# Pool heater "time to heat" and "cost to heat" calculations >

#### Turbota

##### Active member
Here is a way to determine how long it will take your heater to heat your pool water, and how much it will cost you if you are using an electric heater.

This of course won't tell you how much it costs to keep the temp at a certain level once warm, but it will give you an idea as to how long it will initially take to heat the pool.

This is based on my in-ground pool, but you can figure in the parameters of your individual pool to get your correct numbers.

Be advised .. This is only an estimate .. These calculations only reflect the required heating of the water, and do not take into consideration the heating of the cold pool walls or any associated equipment in the pool.

If you don't agree to these calculations, please feel free to respond.
_____________________________________

9,000 Gal. Gunite Pool
104,000 BTU Electric Heat Pump (5.5 KwH power consumption)

Determine Time Required to Increase Water Temp 1 Degree F:

1. Multiply the total gallons of water in the pool by 8.3
This will give you the total weight of the water
(9,000 gal x 8.3 = 74,700 lb)

2. Divide the weight of the water by your heater's BTU
This will give you the hours it takes to raise the water temp 1 degree F
(74,700 lb / 104,000 BTU = .72 hrs)

Determine Total Time Required to Increase Water Temp the Desired Amount:

1. If the current water temp is 65 degrees and you want to raise it to 80 degrees (15 degree increase) ..
Multiply 15 degees by the time it takes to raise the temperature 1 degree F
(15 degrees x .72 hrs = 10.8 hrs)

Determine the Cost of Electricity you Will Use:

1. If your heater uses 5.5 KwH, and with a 10.8 hr run time, it will use 59.4 KwH of electricity
(5.5 KwH x 10.8 hrs = 59.4 KwH)

2. Using 59.4 KwH, and a cost of electricity in your area of 12 cents per KwH, it would cost \$7.13
(59.4 KwH x .12 cents = \$7.13)

#### JasonLion

TFP Expert
Platinum Supporter
Re: Pool heater "time to heat" and "cost to heat" calculatio

In most cases this will be wildly optimistic for a heat pump, and mildly optimistic for a gas heater. Heat pump efficiency varies dramatically with the air temperature, only reaching the listed BTU number when the air is quite warm. Gas heaters are more consistent, but only about 80% efficient, which needs to be factored in. Also, there will always be heat loss to the environment, increasing the amount of heat required, which increases the heating time and cost.

I have a fairly small gas heater (180k BTU) and a 19k pool. During a sunny day the heater performs at something close to it's listed output, but at night in the spring and fall it is lucky if it can maintain the temperature at all, let alone increase it.

#### Turbota

##### Active member
Re: Pool heater "time to heat" and "cost to heat" calculatio

Thank's for the comment and the additional info ... I just posted this so someone might get a ballpark idea as to how long it might take them to heat up there pool and a basic cost for electricity.

I was initially interested in this because I have never used our pool heater.

The one thing nice about living in S. Florida is that the pool water never gets much below 65 degrees even in the dead of winter (like right now). I would think that starting with water temps of 65 or higher and ambient air temps in the 70s during the day, these heat pumps would more than likely operate at fairly close to there listed BTU, and the numbers I listed above might be close to 'real world' conditions here in Florida.

As for someone trying to heat an outdoor in-ground pool in Minnesota in Feb ... might be better just to use the pool as a small skating rink! (certainly my numbers I listed above would be pretty much worthless).

#### ps0303

TFP Expert
In The Industry
Re: Pool heater "time to heat" and "cost to heat" calculatio

With the weather/temps we have had in FL this winter, most people I know that have heat pumps have only been able to get water temps to around 80. Now once the temps get above 85 outside, then you get some heat in the pool.

#### bk406

##### Well-known member
Re: Pool heater "time to heat" and "cost to heat" calculatio

ps0303 said:
With the weather/temps we have had in FL this winter, most people I know that have heat pumps have only been able to get water temps to around 80. Now once the temps get above 85 outside, then you get some heat in the pool.

And now the florida heat pump guru's know what we folks up north have said about them for years.

#### stimpsonjcat

Bronze Supporter
Re: Pool heater "time to heat" and "cost to heat" calculations >

I finally heated up our new pool last night as I was finally starting to install the fast-lane equipment.

My empirical math had our pool warming at a 0.24 degrees per minute rate. I heated it from 61 to 85 degrees. It had lost 5 degrees in 12 hours when I checked it this morning.

I will be backwards calculating time to get to 80 degrees from any starting temp down to about 60 so that the PLC can start the heater before I get up in the morning by the correct offset.

#### mdrejhon

##### Well-known member
Re: Pool heater "time to heat" and "cost to heat" calculations >

Check link in my signature below.

We have become experts in thermal management lately and ran our pool in January in Canada with a sub-300-dollar heating bill (which included heating house and cooking too!)

For near-freezing weather, trick is full throttle heating without thermostat cycling, turn off pump when not heating, never let water circulate through cold pipes unless heater is running too, keep pool covered when not in use (quick roll on-off cover system). Overnight Temperature decline dramatically slows with the above tricks. Sometimes as little as 1-2F cooldown in 8 hours.

While I am natgas, the same advice of lowering cost of heating (especially in colder weather) and slowing down cooldown, applies to propane, ground pump, electric, etc.

#### zolakk

Bronze Supporter
Re: Pool heater "time to heat" and "cost to heat" calculations >

I don't know if it holds true for everyone on natural gas but the guy from the gas company here that came to upgrade our meter told me that as a rule of thumb 100k BTUs/hour for any gas appliance = 1 Therm = \$1 (for Las Vegas anyway, obviously YMMV on price) so my 400k BTU heater would cost \$4/hr to run, so I was aware of the cost upfront of running it and then highly recommended not using it for the pool. Apparently they have gotten people shocked after using the heater for the pool and/or having it run on accident when going on vacation and getting \$\$\$\$ gas bills so I was grateful for the information.

#### Belo

##### Active member
Re: Pool heater "time to heat" and "cost to heat" calculations >

found this old thread trying to get a ballpark on heating my pool via ng. Surprised there isn't some free calculators out there.

#### Joshii

##### Well-known member
Re: Pool heater "time to heat" and "cost to heat" calculations >

If I dump a whole 20lb propane tank into heating my pool it will go up 3 degrees. For \$17 bucks. Heating a pool is scary.

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#### pooldv

TFP Expert
Platinum Supporter
Moderator Emeritus
Re: Pool heater "time to heat" and "cost to heat" calculations >

1 btu will heat 1 lb of water 1 degree. Calculate how many lbs of water you have and use 80% of the btu output of your heater to figure out how much it will heat your pool in 1 hour.

#### Belo

##### Active member
Re: Pool heater "time to heat" and "cost to heat" calculations >

1 btu will heat 1 lb of water 1 degree. Calculate how many lbs of water you have and use 80% of the btu output of your heater to figure out how much it will heat your pool in 1 hour.

I built a spreadsheet, my comment was that there are a lot of "free html style calculators" out there. Just surprised some pool site doesn't have one where you just plug in btu's of your heater, \$/therm and gallons of water. Yes i built my own and as an engineer it's something simple for me. But many americans are pretty math disabled lol.

#### pooldv

TFP Expert
Platinum Supporter
Moderator Emeritus
Re: Pool heater "time to heat" and "cost to heat" calculations >

Oh, I see. True that!

#### danpik

TFP Guide
Re: Pool heater "time to heat" and "cost to heat" calculations >

The formula is pretty simple.

Take the gallons of the pool and multiply by 8.3. That gives you the weight of the water. Take 20% off of the BTU rating of the heater. (example, 400,000 BTU heater - 20% = 320,000 BTU's)

Divide the BTU number by the weight number and that will give you the number of degrees of temp rise per hour under ideal conditions.

Example: 5000 gallon pool with a 200,000 BTU heater

5000*8.3 = 41,500 lbs of water

200,000-20% = 160,000 BTU's available to the water/hour.

160,000\41,500 = 3.8 deg/hour of temp rise.

Keep in mind this does not account for any heat loss due to air temps being colder than the water or heat gain from the air being warmer than the water.

If you are heating with Nat gas, your billing will give you a cost/therm for the fuel. To make the math easy, Lets assume it is \$1.00/therm. (a therm, sometimes referred to as CCF (100 cubic feet) is approximately 100,000 BTU's)

Using the above heater at 200,000 BTU's of input, it will consume about 2 therms/hour for a hourly run cost of \$2.00.

Dan

#### packrat555

##### Member
Re: Pool heater "time to heat" and "cost to heat" calculations >

here is a decent page to calculate time to heat. I am only interested is heating a 600 gallon spa to go from 70 - 100 degrees with a 360K btu raypak cupro it take 25 minutes. The biggest raypak would seem to be way overkill for a small spa but if I went with 180K it takes 50 minutes who wants to wait nearly 1 hour for a spa to heat up?

Pool Heat Time Calculator

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