Cold Plunge Pool... Geothermal Cooling???

[mjohare]

Hello TFP,

First time poster and soon to be first time pool owner. It is so nice to find a site that is user supported like this.

We are installing a small pool in our small backyard (10' X 8' X 5' deep, vinyl liner pool). Our location is Ottawa, Ontario. The main use of the pool will be as a cold plunge pool that we can jump into after a hot sauna. The ideal temperature for our cold plunge pool will be somewhere in the 10-15C range (50-60F). Ideally we would like to be able to use the pool year round. For the cold months we are installing a gas heater (200,000BTUs). I'm going with gas rather than a heat pump because I want to be able to occasionally crank the pool temperature way up for an almost hot tub like experience, even in winter (although most likely we will close the pool from Dec to Mar to avoid insanely high heating costs and electricity costs from running the pump 24/7 to prevent freezing). Mostly, however, it will be for cold plunging, especially in the summer.

My problem is that without some sort of cooling ability the pool will be well above our desired temperature from June to late August. I've read that a heat pump and/or chiller can do this, but I have neither the room nor the money for yet another pool device. My idea is to use the relatively constant ground temperature as a cooling source. The ground temperature at a depth of 3m (about 9 feet) never gets above about 10C, and even at a depth of 2m (6 feet) the temp never gets above about 12.5C (see Fig 1 below). The idea is to dig a couple of trenches around 8 feet deep and bury some loops of polyethylene pipe (much like for a horizontal loop geothermal system as in image 2 below) and then run the pool water through this. I should be able to get a total of about 200 feet of pipe burried. The backyard is being dug up anyways for other landscaping work, so it is not a major task to dig a couple of trenches.

Does anyone know if I would get effective cooling of the pool water in this way?
Can a typical pool pump handle pushing water through this much pipe? It would only go through this system when cooling is needed, so about 2 months of the year.

Any thoughts would be greatly appreciated.

I know some will think this is frivolous to want to cool the water for 2 months of the year. But to me spending all this effort and money on building the pool is kind of wasted if the water is not refreshing during the hotest months of the year. And the prospect of going from a hot sauna to a warm 'bathtub' is quite unappealing.

I have also attached a pic of the pool in progress.

Thank you!!

 

[Bama Rambler]

Seems as if you've done a good bit of research and I commend you.

Your idea of cooling the pool is sound, but I'd go as deep as possible with the trenches. I might even consider boring wells instead of trenches, to get the delta temp as high as possible, though the cost might be prohibitive.

If you go with trenches, get as much piping in them as possible. I'd also plumb the loops parallel so the pressure drop to the pool pump isn't great. Of course you can always install a 3-way valve to divert only part of the flow through the loops.

 

[mjohare]

Thank you Bama.

I looked into boring some deep holes but access in my small urban lot was too tight for the drilling equipment. Not to mention that cost was indeed prohibitive.

A three way valve sounds like a good idea. I'll look into it.

Anybody else have any thoughts?

 

[ajw22]

What you described is a geothermal heat pump cooling system.

Ground source heat pump - Wikipedia

en.wikipedia.org

Most systems use a loop for the geothermal underground piping that flows through a heat exchanger to heat/cool the appliance, in your case your pool water. I would not flow your pool water directly through the geothermal loop. Keep them isolated and independent.

 

[Bama Rambler]

Allen, why wouldn't you run the pool water through the loop? If you put an exchanger in the circuit you're just going to use water to one side of the exchanger and pool water on the other side. Maybe I'm wrong, but to me the only thing you're doing is adding the losses created by the exchanger, adding the cost of the exchanger and adding the cost of a circulating pump and associated equipment. If you spring a leak in the loop you're still going to have to dig it up to find and fix it.

Like I said maybe there's something I'm not seeing or thinking about.

 

[ajw22]

Allen, why wouldn't you run the pool water through the loop?

First, @mjohare needs to winterize his pool in the winter. Winterizing an underground geothermal loop can be difficult and hard to know if it is clear of water. A closed system geothermal loop uses glycol or refrigerant that does not need to be winterized.

Second, the optimum flow rate for heat transfer on the geothermal loop will be different then the pool water flow. Getting the geothermal loop with pool water circulating through it will take continuous tinkering of valves and flow rates as the pool flow varies. With a closed geothermal loop the circulation rate can be set once.

Third, we know how nasty pool water can get if it is neglected. And we have seen where sand, DE, debris, whatever can get past the filters at times and clog pipes. If the geothermal loop is on it can become difficult to purge and clean the loop.

I think running pool water though an underground geothermal loop is a hack that will not last more then a few seasons. Too many things can go wrong. It is not in the spirit of having a Trouble Free Pool.

A geothermal system with a closed loop and a heat exchanger keeps the underground loop isolated from any pool problems and does not need to be winterized. And any problems with the heat exchanger are above ground and can be repaired.

It is up to the OP if he wants a science experiment or a quality setup.