Replies

1. Expert Member
   Dana Dorsett | Jan 22, 2016 01:53pm | #1

   Yes- set it and forget it, let it modulate up & down with the load. At Seattle's average winter temps it'll have very good efficiency if modulating. If keeping it at 70F in the main areas makes the bedrooms too warm for sleeping you can adjust the temp in the bedrooms downward by keeping the doors closed part of the time, but on sub-freezing nights you'll probably want to keep the bedroom doors open.

   Most reasonably tight 1500' houses would have a whole-house load a bit lower than that at Seattle's mid-20sF outside design temps, let's call it 15,000 BTU/hr @ 25F outside, 70F inside, a temperature difference of 45F. That's then 15000/45= 333 BTU/hr per degree-F below 70F. The GE15 is good for about 16,000 BTU/hr @ +17F, and can modulate down to about 5000 BTU/hr @ +47F. A 47F the load would be about (70F - 47F) x 333= 7659 BTU/hr, which is still above the minimum output of the unit- it'll still have some modulating room to work with even during shoulder-season temperatures.

   Assuming that truly is your approximate design load, the GE15 is right sized for your house, and should give you decent modulating performance for almost the entire heating season, and you're as-used efficiency will meet or even slightly beat the nameplate HSPF 10.0 (COP= 2.9).

   https://www.goductless.com/hvac/pdf/mitsubishi/mitsubishi27-sb.pdf

2. 
   stk | Jan 22, 2016 04:26pm | #2

   Thanks for your insight, Dana. I will set it and forget it. I had an energy audit done that included (I'm told) a Manual J of about 39K BTUs/hr. I am hoping the Mitsubishi will cover me on days when we are in the low-30sF or higher. To be honest, the cost of these units has kept me on the sidelines, but I was able to find a package price that included the MSZ/MUZ unit. I wanted an FENA unit, but I was getting sick of waiting and jumped in. This is in lieu of a gas furnace replacement, so maybe a little victory on that front. I will keep the current furnace as a backup and maybe tear it out in a couple of years, when I get a track record with the mini split. Thanks again!

3. Expert Member
   Dana Dorsett | Jan 22, 2016 04:53pm | #3

   A heat load of 39,000 BTU/hr @ 25F for a 1500' house would be basically an uninsulated house with single pane windows. Is that your house?

   As a point of reference, that's about the same load of my sub-code 1920s 2400' house + 1500' of insulated conditioned basement at 0F.

   That's about 25 BTU/hr per square foot of conditioned space, which would be a typical heat load ratio of code-min construction in Fairbanks AK at an outside design temp of about -50F.

   If your house is really that leaky and uninsulated, it's definitely worth fixing up the house. A tight 2x4/R11 house with clear glass double panes (not low-E) and R25 in the attic would come in at about 10 BTU/ft @ 25F with an insulated basement or insulated floor, maybe a bit more if it's an uninsulated vented crawl space, but not 25 BTU/ft-hr.

   With your ZIP code, a wintertime gas bill with the fuel use and the EXACT meter reading dates and the input BTUs and DOE output BTUs of your gas furnace, it's possible to determine an upper bound on the actual whole-house load.

4. 
   stk | Jan 22, 2016 06:03pm | #4

   Nov 12 2015 - Dec 13 2015 - 128.736 therms
   Zip 98133
   90K BTU gas furnace, 80% efficient, downflow
   Return duct in unconditioned attic, duct uninsulated, joints sealed w/mastic
   Supply ducts wrapped w/insulation but joints unsealed
   Underfloor fiberglass insulation between joists, but improperly installed w/dead space between insulation and underside of floor

5. 
   wjrobinson | Jan 22, 2016 07:34pm | #5

   S K, Joe L. says at building science that airspace is correct as to floor insulations. It's all about proper perimeter insulation etc.

6. 
   stk | Jan 22, 2016 08:30pm | #6

   Forgot to mention that exterior walls are uninsulated 2X6 w/thick cedar lap siding. Double-paned vinyl windows.