pool heating-what i've learned that may help you

had an old teledyne laars 150,000 btu heater up until a month ago (by old, i mean 23 years..it served us well) so i replaced with a 125,000 laars jandy heater. my thinking in going with smaller size was largely based on size of my pool, what charts told me i would need for the temperature rise i wanted and the expense difference between a 125K and the next step up which is 175K.

my pool is about 430 square feet and based on that size, i figured the 125K to be more than adequate as i never did have to "crank" up the old one. the new heater is more efficient as well. my goal was a 1 degree rise in temp per hour.

my actual experience is i am getting on average, less than a 1 degree rise per hour. why? because the charts i read prior to my post purchase research did not tell me when they are referring to temp rise, they are relating it to air temp, not the current pool temp.

for example, yesterday afternoon when the air temp was about 70, my starting pool temp was 63 farenheit (into cooler nights now but still warm days...between 65 and 85 is the forecast for next week) my heater up until 10pm was able to rise the temp about a degree per hour. by 10pm, the outside temperature was 59 and this morning it is 40 degrees) the pool temp has risen only 5 degrees in 8 hours so about .5 degrees per hour overnight.

over 17 hours, the pool temp has risen 12 degrees so about 2/3 of a degree per hour average. not quite my target.

now, that also does not account for wind and evaporation as it was calm last night and i do have a solar pool cover on (thank god)

the better charts i've run into will factor in windspeed, not "ideal" heating conditions but i'm assuming do not take into account having a pool cover or not. they also will show the btu rating needed for "startup" which is longer of course than "maintenance". they also will state the heatup time is over a 24 hour period.

the main thing i'm concerned with is being able to get the pool up to swimming temp for the whole season (basically mid may to mid september for the pool season in my area)

it turned out i think that i was lucky that the 125K will be "adequate" for the task, but that's based on the big factor of conditions generally being perfect. low to no wind, pool cover on, no desire to push the season further than mid september.

i totally did not take into account that while we still get hot days in late august and september, overnights the temperature has dropped from double digits to single every night.

to summarize,my tips if you use a heater would be:

a) take air temperature overnight into account. a big factor.
b) buy a pool cover. right now, there's many online pool stores that have "fall deals" on. good time to buy.
c) consider carefully if your goal with a new heater includes wanting to extend the season for your pool.
d) remember, the charts are based on air temp rise, not current pool temp. if the pool temp is 71 and air temp is 59 and you think, well, my heater is rated to give me up to a 20 degree rise in 20 hours and my pool temp is 71..heck, i'll be at 85 in 16 hours..don't bank on it.
e) take into account the least desirable conditions...colder weather, wind, no cover..and plan heater size based on that..then you'll KNOW that you're heater will be more than adequate in the generally ideal condition "summer" season.

my greatest fear would be buying a heater that will not bring my pool to desired swimming temp (in our case, 85 works) when i want it to.

my heater will do what i need it to do, but barely knowing what i know now. in hindsight, i would have bought the 175K.

live and learn!

A better way to size a pool heater is by pool volume. Surface area is a good indicator for heat loss but volume is much better for heat gain. 1 BTU will raise the temp of 1 lbs of water 1 degree F.

A 125KBTU heater should raise the temp of 125,000 lbs of water one degree in one hour. This is about 15,000 gallons. However, as you pointed out, you do have to take heat loss into account and when the air temp is much less then water temp, you will lose quite a bit of heat per hour. Just curious, what is the volume of your pool?

hey mark..i figure it to be about 11,000-12,000 gallons. it took from 6am this morning until about 10am for it to get from 75 to 77. no sun on it until about 10:30am and now it's heating up at closer to the 1 degree per hour rate again..the overnight was a killer, no question. i originally was figuring in 24 hours it would go from 63 to 85..i think it will take about 30 hours say. (based on that thinking..8 hours to get from it's 77 to 85 during daylight sun on it which would seem to be about right given what was happening yesterday afternoon when i started it) i know from experience after i got my pool cover that the maintenance when up to the heat i want should be decent, but i don't know with cooler overnight temps how much i will lose yet even with the cover on. clearly it will cool off..how much? i should find out tonight. when it was staying 20+ outside overnight, it would stay at the 85 range by itself and with cover on for 18 hours a day without the heater having to turn on..but, that season is gone..it's "fall" relative to pool time in alberta:)

Well, I have solar and I can tell you that without a cover and depending on air temp at night, the most I have lost is about 8 degrees over night and typically it is around 6 degrees. The solar makes up all of that plus some but it does create large swings in pool temp over the day especially when nights are cold. At this point, I just don't want to fuss with a cover but I know that would help.

thanks mark..i will report back tomorrow morning the closing temp tonight based on it being heated up versus temp in the morning. given we are still going to swim in it for the next 3 days, i can't turn the heater off and want to maintain the temp for now, but it will be interesting to see if the heater can maintain that 85 temp with a cold overnight temp (cover on) or if it's going to have lost some ground. i would be very surprised if it's under 80 by tomorrow morning, even with heater cycling and cover on..we'll see!

well..last night, here's how things worked out. i'm quite happy and thrilled with myself that i invested in a pool cover.

heated my pool up from a 63 F startup temp to 82. (it took 27 hours so i think that's about a 2/3 degree average rise per hour. i let the heater and pump that point go into my desired 6 hour cycle with the temp on heater being left at 82. the pump/heater were on until 2am with the heater cycling on and off as necessary to maintain 82.

at 7:30am this morning, i manually started the system (you can't see temp on heater until the heater is powered on). i was initially shocked to see the temp at 63 and thought..ouch...i'm back to square one..seconds later, it jumps to 76..i'm thinking..6 degree heat loss from 2am to 7:30am..average overnight temperature was about 55..6 to 8am this morning it actually dropped to about 42! within one minute of circulation it popped up to 80...i thought ok..now, we're talking! 2 degree overnight loss was awesome..within 1/2 hour it was up to 81...

so, what this tells me is that my heater while not bringing the temp up in fall by 1 degree per hour as intended, is very easily able to "maintain" the temp once up to optimum temp.

it also tells me that a solar cover provides a huge benefit..i don't know what the temp drop would have been without it but i have to guess at least 8 degrees versus 2 from what i read.

the best investment in my "redone system" for cost efficiency will no doubt have been the solar cover.

i'm now into the cycle and until we decide to close will check the temp each morning and use that as a guage..the key for me will be how much it has to be heated up post the overnight "shutdown" to maintain 82. it would be nice if i can keep it running to end of september. after that point, the daytime airtime is too cold for my kids to have interest. that will also be the key..if it's in the teens post 20+, i know my kids won't want to bother.

when i first got the pool, i kept it open until mid-october and it got to the point where i was heating it for 12 hours to maintain temp (old heater, no cover) for 1 hour of swimming 3x per week. just isn't worth it in my mind at that point!

Hi Tenax,

I haven't owned a pool that long and don't have a gas heater, so please understand that the thoughts I give are just that....thoughts.

First off...I'm glad to read that things seem like they're going to work out for you. But I have to wonder if a call to the pool store (where you purchased the heater) may be in order. If you give them a call and explain that you've tried the heater out...and while you are happy with the heater by itself...that you made an error in sizing it to your pool and that it doesn't do a sufficient job. Would they be willing to take this (like new, I'm supposing) heater in for an exchange on a larger unit.

If you could do this while the heater is still new...it may save you some money in gas consumption from month to month.

In addition to the solar cover AND whichever heater you decide to stick with. Have you given any thought to running a solar heater as well? This way you can supplement the gas heater when the sun is available. Just a few thoughts.

good luck...and happy swimmin',
dan

the heater was bought and shipped freight from the u.s.(less than half the price versus buying it in canada) so that's not an option.

since i've started this thread, i've obviously picked up more experience. i would never end up running my pool past mid may to end of sept anyhew..the debateable time is the from mid to end of september..

heater is turning out to be fine for my purposes otherwise. it was able to get up to the optimum temp i want for swimming and beyond in spite of cool nights. (maybe a silly fear, but i had a fear nonetheless that it wouldn't be able to heat the pool up to 82 or potentially 85) it can do it..it's actually better if i can't heat it up reasonably to like 95 as that's where my wife would want it at all the time..82 is good enough for the kids and i, she's fine with 84, 85..even though she says she would like 90+. i said if you want a sauna, pull out the bank book, but that's not the intent of the pool!

i considered solar heating before i purchased this heater..to totally get away from ngas..it was not feasible unless i put the solar grid on the ground for the square footage required. (something like twice your pool size is a good measure)

but now that you mention it, i wonder if anyone here uses a combination grid or if that's possible? i do have one roof with a south exposure I believe that's what you want..or is it west?) anyway, i do have a roof that faces south that is about 400 square ft..that's why i didn't consider solar..but if i did that size, it would be enough heat for half the pool and it's on the back side of my yard by the pool so it would be feasible to run pipe...not sure my new pump would have the suction power. i have a 3/4 hp i could theoretically run as well as the main one..hmmm..i have to get more info:)