[Bldg-sim] System 2 PTHP Compressor Lockout, G3.1.3.1, & MIN-HP-T

[Robby Oylear]

http://www.ashrae.org/File%20Library/docLib/StdsInterpretations/IC_90-1-2007-9.pdf

This ASHRAE 90.1-2007 interpretation states that Appendix G does not
mandate heat pump compressor cutoff or defrost cycles, and recommends the
manufacturer's control recommendations be followed.  The consensus from
PTHP manufacturer's seems to be no compressors below 40 deg F.

I will put together an interpretation request to specifically address
System 2, but by extension of the logic used in this interpretation it
seems acceptable to set MIN-HP-T to 40 deg F.

-Robby

On Fri, Aug 17, 2012 at 1:52 PM, Robby Oylear <robbyo at rushingco.com> wrote:

> Don’t mind me as I continue to talk to myself here, but I just received
> confirmation that EnergyPro utilizes 0 °F for their MIN-HP-T setpoint for
> LEED & Title-24 compliance.****
>
> ** **
>
> Robby Oylear, PE, LEED AP****
>
> *Mechanical Engineer*
>
> *Senior Energy Analyst*
>
> * *
>
> *D* 206-788-4571 ****
>
> *www.rushingco.com* <http://www.rushingco.com/>****
>
> ** **
>
> *From:* Robby Oylear [mailto:robbyo at rushingco.com]
> *Sent:* Friday, August 17, 2012 12:10 PM
> *To:* 'bldg-sim at lists.onebuilding.org'
> *Subject:* RE: System 2 PTHP Compressor Lockout, G3.1.3.1, & MIN-HP-T****
>
> ** **
>
> I’m not sure how much this is worth, but eQUEST’s LEED compliance analysis
> module when set to residential with electric heat leaves the PTHP MIN-HP-T
> at its default value of 40°F.****
>
> ** **
>
> Robby Oylear, PE, LEED AP****
>
> *Mechanical Engineer*
>
> *Senior Energy Analyst*
>
> * *
>
> *D* 206-788-4571 ****
>
> *www.rushingco.com* <http://www.rushingco.com/>****
>
> ** **
>
> *From:* Robby Oylear [mailto:robbyo at rushingco.com <robbyo at rushingco.com>]
> *Sent:* Friday, August 17, 2012 8:43 AM
> *To:* 'bldg-sim at lists.onebuilding.org'
> *Subject:* System 2 PTHP Compressor Lockout, G3.1.3.1, & MIN-HP-T****
>
> ** **
>
> 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.****
>
> ** **
>
> From ARI 310/380-2004:****
>
> ****
>
> 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****
>
> *Mechanical Engineer*
>
> *Senior Energy Analyst*
>
> * *
>
> *D* 206-788-4571****
>
> *www.rushingco.com* <http://www.rushingco.com/>****
>
> ** **
>
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