GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter 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

Heat pump head unit locations

user-6318603 | Posted in Mechanicals on

Hello all,

I’m currently building a small house in Vermont and I plan to heat it with mini splits. It’s a 1300 square foot 2 bedroom ranch with an unfinished walkout basement. (See attached floor plan image.) The walls are 2×6 with fiberglass in the cavity and 2″ of foam on the outside. The below-grade walls in the walkout basement will have 2×4 with fiberglass. The above-grade walls in the walkout will have the same assembly as the main floor.

I’m currently planning to have two outdoor heat pump units, one for the master suite, and one for the living room.

I’m also planning to use a heat pump water heater in the basement. I will be using an HRV to control air quality since the house should be very well air sealed. The HRV will most likely exhaust from the two bathrooms and the laundry room, and supply to the two bedrooms.

I’m thinking about adding another small head unit in the second bedroom. This is to ensure that it stays plenty cool for sleeping in the summer, and to help offset the cooling effect of the HRV supplying fresh air to the bedroom.

I’m thinking the living room head unit and the bedroom #2 head unit could be connected to the same outdoor unit.

The folks at Efficiency Vermont, as well as my builder and his HVAC contractor are recommending I add another heat pump head unit in the basement, since there will be no waste heat from a conventional heating system such as a furnace. They are recommending this so that the floors in the living spaces above the basement will be warm, and also to provide heat for the heat pump water heater. Perhaps this head unit could be connected to the same outdoor unit as the master bedroom head unit.

These recommendations make sense to me, but I’m wondering a couple things that I was hoping to get some additional opinions on:

1) It seems somewhat counter-intuitive to heat the basement and then use that heat to heat our water. Should I just use a conventional electric water heater instead or will the “heat pump plus heat pump water heater” still be more efficient for the task?

2) Is it overkill to have three head units in the living spaces of such a small ranch? I.e. are my concerns about bedroom #2 not staying as warm/cool as the rest of the house unfounded?

Thank you for any input!

-Ross

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.

Replies

  1. GBA Editor
    Martin Holladay | | #1

    Ross,
    Before we get to the heat pump issue, it's worth noting:

    1. Two inches of exterior rigid foam aren't enough in Vermont to keep your wall sheathing above the dew point during the winter. You need 2.5 or 3 inches of rigid foam. More information here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    2. You can't insulate the interior side of your below-grade basement walls with fiberglass batts stuffed between 2x4s. For more information, see How to Insulate a Basement Wall.

  2. GBA Editor
    Martin Holladay | | #2

    Ross,
    Putting a minisplit head in Bedroom #2 is a judgment call. I wouldn't do it. You can always install a small electric resistance heater there in the future if the need arises.

    Concerning your basement: You only need to heat your basement if you want to have a warm basement. Without a minisplit head down there, it will definitely be cold in the winter. (The heat-pump water heater will make sure of that.) But a cold basement isn't necessarily a bad thing if you aren't living down there.

    Here's what Alex Wilson wrote: "With the cold weather we’ve had (as I write this, it’s about 2 below), our basement [equipped with a heat-pump water heater] has stayed pretty cool: typically 50°-54°F, though with the exceptionally cold weather we had a few weeks ago during a time of heavier hot water usage, the temperature dropped as low as 47°F. Right now, it’s 52°F."

    For more information, see Heat-Pump Water Heaters in Cold Climates.

  3. user-6318603 | | #3

    Thanks Martin ... you and I had recently discussed our rigid foam situation in the comments section on one of the articles on the subject. We're going with 2 inches of XPS which supposedly just about meets the 11.25 R value requirement for zone 6 given that its R value increases slightly in the cold weather to 5.5 per inch. We were originally going to go with polyiso until we learned that its R value falls in cold weather. We also talked about ensuring that we keep our indoor RH under control, with the HRV helping a little bit with that goal. I purchased a hygrometer to help keep an eye on the RH in various parts of the house once we're living in it.

    Regarding adding a heat pump head unit in bedroom 2, are you thinking a resistance heater makes more sense economically because we presumably wouldn't need to use it very often? I guess the downside there is that we wouldn't get the AC functionality that the head unit would offer. However, in such a small house, maybe that's not an issue if the door is kept open most of the time. We have a young teenager that will occupy that room so she'll probably want to keep the door closed most of the time. :)

    Regarding the basement, we do plan to use it for projects from time to time. My wife and daughter are both very "crafty" so I think they'll be doing a lot of projects down there, so we'll probably go ahead and heat it.

    I will look into the below grade insulation question ... I could be wrong about the type of insulation the builder is planning to use there. I will take a look at the link you posted.

    Thank you very much for the input!

  4. STEPHEN SHEEHY | | #4

    Ross: If you plan on XPX, look into reclaimed material, which is widely available in New England.

    My house in Maine is a little bigger than your house. One minisplit heats it pretty well, although we have a second minisplit in the master bedroom that we use occasionally. As for your second bedroom, how often do you need cooling in Vermont? I'd say we use our minisplit for cooling maybe six or eight days a year. We usually just open the windows. Unlike some parts of the country, it's pretty rare to have night temperatures so high that the house won't cool down by just opening windows. Our spare bedroom just has an electric resistance heater that we almost never use.

  5. user-6318603 | | #5

    Thanks for the tip on reclaimed XPS! I will definitely look into that.

    Regarding cooling days, I lived in a small 2 story cape in Vermont for the last 11 years, and we ran window AC units at night throughout the summer months. (The bedrooms were upstairs obviously.) All three of us prefer to sleep in a cool, dry house. We didn't run AC on the first floor, but there were many days during the summer when we wish we had a window AC unit on the first floor.

    Maybe it won't be as much of an issue on a single level ranch? I don't know ... I just remember that there are plenty of nights in the summer when it is still warm and humid during the night, making it impossible to sleep comfortably. So we definitely want some AC in bedroom 2 ... it's just a question of whether or not the head unit in the living room will provide enough cooling for bedroom 2.

  6. GBA Editor
    Martin Holladay | | #6

    Ross,
    Ceiling fans.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |