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Passive geothermal loops & in floor hydronic cooling

Peter_R | Posted in Energy Efficiency and Durability on

We’re in the process of building a new home on Harrison Lake near Vancouver (climate Zone 4C).   Our heating/cooling system consists of a Chilltrix CX34 air to water heat pump for the in-floor hydronic heating (first floor only), as well as 3 Minotair Pentacare-V12 units for heating/cooling and ERV/dehumidification.  The house will have R30 walls and a R60 roof and will be sealed to below 1.0 ACH @ 50 Pa.  

We had to excavate down over 8′ in a lot of areas, which will soon get backfilled.  The temperature this deep in the ground during our limited cooling season should remain below 60 deg F, and possibly closer to 55 deg F.  

I am asking myself: “why not run 1200 ft of PEX loops before I backfill?”  In the cooling season, a pump could circulate fluid through these loops to cool the hydronic buffer tank. 

Looking at the calculations for our hydronic system, with 90 deg F water temperature and a 72 deg house temperature, the engineers are predicting 22,000 BTU/hr of heat transfer, or about 1,200 BTU/hr per degree difference between ambient and the fluid temperature.  If the PEX loops in the ground could cool the fluid down to 63 deg F, then I’d expect to be able to remove about 11,000 BTU/hr.  

In my area, the dew point outside even on a hot summer day is rarely above 60 deg F.  So it seems that if I kept the fluid temperature not too low, condensation would not be a problem except in the most unusual case (e.g., someone leaves the door open all day on that extremely rare muggy day when the dew point is 70 deg F).  

It seems that I have a heat sink that I could dump about 11,000 BTU/hr in for only the cost of running the circulating pump and the up-front cost of 1200 ft of PEX tubing.  11,000 BTU/hr is not enough to cool my house on the hottest days, but I bet it would be all the cooling I need for _most_ days.

Is this a crazy idea or does it make sense?

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    It MIGHT work. There are issues with the ground gradually warming up as you pump heat into it, and the relatively low temperature differentials with passive systems. I’ve done a few systems somewhat similar to this, but always using cooling ponds where evaporation helps cool things down. I also usually have MASSIVE amounts of heat to dump since I’m doing cooling for datacenters. One thing I want to try sometime is a passive loop under a parking lot to thaw snow. Such a system will increase plant efficiency, and also eliminate the need for plowing and salting the parking lot so less waste! A Win-win.

    For your system, I think your biggest concern is how much will it cost to put in the passive loop, and are you willing to spend that much on an experiment? The only potential downside I can see is that you might not get as much benefit as you want out of the loop, so the cost of it might end up essentially being wasted money. I suspect it WILL gain you a little cooling, but maybe not enough to really justify the expense of the installation.

    Bill

  2. Expert Member
    AKOS TOTH | | #2

    I think the better use for the ground loop is to pre-heat/cool the air going to your HRV.

    Even there it is hard to justify the power of running the pumps except in the coldest climates and ROI on just the equipment cost is probably in the never range.

    Having said that, I would still do it as it just feels like the right thing to do.

  3. frasca | | #3

    Do it! sounds fun and it might work.

    1. Expert Member
      BILL WICHERS | | #4

      You’re thinking like me here Malcolm :-)

      This is a fun experiment and I’d definitely try it if it wasn’t too crazy expensive to consider. Then write an article about it for GBA so we can all read about how well it worked and any issues that came up during the build.

      Bill

  4. Expert Member
    NICK KEENAN | | #5

    Here's a paper on hydronic cooling:
    https://www.nrel.gov/docs/fy14osti/60057.pdf

    I'm not sure the passive part is going to work. Heat flow is all about temperature gradient. Hydronic systems usually quote average water temperature, which is the average of incoming and outgoing water temperature. Your average temperature is going to be higher than the incoming temperature.

  5. Jon_R | | #6

    I have a ground cooled basement. Ignoring humidity, it works well.

    Once you have a loop, might as well go a little further and add a water-water heat pump for those cold periods when it will beat the Chiltrix COP. It will also reduce the ground temp, improving Summer cooling performance.

    Also note that with intermittent use of a ground loop, a large buffer tank improves performance (lower delta-Ts and less pumping losses per btu moved). Only an extra thousand $ (and some space). It also allows time of use optimizations.

  6. Peter_R | | #7

    Thanks for the replies, Zephyr7, Akos, Maximilian T, DCContrarian, and Jon R! This is encouraging.

    I'm going to bounce it off my builder and if there's nothing I'm missing that would make this a lot harder than I'm expecting, I'll do it. I'm quite curious how well it will work.

  7. nungaman | | #8

    Hi Peter,
    I'm also building a similar house in the lower mainland.
    Would you be interested in sharing you experience with me?

    Im doing 1600 sq ft ATWHP hydronic heating and perhaps
    electric boiler for DHW. Perhaps a Minotair
    air exchanger with a built in heat pump.
    Thanks,
    paul

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