Heater use - mathematician needed

[Sunbaby]

Ok guys I've searched and found all sorts of threads on how to determine how much it costs to heat a pool and how long it takes. BTUs, therms & other terms I know nothing about. I can't figure it out.

So, please help a gal out. I simply need an estimation on when I should start heating my pool for a party.

~8000 gal (we thought it was more, but my Chems work best at this estimation.)
Propane
Mastertemp 400
Water temp 72-74
Outdoor temp 65-80
Desired temp 85? - I guess?

I'm not concerned about the cost (well, not true, scared to see my cost, but it's the first time to heat the pool, I'll call it a learning experience)

My hot tub goes from 65-100 in like 45 min to an hour? I've never timed it, it's always hot by the time I get out there.

4 hours? 8?, 12?

 

[MarianParoo]

Hmmm. It kinda comes fown to pool volume, heater size, efficiency, type of gas, etc. When it comes to heat for a party, I don't worry too much about the cost. That's just me.

However, I replaced my heater last year with a more energy-efficient one, and I ran it for an hour, tested temp, another hour, tested temp, etc, for four hours. Then I got a rough idea of timing. Now I can calculate prior to a party.

 

[JamesW]

You should gain about 5 degrees per hour, not accounting for heat loss. If you allow for 1 degree per hour heat loss, then you should get about 4 degrees per hour.

Losses can vary quite a bit depending on wind speed, water temperature, air temperature, humidity, covered or uncovered etc.

 

[jblizzle]

Rough estimate, I would say you might add 10 degrees to the pool in a bit over 2 hours.

8000 x 8.2 x 10 degrees = 656 kBTU

Heater assuming 80% efficiency adds 320 kBTU per hour.

Although not counting losses to the environment.

I think 4 hours would be plenty of a head start.

 

[pooldv]

85 is a good temp. My calculation says your pool is about 15k gallons. To raise 15k gallons 10 degrees will take just under 4 hours, a tad over 5 hours to heat it 13 degrees. Maybe 6 hours if you are starting to heat earlyish in the morning when it is cooler.

Here is the easy math formula
15,000x8.3 = 124,500 lbs
124,500÷320,000btu = 0.389 hours per degree
0.389x13degrees = 5.057 hours

320,000 btu is your 400k heater at 80% efficiency

 

[Sunbaby]

You guys are awesome!!! Thank you for your help!

 

[danpik]

I will agree with Pooldv...almost. My calcs assuming an average depth of 54" is 16,700 gallons. Not enough to disagree over. His may be more accurate if he did some more accurate estimates on volume based on the depths of each part of the pool. Either way, 15,000 - 16,000 gallons is a more realistic number for your pool.

As far as cost goes to run the heater, this is based on two factors. BTU input of the heater and cost of propane in your area. A gallon of propane will yield roughly 92,000 BTU's. Your 400 will consume 400,000 BTU's/hr or about 4.3 gallons of propane/hr. Now we know how much propane you are using per hour, all you need to do is multiply that by the cost per gallon you paid for it

 

[ps0303]

Turn it on the day before to make sure it works! LOL

 

[Sunbaby]

Part of the difference in size is due to my giant tanning ledge. It takes up an entire end. On paper it was 12' X 7' but my PB made it curvy to flow better. I could probably get a better estimation if I started the pool length at the end of it. I'll try to measure and get a more accurate measurement.

My PB said it was probably 14000 so I started there when working my chemicals. I kept adjusting till I got to 8000 - it works here well.

Thanks again for the input.

 

[pooldv]

Aha, the sun shelf! Well if Poolmath says it is 8k and various chemical additions support it then you can cut my number just about in half. That is a great way to figure your true pool gallons too.

 

[borjis]

curious in the formula where is the 8.2 & 8.3 derived from?

 

[danpik]

curious in the formula where is the 8.2 & 8.3 derived from?

That is the universally accepted weight of water (8.3). When dealing with BTU's gallons are only part of the formula. A BTU measures the temp rise in a pound of water... Energy it takes to raise 1 lb of water 1 deg F

 

[pooldv]

Water weighs less in AZ because it is so HOT!

 

[jblizzle]

I pulled 8.2 off the top of my head it was close enough.

 

[danpik]

I pulled 8.2 off the top of my head it was close enough.

when you consider all of the other variables that can affect how a pool heats, 0.1 lb of water is pretty insignificant

 

[duraleigh]

Sorry, a bit of a hi-jack here,

My PB said it was probably 14000 so I started there when working my chemicals. I kept adjusting till I got to 8000 - it works here well.

That would worry me to death. It's a huge discrepancy that he would miss it that far.

 

[Sunbaby]

That was in the planning stage. We changed quite a bit once we saw the layout

 

[rphpool]

My PB est ours would be 18k. When we filled pool we took a water meter reading at start and went about our day. Before you know it , it was full. I went and recorded final water meter reading, did a littele math and it was 8500 gallons !!!! We to have a big sun shelve as well as seating ledge around half the pool. We go from 3.5ft - 5 ft.

 

[borjis]

Had this as well, pool was documented at 16,500 but 13,000 in actuality.

thanks for clarifying the formula, this year will by my first with a gas heated pool.

I had solar at the last place only and it worked very well, so I'll probably ditch the heater when it's
service life ends.