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eQuest PkgdVarVol Cooling and Heating Setpoint Issue

asked 2021-08-19 12:33:54 -0500

Jono gravatar image

Hi Everyone,

I noticed something curious today when using eQuest 3.65 b. 7173. I setup a Pkgd Var Vol system to serve a single zone. The system has no reheat coils or baseboards, so only delivers heating and cooling through the main coils.

I set a setpoint of 72F during heating and 75F during cooling. Curiously, when I ran an hourly report, I noted the system was HEATING the zone based on the COOLING setpoint (75F). Has anyone seen this before? I'm stumped as to why the software would grab the cooling setpoint.

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answered 2021-09-03 11:07:43 -0500

Drew Morrison gravatar image

Jono,

I can't say I have a complete answer, but I do have a few suggestions for troubleshooting (at least, where I'd start to tackle the issue).

The first thing I'd do is determine confirm that the system is actually in heating while the thermostat is trying to hold the cooling setpoint. The hourly report block for the system has two output variables - "Total central heat coil output" and "Total central cool coil output" that represent the heat added or extracted by the central coil. Since your system has only one coil, at least one of these two values should be zero at all times. Another thing to check is the "Current hour heat extraction rate" in the zone report block. It's been a while since I've looked at it, but I believe if that value is negative the HVAC system is heating the zone (and positive if it's cooling). Though I'll have to warn you that my troubleshooting methods sometimes raise more questions then they answer, so if you follow these suggestions you automatically agree to my indemnity clause...

One more thing I'd consider is if the HVAC system is really appropriate for the application. The Packaged Variable Volume system (abbreviated PVAVS in eQuest terminology) is really more of a VAV system with DX cooling in place of a chiller. If you're really setting up a system to serve a single zone, then Packaged Variable Volume Variable Temperature (PVVT) or Packaged Single Zone (PSZ) might describe the actual system better. eQuest help also has descriptions of each of its system types in its help library, which you can find by right-clicking on the "System Type" pulldown in HVAC system box, navigating to "Topic Help", and picking any of the Air Side System Types options that show up. (You might already know about this, but just in case.) I've found these system descriptions really helpful when I'm trying to figure out how different systems are controlled and how they respond to zone demands.

Good Luck! Drew

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Hey Drew, thanks for your comment! These are good suggestions. It turns out I had a min supply temp schedule at the system level that I missed - Silly me! Glad it's my mistake and not a bug.

I have always avoided PSZ systems because I noticed at times when the system fans are off the baseboards do not operate, yet the zone temps / underheated hours do not move. As a result, heating energy will be underestimated and the energy model will not warn you with UHHs. This is why I always use PVAV.

Jono gravatar image Jono  ( 2021-09-10 13:06:49 -0500 )edit

Glad you figured it out! Chalk up another troubleshooting outing in the win column. I appreciate your observations on PSZ systems - the baseboard issue is a feature I wasn't aware of that I'll hopefully be on the lookout for in the future.

Drew Morrison gravatar image Drew Morrison  ( 2021-09-10 13:47:20 -0500 )edit

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Asked: 2021-08-19 12:33:54 -0500

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Last updated: Aug 19