I'm curious how others have been interpreting G3.1.3.1 and when to lock
out the compressors on PTHP units in the Baseline. I have been setting
the minimum heat pump operational temperature (MIN-HP-T) in such a way as
to allow the heat pump to operate at all hours and allow eQUEST to adjust
the capacity of the heat pump with its own capacity curves. This allows
the heat pump to run simultaneously with the supplementary electric heat,
albeit at a reduced capacity.

Realistically, no PTHP manufacturers allow their heat pumps to operate
much below 35 deg F. I have heard from other modelers that they lockout
the compressors at 40 deg F in the baseline. For those of you out there
who take this approach, how do you justify it? Has the question come up
in any LEED reviews?

The following is my rationale for not locking out the compressors at 40
deg F:

ASHRAE 90.1-2007 section G3.1.3.1 states:

?G3.1.3.1 Heat Pumps (Systems 2 and 4). Electric air-source heat pumps
shall be modeled with electric auxiliary heat. The systems shall be
controlled with multistage space thermostats and an outdoor air thermostat
wired to energize auxiliary heat only on the last thermostat stage and
when outdoor air temperature is less than 40?F.?

So the section clearly references System 2 ? PTHP?s. Then the question
you could ask is what does ?energize auxiliary heat only on the last
thermostat stage? mean? From USGBC?s Advanced Energy Modeling for LEED
guidelines, page 38:

The guideline says to operate the compressor as low as 17 ?F (low
temperature rating condition using heat pump ARI 340/360 test procedure).
Now PTHP?s are not rated per ARI 340/360 ? they use ARI 310/380.

17 ?F is also utilized for PTHP?s.

I've searched the CIR database and could not find any references to this
specific section.

Robby Oylear, PE, LEED AP