts blowing full hot again. In other words, the HVAC is constantly fighting itself to stay on temperature. Needless to say, the tenants are not happy that the fan will be alternately blowing full hot and then full cold air on them whether it's during the highs of summer or the lows of winter.
So I was hoping to offer them an electronic thermostat that can control the two pneumatic valves (hot and cold) independently. Obviously, this would mean separating the two valves from the single pneumatic line from the thermostat and running a dedicated pneumatic line to each of the valves.
Does such a thermostat exist that is also relatively economical? I've only worked with fairly modern commercial heat pumps and furnaces, so this is my first experience with an old pneumatic system.
PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
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Post Likes - 4 Likes, 0 Dislikes
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This assumption you state is incorrect. The problem is that you do not understand the design, nor how what you have in front of you deviates from the design intent.
PHM
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Originally Posted by
SpyGuy
It's not broken. Its working as designed. The problem is that the design is crap.
PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
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Not to mention they are a pain in the butt to program.
Originally Posted by
2sac
That's probably the same stat JCI used to sell and has since discontinued, and it was twice the cost of a standard DA/RA stat
I do a triple evac with nitro to remove non condensables.
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Think that is what I said in post #10.
Originally Posted by
Poodle Head Mikey
What you describe is not how it was ever supposed to work.
It's not either/or - not an on/off thermostat - and if it is the people who last worked on it created that obviously incorrect situation.
There is a dead-spot in the middle of the actuation pressure range. One valve slowly closes - then: dead spot at set point with both valves closed. If the space temperature overshoots past the set point far enough to require the other valve to begin to open - sumting wong boss.
You don't need more control - you need less. <g>
PHM
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I do a triple evac with nitro to remove non condensables.
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What am I? Clairvoyant? How would I know what's in post ten from reading what's in post one? <g>
Actually when I got to your post I was going to credit you with beating me to the finish line but for some reason, long since forgotten, I didn't. <g>
PHM
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Originally Posted by
BBeerme
Think that is what I said in post #10.
PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
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You might be Clairo Voyant. I have no clue how you choose to respond to a post.
All I know is what I see. Your choice on what I see.
Originally Posted by
Poodle Head Mikey
What am I? Clairvoyant? How would I know what's in post ten from reading what's in post one? <g>
Actually when I got to your post I was going to credit you with beating me to the finish line but for some reason, long since forgotten, I didn't. <g>
PHM
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I do a triple evac with nitro to remove non condensables.
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It is so sad how the beautiful world of pneumatic control has been lost. Funny, when something is mis-understood, it is condemned.
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Post Likes - 3 Likes, 0 Dislikes
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In a technical forum especially; I read posts in sequence. I tend to answer the first post/question without going further because I don't want to be influenced by what others might post.
Plus; it's too easy to get sidetracked off to a subsequent question or topic tangent and then never actually response to the original guy's situation. <g>
I may well Have to become Clairol Voyant - I'm not at all happy with my present shampoo.
PHM
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Originally Posted by
BBeerme
You might be Clairo Voyant. I have no clue how you choose to respond to a post.
All I know is what I see. Your choice on what I see.
PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
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Post Likes - 2 Likes, 0 Dislikes
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Let's have a show of hands.
Will Spy Guy take up the small task of learning some basic pneumatic control logic and application? Or that of enlisting the help of someone who already knows it?
Or will he continue searching for someone who will help him justify selling the customer on some electronic thing they don't need and which won't resolve their issue while having an infinite payback period?
Did he want an answer to a question?
Or just an agreement with an assumption?
PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.