Sizing a heat exchanger for a pool

[MrEd]

I just started thinking about getting a pool heat exchanger, and started checking them out (on ebay mostly).

Not sure how you go about sizing one though - most of the charts seem to give you a pool-size to heat exchanger lookup table and then you are supposed to pick - easy enough...So for my pool, which I estimate as 35,000 gallons I should need an exchanger rated for about 400K.

But if I already know that my tarm can put out a max of 200K BTUs, do I get a heat exchanger that is sized for what my heat source can put out, or a heat exchanger sized for the heat demand? How do those two things come into play?

 

[TCaldwell]

mr ed, are you sure 35000 gal? if you do not have storage it should be sized to the boiler output, if you plan any other demand during the pool heating, ie. dwh you might size the shell and tube hx a little smaller yet. i have a 22000 gal pool heated by a 250k btu hx supplied by a garn. be prepared to consume some wood and swim guilt free

 

[MrEd]

The 35K is a pretty good estimate...its a 20x40, but with a much larger than normal deep-end (increasing the avg depth).

I am curious how yours works? Do you find you get noticeable improvement in temp for a reasonable amount of wood? or are you burning 12 hours to go up just a couple of degrees? I don't want to burn 5 cords of wood to get an extra week or two of swimming in....

 

[sgschwend]

Wouldn't it be great if solar could do that job for you? I wish solar would work in my location.

You will only need about 7 million BTUs to raise the water up 20 degrees. That is about 1/3 to 1/2 of a cord a day depending on the wood species and how dry it is.

 

[TCaldwell]

mr ed,i got tierd of paying the propane man every 10 days, to keep the pool up to a temp the GIRLS would use, my control is basic but it works, the on/off control is a 15$ intermetic 6 hr spring wound timer that opens a zone valve and energizes the circulator to supply the shell and tube hx. with the garn ,i would fire with a starting temp of 150+ and turn the timer to 4 hrs, dump close to a million btu into the pool and still have enough in storage for dwh. when i was buying pool gas at approx 1.75$/gal, it still would cost 3K$ a season. the heating value relationship of propane at 1.75$ for a seasoned cord would be 365$, basically the 3K of propane has been replaced with 5 cord of wood and a one time parts cost of 1000$

 

[sgschwend]

I don't know much about these and you may already have one, but a pool blanket can save you a lot of heat loss. The folks selling these say they can save 3/4 of the night time heat loss. Which in your case would be 2-3 million Btu.

 

[flyingcow]

Got 18x36 in ground. About 28,000 gals. Pool cover works very well, especially up here, wouldn't go without one. Eventually hooking it up to my wood boiler. this thread will be good. But i heat my pool now with propane. Usually turn it on a few hours before pool use. Don't go thru 75 gals a year. this unit will bring the temp up 1 and 1/2 degree per hour, on a cloudy day. But every one seems at happy at 78. Will be going solar soon, whether its DIY or buy a system, don't know. Good luck.

 

[Gooserider]

You might also try asking from the other end of the equation - I really find the Trouble Free Pool website to be a great place - they are the Hearth.com of the pool world, and could probably help out from the pool designer end of the question. (They are also the go-to people for pool chemistry - since I started doing the TFP method, I have had perfectly clear water, while spending far less effort and money on chemistry...)

I haven't read a lot on the pool heater stuff over there, but essentially it's largely a question of time - almost any size HX will warm the pool eventually, the question is how fast do you want to do it... The bigger the HX the faster it will heat the pool...

My own guess would be to size for the output of the boiler, as that is really your limiting factor...

Gooserider