eQuest - VAV AHU serving zone VAV units with HW reheat

asked 2015-03-03 10:46:44 -0500

heather0638

21 ●4

updated 2015-04-13 13:55:04 -0500

Neal Kruis

4675 ●9 ●17 http://bigladdersoftware.com/

Hi all,

I've spent countless hours trying to troubleshoot unmet heating hours, as well as determine why I have simultaneous heating and cooling. I suspect that it's a controls issue, but I cannot figure out what the issue is. The unmet load hours isn't crazy significant--around 450 hours--however, the simultaneous heating and cooling is what is bothering me the most. I want to make sure that I'm modeling the building as accurately as possible, as opposed to building it with questionable "workarounds" to get rid of unmet hours.

Some background on the model:

I'm using eQuest 3-64 for LEED-IDC v4. The model in question is the proposed model.
Some spaces in the building aren't in scope. For the model these thermal zones were moved to two thermal zone groups I made: "Not Conditioned" (not owned by tenant) or "Whiteboxed" (owned by tenant, but aren't being developed as part of the LEED scope). These spaces were specified to be unconditioned in the internal loads section and in the air-side HVAC section. The "Surface Type" of the floors and ceilings of the in-scope spaces that were adjacent to non-scope spaces were specified to be adiabatic. All system equipment capacities have been downsized based on in-scope area per the LEED guidelines.
The building has IDF/MDF rooms, which are cooled via CRAC units (I'm assuming this means that there will be cooling all year round, but not to the extent at which it is). There is also a commercial kitchen and I have accounted for the resulting sensible heat load to the space.
Most of the ventilation is provided by three VAV AHUs. For the model I combined these into one AHU because they serve one medium pressure loop. All of my thermal zones with unmet hours are served by this main AHU.
The AHUs have HW heating in the winter to temper the air, but most of the heating should be provided at the zone level, which have HW FPBs/VAVs with HW reheat, as well as HW baseboard heating.
Spaces with a roof heating load should be served by FPBs, while most other spaces are served by VAVs. Right now the air-side system type is modeled as "Variable Air Volume" with VAV boxes only and baseboard heating.
The design space temperature is 75F in cooling and 70F in heating. The main AHU is scheduled to be off at night and the zone fans are to cycle during this time.

Things I've tried:

Changing "HMax Flow Ratio" from default of 0.5 to 1.0. I did this incrementally in steps of 0.1, but ultimately found that 1.0 was the best solution.
Specifying a capacity for the perimeter baseboard heating and changing the control setting from "Outdoor Reset" to "Thermostatic." This reduced unmet hours, but the only way to optimize the model was to only do this in zones with unmet hours. When I did this to all perimeter zones, the ...

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answered 2015-03-03 14:26:49 -0500

Benjamin
1145 ●1 ●10 http://ca.linkedin.com...

updated 2015-03-03 14:31:01 -0500

1) You want to use the PIU system if you have fan-powered boxes in your VAV system. You need to be able to capture the fan energy used by those boxes, and this is the only system that can capture that fan power. This will not resolve your unmet hours, but this is the correct system for modeling FPBs.

2) A throttling range of 2F is very tight, particularly with a VAV system. You may want to read the help files in eQuest in regards to throttling ranges for VAV boxes - 4F is very acceptable, even 6F would probably pass muster (as indicated by the eQuest help files). All of our eQuest models use a throttling range of 4F.

3) Unconditioned zones next to conditioned ones absolutely can have an impact. If this is what was happening, the zones next to the unconditioned spaces would have the underheated hours - is this the case?

Baseboards should be, in almost all cases, specified as Thermostatic. This means that the baseboard will max out it's capacity before calling for heat from the main system. Presumably, if baseboards are installed, you'd want these to do the majority of the heating in each space. You should also be specifying a capacity (or -1 for no capacity) - if you don't put a value in, the baseboard will autosize and may give you heating where you don't want it. Check your SV-A report to see what baseboards you are getting.

With that said, it seems counter-intuitive that you would have both heating installed in the VAV boxes, and also have baseboards installed as well.

In general, I'd look at how you've specified your heating & cooling at the system and zone levels. It definitely sounds like a controls issue, if you've got multiple zones simultaneously heating and cooling. It'd be easier to provide more focused answers if you can post a link to your actual model...