I have a 4 ton Trane WSHP that I operate with a well open loop. I also have a 15,000+ gallon swimming pool. I am researching using the pool as a loop possibly during parts of the year. I was thinking of using a plate type heat exchanger with a circulating pump and being able to switch back to the well if the water tempature in the pool got to high. My idea behind this would be to gain some heat for the pool, avoid some wear on the well, and reduce water usage. Any thoughts on this?

This could work very well if properly executed, or not so well if done poorly.

This has been on my "try it if I get a chance" list for awhile.

In round numbers, a 4 ton WSHP operating 12 hours in a day (typical in hot weather) would raise the temperature of a 15kgal pool by 5-6 degrees. The question would be whether the pool would dissipate a similar amount of heat each night. Salient variables include:

1) Surface area of pool
2) ambient wet bulb temperature
3) presence of a screen enclosure
4) pool surface exposure to wind
5) allowed pool temperature upper limit

Ancillary effects may include increase in pool chemical consumption and increased evaporative loss of pool water.

There are internet-based pool water heating energy calculators. Aqua-Cal has one on their site for sizing heat pump pool heaters. You can essentially run the numbers in reverse, since operation of a heat pump to cool a house and heat a pool is functionally identical to operating a dedicated heat pump pool heater, except that the controlled variable is house temp instead of pool water temp.

In general, I'd expect that a pool in a screen enclosure or otherwise shaded in a mild-to-hot climate would benefit from such an arrangement. An unshaded pool in a really hot climate would likely overheat, although a pool in an arid climate would cool rapidly at night via evaporation, although excessive evaportation raises water bills and concentrates hardness elements in the pool.

Make sure heat exchanger is plumbed pool side.
A failed het ex or plumbing circuit in the basement, means your pool empties itself there!

Currently the pool is uncovered but plans are for a screen enclosure. I was thinking on a DIY setup with a plate type exchanger. I need to find some information regarding sizing for the heat exchangers. Nothing ventured, nothing gained.

A pool that empties into the basement has simply morphed into being an indoor pool...could be quite convenient in winter.

Plate heat ex info is out there on the web. They can be suprisiingly small for the flows and heat exchange rates they accommodate.

No plate HX on a pool. You have far too much flow rate through the pool filter/pump to use a plate HX. The pressure drop would be way too high. There are HX designed for pools. Try this http://www.pahlen.com/our-products/heating-heat-exchangers/heat-exchanger-hi-flow-titanium.html    The titanium stainless HX is good for salt water pools as well.

It has to be plumbed after the filter on the way back to the pool.

I was actually thinking of using a separate smaller pump for the pool side of the heat exchanger. I think it would be a waste of energy to run the pool pump, but have been looking at the high efficiency variable flow pool pumps as another option. I would like to do a rough plumbing setup to test it out for a while and see how it works.

You have to run the pool pump for a certain amount of time anyway. HPs need very specific flow rates and temps to keep the pressures within limits so having a HX inline with the pool pump and a dedicated pump from the HP to the pool HX will keep it within spec. I don't know how the variable speed pool pump will work in this situation so I will do some investigation. HPs have the best electrical performance when there is a close Dt (in/out temp) such as 6-8F. Trying to get a lot more will be problematic and a variable speed could create this condition.

Your pool water is used instead of well water. Why add another heat exchanger?

Posted By jonr on 02 Jun 2012 01:45 PM
Your pool water is used instead of well water. Why add another heat exchanger?

Because there are things in the pool water which can be quite corrosive. And yo don't want to expose your heatpump heat exchanger to that.

I read a lot about pool waters effect on copper even with the salt and most information I found said that the ph balance was the most important factor and the chlorine and salt levels are low. My thoughts are that it's easier to replace the exchanger if it fails. I will try to track the heat pump run time to see how it compares to the pool run time. Currently the pool operates about 5 hours a day to turn it and keep the chlorine level up. Of course some of that is adjustable and could operate longer without problems.

The concerns of overheating the pool can be reduced by adding a waterfall or fountain to the pool .It will have the same effect as a cooling tower.The evaporative effect can be tremendous in cooling the pool water. Another alternative is to use nocturnal solar to reduce pool water temperature.

If the HP exchanger fails it'll let refrigerant and oil into the pool water and then pool water into the HP compressor - very ugly outcomes.

OTOH, I believe I've seen a Titanium coax heat exchanger option for WaterFurnace heat pumps. I've never priced one.

I agree that a falling water feature would cool a pool, especially if operated after dark when drybulb temperatures approach wet bulb temperatures

f the HP exchanger fails it'll let refrigerant and oil into the pool water and then pool water into the HP compressor - very ugly outcomes.

What kind of water in a compressor isn't ugly?

I found a few titanium coax hx on eBay in the $400 price range with 1.5 plumbing connections for the pool side plumbing. It was a 85k btu unit. Any ideas on how to size it? Most of these are marketed to the wood burner crowd. Seems like it may be worth it if I only need the heat exchanger and a circulating pump for the heat pump side. Then I was thinking of somehow connecting the well to the hx also, so that I can switch between the two and not have to change anything on the heat pump side.

Keep in mind that they are usually sized for an 80 degree F delta T and 180F entering water temperature. Something not appropriate with a heatpump. Rule of thumb: Flow as high as possible, pressure drop as low as possible, delta T as low as possible.

Posted By gregs on 03 Jun 2012 10:37 AM
I found a few titanium coax hx on eBay in the $400 price range with 1.5 plumbing connections for the pool side plumbing. It was a 85k btu unit. Any ideas on how to size it? Most of these are marketed to the wood burner crowd. Seems like it may be worth it if I only need the heat exchanger and a circulating pump for the heat pump side. Then I was thinking of somehow connecting the well to the hx also, so that I can switch between the two and not have to change anything on the heat pump side.

Did you try the Pahlen HX? It is made for pools and it can be sized for HPs. If you pick one we can help with the sizing but I would look for one in the 250-300MBTU range because the temp drop from the HP is low compared to a boiler so you will need a proportionally bigger HX to cover it.

I will contact pahlen and see what's available and how it's priced. So with the lower temperature differential you need more surface area in the hx?

Posted By gregs on 03 Jun 2012 03:43 PM
I will contact pahlen and see what's available and how it's priced. So with the lower temperature differential you need more surface area in the hx?

Absolutely. Heat transfer is a function, mostly, of dT and flow rate. 20deg dT X 500(a constant) X 1gpm = 10,000btu. Half the dT and you get half the heat transfer. Any HX specs will give you the flow rate and dT that they base their performance on. Check it against you HPs performance and you can get a rough idea.