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Community and Q&A

PH Cooling Options in High Mountain Desert?

user-1066644 | Posted in PassivHaus on

Heating my Salt Lake City based 1010 PH project [1], with 5765 heating degree-days (F), does not seem to be a problem with largely passive solar gain; however, the 1047 cooling degree-days (F) are a concern [2].

The first floor and second floor are spec-ed with concrete floors for thermal mass, thus stabilizing temperatures. With concrete floors in an arid high-mountain desert, the obvious option is to incorporate hydronic cooling into the concrete slabs, dissipating the heat into the surrounding ground via ground loops. The ground is at 56F ambient but even after a long summer of hot weather, it should be able to address the cooling needs.

Are there other, perhaps better, options given that the heating in this is home is done with ventilation-only systems (so, no re-circulation)? Passive night-time flushing is designed-in, but perhaps an active night-time flushing should be incorporated?



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  1. user-1066644 | | #1

    I should have mentioned that the design heating capacity is 8 kBTU/h and design cooling capacity is 10.6 kBTU/h.

  2. GBA Editor
    Martin Holladay | | #2

    I probably sound like a broken record, but all your house needs is one or two ductless minisplits.

    Be careful with your plan to circulate hydronic fluid through your slab in the summer. The energy required to run your pump may be more energy than the thermal benefit justifies. Remember, the longer you run the pump, the warmer the soil near your buried pipes.

  3. user-1066644 | | #3

    Hi Martin @ 2: thanks for the quick reply (again!). I must admit, I'm a bit biased against the ductless mini-split because of aesthetics. I've done a bit of searching but I have not found a good source for how to calculate the cooling loop surface area to allow for sufficient cooling - do you know of any sources?

  4. GBA Editor
    Martin Holladay | | #4

    I don't know of any resources to help you size your cooling loop. There are many unknowns when it comes to designing such a system; the biggest unknown, of course, is the temperature of the fluid in your tubing. If you have no cooling equipment, and you intend to dissipate the heat in the fluid by running the tubing in buried earth loops, the temperature of the fluid will gradually rise over the summer. (This is due to two factors: the soil will gain heat from above due to the effects of the sun and high summer air temperatures, and the soil will gain heat from the tubing.) The cooling system may work on May 1st, while being unable to perform any cooling at all on August 1st.

    These systems are problematic because of condensation on the floor. Once you make your floor cold, you will get dew on it, making walking slippery and dangerous, unless you keep your indoor humidity very low. If you have an airtight home, your indoor humidity levels will probably rise when you take showers, do a lot of cooking, or water your house plants.

  5. Expert Member
    Dana Dorsett | | #5

    You can do it all with a Daikin Altherma air-to-hydronic heat pump (with mini-split technology on the compressor/condenser end) but the price tag might make you feel a lot better about the aesthetics of a pretty-good 1-ton mini-split. ;-)

    The likelihood of dew forming on the chilled floor in a house with loads that low are extremely low. Summertime dew points in the Wasatch Ior even Salt Lake) rarely break 60F and average in the mid to high 40s. In a PassiveHouse with chilled slabs the surface temp of the slabs would never need to be much below 70F to keep it under 78F (which is a comfortable temp when the dew point is 45-50, even 55F.) With an HRV you should b ventilate at a sufficiently high rate to keep the dew point of the interior air below 55F when the outdoor dew point is 45F without a huge boost to the sensible cooling load, but even if you let the indoor air reach 65F for a dew point, you won't have condensation on the floor (but you may end up with mold conditions under rugs if that's the way you run it all summer.)

    With an infinite source of 56F water you'd get maybe ~10F delta-T out of it before it started losing it's punch on cooling the slab, at most 15F (but let's make it 10F to make the math easy, eh? ;-) ) To handle a load of 10,000BTU/hr with a 10F delta that's 1000lbs of water/hour, or about 2 gallons per minute. If the water doesn't have to be pumped up hill you're looking at about 100 watts of pumping power to get that through a bunch of PEX in the slab. That would deliver an effective coefficient of performance of about 30, which would be VERY good. But if you're pumping it from a 100' well you're in pretty tough shape and a mini-split would beat it.

    The pumping it though a buried slinky-style closed loop in the ground (geo-style) you're probably looking at ~300 watts of pumping power and the mini-split still wins on average, but maybe not at the very peak load. The size of the closed loop necessary to deliver that much cooling depends a lot on the soil type & moisture (you can do a lot better if the slinky is below the water-table, but that's probably not happening, eh?) You'd have to consult a real geo-pro to know the total amount of pipe you'd need and the pump sizing etc- it's still a real hydronic and geo design, and something you'd want to do the real math on if you hope to reach mini-split efficiency with it. Without a heat pump on the system the delta-T between the return water and the ground temp in the slinky loop is pretty small, and the size of the ground loop will need to be several times that per ton of cooling than you would need with a GSHP where you can feed the loop water that is 50-80F above earth-temp rather than 10-15F above.

    The installed cost of a 1-ton mini-split would likely cost less than the design + installation of the ground loop too.

    In high mountain areas with reliable diurnal temperature swings nighttime ventilation schemes work pretty well. Find your real location via zip code or nearest small town, and plug it in to Weatherspark to scope out the nightly lows, etc, and the actual summertime dew points:!dashboard;q=salt%20lake%20city%2C%20USA

    (Different graph options are selectable, and the bottom slider changes the scale, so you can look a a whole year and use the cursor to eyeball the averages & extremes, etc.)

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