Passive geothermal loops & in floor hydronic cooling
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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