Electic heat pump minimum operating temperature

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What 90.1 PRM Section(s) are relevant to this issue?
3.7.6.5 Heat Pump Compressor Minimum Operating Temp

Explanation of issue:
The PRM indicates 17°F as the minimum HP operating temperature.  However, we 
have a lot of evidence that suggests this is not a reasonable baseline 
assumption, or a ASHRAE/federal efficiency requirement for PTHP systems.

Proposed resolution:
Since there is not federal or ASHRAE requirement specified for this equipment 
category, I would recommend revising to a value between 35 and 40°F to be more 
consistent with current PTHP technology. 

Original issue reported on code.google.com by da...@360-analytics.com on 15 Sep 2014 at 5:46

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Also, make sure that any requirement applies to "AirSource" heat pumps - for 
WaterSource we are using a minimum OAT of -20F, and that may not be low enough 
for 90.1 which may be applied in places where OAT drops below that, such as 
Alaska.  

Original comment by rhedr...@archenergy.com on 15 Sep 2014 at 5:50

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I changed the min OAT for WaterSource to -50F

Original comment by rhedr...@archenergy.com on 15 Sep 2014 at 6:09

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Table 6.8.1B specifies a COP of 2.05 at 17F, which indicates that heating is 
done by the compressor at low temperature as well-
we also looked through some manufacturers literature, which specifies minimum 
operating temperature for HP compressors at 25F.

http://www.trane.com/download/equipmentpdfs/ptacprc001en.pdf
 and 
http://products.geappliances.com/MarketingObjectRetrieval/Dispatcher?RequestType
=PDF&Name=73791_zoneline_data_manual.pdf

We'd like to go with 25F as the min operating temp for HP compressor- does that 
sound good?

Original comment by supriya....@pnnl.gov on 17 Sep 2014 at 6:09

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Table 6.8.1B is not applicable to PTHPs; the efficiency requirements of these 
units is specified by Table 6.8.1D. This table does not specify a minimum 
low-temp efficiency rating, and AHRI 310/380 indicates this rating point is 
optional.

In the Trane manual, Table PD 3, Note 2 indicates:

Heating capacity and efficiency is based on unit operation without condensate 
pump. Unit automatically switches to electric heat at 25°F outdoor coil 
temperature.
Depending upon relative humidity conditions, this will occur at approximately 
35 degrees outdoor ambient temperature

In the past, we've surveyed various manufacturer's data and found that the 
compressor lockout varies from 25F outdoor coil temp, up to 40F (assumed 
outdoor temp).

I think the appropriate PRM number should be between 35 and 40F.  40F appears 
to be the absolute highest temperature allowed by 90.1, so that is arguably the 
temp to use, since a manufacturer could produce and sell a unit that operates 
this way and still comply with 90.1.

Original comment by da...@360-analytics.com on 17 Sep 2014 at 7:14

• Changed state: Discussion

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Yes, you're right- 25F OAT isn't justified. Please go ahead and use 35-
So instead of 17 (or 25) , the lower compressor shut off should be 35, so that 
electric resistance comes on for temperatures below 40- and runs alongside the 
compressor, the compressor shuts off at 35, below which its only electric 
resistance

by the statement- '40F appears to be the absolute highest temperature allowed 
by 90.1, so that is arguably the temp to use' - are you referring to the 
resistance heat lock out specified in G3.1.3.1?

Original comment by supriya....@pnnl.gov on 17 Sep 2014 at 9:43

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G3.1.3.1 is one reference, 6.3.2(h) is the other. 

Original comment by da...@360-analytics.com on 17 Sep 2014 at 10:03

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I think 35F is definitely the low end.  

In some research Lukas did on this for another project, he found that most all 
the manufacturers indicate that once the compressor is locked out, which is 
stated to be approx 35F OAT, the HP will not run again until the OAT increases 
to 40-45F. This behavior is not readily simulated, so combined with the fact 
the only definitive temperature limit stated in 90.1 is 40F, we have used 40F 
in our PRM (LEED) analyses to date.  

I know USGBC is just now starting to pay attention to this, so resolution here 
may be help that program as well. 

As another point of reference, the 2012 Seattle Energy Code C407 simulation 
approach indicates 35F is assumed for baseline PTHP lockout, but it doesn't 
speak to what temperature the HP needs to turn back on.

Original comment by da...@360-analytics.com on 17 Sep 2014 at 10:18

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Here is a link to the Trane PTHP catalog, see Table PD-3
http://www.trane.com/download/equipmentpdfs/ptacprc001en.pdf

Original comment by da...@360-analytics.com on 25 Nov 2014 at 10:30

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Carrier's vertical PTHP appears to have a lockout ~40F
See pdf page 2,and further explanation in Controls section, pdf page 14
http://dms.hvacpartners.com/docs/1005/public/00/52v-3pd.pdf

Original comment by da...@360-analytics.com on 2 Dec 2014 at 9:45

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I checked with the contact from AHRI on this. He claims that these vertical 
units are different from PTHPs. Here is his response.

Michael, 

We often refer to those units as VTAC or VTHP (vertical terminal….). 

Industry volume on these are fairly small (relative to normal PTAC/PTHP).  I 
bet Dick will say Carrier PBM’s this unit instead of makes it themselves, 
otherwise they would use refrigerant heat below 40F.

Original comment by bikerose...@gmail.com on 2 Dec 2014 at 11:18