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1. Expert Member
   Dana Dorsett | Nov 03, 2015 04:00pm | #1

   So? Several years ago when the NEEA bench tested an FE12NA they couldn't match the HSPF efficiency, but monitored in-situ in occupied houses a fleet of 10-12 FE12s came close to hitting the numbers even in a zone 6B climate, even though the HSPF test presumes a zone 4 type of climate.

   The numbers are never precision numbers no matter what. Sizing it for the seasonal loads so that they modulate MOST of the time is a critical (and often missing) factor. With modulating equipment of any sort it's possible to see large differences between as-used efficiency and test-bench efficiency. But if I had to guess, when used as the primary heatng source most installed units are oversized to the point of not modulating.

   Over the past couple of months I've been reviewing and advising a family in Vineyard Haven MA on various heat pump and mini-split proposals. Based on recent experience, most of the people installing the stuff can't (or won't) do math unless there's a $ in front of it, ( and even then will bend over backward to screw it up. :-) )

   The total design load for the whole house is a bit over 30,000 BTU/hr, most proposals are for between 4-6 tons of compressor, some with pretty lousy turn-down ratios. Most would only be in a modulating range when it's below freezing outside. The binned hourly temperature average in a typical January is above freezing, which means these systems would almost NEVER be modulating, only cycling, taking a toll on both comfort and efficiency (and the upfront wallet vacuum-cleaning.)

   Some take-aways from that experience:

   * Multi-split compressors and heads can't modulate low enough to operate efficiently in new code-min construction with a one head per room approach, but some (certainly not all) mini-ducted versions can be reasonably sized in US zone 5 climate.

   * Always run an independent Manual-J load calculation, and review the specs on the proposed equipment carefully for both maximum and minimum modulated output.

   * It's better to undersize something slightly and have it modulating into the high 40sF or even the mid-50s than to oversize it and only modulating during the coldest week of the year.

   * Most mini-splits have too narrow a modulating range, making it difficult to size them. (The notable exception is the Mitsubishi FH09NA, which has about a 11:1 turn down ratio- love that little mini-split!)

   As of this morning that family was close to pulling the trigger on a proposal, but for some reason the contractor wants to install the mini-duct cassette & ducts in the attic rather than in the unfinished basement. They pushed back, and this afternoon the same contractor re-bid the thing with 1-head per room multi-split, with a compressor with a minimum modulated output that is more than half the design heat load (sigh...) I've seen the place, and don't quite get what the problem with routing the ducts in the unfinished basement would be. It's only marginally more difficult than installing ducts in the attic.