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Community and Q&A

How to release stored heat in floor’s thermal mass

bunney | Posted in Mechanicals on

Years in the planning, five months in building, we are now home at last in our 1,100 sq passive home at 46 degrees North in North Central Minnesota.

Fujitsu Halcyon mini split heating and cooling.Heat load is around 26K BTU

Q 1. Am I correct assuming that I have to set the the two mini splits’ thermostats at a night time temp that is lower than indoor temp before the concerete floor’s thermal mass will release its heat?

Last night I set mini splits at 66 degrees — guessing that that setting would be cooler than floor.

Indoor temp was 68 this morn when I got up. I then called for 72 degrees on our primary mini split. It blew moderately cool air for about an hr. I raised thermostat to 74. Mini split soon started blowing warm air.

I contacted installer. He reported that mini splits don’the like that big a setback temp at night. He said the systems works best if I just set desired temp and leave it alone.

Q 2. How then do I get the floor to release its stored heat at night?

Q 3. What’s the best way to measure concrete floor temp?

It seems to me that I would have to have a probe set into the center of the floor to get a pertinent temp, as opposed to surface temp.

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  1. bunney | | #1

    First sentence should read "... passive solar ..."

  2. morganparis | | #2

    Best way to get the floor to 'release its stored heat' is to turn off the mini split and open the windows. When the thermometer gets down into the forties you should notice the benefits. Sorry if this sounds facetious but you're really not going to get any appreciable heat return from your floor until the room temperature falls well below most people's comfort levels. This doesn't mean the slab won't have some benefits in stabilizing interior conditions, especially in hot summer weather - it'll be better at keeping you cool in summer than warm in winter.
    If you're not prepared to go that hardy route there's good news. Fire up that wood burner good and hot on a chilly night and it'll raise the local slab temperature a good bit ABOVE room ambient. Then after the fire goes out the slab will return some of that elevated temperature to the room and reduce the calls on your mini split. But for goodness sake follow the mini split maker's recommendation and leave it at a constant set point, wherever that may be, with no night setback. It'll run far more efficiently that way.

  3. charlie_sullivan | | #3

    You are correct that heat won't flow out of the slab into the room unless it's warmer than the room. But if you try to help that process by cooling the room each night, you will force the slab to start each day cold, and then best case, by the end of the day the slab will only be a little cooler than the room. You want the slab to start the day at about the room temperature you want, so that by the end of the day, it's a little warmer than the room temperature, and is releasing heat to the room without your needing to change the room temperature.

    The good news is that if you do it that way, you can leave your minisplit thermostat alone it it will stay happy and efficient.

  4. Dana1 | | #4

    Mini-splits can be operated in a setback mode, but they run at a higher speed/lower efficiency on the recovery ramp, which almost always ends up using more electricity than a "set and forget" approach, tweaking it up or down only a degree or two at a time for comfort as needed. But for the highest efficiency let it modulate, tracking the load- setbacks and ramp-ups use more power.

    A $50 pistol-grip infra red thermometer works fine for spot checking the temp of the slab in multiple places, or checking the temp of the ceiling, for that matter. It's not a precision instrument, but it's not bad, as long as the IR emissivity of the materials are all in the same range. (It'll lie to you big time about the surface temp of shiny metal, for instance.)

  5. davidmeiland | | #5

    The slab will radiate heat to any object that is colder than the slab, so if for example the slab is 64 and the window glass is 60, heat will flow from the slab to the glass, even if the air temperature is 68. At the same time, the air in the room might be transferring heat to the slab and the glass, depending on the air temperatures adjacent to those surfaces. Trying to evaluate when heat will flow, and from where to where, is somewhat complex. You have to take into account radiation, convection, and conduction, and in a house it's an always-changing process.

    I wouldn't focus on the slab. It will serve as a stabilizer of indoor temperature. If it's being heated by the sun to a significant degree, it's doing some passive solar heating for you and that's good, at least during heating season. Sounds like your main issue might be the mini-splits and the air temps they're providing. They probably want to run long and slow, and maybe you want them to run hotter when playing catch-up. Talk to the installer about whether there are adjustments that will alter the performance in a desirable way, although you might sacrifice some efficiency in the process.

  6. vensonata | | #6

    Some confusion here. A mass slab with a "solar passive house" is intended to stabilize the natural swings in solar heating. If you are heating with mini split heat pumps they are intended to provided stable temperature. In the latter case the slab just absorbs heat initially and remains at temperature through the whole heating season. In other words it is somewhat useless.
    If you decide to ride the solar only, then turn off the heat pumps, or leave them at 60 F and let the floor mass do what it is intended to do. Just get used to living with an 8 degree swing in temperature! Or you can use the passive strategy in the shoulder seasons and then turn on the heat pumps only in December January...which is unfortunately when they are least effective.
    And then there is that nice wood stove I see in the picture! That is also quite appropriate in conjunction with a thermal mass building. Bring the heat up during the day when it is convenient to feed the stove. End the day at 75 F and let the stove go out in the night. In the morning it should not be less than 63 F. Now start the fire. So basically there are two systems with entirely different theories behind them. They don't really work together.

  7. user-304075 | | #7

    I leave my mini-split off.
    I am in the Pacific Northwest so it may not be as cold here but we are getting into the 40's at night. The house is built to "not quite" passivhaus standards.

    So far I have never turned on the heating in the house and interior temperatures has never gone below 68 degrees.

    You can see from the graph attached that the sun heats the house in the daytime and energy is released at night. I will turn the Mini-split on once the house cannot keep up with the cold weather.

  8. kevin_in_denver | | #8

    I've lived in a house like this for 11 years in Denver. The main difference is that Denver is sunnier.

    In October, EVERYTHING should be OFF.

    See what happens for the next two weeks, and you'll start understanding your house better. The slab stabilizes the temperature, as David said.

    Of course, every person has different temperature preferences, but when your walls & ceiling are well insulated, the house isn't drafty, and you don't live adjacent to the windows, I think most folks will find a temperature range of 63-68F to be comfortable. The old standard 72F setpoint was for poorly insulated drafty homes.

    That means in October - November, you set the minsplits in heating mode to about 66F. They will rarely come on.

    A passive solar house like this is designed to have daily temperature swings, and the slab helps to minimize the amplitude of those swings.

    If you have family members that can't tolerate a 4-8F daily swing, it's not a problem, you just won't save quite as much money. The sun will still heat your house if it starts the day at 72F or 62F.

    The slab surface temperature is within a couple degrees of its average temperature, so get one of these:

  9. charlie_sullivan | | #9

    The thermal mass is not useless with the minisplit.

    If you leave the minisplit at 70 F, as you get more solar gain through a sunny day, the amount of heat the minisplit supplies will go down. On some days, the minisplit will continue to put out a little heat through the whole day, and the temperature stays at 70. Those days, you are taking advantage of the solar gain, but the thermal mass is doing nothing.

    But on days where you have more solar gain, the temperature will go on up past 70 F, and the minisplit will shut off. Without thermal mass, the building would rapidly get too hot (say 80 F), and then rapidly cool down in the evening, so the minisplit would need to turn on again shortly after sundown. With thermal mass, the temperature swing will be much less (perhaps only up to 73 F), and will stay above 70 for some time after sundown. The thermal mass prevents the temperature from getting uncomfortably hot, and helps keep the minisplit off longer. You get that benefit even if you leave the minisplit on, because it's controlled by a thermostat.

    The indication that your thermal mass isn't working would be if the temperature got up well above the minisplit setpoint and dropped back quickly on sundown. If you are never getting at all above the minisplit setpoint, that's an indication that you don't have a lot of solar gain, not a problem with the thermal mass.

  10. wyobunney | | #10

    Solid insight from all. Thank you.

    Window coverings are another essential element to retaining our hard-earned winter heat and holding back the summer sun. We are going with the symphony blackout shades, double cellular with sidetracks. Alpen 700s series windows.

    * James, we will for sure be taking advantage of the clerestory windows during summer nights to flush heat out of the floor.
    * Dana, thanks for the introduction to emissivity. Important variance to know.
    * Stephen, good graph. I don't have a software solution to collecting data - but I imagine I'll be tracking temps in our great room
    * Ven, good strategy for the wood burning stove
    * Kevin, infrared thermometer sounds like it could even be fun
    * David, Charlie and others, good description of the properties of solar slab. My expectations are more tempered now.

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