VAV Reheat Coil Sizing

asked 2016-12-12 22:32:10 -0600

sspielman
251 ●1 ●6

updated 2017-08-28 09:19:31 -0600

__AmirRoth__

4436 ●5 ●17 http://bleedinggreenna...

I am having a problem with heating unmet hours on a VAV system that appears to be an issue with the reheat coil. The problem only occurs in internal zones where the heating load on the coil is due to reheat only. On exterior spaces where there is envelope load there is not problem. The issue is most obvious in large spaces where ACH are high compared to internal loads. The VAV box is supplying air near 55F and not reheating to meet the temperature setpoint. It appear that the coil is not being sized for the reheat load and only for whatever exterior load exists which is none.

Here is my sizing:zone object C:\fakepath\Capture.PNG

Here is my sizing:system object C:\fakepath\Capture1.PNG

The loop set up is simple with a HW loop operating at 130F with a 40F delta T serving the air loop.

Does anyone have insight into why the coil is being sized incorrectly and I am seeing unmet hours?

answered 2016-12-14 07:29:07 -0600

Archmage

7185 ●3 ●11

This is fairly common issue. The zone's design heating load is used to size the reheat coil and if that load is tiny or zero the reheat coils do not come out with enough capacity.

One way of getting some design heating load is to use OtherEquipment (now with the fuel type set to None) with a negative value for the power. A negative internal gain will provide cooling to the zone and thus a design heating load. I use something lik e -20.0 W/m2 or so. Use the OtherEquipment schedule so that the loads occur only during heating design days by using the day type called WinterDesignDay.

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Thank you for the feed back. I guess the work around is providing a dummy load in the space on the heating design day. Is there no way to include a minimum ventilation reheat load on the heating design day?

sspielman ( 2016-12-14 13:52:21 -0600 )edit

@Archmage, I tried to add the negative other equipment definitions, it seems it is only allowed to add "Design Level" with unit "W", but not "Energy Per Space Floor Area" with unit "W/m2"?