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Autosizing of heating coil UA failed for Coil:Heating:Water

asked 2019-02-10 19:35:24 -0500

mldichter's avatar

updated 2019-02-11 13:46:49 -0500

I'd appreciate some advice on appropriate object choice. I have a building with a group of 5 zones, 4 perimeter and 1 core. I have a DOAS, just a fan for ventilation with no heating or cooling, that serves each of the 5 zones. I want to use a Coil:Cooling:Water and Coil:Heating:Water in a central unit that conditions the air in all 5 zones. Then I would have a reheat gas coil for the registers in the perimeter zones and the core zone without reheat since that will be the controlling thermostat zone.

The DOAS seems to be working fine, but I'm having a big problem with the coils. I'm using an AirLoopHVAC:UnitarySystem containing the Coil:Cooling:Water and Coil:Heating:Water and a fan. (I wanted to use a ZoneHVAC:FourPipeFanCoil, but I think that can only service a single zone.) Then I'm using an AirTerminal:SingleDuct:Uncontrolled for the core zone and AirTerminal:SingleDuct:ConstantVolume:Reheat for each perimeter zone.

In particular, I have a fatal sizing error in energyplus.

   ** Severe  ** Autosizing of heating coil UA failed for Coil:Heating:Water

There are very similar unmethours posts here and here.

There is also an energyplus issue that appears to have been resolved here.

From what I understand, the error is from the water flow in the hot water loop being too low, which the water coil tries to compensate for with a huge UA value. This huge UA value causes the error.

I tried looking through my idf and found that everything for water flow in the hot water loop was already autosized. I haven't been able to find the error, and some of my attempted fixes make many more worse errors!

At this point, I think I either used inappropriate objects, or I entered something really weird in an object and I just can't find it. I am planning on starting over (the idf file is a bit of a mess, which might be why I'm having trouble fixing it) and wanted some advice from the experts here on if there's a likely suspect for my error, or if my choice of objects could be better, or a very similar example file would be great!

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answered 2019-02-12 12:53:37 -0500

mldichter's avatar

Turns out you don't need a AirLoopHVAC:UnitarySystem or ZoneHVAC:FourPipeFanCoil objects to encompass the water coils. You can just put the water coils directly in the hvac loop. The example file 5ZoneAirCooled.idf is a good place to start if anyone is interested.

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Asked: 2019-02-10 19:35:24 -0500

Seen: 545 times

Last updated: Feb 12 '19