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Community and Q&A

Heat Pump Sanity Check – Empirical load versus NEEP tables

itech20 | Posted in Green Products and Materials on

TL;DR; – I would value a gut check about whether I am correctly/realistically interpreting the output I can expect from a given heat pump at a given temperature based on the data from our friends at neep.org .

I’m in the final stages of equipment selection for electrifying our primary furnace/AC system.

In an earlier post, I summarized some preparatory work we did to get the overall load down via targeted foaming and some dense pack cellulose.  Link here:

https://www.greenbuildingadvisor.com/question/retrofit-external-insulation-for-a-brick-foundation-ground-below-grade-interface

The result is that this morning, here in Chicago, it’s a windy and bitterly cold 10F and my Nest based instrumentation is indicating our existing furnace had a peak averaged output, just before the sun came up, of around 19K BTU/hr.  This is for the “downstairs” furnace located in the basement.  We have two independent HVAC systems, but 2 years worth of data suggests that during the winter, the first floor/basement system carries over 90% of the load.  The house is 1500 sf (or 2100 sf if you include the foamed but unfinished basement).

The downstairs/basement furnace is a ducted natural gas unit. The registers all feed into various locations on the first floor which is actually a single “great room” covering the entryway, kitchen, dining room and living room.  (As a single stage 80K BTU/hr unit, it is also profoundly over sized.)

I have for some time, had my eye on a Fujitsu cold climate single head unit with a nominal/rated output of 15K.  Here is its NEEP data page:

https://ashp.neep.org/#!/product/25337

The NEEP data above indicates the unit can actually output 20,500 BTU/hr at 5F (with a COP just above 2).  I’d be perfectly happy to set today’s conditions as my rough design load, and am prepared to cover any additional heat needed at lower temperatures with some combination of space heaters and our wood stove.  My simplified understanding is to compare this to my measured heat demand this morning of 19,000 BTU/hr at 10F and to say “I’m good. Let’s go.”

Am I missing anything?

This unit (and similar sized ones from other vendors offering low temperature operation) seems to be just about the “biggest” thing I can get that still has a good turn-down ratio with desirable/efficient low output operation at higher temperatures. As such, I’m keen to make it work if I can.

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Replies

  1. bfw577 | | #1

    There are many units that have much better COP's and turn down ratios than that Fujitsu. Dig around the NEEP site and you will find some units that have COP's of 2.5-3 at 5 degrees. Take a look at this LG and Carrier for example.

    https://ashp.neep.org/#!/product/25944

    https://ashp.neep.org/#!/product/26721

    1. this_page_left_blank | | #2

      Looking at the full specs, I'm not convinced either of those are better than the Fujitsu. The COP of the LG is not that great at more modest temps in heating mode, and it's abysmal in cooling mode below nominal output. The HSPF is a pretty good indicator of overall efficiency in heating mode, and both the LG and Carrier are lower than the Fujitsu, especially the Carrier at 10.3 is quite poor.

    2. itech20 | | #3

      The improved COP at high output is intriguing. I do recall some members here explaining the differences between HSPF, which is fairly prescriptive and official, versus the more detailed NEEP figures, which AIUI come with fewer strings attached in terms of testing conditions.

      In any case, I'm still keen to hear some feedback on the other half of the question, which is whether anyone happens to know if the "Max" output figures can be viewed as a reliable/sustained output figure. (I suppose one particular concern is that these figures represent the output when heating but don't reflect the unavoidable periods of time where the unit is running a defrost cycle.)

  2. kyle_r | | #4

    Im no expert, but I believe the max output does not take into account the defrost cycle.

    As a reference, I installed a 15k floor mount Fujitsu on the first floor of my home this fall and it has heated the whole house so far this winter. I live near Detroit, 2 story home, Manual J heat load of ~23 kBTU/hr at 5 F.

    I plan to install a ducted unit (9RLFCD) soon for the upstairs, but this is for AC more than heat.

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