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Difference in handling heating load and component sizing, EnergyPlus v7.2 vs. v8.2?

asked 2015-03-15 17:41:02 -0500

jfirrantello's avatar

updated 2020-03-10 09:52:49 -0500

2015 03 25 - Added some new information at the bottom

I am currently using the 1.4 versions of the DOE Commercial Reference Buildings (2004 construction) for research purposes. They are provided as EnergyPlus v7.2 IDF files via the web. I recently converted them to v8.2 using the IDF Updater, changing nothing else. There seems to be a difference in how VAV reheat coils are auto-sized and handled. I first noticed this in the Hospital Commercial Reference Building. (Simulation was run in Baltimore climate and, again, 2004 construction).

Both the 7.2 and 8.2 versions of the file generate error messages like this when run:

** Warning ** Calculated design heating load for zone=CORRIDOR_NW_FLR_3 is zero.
** ~~~ ** Check Sizing:Zone and ZoneControl:Thermostat inputs.

However, only the 8.2 version generates the following error message:

** Warning ** The design coil load is zero for Coil:Heating:Water CORRIDOR_NW_FLR_3 VAV BOX REHEAT COIL
** ~~~ ** The autosize value for maximum water flow rate is zero
** ~~~ ** To change this, input a value for UA, change the heating design day, or lower
** ~~~ ** the system heating design supply air temperature

Looking at the html table outputs, the difference manifests in the coil sizing summary. The 7.2 entry:

Maximum Air Flow Rate [m3/s] Maximum Reheat Water Flow Rate [m3/s] Reheat Coil Sizing Air Volume Flow Rate [m3/s]
CORRIDOR_NW_FLR_3 VAV BOX COMPONENT 1.34 0.000367 0.403147

versus an 8.2 entry:

Design Size Maximum Air Flow Rate [m3/s] Design Size Maximum Reheat Water Flow Rate [m3/s] Design Size Reheat Coil Sizing Air Volume Flow Rate [m3/s]
CORRIDOR_NW_FLR_3 VAV BOX COMPONENT 1.34 0.000000 0.403147

As one can see, the difference is in the maximum reheat water flow rate.

More importantly, this manifests itself in the inability of these zones to be heated, which leads to an unrealistically low heating energy use, and unmet heating hours. I've gone through the IDF files in their respective editors, and can't seem to find a difference in the input.

So the primary question is:

1) How do I get v8.2 to treat the file the same as v7.2 and provide a maximum VAV reheat coil flow rate that is not zero, even though the calculated load is zero? (I'd prefer to find a way that isn't manually inputting explicit values, if possible)

With the related questions:

2) How and why does this happen?
3) Would this affect other things that I need to be concerned about?

---- New Information 2015 03 25

The difference also manifests itself in the number of occupied heating hours where setpoint is not met. For example, in the zone mentioned above, the 7.2 run had 0 hours, while the 8.2 run had ~5000. This leads me to not have confidence in the "new" evaluation of the zone's needs.

Some possibly relevant items from the "Known Issues 8.2" document are:

3580 The sizing algorithms for reheat coils in ... (more)

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answered 2015-03-30 11:44:01 -0500

jfirrantello's avatar

I think I have found the issue, and the solution fixes the zero load in 7.2 as well as 8.2 in the Hospital file. In short, the "WinterDesignDay" category in scheduled loads should specify zero percent load from people, equipment, lights, or anything else that could be taken "credit" for during the design heating calculations. Whatever the changes were in the reheat coil sizing from 7.2 to 8.2 seemed to make the problem more visible.

In the case of (at least) the hospital file, there were low, but non-zero, values for a number of the occupancy and equipment schedules under the WinterDesignDay day. These heat gains offset the envelope heating loss, resulting in no heating. Zeroing these heat gains fixed most of the problem. So, the problem is (un)fortunately as simple as the heating design load calculations being performed incorrectly.

A problem with unmet heating hours in one interior zone remained, and I believe this is addressed in this question, as well as this thread in the Yahoo Group. It seems to be a larger issue with reheat coil sizing for interior zones.

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answered 2015-03-19 09:43:55 -0500

updated 2015-03-31 03:48:14 -0500

I can have a stab at this but I am not sure if this will solve your problem. In the past when we were using reheat coils in a VAV system we would set them to "reverse", I think (the options are normal or reverse), which was the opposite of what the I/O guide said we should do to reheat. Then a version change happened, it could easily have been from 7.2 onwards and "reverse" no longer worked (ie the damper no longer opened and the reheat coil no longer reheated), so we changed them to "normal" and voilà they worked again. Now I might have mixed up which way we changed them (reverse to normal, or normal to reverse) but I would recommend trying in v8.2 to change the reverse/normal option to the opposite of what it was when it was working in v7.2.

As I say this might not solve your problem and might not be related to your problem, if the EnergyPlus is saying there is no load then there is no reason for the reheats to turn on. But it is an easy change to make and it did cause us some problems for a while with the reheat coils so might be worth a try.

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Annie, thank you for the information. I ended up changing the action on the dampers from "normal" to "reverse" in my model. This had the effect of changing the heating load on the zones that already registered having a heating load, but had no change on the zones that showed no heating load.

The lack of heating in these zones also changes the number of hours of setpoint not met during occupied heating from 0 to ~5000, so I can't say I trust the "new" evaluation of the heating load equipment, and am convinced something else must have changed. Will update some of the info above mentioning this.

jfirrantello's avatar jfirrantello  ( 2015-03-25 09:19:53 -0500 )edit

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Asked: 2015-03-15 17:41:02 -0500

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Last updated: Mar 31 '15