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

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

    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!
    660k Commercial Plaster Indoor Pool

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

    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.
    Dave S.
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    Re: Anyone have a guess as to "normal" evaporation on an indoor pool?

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

    A bucket test in the pool still wouldn't rule out leaks in the pipes between the surge tank and returns.
    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.
    Yes. That's too much for an indoor pool, I believe
    Dave S.
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    Re: Anyone have a guess as to "normal" evaporation on an indoor pool?

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

    About .5" per day. I know the gallons is around 5000, and that would calculate out to around .5" although I'm not sure how accurate that is.
    660k Commercial Plaster Indoor Pool

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

    Try marking the level to get an accurate measurement. Are you familiar with a bucket test?
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    Re: Anyone have a guess as to "normal" evaporation on an indoor pool?

    You can calculate evaporation loss explicitly:

    Evaporation from Water Surfaces

    What is the air temperature?
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
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    Re: Anyone have a guess as to "normal" evaporation on an indoor pool?

    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.
    660k Commercial Plaster Indoor Pool

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

    Quote Originally Posted by mas985 View Post
    That makes my head hurt. How do you calculate the increased surface area in the case of waves?
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    Re: Anyone have a guess as to "normal" evaporation on an indoor pool?

    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.
    Mark
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    Re: Anyone have a guess as to "normal" evaporation on an indoor pool?

    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.

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

    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...
    660k Commercial Plaster Indoor Pool

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

    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...
    660k Commercial Plaster Indoor Pool

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

    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.
    Dave S.
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    Re: Anyone have a guess as to "normal" evaporation on an indoor pool?

    Quote Originally Posted by duraleigh View Post
    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.
    +1 (head still hurts)
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    Re: Anyone have a guess as to "normal" evaporation on an indoor pool?

    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...onEquation.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.
    Mark
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    Re: Anyone have a guess as to "normal" evaporation on an indoor pool?

    Quote Originally Posted by mas985 View Post
    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...onEquation.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.
    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.
    660k Commercial Plaster Indoor Pool

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    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.
    Mark
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    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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

    I still think a bucket test would definitively tell you weather or not you had a leak compared to an evaporation loss, which would be managements problem at that point.
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