Multi-room FP-VAV units

[awidiger]

So we did a remodel install a few years ago. The mechanical engineer first tried to get the customer to accept a single FP-VAV unit for a single office/area, but the customer didn't like that option. So instead, he split a single FP-VAV to two or sometimes three offices/areas. Now the customer is getting complaints that the rooms without the almighty stat are not as comfortable as they would like it. Since the controllers we used (Trend IQL-VAV) can only accomadate one thermostat per controller, we are left to design something that switches the FP-VAV unit to its own AHU (basically). The plan so far is to have a thermostat controlling a modulating (I almost think I should switch it over to a two-position) actuator to the room's diffuser. With this I could modify a cfm setpoint on the FP-VAV to make sure that the ducts don't blow out, but I still have my concerns.

Trend LON is not the strongest on allowing third-party LON devices on its trunk (as far as I know, but I am not Echelon certified), so I'm basically stuck programming the damn thing to act on a voting system for the system changeover. Has anyone dealt with something like this? Thoughts, recommendations, horror stories? :anyone:

[orion242]

Here are a few options

1. The sensor is 10K type 2. You could add more by paralleling 2 sets of 2 sensors in series. That would give you 4 sensors that would averaged together. Cheap and easy.

2. Pickup the space sensors on another controller, and send the value over the communication network to the existing controller. Again you could average them or take the min or max and control with that.

3. The controller could be reprogrammed to accept another sensor or two depending on how many inputs are free. This is not an easy process but possible.

4. Adding dampers to the other diffusers and hardwiring stats will be a mess. The VAV is going to try to maintain its CFM setpoint. Closing a diffuser will cause the others to sound like a jet engine. Lowering the cooling CFM max, will likely prevent the unit from meeting the cooling load during the summer.

If they want perfect control, they will have to add VAVs.

[xarralu]

So instead, he split a single FP-VAV to two or sometimes three offices/areas. Now the customer is getting complaints that the rooms without the almighty stat are not as comfortable as they would like it.

Sounds like most any other office building.


4. Adding dampers to the other diffusers and hardwiring stats will be a mess. The VAV is going to try to maintain its CFM setpoint. Closing a diffuser will cause the others to sound like a jet engine. Lowering the cooling CFM max, will likely prevent the unit from meeting the cooling load during the summer.

If they want perfect control, they will have to add VAVs.


Sounds like your best bet

[incontrol]

So the customer is disappointed with his decision and wants to make it your problem.. Oh, and keep the repairs under $100 too.
If they're always overcooling you may want to add reheat. If they're overheating you'll need volume control. Adding a damper may not be a bad idea since a box in reheat mode is not searching for maximum cfm. I can't find the info on it but Moneywell makes a damper, room sensor, and duct stat to switch damper actuator based on duct temp and if the space needs what's available.

[awidiger]

Here are a few options

1. The sensor is 10K type 2. You could add more by paralleling 2 sets of 2 sensors in series. That would give you 4 sensors that would averaged together. Cheap and easy.

2. Pickup the space sensors on another controller, and send the value over the communication network to the existing controller. Again you could average them or take the min or max and control with that.

3. The controller could be reprogrammed to accept another sensor or two depending on how many inputs are free. This is not an easy process but possible.

4. Adding dampers to the other diffusers and hardwiring stats will be a mess. The VAV is going to try to maintain its CFM setpoint. Closing a diffuser will cause the others to sound like a jet engine. Lowering the cooling CFM max, will likely prevent the unit from meeting the cooling load during the summer.

If they want perfect control, they will have to add VAVs.
Might give them #1 as an option. I agree with what you have said about #4, thats why I had thought putting in a programmable controller could help the situation, but to me there is still a possibility it won't work.

But it looks like bringing in more VAV's will be the final solution for what they want.

[awidiger]

So the customer is disappointed with his decision and wants to make it your problem.. Oh, and keep the repairs under $100 too.
If they're always overcooling you may want to add reheat. If they're overheating you'll need volume control. Adding a damper may not be a bad idea since a box in reheat mode is not searching for maximum cfm. I can't find the info on it but Moneywell makes a damper, room sensor, and duct stat to switch damper actuator based on duct temp and if the space needs what's available.
I'll do some searching for it. I had thought about putting in some VVT controllers for the overheating/overcooling rooms, but that's over 100 bucks lol. I'm not too sure beyond getting the Mech. Engineer involved, but he's on vacation :couchhide: