Odd thermostat requirement

Todd0x1

Senior Member
Location
CA
Does such a thermostat exist? Odd setup with an AC unit feeding a pair of VAVs with heaters. The existing thermostats control the VAV with 2 outputs relay and 2-10 modulating. This original setup had the AC unit running 24/7 as it cooled a computer room and the VAVs were branched off it to feed some offices. Now the computer room is repurposed and they replaced the big computer room ac with a small one just to feed the offices. I need to come up with a way to run the VAVs and heat as they ran before, but when the Tstat is on the cooling side I need to be able to send a call for cooling to the AC unit whereas before the installation assumed constant cooled air.

So I need 3 outputs for cooling / heat function plus fan control.
-cooling relay
-heater relay
-2-10v modulating for VAV damper

And there are 2 thermostats, each running a VAV. Either or both need to be able to command the fan and cooling (I guess a couple pairs of relays would take care of this part)
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Brewery Engineering Plant Technician - HVACR Electrical and Mechanical Systems
When you say VAV are you indicating Fan Powered Boxes or DX FCUs? The original setup sounds quite atypical, as well as the one that is now wanted. " AC unit feeding a pair " raises black flags over design air flow and system loading.......
Sounds like possibly a VVT system and you seem to be indicating DX cooling " upstream."
This is truthfully a " Re-design " for an HVAC outfit. I am sure you are aware there are some potential pitfalls in this kind of configuration, especially with DX cooling.
 

Todd0x1

Senior Member
Location
CA
The VAV boxes are a damper and heater. The original setup had some liebert crac units feeding a common plenum for the computer room. That plenum had a duct running to the office area where there are two of these VAV boxes above the ceiling. The change that was done reconnected the office duct to a small DX cooling unit. The cooling unit will provide fan and cooling, the VAVs control airflow and provide heat when necessary. It's only going to be there for 1-2 years and just has to sort of work, so theyre not going to have it engineered.
 

Todd0x1

Senior Member
Location
CA
HA! found an off brand tstat that does what I need. Has Bacnet/modbus, several inputs, 2 analog outputs, 5 relay outputs, and software to program it to do whatever you need.
 

Eddie702

Licensed Electrician
Location
Western Massachusetts
Occupation
Electrician
Usually with a VAV system you maintain a constant 55 degree summer and 60 degree winter discharge temp.The air damper in the vav should be in the minimum position while heating. I would let the stat modulate the damper if the return air rises above 74 you bring on the cooling and modualte the damper open as needed. If the return air drops below 68 drop out the cooling and bring on the heat. You could do this with some Johnson 350s not very expensive. They can output 0-10 volts to modulate the damper and have stage controls you can stack on to control the heating and cooling
 
... It's only going to be there for 1-2 years and just has to sort-of work, so theyre not going to have it engineered.
I wonder if they said the same thing when it was installed in 1970?

Are the VAV boxes equipped with electric heaters? Make sure there's enough airflow over them to prevent burnout when the VAV is dialed down to the minimum.
 

Todd0x1

Senior Member
Location
CA
I wonder if they said the same thing when it was installed in 1970?

Are the VAV boxes equipped with electric heaters? Make sure there's enough airflow over them to prevent burnout when the VAV is dialed down to the minimum.

Thats that saying? "There's nothing more permanent than a temporary repair"

In this case, the original install was in the early 2000s and it was heavily engineered. The temp fix really is temporary as the building is slated for removal.

The VAV boxes have electric heaters and there is an airflow switch interlocked with the heater contactor's coil. Good point on ensuring the airflow is sufficient at the lower airflow setting though, don't really want to be relying on an interlock for normal function.
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Brewery Engineering Plant Technician - HVACR Electrical and Mechanical Systems
I do not quite have a picture of the layout described. I am sure you are aware that the DX system in question requires an absolute minimum airflow to operate. Unless it has unloading capability which is very unlikely, this means you may need a relief damper and you cannot control on space temperature and will have to control on supply or return plenum temperature. Also there can be no mode differences between zones.
No doubt you can make it work.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
Thats that saying? "There's nothing more permanent than a temporary repair"

In this case, the original install was in the early 2000s and it was heavily engineered. The temp fix really is temporary as the building is slated for removal.

The VAV boxes have electric heaters and there is an airflow switch interlocked with the heater contactor's coil. Good point on ensuring the airflow is sufficient at the lower airflow setting though, don't really want to be relying on an interlock for normal function.
In 1917, Stevens Institute of Technology put up a temporary building that was used for training officers for the Navy under some sort of contract for WW I. It was, imaginatively, called "The Navy Building". This temporary structure was finally demolished...in 1984 or so. I had several classes there in the late 70's.
 

Todd0x1

Senior Member
Location
CA
I do not quite have a picture of the layout described. I am sure you are aware that the DX system in question requires an absolute minimum airflow to operate. Unless it has unloading capability which is very unlikely, this means you may need a relief damper and you cannot control on space temperature and will have to control on supply or return plenum temperature. Also there can be no mode differences between zones.
No doubt you can make it work.
I'll see if I can sketch the layout.

Very good points. I think ill be ok with regard to minimum airflow as the dampers would be open when the compressor is running. I found out that besides the two VAVs the DX unit also supplies an electrical room and a bathroom so there is some air bled off there too.

I thought I could control on space temperature, to a point. What am I missing? No mode differences is acceptable.

Looks like ill end up with two tstats connected rs485 to a controller. I can add temperature sensors to the supply and return ducts. With all that, getting it to function properly is just a matter of getting the programming right.
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Brewery Engineering Plant Technician - HVACR Electrical and Mechanical Systems
The main thing to avoid goes like this.....
Any time you have a DX cooling system that does not have capacity control, and you start shutting dampers downstream with other dampers still demanding cooling, you can get into a situation very quick where you have more compressor horsepower running than air flow. This can create a liquid refrigerant floodback situation to the compressor very quick, especially with a fixed restrictor. This is why larger DX - VAV systems control the cooling machery on generally discharge air temp in the main supply plenum.
So you do not want any scenario that could create an underloaded situation which can lead to liquid floodback and fast compressor damage.
You do not want the evaporator running down much below 30F saturated suction temp if it can be avoided and you need at least 20 degrees of compressor superheat ideally. If you have a system that is dependent on the main supply fan running the heat, you cannot have disparity between zones with respect to cooling or heating mode unless you have hot and cold deck multi-zone capability.
Its has to be one or the other. This is why fan powered boxes are better if you are running electric heat because the inlet damper can go full closed and the box will come on and run heat in that zone. So your design has a lot of built in limitations that have limited solutions with respect to the space being conditioned and what is Technically possible. It sounds like you are talented with controls.
 

Todd0x1

Senior Member
Location
CA
The main thing to avoid goes like this.....
Any time you have a DX cooling system that does not have capacity control, and you start shutting dampers downstream with other dampers still demanding cooling, you can get into a situation very quick where you have more compressor horsepower running than air flow. This can create a liquid refrigerant floodback situation to the compressor very quick, especially with a fixed restrictor. This is why larger DX - VAV systems control the cooling machery on generally discharge air temp in the main supply plenum.
So you do not want any scenario that could create an underloaded situation which can lead to liquid floodback and fast compressor damage.
You do not want the evaporator running down much below 30F saturated suction temp if it can be avoided and you need at least 20 degrees of compressor superheat ideally. If you have a system that is dependent on the main supply fan running the heat, you cannot have disparity between zones with respect to cooling or heating mode unless you have hot and cold deck multi-zone capability.
Its has to be one or the other. This is why fan powered boxes are better if you are running electric heat because the inlet damper can go full closed and the box will come on and run heat in that zone. So your design has a lot of built in limitations that have limited solutions with respect to the space being conditioned and what is Technically possible. It sounds like you are talented with controls.
Thank you for the compliment, I only wish I were talented with controls. I have a long way to go and am just learning hvac. (only reason I am messing with this is due to the situation, the fact the building is only here for a couple years, and if I completely scerew it up there is no loss or damage -I would never touch something that I don't know enough about in a normal situation)

This is exactly the information I needed. This is a small system, but ill add a temperature sensor to the supply plenum. I see what you are saying about dampers being closed when cooling. The only time I have to use the dampers is during heat, so I can program the controller to leave both dampers full open during cooling.
 
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