VAV reheat coil behaving unexpectedly and leading to unmet hours

asked 2017-12-10 08:20:38 -0600

updated 2017-12-11 09:05:02 -0600

After many days of digging through documentation and tweaking my model, I'm having trouble resolving a couple of issues related to some of the (autosized) VAV reheat coils in my model:

the supply temperature from the VAV box (as indicated by the variable "single duct VAV reheat supply inlet") to a few of the perimeter zones on the sizing day is below the zone air temperature (and the zone air temperature is below the heating setpoint). So basically the zone is underheated all day. All HVAC system capacities (flow rates, output levels, etc.) are autosized. But I did notice the coil heating rate and delta T across the water side of the problem coils seem to be reaching some inexplicable limit.
the design capacity for said VAV reheat coil (as per the summary table) is about 75% as big as its heating design day heating rate. I'd have expected that the sizing period simulation would have led to a coil capacity that is adequate to prevent underheating.
I have reason to believe that the sizing period may not be accounting for the ventilation-related loads, as I expect. I can only get core zone VAV reheat coils to be big enough (or non-zero actually) if I crank up the People Per Zone Floor Area to ASHRAE standard levels (0.054/m2). In contrast, the whole premise of my model (being used for research purposes) is to explore lower occupant densities.

Perhaps these things are related to each other - perhaps not. In any case, advice would be greatly appreciated!

Here's my model and weather file used.

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Your .idf file is has hard coded links to external schedule files that are not available in the dropbox, so I can't run your model as is.

TomB ( 2017-12-10 20:44:13 -0600 )edit

Sorry Tom. Silly me. I updated the link after changing all schedule:file objects to schedule:compact objects. Thanks for your interest.

LiamOBrien ( 2017-12-11 07:41:27 -0600 )edit

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answered 2017-12-11 02:43:46 -0600

Chandan Sharma

2432 ●1 ●2 ●9

VAV reheat coil undersizing in a known issue. One workaround could be to reduce Central Heating Design Supply Air Temperature (to 14 or 13.5 in this case). That will increase the design heating load on reheat coil and reduce the unmet heating hours in core zone. Other approach could be to add fictitious heating load only during winter design day (Central Heating Design Supply Air Temperature will be unchanged) as illustrated below.

  Schedule:Compact,
    Other Equipment Schedule, !- Name
    Fraction,                 !- Schedule Type Limits Name
    Through: 12/31,           !- Field 1
    For: WinterDesignDay,     !- Field 2
    Until: 24:00,             !- Field 3
    1,                        !- Field 4
    For: AllOtherDays,        !- Field 5
    Until: 24:00, 0;          !- Field 6

  OtherEquipment,
    Level1XGroundLevelX:OfficeSpaceXCoreX Other Equip,  !- Name
    None,                                   !- Fuel Type
    Level1XGroundLevelX:OfficeSpaceXCoreX,  !- Zone or ZoneList Name
    Other Equipment Schedule,               !- Schedule Name
    Watts/Area,                             !- Design Level Calculation Method
    ,                                       !- Design Level {W}
    -20,                                    !- Power per Zone Floor Area {W/m2}
    ,                                       !- Power per Person {W/Person}
    0,                                      !- Fraction Latent
    0,                                      !- Fraction Radiant
    0;                                      !- Fraction Lost

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Hi Chandan: thanks for the suggestion. To as great an extent as possible, I'm looking to avoid workarounds because this is for research and my goal is to determine the correct equipment sizes rather than merely reduce unmet hours.

"VAV reheat coil undersizing in a known issue" - what I haven't been able to determine from reading the forums is whether this a bug/shortcoming of the autosizing algorithm or whether my model is the issue. I still have faith (and am desperately hoping) that my model is the issue.

LiamOBrien ( 2017-12-11 07:45:22 -0600 )edit

Hi Liam, I have notice this problem as well. From my experience, the reheat coils are sized to heat only the air required to heat the space, and not sized for reheat of a minimum air change rate, or minimum allowable box flow. I would recommend making a spreadsheet to manually calculate your required reheat load, then you can use a measure to add (or manually add) a "fictitious heating load only during winter design days" as Amir suggested above. However, this is not a "fictitious" work around. I appears that in many cases energy plus does not properly calculate the reheat load.

sspielman ( 2017-12-11 09:15:49 -0600 )edit

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answered 2017-12-17 04:04:16 -0600

LiamOBrien
81 ●3

It seems that sspielman's suggestion (to calculate cooling impact of delivering preheated air to the zone directly) and then compensating it with a negative fictitious load eventually worked. "Eventually" because it also took quite a bit of tinkering with the OA controlller to understand why OA was a bit higher than calculated directly from the outdoor air design specification. I also had to add a bit of margin (~20%) to the fictitious load. Anyway, I got 0 unmet hours after 500 runs with semi-random occupant schedules. So I can move onward!

Thanks everyone for your help!

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