Replies

1. Expert Member
   Dana Dorsett | Nov 04, 2015 02:16pm | #1

   Got a ZIP code?

   I've given up on letting HVAC installers call the shots. Run your own (or independently contracted) heat load numbers, figure out which solutions work, and tell THEM what you need, sticking to your guns.

   The minimum modulated output of multi-split compressors are often well over half the design temp heat load of the zones it serves even in code-min construction. eg: A three-head Fujitsu has a minimum output @ 47F of 6800 BTU/hr, but can deliver something like 17- 18,000 BTU/hr @ -5F. That 6800 BTU/hr is WAY more than the heat load of three typical code-min bedrooms @ 47F. If it's serving three bedrooms with design loads of 3500 BTU/hr @ +5F, both the heads AND the compressor will be cycling like crazy except during the coldest hours of the coldest days of the year, modulating only rarely.

   http://www.master.ca/documents/Fujitsu_Submittal_AOU24RLXFZ.pdf

   If multi-splits don't work in code-min houses (and I submit that they usually don't, if it's one head per room) it'll never work well in a PGH.

   Mini-duct cassettes and splitting the output between 3-5 rooms is more likely to work than a multi-split solution, and easier to sell to code officials that it will in fact heat every room adequately compared to the one or two mini-split head for an 8-12 room house (which sometimes works, sometimes not.) The Fujitsu xxRLFCD series will all modulate down to 3100 BTU/hr @ 47F independent of the size, which might be about the whole-house heat load of a PGH at that temp.

   So, start with a room-by-room head load calculation at your 99% outside design temp, and either re-calculate or interpolate what the loads are at +47F. When you have that information nailed down you can then start looking for a more optimal solution. You are the designer, not the HVAC installer, and it's up to you to get it right, but you're much more likely to come up with something that works efficiently and is sized reasonably for both peak & average loads.

   Then it becomes a matter of finding a contractor who will actually install it for you to your specifications, which isn't always easy. You have to be really blunt- tell them that you've done the math, and aren't interested in seeing alternative proposals unless it physically can't be done (that sort of works, at least sometimes. :-) )

   After recently walking a friend through the process she commented:

   " I will say this heating cooling industry is a racket that a commoner such as myself has been put through the hoops to simply get this done ...., I have learned some and respect all others knowledge in an area I do not have a clue .... "

   ... to which I responded that the only people more clueless than the customers are most of people specifying and installing the stuff. That's sad, but more true than not, especially when the client understands a little bit about efficiency.

   This is not limited to mini-split installers- it's endemic in the HVAC biz. Maybe one in 20 will even offer to do a Manual-J, and only a few of those will do an adequate job of that calculation, which is why hiring the load calculations out to an energy nerd is a critical first step.

2. 
   Aledoux13 | Nov 04, 2015 08:35pm | #2

   1887

3. GBA Editor
   Martin Holladay | Nov 05, 2015 04:18am | #3

   If any GBA readers have any ideas, Arthur appears to live in Wilmington.

4. 
   brp_nh | Nov 05, 2015 08:55am | #4

   Both Mitsubishi and Fujitsu have find a contractor listings on their websites:
   http://www.mitsubishicomfort.com
   http://www.fujitsugeneral.com/contractors/search/states/MA.htm

   Fujitsu even has a ranking system, Mitsubishi has what they called Diamond contractors. I know that this doesn't necessarily mean they are great installers, but it's a start. I know the warranty is increased when you use a Mitsubishi Diamond contractor.

   We used heat load calculations by our HERS rater and just told the installer what we wanted, which was one mini-split on the first floor. He was somewhat skeptical and made a note on the invoice that we specified the system specs, to cover himself.

5. Expert Member
   Dana Dorsett | Nov 06, 2015 11:09am | #5

   Wilmington MA is within the service area of literally dozens of competent Mitsubishi installers, a handful of Fujitsu installers, and a least one Daikin installer. But don't leave it to any of them to specify the equipment- they'll all screw it up (except for those with dumb luck. :-) )

   Follow the prescription:

   1: Do your own heat load calculations, at both your outside design temp (about +5F in Wilmington) and at +47F. The +47F is a useful HSPF test specified outdoor temp where both maximum & minimum output capacity is tested, usually included on the efficiency test submittal pages downloadable from the manufacturers' sites.)

   2: Figure out what's going to be reasonably sized for the load and still be capable of modulating when it's in the 40s F, based on the minimum output numbers relative to your +47F load.

   3: Tell the installer in no uncertain terms EXACTLY what equipment you want, and don't back down unless there are practical reasons why it can't or shouldn't be done.

   If you leave it open ended you'll end up with dozens of proposals that don't make any sense, even from the best of them.

   I've yet to meet a mini-split installer in MA that even runs Manual-Js, let alone paid attention to the minimum-modulation numbers, (but the sample size is small.) A mini-split installer in western WA a few years ago CLAIMED to have run a Manual-J (without sharing that with the client), but the whole-house numbers didn't hew very closely with a sketched out IBR load calc I did on the place. It made a difference on which equipment was chosen.