Heater Outlet Temperature?

[HeyEng]

Well, today is the day when I fired up the heater on our "new to me" pool to see if it works...and well...I think it does.

Pool temperature is currently 58 degrees. I am not looking to get it in a "we can swim now" range but more of a good ops checkout to see if there are any issues. Anyway, I pulled off what panels I could and found that everything looked normal. The previous owner said that she hadn't used in a "quite a while" because of the expense...so I was expecting the worst.

Anyway...it appears to be heating and there is heat exhausting from the heater but it seems like the water temperature at the returns is still freaking really, REALLY cold! Granted it's only been running for an hour, but I would think that a 400,000 BTU heater would have a bigger effect. Maybe I am missing something in the "science" of heating the water; so, is this normal?

 

[JamesW]

Assuming an 84% efficiency, a 400,000 btu/hr heater will deliver 336,000 btu to the water in 1 hour.

At 40 gpm, the total amount of water going through the heater in 1 hour is 2,400 gallons or 20,016 lbs.

That will create a temperature rise from inlet to outlet of 16.8 degrees Fahrenheit.

Gpm.....rise....outlet
40........16.8......74.8
60........12.6......70.6
80..........8.4......66.4

 

[HeyEng]

Assuming an 84% efficiency, a 400,000 btu/hr heater will deliver 336,000 btu to the water in 1 hour.

At 40 gpm, the total amount of water going through the heater in 1 hour is 2,400 gallons or 20,016 lbs.

That will create a temperature rise from inlet to outlet of 16.8 degrees Fahrenheit.

Gpm.....rise....outlet
40........16.8......74.8
60........12.6......70.6
80..........8.4......66.4

That definitely puts the science in it! Thanks for the input. I don't know my flow rate, but it seems to be pretty high. I guess I will let it run for a few hours and see what it does.

The display has "heat" "pool" and "stand-by" all illuminated but I would think that "stand-by" would be extinguished, but the Hayward manuals don't really speak to this. It's running through automation and there are no error codes, so I am guessing this is ops normal.

 

[JamesW]

For your pump on high, I would expect at least 60 gpm. So, the temperature rise probably will be in the 10 to 12 degree range, which won't necessarily be obvious.

You should get about 2.9 degrees temperature rise per hour for the general pool water.

 

[HeyEng]

For your pump on high, I would expect at least 60 gpm. So, the temperature rise probably will be in the 10 to 12 degree range, which won't necessarily be obvious.

You should get about 2.9 degrees temperature rise per hour for the general pool water.

Thanks so much for the info...it's greatly appreciated. At 2.9 degrees that would have me at 80 degrees in about 8 hours! So...if it will actually heat this fast, then I think I might give it a REAL trial run and be taking a dip around 8'ish tonight!!!

 

[JamesW]

Note that as the water temperature increases, the pool will begin to lose heat due to evaporation, radiation, conduction and convection.

So, the 2.9 degree rise per hour will only be accurate during the beginning and will slow down some as the water temperature rises. A cover helps quite a bit.

 

[HeyEng]

Note that as the water temperature increases, the pool will begin to lose heat due to evaporation, radiation, conduction and convection.

So, the 2.9 degree rise per hour will only be accurate during the beginning and will slow down some as the water temperature rises. A cover helps quite a bit.

Ah...trying to spoil my fun, eh? Well, it's supposed to rain tonight anyway, so if I can't get it quite warm enough for a test run, then I guess that's OK.

 

[ajw22]

In the initial heat up of a cold pool you lose a lot of heat to conduction as you warm the shell and surrounding ground. Once you have it up to desired temperature then the ground helps as a heat sink. So initiall warm up takes more time then you expect.

 

[HeyEng]

In the initial heat up of a cold pool you lose a lot of heat to conduction as you warm the shell and surrounding ground. Once you have it up to desired temperature then the ground helps as a heat sink. So initiall warm up takes more time then you expect.

Our current 24 inch soil temp is 50 degrees, so I can definitely see how this would slow the heating. Since I am farting around in the yard today, I am keeping a log of temperature increases each hour. It's really just for entertainment since there are so many variables to heating/cooling of the pool water.

 

[HeyEng]

@JamesW Your math was right on. After 12 hours of heating, it climbed to 85 degrees...which gave me an average of 2.8 degrees per hour. The air temp got down to 50 last night and we had a decent amount of rain this morning (which made the pool steam as much as a hot tub!) so I'm sure it's not that warm right now. Our utilities are pretty cheap here and I figure it cost about $22 to heat it yesterday but the few hours swimming last night was very much worth the money!

Edit: Final pool temp was 85, and in the 12 hours since the heater was turned off and half an inch of rain (and a low temp of 50), the pool is now down to 74....so no swimming today!

 

[JamesW]

Excellent. Glad that it worked out well for you.

Thanks for the update.

 

[sktn77a]

@JamesW Your math was right on. After 12 hours of heating, it climbed to 85 degrees...which gave me an average of 2.8 degrees per hour.

Interesting. We have a 15,000 gal pool and a 250,000 BTU heater and get about 1 degree per hour. A little less than yours. Maybe our 8 year old Jandy Lite 2 heater isn't as efficient as yours.

 

[HeyEng]

Interesting. We have a 15,000 gal pool and a 250,000 BTU heater and get about 1 degree per hour. A little less than yours. Maybe our 8 year old Jandy Lite 2 heater isn't as efficient as yours.

Well, we did have a lot of sun yesterday and it was shining on the pool for a good 6-7 hours and the ambient high temp was 73, so I know that helped it some. Also, the soil temps aren't too cool, so that probably helped a little bit. I initially figured about 1.5 degrees per hour...but was delighted with the almost 3 per hour.