What an interesting idea. Almost like a ground source system, but using inexpensive air-to-air equipment. My question, and I think it would be hard to know the answer, is how much of the temperature of the underground gym is due to the ground around it, and how much is due to the fact that you have a lot of air trapped in there.

For summer cooling, I'm thinking this would probably work better than for winter heating, because in the summer you have cooler times in the evenings and on cooler days when the system would run less, and the underground room would have a chance to dissipate its heat. Plus you could even have an exhaust fan/air intake system that would automatically exhaust the underground air anytime the outdoor air was cooler than the underground air.

But I'd worry a little bit about the heating, because you're only sitting at 32 F now, so you're not going to have to remove a whole lot more heat from this room before it starts to get to where a heat pump won't work that well. Also, the living space you're proposing to heat is not too much smaller in cubic feet than the underground gym, so dropping that downstairs temperature appreciably might not take too long.

We set up a home built inside of a big steel barn (if you will) with a heat pump system indoors. Very efficient and never goes to backup heat strips. The people are very happy with it. Here, local rates are .11 kw, but if you go heat pump and have a separate meter installed it falls to .05 ...that is cheap a/c in the summer too...putting in more all the time.

I did it once for an area that served as a service counter with a lot of traffic. The building was a former railroad storage facility, and the condenser was in the basement (not used except as occasional rifle range) that had a constant temp of 50-60 degrees year round. It worked well was efficient and nobody ever ran into it with their car.

I had it up on blocks in case of flooding

I love your idea. I personally believe the immense amount of of material surrounding that gym would be more than enough to cool the condensing unit.

Have you ever considered venting some of that cool gym air into your living space? The only problem with that is the possibility of musty odor and condensation on the gym walls. As far as heating goes, yes it would become a very cold room, but I'll bet that mass could handle it, but I think what ky114 said about the heat pump becoming inefficient in those frigid temps is a very valid point. I'm just wondering how much of the heat stored in that mass during the air conditioning season can be recovered by the heat pump in the winter.

What a great walk in refrigerator you have there. What you have almost sounds geo-thermo.

Boy! Does your house sound terrific! I'd love to see it!

Jim

Look at the TVA (Tenn. Valley Authority) super home designed in the 1970's or 1980's. It utilizes a area of rock underneath of the house for heat and cooling.

I posted this morning, and at the end of this day I have had 5 encouraging and informative responses. Thank you for your time and insight. Thanks ky114 for the idea of exhausting hot air from the gym in the summer. Thanks tinmantu and z1700 for your examples. Thanks for raising the concerns about winter temps in the gym. If I can take better advantage of that 50 degree (+/-) constant temp of the ground below 6' I'll fare much better. There are places along the upper 1/3 of the walls that I can insulate to bring the winter temps up. All of you have opened new lines of thought for me, and I thank you again. If you have additional ideas or suggestions.. even negatives, I'll be happy to hear them. We all know that problem-solving is a process of sorting through lots of material, so the more ideas the better. When all of the "ducks" waddle into place, I'll set up a link,record my observations and do pictures.... Dave

The main issue I see here is that it will only take about 691 BTU to raise or lower the 38000 cu/ft of air 1 degree.

So in the winter even a a modest heat pump for your size living space could be pulling in excess of 48,000 BTU/h out of that air space. Dropping the air temp in there 69 degrees per hour.

Of course the air in that room in in contact with the concrete walls and floors that are going to act as a heat exchanger with the earth and heat will conduct into the air from them at some BTU/h rate that would need to be greater than the amount being pulled out or you would get an ice box down there real fast. This gets into the thermal transfer properties of air to concrete along with air flow around that room to make contact with that concrete which is beyond my expertise. I can say based on Fourier's Law 6" solid stone concrete floors and walls of that sq ft should be able to conduct about 112,000 BTU's from one side to the other with a 10 degree difference, but that must end up in the air and blowing over the coil to make up for the 48,000 pulled out otherwise the temp will just keep dropping below usable range.

If you do it you might get lucky bascially using a air source heat pump as a ground source heat pump, but if the room can't handle the heat exchange load you will just turn it into a ice box in winter and a hot box in summer without getting enough for you living space out of it.

Yes you can do this I have installed and worked on several stadium systems where we put the condensors in the area beneath the bleachers we experienced extended lifespans of the equipment and increased efficiency as for issues in heating you say it stays 32 at minus 20 that is just a small time window of your entire heating season the great majority of your season is probably above 20 degrees and your heat pump if properly installed will run very efficiently between 35 and 55 degrees.