GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Reducing energy further in Michigan home

greenhomeinst | Posted in GBA Pro Help on

We got a 2003 2 story home with a walk out basement. Just got a department of energy home energy score of eight out of 10 and it is predicted to use around 33,000 kwhe a year.

The home is in climate zone 5 and seems to be pretty tight ( 4.5 ach) and insulated beyond the code code at that time.

We just did all LEDs and are getting motion sensors next.

The home has a good southern and eastern pitch to get 5 to 8 kW of solar someday but I know there is more efficiency to get out of it.

At this point the heating, cooling as well as water heater are all pretty old as they are the original one from 13 years ago and look like some of them ready to go here soon though even if not they’re fairly inefficient. The house is on natural gas. We could simply just get an energy star sealed combustion gas tank water heater, high efficiency 2 stage furnace and maybe a seer 15 AC but 1) we want to avoid using gas and 2) what to try something else.

Considering doing an integrated heat pump for heating and cooling, right into the existing ducts ( they are not sealed but are in conditioned space) and then either a heat pump water heater OR a few point source water heaters.

My biggest concern is those potential days it gets so cold that the heat pump(s) fail. Geo sounds infesting but just costs too much.

Any recommendations?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Marc Rosenbaum gave a presentation a while back on a group of houses on Martha's Vineyard that used an average of 9,061 kwh/year each.

    Your house is using between three and four times as much electricity as these houses -- and you are also using natural gas, which the Martha's Vineyard homes didn't even have. (They were all-electric houses.)

    So the first order of business is probably to determine why your house needs so much electricity.

  2. GBA Editor
    Martin Holladay | | #2

    Two other points:

    If you are unsure what type of energy-retrofit measures you should pursue next, you might want to hire a home-performance contractor or energy rater (ideally, once certified by RESNET or BPI) to perform an energy audit. As part of the audit, you will be given a list of recommended measures to lower your energy bills.

    Second, one of your worries -- "My biggest concern is those potential days it gets so cold that the heat pump(s) fail" -- is groundless. Several manufacturers of air-to-air heat pumps, including Mitsubishi and Fujitsu, made equipment that works when the outdoor temperature drops to -13°F or even -20°F.

  3. greenhomeinst | | #3


    I converted mbtus to KWHe so that's total, gas and electric.

    We did get an audit as I mentioned we had a Home Energy Score of 8. The audit of course recommends conventional systems. I realize those systems are rated down to -13 but it still makes me nervous. Thanks for the vote of confidence, though, on it.

    Has anyone seen these heat pump systems work well integrated into existing ducts?

  4. GBA Editor
    Martin Holladay | | #4

    Ducted minisplits are designed to work with low-static-pressure ducts -- in other words, with ducts that are larger than the ducts connected to most American furnaces. A ducted minisplit system needs ducts that are large, short, and that include as few elbows as possible.

  5. Dana1 | | #5

    At 4.5ACH/50 the house is 50% above the IRC code maximum for new construction, and there is likely to be some cost effective air sealing left.

    If the existing furnace is something like 150,000BTU/hr, it's at least remotely possible that the ducts could be used by a mini-duct cassette, and it's almost certainly with the range of a 2.5 (MVZ-A30AA4 ) or 3 ton (MVZ-A36AA4 )Mitsubishi MVZ air hander capable of handling the current pre-improvements existing heat load:

    The turn down ratios on the bigger deal air handlers aren't as wide as the better mini-duct cassettes, so it's important to run the heat load numbers carefully and right-size it for the "after improvements" picture.

    With some re-designs & tweaks on the existing duct system it may be preferable to go with 2 or 3 mini-duct cassettes, zoned by floor, rather than a single centralized duct system, which usually has floor-to-floor temperature balancing issue that change with the seasons.

    If the foundation of the walk basement out doesn't have at least R8 continuous insulation it's probably going to be cost effective to bring that up to current code min as long as it doesn't involve gutting nicely finished living space.

    There are now 100s (or 1000s?) of existence proofs that cold climate mini-splits can still deliver the goods at negative double-digit temperatures. For purposes of estimating your 99% outside design temperture, what is your location?

  6. greenhomeinst | | #6


    Yes. We could probably bring back the cellulose in the attic and air seal under it as well as portions of the rim that I can get to. I might be able to do injected foam in the walls too to go around the fiberglass but am concerned about VOC's with that.

    We are basically in Grand Rapids Michigan.

  7. Expert Member
    Dana Dorsett | | #7

    The 99% outside design temp in your area is about +5F. It gets down to negative double-digits sometimes, but only 1% or 87 hours out of a year are below +5F, making it a more relevant temperature bin for designing heating systems. See:

    Design temps in positive single digits is pretty easy for mini-splits these days.

    Of the 33,000 kwh/annum total I expect 5000-6000kwh is for something other than heating, leaving something like 27-28,000 kwh/annum for the HVAC energy budget. In an average year Grand Rapids sees about 6500 heating degree-days (see: ) using base 65F as the presumptive heating/cooling balance point, so on the high side that's about 28,000 kwh/6500= 4.31kwh per heating degree-day, or (/24=) 0.1795 kwh per degree-hour. With an outside design temp of +5F and a balance point of +65F there are 60F heating degrees, and the approximate heating load of 0.1795 x 60F= 10.77 kilowatts.

    Converting kilowatts (international units) that to BTU/hour (British Thermal Units) is a simple ratio, 1kw = 3412 BTU/hr, that becomes 36,747 BTU/hr, which is within the +5F output of 3 tons of cold climate mini-split (withing the output of 2.5 tons of some), if that's the way you wanted to go with it.

    It's often possible to dense-pack or tighten up existing insulated walls with blown cellulose rather foam, without major re-work or gutting, depending on just what's in the walls already. Cellulose (even at low density) is quite a bit more air-retardent than low or mid-density fiberglass, and when blown into the cavities under pressure it follows the path of the escaping air, finding and substantially plugging the air leaks.

  8. Expert Member
    Dana Dorsett | | #8

    BTW: Since you have a heating history on the place, you can get a much more accurate heat load estimate out of it with a fuel-use based analysis, which uses the existing furnace as the measuring instrument:

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |