Replies

1. GBA Editor
   Martin Holladay | May 23, 2014 02:03pm | #1

   Gina,
   Has anyone performed a heating load calculation or a cooling load calculation?

2. 
   seattlehouse | May 23, 2014 02:11pm | #2

   One company did and the house heating load was 20064 and cooling load was 9915.

3. 
   mackstann | May 23, 2014 02:29pm | #3

   I'd strongly consider a Daikin Quaternity. It is unique in controlling humidity independently from temperature. On these muggy spring days in Portland, I find myself wishing I had gotten one instead of my Mitsubishi. Other than that the Mitsubishi is great though, and the new FH models made a big jump in efficiency.

   Personally I would go for a 1.5 ton unit so as much of the house's load is carried by the mini split, not resistance heat. This should be more economical in the long run.

4. 
   seattlehouse | May 23, 2014 02:47pm | #4

   Although we would love to have an even better system, since we're selling the house next year, we just wanted something to replace our older electric furnace which still works great but is really inefficient. We were hoping we could get something decent without spending a lot more than we had budgeted, which is why these were chosen by the installers. Out of these options and considering our situation, what would you recommend?

5. 
   davidmeiland | May 24, 2014 07:56pm | #5

   I would go with Daikin or Mitsubishi (of those you have named) and I would probably choose more by my feeling about the installer/contractor than the exact price. It probably makes sense to put in the 12k unit and electric heaters in the bedrooms (they will be more efficient than the electric furnace assuming you have typical leaky ductwork in the crawl space and/or attic). You'll spend less on electricity if you discontinue use of the furnace. I assume you have figured out what you can get in terms of free or subsidized energy audit and rebates/incentives on equipment, etc., from PSE or whoever your electric utility is.

6. Expert Member
   Dana Dorsett | May 28, 2014 05:17pm | #6

   A heat load of 20K for a 750 square foot house seems high at more than 25 BTU per square foot. A tight code-min home that size would be well under 15 BTU/ft^2 at Seattle's ~25-27F 99% outside design temps (http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf) ,maybe even under 10 BTU/ft.

   If the place is reasonably tight, has an insulated foundation, and has at least U-0.5 clear-glass double-panes (or storm windows over single-panes) it's unlikely that your true heat load is going to be that high, but it could conceivably be as high as 15KBTU/hr if your windows are old-school aluminum framed single panes and you have no foundation insulation or it leaks a LOT of air. (I have a relative living in a way sub-code place 25% bigger than yours in Port Orchard that started out with a heat load of about 22K @ 20F before swapping in U0.30 windows for the U0.7-ish aluminum double-panes.) My own sub-code house has more than 3x the conditioned space, but my heat load @ 25F is under 24,000 BTU/hr- it's difficult to conceive of a 740' house that would have a true heat load anywhere near mine if it has windows with glass in them, and doors that close.

   The output of your option 4 Hyper Heating MSZ-FH09NA (or FE09NA, if they're giving you a deal on older stock Hyper Heating units) at 25F is over 13,000 BTU/hr, which is probably already more than your true heat load (unless it has thermal deficiencies worth fixing.) Any of the 1-ton units would be overkill. If somehow the 3/4 ton Mitsubishi doesn't keep up, you have some low-hanging fruit to pluck on the heat-leak front, and fixing those issues would provide more comfort than a bigger mini-split would.

   IIRC PSE gives you an $800-1000 rebate when installing a mini-split in a house heated with an electric furnace or baseboards.