Draining plaster pool in summer heat

[palmspriings]

My pool is in desperate need of draining, however, it's 100 degrees daily and will only get hotter. Researching this site, I came upon a few posts that recommended draining and refilling simultaneously to ward off potential plaster cracking. There are two schools of thought:

The pool water is warmer than the tap water, therefore pump out from the top, refill from the bottom

The pool water is heavier than the tap water, therefore pump out from the bottom, refill from the top

Does anyone have first hand experience with this?
Thanks!!!

CH 1200
TA 110
pH 7.5
TC 11
CC 0
FC 11
CYA 120

 

[jblizzle]

I think you got that backward.
If the tap water is colder than the pool (which is likely), then you should fill on the bottom and remove from the top.

There are certainly a few members who have done this. And you definitely need to replace a lot of water to get that CYA and CH down.

 

[JVTrain]

My position is all based on temperature. Cold water will tend to stay on the bottom, warmer water on top. The cold tap water will stay on the bottom as long as there is no intentional circulation. Drain the water off the top will remove the warmer pool water without mixing. Because physics!

However, water with dissolved solids is denser than pure water. Pool water will have more dissolved solids than tap water. The pool water will be more dense. Because physics!

You can make either argument but from what I've read here and other experiences, the temperature is a MUCH greater factor for short term gradients.

 

[JasonLion]

If you do the drain/refill with the pump either off or pumping to waste, so the pool water does not get throughly mixed, this approach can be quite efficient. If the water does get throughly mixed it still works, but you are wasting some water compared to a straight drain/refill.

 

[SwimAustin]

From what I can see, there's no easy rule of thumb. Water at 80 degrees with 2000 ppm TDS has about the same density as water at 70 degrees with 0 ppm TDS. Those are conditions somewhat near the poster's water.

I used this calculator to generate a plot.

Get the TDS and temperate for both fill water and pool water and plug them into the above calculator and see which is heavier. It could go either way. If I had to bet, I'd bet your fill water is heavier, but it is worth taking the time to check.

 

[palmspriings]

Thanks for the responses. That's a great calculator SwimAustin.......looks like the density is negligible, so I will drain from the top and fill to the bottom.
Thanks Again!

 

[duraleigh]

Well, if the your fill water had 0 tds, that might make sense but I think you will find using the temp differential alone will be effective.

If you can't decide, simply drain and refill in 1/3 to 1/2 increments.