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Thread: Pool Heater Question...

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    carlos31820's Avatar
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    Pool Heater Question...

    I have a question regarding something I read on the website for my gas pool heater. The website states "The Pool Sizing Guide shows the maximum pool surface area our Legacy models will heat based on the temperature difference between the air
    temperature during the coldest month the pool is used and the desired pool temperature.
    ".

    This weekend our temp is rising to the mid 70s. We're contemplating firing up the heater and I was wondering why the heater sizing guide works off of the difference between air temperature and desired water temperature instead of the difference between actual water temperature and desired temperature.

    Lets say air temp is 70F and I want water at 85F (15 degree difference) but actual water temp is 50F (35 degree difference). Why does is the heater sizing guide based on air temp instead of actual water temp? I apologize if its a dumb question...
    16x36 Vinyl Pool - 22.5K gallons.
    Jandy Stealth 1hp pump - Jandy CS-150 filter
    Jandy Legacy 250k heater - Jandy Ei SWCG
    Jandy Watercolors LED light
    Paramount Vanquish in-floor cleaner

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    Re: Pool Heater Question...

    As you can see, there are all kinds of ways that one can look at how a heater will perform. I think what it boils down to in your guide is that the ambient air temperature will determine (among other factors such as wind) how fast heat will be lost from the pool. The idea is that a bigger surface area will lose more heat than a smaller one. If the heater is too small, the amount of heat lost over a given time may exceed the BTU being put into the pool. In that case, running the heater would be a futile cause.

    IMO, those heating guides arent worth that much. They adjust the heater size based on the sq footage of the pool. The reality is a 400k heater is really the best buy. They use more gas than a smaller one, but you run it for a shorter period of time, so the actual gas consumed is about the same.
    14,000 gallon IG, Vinyl. Hayward 3/4 hp superpump, Penatair IC40 SWCG, Pentair automation, Hayward sand filter, Aqua Comfort heat pump, Hayward 400k Lo-Nox LP heater.

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    JasonLion's Avatar
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    Re: Pool Heater Question...

    The starting water temperature is important if you want to know how long it will take to warm up the pool, but it has no effect on the warmest you can eventually get the pool. That heater sizing chart assumes that you leave the heater on all the time. The warm up from cold time is assumed to have already been taken care of.

    When the water is cooler than the air the heater will heat the water very efficiently and relatively quickly. At the start, heat lost through the surface of the pool will be relatively small. As the water gets warmer the amount of heat lost through the surface of the pool will increase and the rate of temperature increase will slow down. Eventually you get to a point where heat is being lost just as quickly as the heater can put it in, at which point you have reached the heating capacity limit of the heater.

    As bk406 said, you almost always want the largest heater you can get. So that chart isn't very interesting.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    carlos31820's Avatar
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    Re: Pool Heater Question...

    Thank you for the replies. Im also reading that its much more efficient to put a solar blanket on the pool to minimize heat loss.

    We had to go with a 250k heater due to the size of my meter and the fact that the meter was already over 100 ft away from pool. The 100ft of gas line almost broke the bank. At that point I didn't want to go through the extra expense of upsizing the meter as well. We anticipate we'll only use the heater a handful of times per season anyway. Trust me, I wanted to go 400k btu but decided to spend the extra money on additional concrete deck around pool.

    16x36 Vinyl Pool - 22.5K gallons.
    Jandy Stealth 1hp pump - Jandy CS-150 filter
    Jandy Legacy 250k heater - Jandy Ei SWCG
    Jandy Watercolors LED light
    Paramount Vanquish in-floor cleaner

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