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Ductless minisplit to cool antique 3 level house?

slateandall | Posted in General Questions on

We have an antique house, circa 1900, with basically original envelope: single glazed double hung windows (with basic storm windows added), probably little insulation in the walls (where insulation is visible in the attic it is a wool-based material). 

Total square footage is 4000 sq ft in 3 levels above the basement. We’re in southern New England near the coast. 

HVAC contractors we’ve seen so far are in favor of whole house high velocity AC, in the range of 6 tons to 7.5 tons and a cost of 50K (not including electrical and finishing).  They tell me minisplits would be even more expensive up front. 

I’d like to explore minisplits for:

(1) ability to use only where needed;
(2) lower labor cost;
(3) ability to install only where needed;
(4) ability to expand over time. 

As a bonus, it might be nice to use it to heat 1 or 2 bedrooms while letting the downstairs drop to low 60s at night. (hot water heat). 

Am I totally off here in wanting to explore minisplit cooling?  I just did a bit of research and saw one 3.25 ton unit with 5 heads for under 4K. I might be picky about the heads (will want cassettes in some locations) but hard to see this ballooning into a 50K project. 

Any advice most appreciated!

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Slate,
    Q. "Am I totally off here in wanting to explore minisplit cooling?"

    A. No.

    Q. "Any advice most appreciated!"

    A. You should delay the installation of any HVAC equipment until you've made improvements to your home's thermal envelope (air sealing work, insulation improvements, and the installation of storm windows).

  2. User avatar
    Dana Dorsett | | #2

    To Martin's list I'd add...

    Have a competent third party run an aggressive (per the instructions) Manual-J room by room heating & cooling load calculation on the "after planned upgrades" condition of the house.

    It's unlikely that even before up grades the cooling load is anywhere near 6-7.5 tons for an antique 4000' house that probably DOESN'T come with huge sweeping "sunset view" west facing picture windows (the way some mid-century modern houses do) to drive up the cooling load.

    In coastal New England there is a preponderance of latent cooling hours over sensible cooling, and a 1% design temp in the low to mid 80s: https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf. A Manual-J on the house as-is will probably come in something like 3.5-5 tons of peak cooling load, maybe 3-4 tons if you replace the storm windows with tight low-E storms (which in combination with wood sash single panes performs about like a code-min replacement window at a fraction of the installed price.) Harvey's Tru-Channel storms are the tightest in the biz, and they have a low-E glazing option, but the Low-E Larson's sold through box stores aren't bad either. Air sealing the attic floor and basement are critical for getting the latent loads under control (and would be triply important during the heating season). If it's balloon framed that means blocking both the bottoms & tops of the wall cavities all exterior walls (and sometimes partition walls in some of those houses.)

    With multi-splits you need to be very careful about sizing the heads proportionally to their loads. A half-ton head in a room with a tiny 1500 BTU/hr design load will overcool the zone even when it's off if the other zones are right-sized for the loads. The multi-split compressors don't modulate continuously with load, instead operating in steps based on the size of the zone calling for coolant. There is always a modest amount of "extra" that gets distributed to the other heads, even if they're "off", to avoid returning liquid coolant to the intake side of the compressor. For tiny-load rooms even without the blower running the coolant running in the coil can overcool (or over heat, in winter) the room.

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