Trace-users Digest, Vol 46, Issue 1

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Hi John-

Wouldn't it be nice if ASHRAE had just defined unloading curves like they
should have? Anyway, I'll take a stab at this...

It would've been nice for the reviewer to site his or her source. I think
the document being referred to is this:

Cutting straight to the issue:

The TRACE default 90.1-2007 heatpump shuts off at the minimum operating
temperature of 40 degrees F and switches to backup heat. There's a lot of
ways you could create custom biquadratic curves so that between 40 and 17
you have a "series" arrangement - but that won't solve the problem (because
the loading curves would need to change from project to project anyway,
given that there are no defined unloading curves!)

Here's what would work:

1) Copy the 90.1-2007 equipment in the TRACE library and reset the minimum
operating temperature to 17 degrees
2) Use this as your new heat pump
3) Run the simulation with this new equipment
4) From the equipment energy consumption report, get the total kw-hr in
heating mode for the heatpump
5) Same report - Get the total KW-hr of the auxiliary electric resistance
6) Go into the visualizer and get the total KBtu-s on the heating coil for
the heatpump systems

7) Take the total Kbtu-s divided by the total kW-hrs (from 4&5) and that is
your current HSPF (It needs to equal to or higher than the HSPF in 6.8.1)

8) If the HSPF is lower than required by 6.8.1, adjust your efficiency of
your heat pump (or alternatively of your Electric resistance equipment) and
reiterate until you get an HSPF equal to or greater than that in 6.8.1.
This is textbook "Thermodynamically similar" - so that should work.

Obviously, it is best to get the HSPF to be EXACTLY the same as specified
to maximize your energy in the baseline building.

Have a great weekend

Bob Fassbender

On Fri, Aug 3, 2012 at 3:06 PM,


Bob's picture
Joined: 2010-06-30
Reputation: 475

Classification: UNCLASSIFIED
Caveats: NONE

I know there are many ways to get rid of unmet load hours. What I want it to look at the output in an excel format and see the unmet load hours. How can I see the actual numeric output? I have many systems so I'm not even sure which system I need to trouble shoot.

Right now I'm making multiple copies of the project and in each file deleting all systems but 1, so I can identify the offending system.

Classification: UNCLASSIFIED
Caveats: NONE

Jeurek's picture
Joined: 2010-10-07
Reputation: 0


The best way to look at unmet load hours in Trace is to use the Building Temperature Report. This will show you which rooms have unmet load hours, and the output will be sorted by system. Therefore your step of creating multiple models is time consuming and not required. The image below shows the report checked in the output.

Also, this report will show you when the unmet hours occur for the rooms that have unmet load hours. This report makes it quick to focus on spaces with unmet load hours.

[cid:image001.png at 01CD73E5.6FA6A670]

[cid:image002.png at 01CD73E5.AFE26FD0]


Scott Parker

sparker's picture
Joined: 2010-10-13
Reputation: 1