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eQuest: insufficient AHU heating coil capacity

asked 2015-04-20 14:55:03 -0500

jkjenner gravatar image

updated 2015-07-10 20:59:32 -0500

I'm attempting to model a school in eQuest and I'm having some trouble with unmet hours.

The building is located in a cold climate (Calgary, Alberta) and I've selected the "Pkgd Var Vol" system type. The problem seems to have something to do with with way that I've set up this system. In ATTN report I'm getting a warning like so:

PREHEAT/HEATING-CAPACITY in SYSTEM AHU-1                           
             is too low to provide the requested supply temperature for
             the calculated mixed air temperature using total ZONE loads
             (on LS-A/B) and SYSTEM design parameters, plus outside air
             conditions (on LS-C), and specified capacities & air flows.
             Requested temperature is  55.00 calculated is  11.80
             Check HEATING-CAPACITY, HEAT-SET-T, PRE-HEAT-T, MAX-SUPPLY-T,
             DESIGN-HEAT-T, and ZONE loads for consistency.

I can eliminate the unmet hours by jacking up all of the baseboard and reheat coil capacities but this results in a completely absurd level of heating energy consumption, and it also doesn't eliminate the above error.

I've noticed that manually setting the "Hot Deck Max Leaving Temp" to 95 F seems to correct the above error message, but doesn't fix the underlying problem of the unmet hours/absurd energy consumption. I've also tried manually entering an extremely high value in the "heating capacity" field of the "Heating"/"Coil Cap/Control" menu of the system and it doesn't have any effect.

A couple of strange things I've noticed that might provide some clues as to what the problem is:

  • the value listed for "heating capacity" in the SV-A report is 0 (unless I enter a Max Hot Deck Leaving Temp as described above)
  • the minimum zone temperature listed in the SS-F report for the underheated zones is just ridiculous--like -253.1 F for one zone.

Any ideas or suggestions would be very, very greatly appreciated.

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This is likely an error in the way you set up the PVVT indeed. Please open your .inp file, locate the section were the AHU is described (starts with "Name of AHU" = SYSTEM). Then edit your answer to add this section. It'd be great if you could also include the first one or two zones that are served by this system. (you could also post a link to the full .inp if you want it to go faster) Also, how many zones are you serving? Do you have unmet hours in all of them?

Julien Marrec gravatar imageJulien Marrec ( 2015-04-21 02:06:30 -0500 )edit

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answered 2015-04-21 08:23:22 -0500

eQUEST by default does not create a central heating coil in variable volume systems. The default assumption seems to be that the mixed air temperature will be sufficiently warm that all heating can be done by the VAV boxes and/or baseboards in the zones. However, this assumption does not apply to cold climates! As you have noted in your question, setting the "Hot Deck Max Leaving Temp" to 95F corrects the above error message by forcing eQUEST to add a central heating coil to the system to heat the supply air. If you look at the DOE-2 Dictionary you will see that this activates the keyword "HEAT-SET-T = 95".

If you are jacking up the baseboards and the reheat terminals to address the unmet heating hours, it is possible that the boiler is autosizing to a much higher capacity than is necessary. If the boiler is oversized, this could cause the seasonal heating efficiency to be low and the heating energy consumption to be exaggerated. I recommend that you set the "Hot Deck Max Leaving Temp" to 65ºF (or whatever your VAV system supply air reset temperature is in heating mode), and reset the capacity of the VAV reheat boxes and baseboards to default to autosize their heating capacity. Run the model and look at the .SIM file to see what is happening with the unmet load hours and the boiler sizing. The SS-K report will show the unmet hours by zone which should allow you to figure out which zones are causing problems. I recommend using baseboards set to thermostatic control. The PV-A and PS-C reports will tell you what is happening with the boiler sizing and utilization. These reports should give you enough information to manually size the boiler near the peak heating load. Hopefully this will address the unexpectedly high heating energy.

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Very helpful, thanks a lot Daniel.

I'm wondering: is there a document somewhere that maps eQuest's inputs to the corresponding DOE-2 keywords?

jkjenner gravatar imagejkjenner ( 2015-04-21 10:26:17 -0500 )edit

You're most welcome. I'm not sure if there is such a document, I usually figure this out by right-clicking in the entry and going to "Item Help". So for example, if we are looking at the "Hot Deck max Leaving Temp", if you right click in the box where you would enter the temperature, and go to "Item Help" it will usually show you the DOE-2 name, in this case HEAT-SET-T. It should also open the item help in a help window, but I can't always get this to work on my computer so I usually just keep a PDF of the DOE-2 Dictionary open.

Daniel gravatar imageDaniel ( 2015-04-21 10:47:38 -0500 )edit

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Asked: 2015-04-20 14:55:03 -0500

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Last updated: Apr 21 '15