Replies

1. Expert Member
   Dana Dorsett | Sep 24, 2018 11:25am | #1

   Thermostats can keep it from overheating the house, but is it overkill?

   Probably.

   Getting to the optimal heating solution starts with a careful calculation of the heat loss at the 99% outside design temperature.

   The heat loads of PassiveHouse type dwellings are so low that even on the coldest days a radiant floor heating the place isn't going to be much above room temperature. Reducing the amount of floor area that is an active radiator can offer some of the barefoot comfort with a noticeably warmer section of floor, but choose wisely where you put it. With high-R houses it's fairly common to be able to use one point-source of heat per floor, letting natural convection distribute the heat. If you make the entire floor a radiator the floor temperature required to deliver the heat is very low, too low to make much difference in overall comfort.

   Many people opt for a small air-to-air heat pump solution when the heating & cooling loads are very low since it's cheaper than radiant floors, and the comfort difference is negligible.

2. GBA Editor
   Martin Holladay | Sep 25, 2018 07:23am | #2

   Holly,
   Here is a link to a relevant article that may help you make a decision on this question:
   "All About Radiant Floors."

3. 
   Markiz_von_Schnitzel | Sep 25, 2018 10:51am | #3

   Dana, you say that comfort difference is negligible under those conditions.

   Are you speaking from experience, or are there some satisfaction studies or comprehensive measurments done?

   Point source, if it could deliver approximate comfort, has huge benefits.

   But most sources online, including GBA (for example minisplit remote thermostat discussion), mostly say that comfort is not forte of minisplits. Some people (myself NOT included, but my wife definitely) have a great dislike for any noticeable air movement, and to get temperature right seems to be too complicated.

1. Expert Member
   Dana Dorsett | Sep 25, 2018 02:25pm | #5

   In "satisfaction studies" 5% of all people are NEVER comfortable (per Joe Lstiburek). :-)

   While radiant floors can be more comfortable at lower overall room temperatures, field surveys indicate that people with radiant floors don't actually run the room temperatures lower. In a high-R /very-high-R house the energy cost of simply raising the temperature a degree or three to reach similar comfort levels is pretty tiny.

   PassiveHouse principles isn't exactly the same thing as PassiveHouse. Without the heat load numbers and the size of the radiating surface it's really hard to say whether point source would be risky.

   On a deep energy retrofit I was involved with a handful of years ago in a location with a 99% outside design temperature of +5F/-15C point source heating (one ductless mini-split per floor) worked out just fine. The house had U0.18 (and not particularly high-gain) windows, and not a lot of window area in the rooms doored off from the ductless heads. The 99% design heat load for each doored-off room was under 1000 BTU/hr.

   The only comfort issue in that house was related to cooling, not heating- a top floor bedroom gets too much early AM sun in late June/early July, (despite the windows facing well north of due east) causing AM overheating if the bedroom door was closed in the morning. (The sun rises about 5:15-5:30 during the weeks surrounding the solstice, about 30 degrees north of due east. ) Shades helped.

1. 
   Markiz_von_Schnitzel | Sep 27, 2018 09:56am | #8

   Thanks, Dana

   It's not much to go by (just one example), but enough to get me on a track.
   <1000btu/hr (<0.3kw) does seem like a very tiny load though for -15C.
   That said, there is always cheap electric heaters to compensate for these 2 or so weeks of extreme cold (for my area at least) :)

   I would still wish there was more field data regarding this. Hopefully in ~5years time, some of these ductless minisplit fans will have gotten a more detailed data.

   And if minisplit market is growing fast (as I think it is), I guess there is not much incentive for manufacturers to attack even wider market with commercials like "comfort for cheap", backed by real life data.

4. 
   Jon_R | Sep 25, 2018 11:36am | #4

   There is no doubt that a zoned source in every room provides more control and comfort than a "point source per floor". But I'm not aware of any tools that accurately predict how well a centrally located source will heat some back room at the end of a hallway. It certainly won't give you the option of keeping that back room a couple of degrees warmer than the central room.

1. 
   Markiz_von_Schnitzel | Sep 25, 2018 02:58pm | #6

   Well I don't necessarily need to keep it a different temperatures. But approximately the same would be nice.

   Maybe there is no tool, but there are plenty (?) of superinsulated homes being heated by minisplits. So I thought maybe someone from those has been keeping tabs on room to room temps. With known layout, maybe much could be deduced :)

5. 
   STEPHEN SHEEHY | Sep 25, 2018 02:59pm | #7

   The cost savings of installing minisplits instead of radiant floor heat can be used to buy better windows. You'll probably be more comfortable, since you'll minimize the cool drafts coming off cold glass. You'll also save energy.