[Trace-users] Trace-users Digest, Vol 46, Issue 1

Bob Fassbender bobfass11 at gmail.com
Fri Aug 3 15:27:57 PDT 2012

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,
<trace-users-request at lists.onebuilding.org>wrote:

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> Today's Topics:
>    1. Heat Pump Auxiliary Heat (UNCLASSIFIED) (Eurek, John S NWO)
> ----------------------------------------------------------------------
> Message: 1
> Date: Thu, 2 Aug 2012 21:49:17 +0000
> From: "Eurek, John S NWO" <John.S.Eurek at usace.army.mil>
> To: "trace-users at lists.onebuilding.org"
>         <trace-users at lists.onebuilding.org>
> Subject: [Trace-users] Heat Pump Auxiliary Heat (UNCLASSIFIED)
> Message-ID:
>         <
> E777FD5A41D3B04A83296213737A85D70106B5D5 at EIS-MB05WPC.eis.ds.usace.army.mil
> >
> Content-Type: text/plain; charset="iso-2022-jp"
> Classification: UNCLASSIFIED
> Caveats: NONE
> Short question:
> What is the easiest way to have the electric heat to provide heating for
> packaged rooftop when the outside temperature is less than 40 degrees?  Or
> is this even what the review is looking for?
> The full comment from my good magnanimous friend from LEED.
> 6. It is unclear if the packaged rooftop heat pumps in the Baseline model
> were modeled according to Section G3.1.3.1, which requires that the
> electric air?source heat pumps are modeled with electric auxiliary heat
> that only energizes on the last thermostat stage and when the outdoor air
> temperature is less than 40 degrees F. This means that the heat pump and
> auxiliary heat should operate together at low temperature conditions, with
> the compressor as the lead machine. The outside air cutoff temperature for
> the compressor must be no greater than the temperature associated with the
> low-temperature heating efficiency requirements of Table 6.8.1B (17 degrees
> F). Note that for packaged heat pump units smaller than 65,000 Btuh, the
> HSPF rating accounts for electric auxiliary operation, and includes test
> conditions at 17 degrees F. Please indicate the modeled characteristics of
> the electric auxiliary heat in Table 1.4 including the temperature at which
> the auxiliary heat engages, and
>   the outside air temperature cutoff for the compressor. If the compressor
> low temperature cutoff is modeled as greater than 17 degrees F for packaged
> heat pump units smaller than 65,000 Btuh, describe how the Baseline
> efficiency of the heat pump was modeled to reflect the HSPF rating
> including auxiliary heating energy.
> John Eurek
> Classification: UNCLASSIFIED
> Caveats: NONE
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> End of Trace-users Digest, Vol 46, Issue 1
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