Gas Fired Rooftop with Electric Reheat

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How best to model this? It looks to me like the reheat coil gets assigned to the main heating plant so the energy allocation is screwed up since the main heating plant is gas but the reheat coil is electric. I was attempting to use CV with Terminal Reheat but not seeing the light. Any suggestions?

Anonymous's picture
Anonymous

The Trouble you are encountering is that the "main" heating coil is one and the same as the reheat coil. What you call "main", TRACE 700 calls "Preheat". So, assign your preheat coil to the gas heat and the "main/reheat" coil to the electric and you will be modeling as intended. 

 

To better understand the coils names in TRACE, go to the schematic tab under "create systems", which will explain that the preheat coil is the "central" coil, which you are calling the Main coil (but TRACE considers the reheat coil to be the main coil)

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Joe
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Joined: 1969-12-31
Reputation: 400

Ah, were it to be so simple.  No the issue is with a constant volume system (of any type) the reheat and main coil cannot be separated.  (See "Systems Library -Table B6).  Modeling as you suggested with a constant volume system with terminal reheat will result in all heating energy being electric and the preheat coil having no energy assigned as the main heating coil gets invoked first.

I suspect if I were to use a BPVAV system it would work as you suggest but I would have to trick the VAV portion to reflect my constant volume system.

This is an existing sytem by the way otherwise I wouldn't be trying to model it nor would I suggest it as a proper way to condition the space.

Anonymous's picture
Anonymous

Okay, I thought you were asking a newbie question. It is the same issue with the inseparable coils with VAV also. 

Here's where we need to start: What exactly do your system's reheat boxes do?

See, the typical control scheme is that the "preheat" coil heats the mixed air to say 55 degrees and then the Reheat coil heats it to room neutral and the main heating heats it to say 90 degrees (of course it is one coil in reality but not to TRACE's algorithms).

I am guessing that your system does not do this but instead the central coil heats to room neutral and then the reheat boxes do the rest (based on your comments about the system being a bit asinine)

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Joined: 1969-12-31
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I suspect that the original air handling units are oversized. There are large overhangs on the spaces that I suspect weren't taken into account during the rule-of-thumb sizing of the units. So the end result was short cycling of the units and uncomfortable humidity levels in the spaces. The "fix" was to add electric reheat coils in the main ductwork and operate those off a (humidi)stat and lower the cooling setpoint. Attempt was to trick the units to stay on longer with the reheats kicking in for comfort. The control is a confusing mess frankly.  I surmise that the humidistat was used to control the units in oder to only energize the reheat coils during the summer months with increased relative humidity ; in the winter the electric reheat would essentially stay off. I realize it is not easy to model systems that don't operate as expected but the customer needs to show the local utility the expected kWh savings to receive an incentive. Frankly, this should be a slam dunk control replacement and properly size the units (nearing the end-of-life as well with EERs in the 7 and 8). 

I did create a base utility and tweak the schedule to match my assumed operation.  It is more on/off operation than being triggered by space humidity levels.

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