Consequences of increasing water temp on community pool

[dankasprick83]

Hey all,

I spend most of my time on the spa side of the forum but am putting together a proposal to add a new heater to our 60000 gallon gunite community pool in southeast Michigan. Our current heater can only get it to 82 and we are looking for more like 85 or so.. Those in opposition are going to take any angle they can to block the proposal and one thing that's come up is increased consumption of sanitizer etc. Can anyone speak to any estimate of what kind of increase in consumption we are looking at with an additional bump of only 4 degrees or so? I understand algae and bacteria can proliferate more rapidly in warmer water but wouldn't expect a drastic change with a 4 degree bump.

We have a commercial pool company that takes care of the pool daily but I've been less than impressed with their answers about other issues and would love to have some background before speaking to them more on Monday.

As far as I know the commercial pool requirements for FC don't change when jumping from 82 to 85.

 

[ajw22]

What sanitizer is used in the pool?

Free Chlorine - Further Reading

www.troublefreepool.com

Does Chlorine Consumption Change with Water Temperature?​

Warmer water losing more FC due to sunlight than colder water is not true.

Chlorine loss from the UV in sunlight does not depend on temperature because it only depends on the number of photons per area entering into the pool and on the concentration of chlorine. The photons of light are traveling much faster (at the speed of light) than the molecules containing chlorine so the temperature which relates to the speed of those chlorine molecules is irrelevant. From the point of view of the photons, the molecules of chlorine are essentially standing still so the cross-section of those molecules which is the area with which the photon has a quantum probability of reacting with the molecule is independent of the temperature and only related to the concentration of such molecules in the water.

Chlorine consumption that is related to temperature is for chemical reactions with chlorine such as oxidizing pool covers, bather waste, pollen, leaves, algae, etc. And yes, algae grows faster in warmer water but such consumption won't matter if there is sufficient FC/CYA since algae will get killed before it can reproduce so the rate will be based solely on the rate of blown-in algae spores which is usually not measurable (pollen, on the other hand, can be voluminous as can pods and other material dropped from trees).

A pool with a solar cover that is opaque to UV would lower the loss of chlorine from sunlight that is not temperature dependent but would increase the loss of chlorine from oxidizing the cover which is temperature dependent.

There can be a temperature dependence on the subsequent chemical reactions that occur after the photochemical reaction occurs.[5] But for the breakdown of chlorine by UV, it's pretty much all over with such breakdown since the probability of having the OH• and Cl• reform compared to other reactions that lead forming chloride and oxygen gas is low and not temperature dependent. At most, the intermediate concentrations of some intermediate species such as hydrogen peroxide may be higher at lower temperatures, but by that time it's too late and the chlorine is already on it's way to becoming chloride.

There can be a small dependence on temperature in terms of the rate of relaxation from excited states so there is a small but negligible temperature dependence on having the HO-Cl vibrational state be excited and not break apart as often because of low temperature, but it takes a very low temperature before that effect would be seen in practice.

 

[dankasprick83]

Chlorine is the sanitizer in the pool.

 

[JamesW]

The extra cost of heating will far outweigh any extra chlorine use.

Chlorine consumption by sunlight is mostly due to the frequency/wavelength and intensity of the UV light that hits the hypochlorite ion.

The UV index X Time is the most indicative of expected chlorine loss.

The primary reaction is this.

2OCl^- + UV --> O₂ + 2Cl^-

The chlorine takes 2 electrons from the oxygen.

The chlorine is always pulling on the electrons, but the oxygen does not want to let them go.

As a photon of UV hits the electron, the electron gets farther from the oxygen atom's nucleus and this allows the chlorine to take the electron, thereby oxidizing the oxygen.

Warmer water can increase the reaction, but I am not sure exactly how much effect this has.

 

[JamesW]

Exposure to UV and temperature increase the chemical reaction rate of chlorine oxidizing the oxygen atom.

Below is for bleach, but the general principle applies.

https://castlechem.com.au/sds/Sodium-Hypochlorite-Stability.pdf

 

[mgtfp]

You can estimate the temperature dependant chlorine use by chlorine oxidation processes like that:

Run an Overnight Chlorine Loss Test. This will tell you how much chlorine your pool uses without exposure to light over the time of your OCLT. Let's say your OCLT was over a period of 8 hours. Multiply the FC loss over 8 hours by three to get the UV-independant loss over a 24 hour period.

You can roughly say that the rate of these temperature dependant chemical reactions doubles every 10 degrees. So a 4 degree increase will speed up the reaction rate by a factor of 2^4/10=1.32.

Let's say your overnight loss over 8 hours at 82°F was 0.5ppm. That gives a UV-independant loss over 24h of 1.5ppm. At 86°F this will scale up by a factor of 1.32 to about 2.0ppm.

This increase should pretty much vanish in losses to UV and bather waste, which should not change much.

As James said, the main cost increase will come from the extra cost for heating. But even that could actually get at least partially compensated by a new heater hopefully being more efficient than an old one.