How do you determine the correct gas pool heater size for an above ground pool?

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This a bit involved, so work through this with me ...
The easiest way is to contact InyoPools online or call them 877-372-6038

The goal is to have a heater that has enough capacity to heat the pool to your desired level in a reasonable amount of time. Maintaining that temperature is a lot easier once the pool is up to temp.
First, the capacity of most pool heaters are rated in BTU's or British Thermal Units. One BTU is the amount of heat required to raise the temperature of one pound of water, one degree F. And since there are 8.33 pounds per US gallon it takes 8.33 BTU's to raise one gallon of water, one degree F.
Now let's calculate:
1. Determine the number of gallons in your pool (G).
2. Determine the amount in temperature that you want to raise the pool temp (the easiest way to figure this is to use the air temp as the minimum and the desired pool temp as the maximum) (Delta T).
3. Calculate the number of BTU's needed (BTU) per gallon of water by multiplying step 2 x 8.33. Multiply this number times the gallons of water in your pool (G). This is the number of BTU's to raise your pool temp from the minimum to desired temp.
4. Determine how FAST you want to be able to go from the minimum temp to the desired temp (in hours) (H).
5. Divide the total number of BTU's from step 3 by the hours in step 4 to yield the BTU'S per hour that your heater will need to deliver.
6. Multiply Step 5 by a error factor of 20% and add to step 5. This is approximately the size of heater that you will need.
1. Assume pool volume, G = 10,000 gallons 2. Assume 80F desired pool temp and 60F air temp. Delta T = 80 - 60 = 20F 3. BTU's per gallon x Delta T: 8.33 x 20 = 166.6 BTU's per gallon. BTU/Gal x Gallons (G): 166.6 x 10,000 gal = 1,666,000 Total BTU's. (Wow, seems like a lot!) 4. Assume I want to be able to warm the pool in two days of continuous operation = 48 hours. 5. Total BTUs / Hours: 1,666,000 / 48 = 34,708 BTU's per hour. 6. (BTU/Hr x 20%) + BTU/Hr: 34,708 x 0.20 = 6,941 BTU's 6,941 + 34,708 = 41,649 BTU's per hour.
Now, most pool heaters come in a round number of BTU's like 50,000, 100,000, 150,000, etc. You should select the closest size to your needs (usually on the higher side is best).
Hence, for this example, a 50,000 BTU heater would be more than adequate.
Obviously, you could also work these calculations backward to determine what Delta T you could handle given a certain size heater and pool size.
... SUGGEST YOU NOT SIZE A HEATER BASED ON THE ABOVE ARTICLE !!! So sorry, I find so many exceptions (wrong statements and conclusions)... This novice article/ suggestion is a nice try but misses the mark and should not be used to size a pool heater.
Firstly there are different design conditions for pool water temps, and different methods, which must be used depending on whether the heater is a gas (i.e. natural, naphtha, propane) or an electric resistance or heatpump.
Most people are likely to use natural gas due to the low cost of this fuel. Heat pumps can be about the same operational costs or cheaper, depending on your gas and electric rates in your area. In my area a heatpump is at a par with the cost to operate a natural gas heater. (Considering a 4.0 C.O.P.)
Calculating a pool heat loss is involved for sure and one must calculate evaporation, radiant losses and convection losses (calculation performed within the evapo-transpiration rate). Conduction can be said to be about 3% as a standard so this calculation is not needed to be input as a variable, but should be added to the calc at some point for the conclusion. All these calculations are done against a varying outdoor ambient (temperature, humidity, wind) condition.
Without a pool cover the losses can be up to 3 times that of using a pool cover (Lower wind across the pool surface is a lower rate of loss). Normally it's a factor of about 2-2.5. In the case of a body of water, the convection (wind) is the "driving force� behind the evaporation portion of the formula's calculation. So one should also pre-determine whether a pool cover is to be used before calculating. (Typically a pool without a cover will require twice the btuh for pool water maintenance (twice the heater sizing) and will naturally take longer hours for initial heating.
Anyone not using a pool cover and heating a pool is ludicrous and will pay for it. Note that with a pool cover, the cover remains on the pool 24 hours a day except for the time the pool is used which can be about 3 hours for most residential pools. Evaporation accounts for about 70% of the pool heat losses so pool covers do a great job as water vapor is trapped below the cover and thus so is the heat (a saran wrapping of 1 mil would do the same as a vapor barrier).
For a gas heater sizing you can design with it in mind to let the pool water temperature drop and turn on the pool heater at whatever day you want to use the pool. This sizing will then be based on the number of hours you want to wait for the pool to heat up. It will also be based on the starting water temp and final swim temperature you want. There will be the water volume to heat plus the ambient losses during the hours of heating, plant room, exposed piping losses, etc. (which can be about 8%) occurring during the heat up time. (This method, I think, is what the above article was trying to get to, but sorely missed de-rating the heater (you have to figure it's 3-5 year efficiency not it's new efficiency), forgot about the on-going heat losses to the ambient and failed to mention adding piping/plant room losses, activity losses, with or without a pool cover usage.
The article above is very miss leading as it does not correctly conclude the proper gas heater size... Heating efficiency; I know some mfgrs. Say they have 99% efficiency, but this is not a practical truth when measured in the field. Plus there is the exchanger losses which build up (water heated over 40 deg C will precipitate iron, magnesium, calcium, etc.) so fouling of the exchanger (loss of BTUH output) does occur at faster rates than with heatpump s. I suggest you always use a 75% output gas btuh against any gas heater mfgr's claim.
For heatpump s it's very different. These are sized to the maintenance of the pool temperature for the estimated hours the pool is covered and estimated hours the pool is uncovered. Swimming activity also is an important equation to add, as this activity accelerates the pool heat loss (accelerates evaporation, plus water losses from swimmers) while un-covered. If anyone tried to size a heatpump on the volume guestimate above you'd end up with a huge heatpump and the initial cost would be prohibited. (You�d never buy a heatpump based on this guestimate using the miss-leading volume method, where-as in some parts of the USA and other world countries; the heatpump is the only way to go). Heat pumps require the correct detailed calculation for the heat loss of a body of water that utilizes the items mentioned above in this article with evaporation calculated from the convection and other factors in a formula that calculates the water pressure difference to that of the air with a database of 24-hour historic ambient low averages.
Having said it's very different for Heat pump sizing methods and calculations, it doesn't have to be... I happened to size my gas heater using the heatpump sizing formula; i.e. the only correct calculation for determining pool heat losses; surface heat loss based formula, and sized my heater with the input data that: the pool stays covered 24 hrs a day except 3 hours a day for swimming. I ended up with a very small 100,000 btuh heater that does the job great.
Now having said this let me also include that heat pumps are also set up differently than gas heaters typically, unless you use the accurate heat loss method described here-in to which I refer and not show..sorry. Heat pumps are set up to be able to run as many hours a day as they need, and to only turn on once a day. (so heat pumps are always simply maintaining the pool temp at a constant, (generally) and these amounts of heat are relatively small. I used this same set-up for my gas heater, so it simply maintains the pool temperature I have set. (always ready for me to jump in whether it's day or 3 am!) You don't need to have the pool heated at 3 am, as the mass of water has a great retention with a cover, so you do allow it to reach your set temp and then wait for the following day; during sunshine (higher ambient temps for higher effeciency) to bring back the set temperature.
To help think about heat loss from a body of water (swimming pool) consider a well, lots of water; say 10,000 gallons and you heat it, and it stays heated for long periods of time as the surface is say only 2 meters across. Now take the same water volume; say 10,000 gallons, spread it 1/2" thick across a field and wait one day: it's all evaporated and gone! The heat loss for the well was minimal, the heat loss for the field of water; tremendous. So surface water calculation is the only true calculation to use for a pool, with a check on the warm up time (if you plan to let the pool water drop a day or so, then raise it, then drop it, etc.) Pools with big volumes and small surface areas lose less heat/ hr and require smaller heaters to maintain the maintenance temperatures.
Take a hot cup of coffee... let it stand in a wind protected place...takes a long time to cool down. Now take the same cup of coffee and start blowing ambient air on it;(like a pool with wind at it's surface) now lots of heat loss, and the temperature can cool up to 2.5-3 times faster..(The wind (convection) accelerated the evapo-transpiration! Add a swizzle stick in the coffee (swimmers in a pool) and you further accelerate the rate of radiant and evaporation losses.
This is why using a "volume based heat requirement formula" as described above is not correct, even if you do add these minor corrections mentioned here-in. The lack of pool cover mention is the articles biggest err...
I'm sorry to not go on with providing the correct calculation formula and many other details, but thought one reading this article should be made aware, this one is not to follow...
Note; most economically sized heating plants will require about 3-3-1/2 days to provide initial warm up (considering about 82-85 Deg F) and be proper for the best operating costs. For myself, I can't see swimming in pool water less than 86 deg F and we keep about 89 Deg F. most of the time!(old folks)
pool man
Interestingly enough, poolman, for rough estimates, the formula at the top of the article is accurate enough. Since I can't buy a 103356.741 BTU heater, the above formula gives me enough information to make an informed decsion on what size heater to buy. Given the length of your response, and the fact that you don't provide your actual formula, I am left to assume you sell pool heaters. Do you service the WNY area???
A thought added by robtig1:
I think that both of these answers have equal merit. Essentially, the raw volume btu calculation needs to be done not based on the delta T (change in desired temperature), except when determining the time taken to initially heat the pool, but rather, the raw volume btu calc needs to be applied to the heat loss principles as mentioned by poolman's response.
In short, I think it is the rate of heat loss that needs to be determined and then take that delta T and multiply etc.. by the 8.33 and volume etc etc... A simple average day experiment will let you know the change in temp you wish to achieve. Just heat your pool up, and see how much it cools down during a 24 or 48 hour period. This will give you a simple version of pool man's very complicated assessment of heat loss through evaporation, line loss, etc..
My purpose of these calculations is to assess the size of a solar heater needed to warm up my pool. I did a simple spreadsheet, and even with a good 12 hours of sunlight optimistic I fear that my input at such a low BTU rating will hardly dent the temperature of my pool. I am calculating on 6,600 btu per hour output on a vacuum tube assembly with 60 tubes. Initially I thought that number to be high, but on my 140,000 litre pool it only warms it up 0.18 degrees F per day. I feel i must be doing something wrong as I should not need an acre of solar panels to heat my pool. I would be happy if I could even add 1 degree per day to my pool. If anyone has any advise on this, I would be happy to hear it!!

The calculations used in the 1st part are the same calculations and numbers used in Ordini's Pools web page. Ordini's continues on their page to give information on surface area and losses, as well as cost comparisons between various available heater styles.

Solar heaters will NOT heat your pool to a swim temp. They are a secondary heat source only. You need 400,000 or more btu heater.

Paul: I have an in ground 12 by 24 foot pool with a 30kw gas heater. It raises the temp about 1 deg C in two hours. BUT in all but high summer [in south u.k] thr ground losses through the pool walls can drop the temperature 2 degress C overnight even with a pool cover and home made 1 inch thick foam slabs laid on yop og the cover.

So.. if your builder didn't put insulation round the pool sides / floor, which mine hasn't got, then [as i do] you either pay the gas bill consequences or only use the pool when the ground temperature has hotted up. This year 2012 its been unusually cool here, i've only just started using the pool in May, purely due to heat loss probs to the ground mentioned above.

And yes I made my own on-top slabs using polystyrene 8 by 4 foot slabs encased in horticultural grade plastic sheet and seam welded with a hot air gun designed for the purpose, around 100 pounds for the gun. So my slabs cost me far less than the 1000 pounds commercial ones would have cost.

oh and btw, its feb 2013, i know now that the gas heater is only transferring 15kw to the pool water [rather than 30kw] this is purely because the heat exchanger i inherited, is running at half its labelled rating, because the manufacturers spec. likes to look good. with an inlet temp. of around 70 centigrade it can only transfer 15kw. so beware, err on the side of choosing, around double the heat exchanger rating, or double check that the inlet-to-pool temperature difference [and water flow rate] will achieve the required heat transfer.

re. robtig1, in wales uk we only really get lots of sun occasionally, and the prob with the collectors is that they produce maximum energy when the sun's already heating the pool anyway. So in midsummer long days yes it can add a fraction of a degree c,,.. but it tends to get lost overnight or by subsequent dull days.

I have about 10 square metres of the black ribbed rubbery collectors, the pool area is 30 square metres. I'd say I need at least 30 sq metres to make much difference over all. and my pool is under a huge plastic greenhouselike structure, the collectors are at 60 degree angle inside that enclosed area, and pick up more heat [or rather, lose less heat to the wind] than if they were outdoors. Over all I'm not impressed by the manufacturers claims of 5 or 9 degree increase. Not in the average uk summer anyway.
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