Thermostat Locations 1

[Buildtech2]

The rooftop package unit is serving to a large office with two external walls with windows, the entrance lobby and an internal office space for which walls are not exposed. The thermostat is located in the large open office which is quickly achieving set point temperature in summer and cutting off the compressor and internal office and entrance lobby is not achieving desired set point temperature because thermostat is cutting of the compressors as soon as large open office is achieving the desired set point temperature and we are receiving complains. To overcome this problem, I have suggested to relocate the thermostat sensor inside the main return duct.

Did anybody came across similar situation. Can we resolve this problem by relocating the sensor inside the main return air duct. please advise.

 

[Lnewqban]

It could help, according to the location of the return grille.
I would suggest re-balancing the air distribution (stealing some supplied volume from areas of the big room that are away from exterior walls and windows), including proper return from the entrance lobby and the internal office space.

The variations in the population of the big office and in the sun loads will eventually make any distribution inadequate for the smaller rooms.

If a re-balance does not work still and cost is not an issue, you could add some zone dampers or booster fans dedicated to the small rooms and controlled by individual T-stats, especially if the roof unit is chilled water.

 

[EnergyProfessional]

You could use thermostats and average them. but the office you describe needs a multizone system (VAV etc.)
right now you make the people happy that have the thermostat. If you move it elsewhere, you make them unhappy.
Also keep in mind in heating you may reverse that happiness again.

sounds like you have a constant volume AHU or RTU.

 

[Buildtech2]

The thermostat will be located in the same office but only its sensor will be relocated to the main return air duct.So they still can play with the thermostat settings in the main main office to suit their requirement and I hope compressors will not cut off until spaces are reasonably cooled.

Yes, this is a constant volume RTU without VAV's

 

[EnergyProfessional]

All you will do is average the temperature. You still will have too warm and too cold areas.

 

[Buildtech2]

You are right EnergyProfessional. Now I have decided to provide two thermostatic location options as follows.

In addition to wall mounted thermostat, a duct mounted sensor will also be installed in the bell mouth of main return air duct. if wall mounted thermostat does not satisfy their requirement, the sensor wire from duct mounted thermostat will be connected in the wall mounted thermostat. Either one of these options may satisfy their requirement and we cannot do anything more than this. The comprehensive solution for this would be to provide bypass type VAV boxes which is not a pert of our original design.

 

[dbill74]

More information = better answers.
How large is the open office area?
Is the RTU fan ON fulltime?
Which direction do the walls/windows face?
How big are the windows? 20%, 40%, 60% of wall area?
Where in the open office area is the thermostat currently located? Near/under a SA diffuser or near the RA grille?
Where are the RA grille(s) located?
What does the air distribution in the open area look like? Is there more air near the windows or is it equally distributed throughout?
Is the open area occupied to design density or some fraction thereof?

The problem with a thermostat in RA duct is that you will need to maintain air movement through the duct at ALL times.

One option could be to add VAV boxes AND a bypass damper.

 

[Buildtech2]

Hi dbill74
My responses are as follows.

How large is the open office area?
152 sqm.
Is the RTU fan ON fulltime?
Yes
Which direction do the walls/windows face?
Exposed walls and windows face NE & SE directions.

How big are the windows? 20%, 40%, 60% of wall area?
The wall facing NE direction has 20 % glass area. The wall facing SE direction has 12 % glass area.

Where in the open office area is the thermostat currently located? Near/under a SA diffuser or near the RA grille?
The thermostat is unfortunately located near to the supply air diffuser which will be relocated near o the return grille. Please refer to attached drawing.

Where are the RA grille(s) located?
Refer to attached layout

What does the air distribution in the open area look like? Is there more air near the windows or is it equally distributed throughout?
Equally distributed throughout.

Is the open area occupied to design density or some fraction thereof?
We found that occupancy in the open area is less than what we considered in the design.

 

[Buildtech2]

Please refer to attached HVAC layout

 

[dbill74]

Seeing the plan now helps.

Relocating the thermostat to the return air will help (only because you have the return air plenum), but there will still be comfort problems. If you want to keep the t'stat on the wall, I strongly suggest moving it to be closer to under a return air grille rather than between two supply air diffusers. Also, locating it in an area that is occupied more closely to the design will help.

Adjusting air balance to have more air from the exteiror diffusers will help. Being that these walls face NE/SE, I'm thinking about 15% should do. I recommend revising your calculations to make the exterior 10 feet or 3m separate zones/rooms for determining air distribution throughout the space.

With RA grilles in the perimeter zone (next to exterior walls) I expect there will be warm spots along the walls.

 

[SAK9]

Couple of bypass dampers that dump supply air into the ceiling void in response to the zone thermostat is also a solution and has the benefit of a VAV system minus the energy savings.