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Heating to Cooling Setpoint

asked 2016-05-10 10:24:46 -0500

Adam Hilton gravatar image

updated 2016-05-10 12:23:22 -0500

I noticed the phenomenon in a model I'm working on and created a test model in an effort to more fully understand what's going on. What I've observed is that even though a zone temperature will be well above the heating setpoint, the heating coil will still provide heat to the loop in order to maintain the supply air temperature setpoint. In other words, even though a zone doesn't need heating, heat is still being added.

Here is a graphical representation of what I'm talking about: image description

I understand why it's occurring, but I was hoping someone could shed some light as to what you do to prevent this. Best I can see is to schedule the availability to be off during cooling months for the heating coil. What is needed is a setpoint manager that works something to the effect of following the mixed air temperature to prevent heating/cooling operations from happening when the zones are within their deadband.

Here's the airloop I'm using to test: image description

I'm using SetpointManager:Warmest with a reset between 55°F and 90°F.

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answered 2016-05-10 16:24:10 -0500

It looks like you've got a VAV system, where typically you're supplying cold air from the AHU, then reheating as necessary at the terminals. In your case, when the SetpointManager:Warmest detects that no zones need cooling (during the evening in summer months), the SAT setpoint goes to 90F, which turns on the heating coil. It seems like an easy solution would be reducing the max supply air temperature down from 90F to something that won't require the heating coil to come on. Typically you might set this max to 60F or 65F. People don't typically allow SAT to float too high, especially where humidity is a concern.

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I agree, but if I'm trying to show potential savings and zone conditions comparing a high SAT and low reheat versus low SAT and high reheat I'd need to be able to set the SAT high. Other than scheduling the HW coil as unavailable during the summer months, I'm not sure how else to accomplish resetting to a maximum of 90°F SAT. I'd prefer to not use the coil availability because then it becomes an iterative process to determine the heating/cooling season. Point and case, the building currently operates under an SAT reset between 55 and 90 degrees based on zone average temperature.

Adam Hilton gravatar image Adam Hilton  ( 2016-05-10 17:18:07 -0500 )edit

I'd like to be able to show the building operator the energy savings/penalty of moving from a central heat focused operation scheme to a reheat focused operation scheme.

Adam Hilton gravatar image Adam Hilton  ( 2016-05-10 17:20:04 -0500 )edit

Is the SAT reset that high to do heating with the AHU, or just so that you can avoid any reheat at the terminals?

aparker gravatar image aparker  ( 2016-05-10 19:41:12 -0500 )edit

I'd have to talk to the building operator to get an answer on that. My guess is that the reheat coils might not have sufficient capacity.

Adam Hilton gravatar image Adam Hilton  ( 2016-05-11 12:28:02 -0500 )edit

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Asked: 2016-05-10 10:24:46 -0500

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Last updated: May 10 '16