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The simulated zone temperature does not meet the thermostat Schedule?

asked 2019-05-16 15:31:48 -0500

Westely's avatar

updated 2019-05-17 09:01:04 -0500

Hi All: I simulate a small office with a fan with coils ( see the attachment), I've assigned the thermostat schedule into zones, and the zone is served by diffusers.

I think the zone temperature should match the thermostat schedule, but it is not. E.g, I set the heating thermostat schedule is 75 F, but the final average room temperature is only 64 F.

I set most values as autosized, and I think if I set the thermostat value and only care about the indoor temperature, I should not worry about the envelope parameters?

It would be nice if you can give me any clue. Thanks~ image description image description

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@Westely your configuration is a little odd. It looks like you are using scheduled setpoint manager on your air loop, so that may be driving your zone temperature, not the zone thermostat. Typically a schedule air loop temperature would be used in conjunction with terminals that have reheat capacity, which yours do not. Lastly, while not related to the specific issue you are seeing, is there any reason for such a high heating setpoint, or are you just experimenting with the model to see if the system could meet that setpoint.

David Goldwasser's avatar David Goldwasser  ( 2019-05-17 09:58:04 -0500 )edit

@David: Thanks David, the air loop is an FCU serving a small office area which is divided into three offices (A, B, C) actually, every office has a diffuser. I use the scheduled setpoint manager to control the supply air temperature from the fan, not the zone thermostat. The zone thermostat is controlled by the cooling and heating thermostat schedule in the 1st picture.

Westely's avatar Westely  ( 2019-05-17 11:13:06 -0500 )edit

And yes, I set the high heating setpoint to see if the zone temperature increase, but it is not. I changed the scheduled setpoint manager (supply air temperature), and the cooling and heating setpoint, but the zone mean temperature keeps 65 F, no matter what I change, it looks like the FCU doesn't work.

Westely's avatar Westely  ( 2019-05-17 11:14:07 -0500 )edit

@Westely the zone thermostat is just a request for what it wants the zone temperature to be. Some system needs to provide capacity to meet that request. If it isn't the air loop though a terminal then you need some additional zone equipment to condition the space. I don't see any zone equipment except the terminal in your screenshots

David Goldwasser's avatar David Goldwasser  ( 2019-05-17 12:43:23 -0500 )edit

@David yes, I want to use the terminal to control the zone temperature. The schedule setpoint manager controls the supply air temperature from the airloop, then the supply air go to the terminal, heat/cool the zone. I think if I set the zone thermostat and the supply air temperature, the CFM from supply fan should be autosized to fulfil the load?

Westely's avatar Westely  ( 2019-05-17 13:13:10 -0500 )edit

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answered 2019-05-21 09:00:56 -0500

@Westely if the schedule temperature for the air flow isn't higher than the thermostat setpoint (enough to make up for heat loos through the envelope) it would explain why your thermostat setpoint isn't being reached. I will point out again that the loop configuration is odd. When a schedule setpoint manager is used on the air loop the terminals are usually VAV and also have heating elements to re-heat the supplied air. If you are going to put multiple zones on a loop with direct terminals you could try to use a single zone setpoint manager on the zone with the most heating demand.

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@David Thanks, it helps a lot!

Westely's avatar Westely  ( 2019-05-22 00:24:37 -0500 )edit

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Asked: 2019-05-16 15:31:48 -0500

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Last updated: May 17 '19