Anyone have a guess as to "normal" evaporation on an indoor pool?

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ringfinger

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Apr 23, 2012
23
I manage a full size 50 meter indoor pool, surface area of 1,250 square meters. We keep the water pretty warm(83-85 degrees) and the ventilation system is quite large and efficient at removing humidity. The HVAC guys tell me humidity is maintained at around 47%. I think our water usage is within the normal range of what we might expect, but those numbers have raised some eyebrows further up the chain from me.

Most of the calculators/estimations I've seen are specialized for outdoor pools facing the wind and sun. Either that or the formulas look like something I'd need to hire an engineer to help me map out. What would you guess is a normal range of gallons used per day/month/year for this pool? I'm just looking for a ballpark figure, like, what numbers would surprise you?

I plan to do a bucket test for leaks, but I'll have to wait til the summer to close down the pool. I imagine with a small bucket and a large pool I'd want a week of testing to be accurate, right?

Thanks!
 
Really hard to speculate because we typically think in terms of how much "depth" of the pool has been lost.

I assume the pool is too active to do the bucket test but could you not do it at night? The results should be deadly accurate and would tell you right away (the first night) what you were losing.
 
I just assumed that with a pool this size a longer test would improve the accuracy. Pumps would need to be off, but I could do a Sunday test when the building is closed.

I'm still interested in any guesses though, for indoor pools in particular. A bucket test in the pool still wouldn't rule out leaks in the pipes between the surge tank and returns. There are lots of 10" pipes underground.

Edit: Would 5,000 gallons per day surprise you? .5" off an olympic pool is 4,100 gallons. Maybe our ventilation system, room temperature, or something isn't quite set right. I just don't want to throw around the L(leak) word yet...
 
A bucket test in the pool still wouldn't rule out leaks in the pipes between the surge tank and returns.
Click to expand...
No, it does not. But if your pool has a leak, it will tell you that and it will tell you overnight.

A guess for evaporation?? An outdoor pool typically loses about 1/4" in depth so it is safe to say you would be less than that. 1/8 inch would seem normal to me. The bucket test will tell you the evaporative loss and also detect any leak you may have.

Do you need a review of how to perform the test?
Edit: Would 5,000 gallons per day surprise you? .5" off an olympic pool is 4,100 gallons.
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Yes. That's too much for an indoor pool, I believe
 
How much loss are you getting now, in inches?

When i read your topic line i thought, you should probably be getting close to no loss. But since you are de-humidifying the room, You have an on going battle. The de-hum is trying it's best to dry out the air, and at the same time, the pool is trying to re-humid the room, especially when there are people in the pool increasing the surface area in the pool with all the waves, not to mention the air temp. That helps with evaporation.

You could be loosing more water (from de-hume compensation) than if the pool was outdoors.
 
Water is 84, room is 76, and nighttime air is down to 60. Just heard back from our HVAC guy on those air temp figures. Humidity ranges from 46% to 52%.

I am familiar with the bucket test and I'll be doing one the next time we get a couple days where the pool is closed.

Yeah, the engineering toolbox formulas are a bit over my head. :mad:
 

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You mean with people in the pool? Generally, that is modeled as an activity factor and it is dependent on the number of people and how rowdy they are. The activity factor will usually range between 1 and 2 so if you want to bound the problem, you just calculate the still water rate for the lower bound and double it for the upper bound.
 
One indicator would be the chemistry. If it's evaporation, chemicals in the fill water would tend to concentrate and increase in the pool. If water loss was from leaking, chemicals in the pool water would tend to fall.

For example, if you had 140 ppm calcium in the fill water, the pool would tend to go down towards that number if the water loss was from a leak. However, if the water loss was from evaporation, the calcium level in the pool would tend to rise away from that number.

Of course, it could be both.
 
Okay, I've put my state college business degree to work and tried my hand at the engineering toolbox formulas.

Daytime
Water: 84 F
Humidity: 50%
Air: 76 F
Air Velocity: .25 (estimated, no idea)

150 gallons per hour evaporated

Nighttime
Water: 84 F
Humidity: 50%
Air: 60 F
Air Velocity: .25 (estimated, no idea)

191 gallons per hour evaporated

We're open for half the day, so I can see 1,800 gallons evaporating during the daytime, and 2,292 evaporating during the half of the day we're closed and room drops to 60 F. That alone, in an unused steady pool, would account for 4,092 gallons per day. Swimmers and splashing could bring that up to the 5,000 gallons per day figure we're seeing on our auto-fill meter.

Any kind soul want to check my work? Please...
 
It appears that our warm water at 84 F, and nighttime room air temperature drop down to 60 F are major contributors to evaporation. Dropping the pool temperature and raising the room temperature could cut those numbers in half. I think...
 
Nice work on the math, ringfinger. It's over my senior citizen capabilities.

I still think that's too much water loss in your pool and I believe the bucket test will show that. I am very interested in your results. Good post and good thread.
 
I agree, I think it is a little too high. I get evaporation rates that are about half that (2500 g/day) from this formulation:

RSPEC - Pool Evaporation Studies
http://www.rlmartin.com/rspec/whatis/PoolEvaporationEquation.pdf

Both are empirical formulations but this one, I have calibrated with my own pool. The reason I gave you the engineeringtoolbox version was it had a web calculator and I had assumed they would give the same answer but it is clear that they give different answers.
 
mas985 said:
I agree, I think it is a little too high. I get evaporation rates that are about half that (2500 g/day) from this formulation:

RSPEC - Pool Evaporation Studies
http://www.rlmartin.com/rspec/whatis/PoolEvaporationEquation.pdf

Both are empirical formulations but this one, I have calibrated with my own pool. The reason I gave you the engineeringtoolbox version was it had a web calculator and I had assumed they would give the same answer but it is clear that they give different answers.
Click to expand...

Well that throws a wrench in things. :(

I just got done speaking with my AFO instructor from a few years back. He suggested that 3/8" evaporation could be normal daily loss on an indoor pool, which would be about 3,151 gallons on a pool my size. He wasn't at all surprised by the 5,000 gallon figure considered the massive difference between our water and air temperature in the room. He suggested the room air and water never be further than 4 degrees from each other.

Also, we don't have a pool cover.
 
Re: Anyone have a guess as to "normal" evaporation on an indoor pool?

It is really air temp with humidity that matters (i.e dew point temp). Even when the temps are the same, you still have evaporation when humidity is less than 100%. So you can also reduce evaporation with higher humidty target.
 

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