The Physics of Freezing and Freeze Protection

mas985

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May 3, 2007
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Pleasanton, CA
Recently, there have been a lot of questions from those living in moderate climates about using automatic freeze protection or running pumps to prevent freezing and what a pool owner should do when temperatures drop so I thought it might be useful to have a thread which dealt with the issue from both a practical and theoretical point of view.

But first a few thoughts about using pumps to prevent freezing. Running the pumps is not fool proof and power outages can obviously be a problem. Also, some pools have a lot of features and some are manually controlled so automatic freeze protection may not always be a viable solution. Plus electricity is expensive in many areas so having the pumps run all night is not always an attractive option. Given all these factors, I am not a big fan of using the pumps to prevent freezing, but there are alternatives for those that are interested.

Also, I wondered how long it would actually take for a pipe to freeze so I did some research into the physics of freezing pipes. Over the years, there have been several empirical university studies and the consensus is that for home plumbing in un-insulated spaces (e.g. attics), the "alert" temperature where pipe freezing and bursting can become a problem is below 20F. Above that temperature, freezing rarely occurs and bursting of the pipe is even rarer. But this is for small copper pipe in attics, not pool equipment so I had to look deeper.

The time it takes a pipe to freeze is dependent on several factors:

Size and dimensions of the pipe: More water in the pipe takes longer to freeze.
Pipe thermal conductivity: Copper is about 2000x more thermally conductive than is PVC.

But then there are also environmental conditions:

Air Temperature: The lower the air temperature is below freezing the faster freezing will occur.
Night Sky Exposure: Radiation losses can have a large impact on the heat loss.
Wind Speed: Wind blowing over the pipe greatly increases the heat loss of the pipe.
Location: Proximity to heat sources (e.g. side of house).

But a freezing pipe does not always result a bursting pipe. In fact, it rarely does. When a pipe freezes it freezes from the outside inwards and as the ice expands, the excess pressure is relieved through the center of the pipe until the pipe freezes solid. This alone will not burst the pipe. However, once the pipe is fully frozen and blocks the relief of pressure, the freezing ice up stream of that section of pipe may not have a path to relieve the pressure and only then does the pipe burst. This process is fairly well described in the following sources:

http://www.spokanecounty.org/data/buildingandplanning/disaster/DIS-FreezeBurstPipe.pdf
http://www.iccsafe.org/cs/PMG/Documents/DIS-FreezeBurstPipe.pdf
http://scifun.chem.wisc.edu/radio/pipefrez.html

So in order to take all these factors into account, I used standard thermal transfer equations to estimate the heat loss of a pipe when exposed to all these environmental conditions. I then ran a few experiments in my freezer to make sure the model was giving fairly accurate results, which it did.

So here are a few examples:



These results assume a minimum night time temperature of 20F, starting at 32F and the time to freeze only includes the time below 32F. So the average temperature is 26F over that time frame. There is some inherent margin included in these numbers because I assumed the water temperature started out at 32F which in most cases it is higher and it ignores the latent heat within the filter water which can act like a heat source for the rest of the plumbing.

Wind has a fairly large affect but visibility to the night sky has an even larger effect. Covering the exposed equipment can reduce the heat loss considerably from both of these factors and freeze times are much much longer. So the easiest thing a pool owner can do is cover the pad equipment and plumbing with a tarp. Adding a heat source underneath the tarp or cover would add enough margin to eliminate the need for running the pump or at least provide enough backup should the automated freeze protection fail.


The heat transfer model is available here for those who are adventurous:

Hydraulics 101 - Have you lost your head?


So if you would like to be able to go without automated freeze protection OR you would just like to have an extra layer of protection, then you can follow some of the suggestions listed below. The temperature ranges and time are using the thermal loss spreadsheet above which are fairly conservative freeze estimates (i.e. assumes 15 mph winds, full sky exposure with maximum radiation losses and ignores latent heat in filters and surrounding structures).

1. Air Temp 32F->25F->32F for <10 hours. Plumbing 1" and above is safe. Tarp or drain for smaller pipes.

2. Air Temp 32F->20F->32F for <10 hours. Plumbing 1.5" and above is safe. Tarp or drain for smaller pipes.

3. Air Temp 32F->15F->32F for <10 hours. Plumbing 2" and above is safe. Tarp or drain for smaller pipes.

4. Air Temp 32F->10F->32F for <10 hours. Plumbing 2.5" and above is safe. Tarp or drain for smaller pipes.

5. Air Temp 32F->0F->32F for <10 hours. Tarp equipment and use an incandescent light bulb underneath OR use heat tape available at your local hardware store.

6. Air Temp <0F. Drain all equipment and plumbing.

Note that when air temps get close to 0F, water temps are likely to be below 40F so there is no harm in draining the equipment and letting the pool go idle since algae is fairly rare at those water temperatures. Also, opening all valves so that all ports have pressure relief to the pool will virtually eliminate any chance of the pipes bursting.

But just so you know, I do practice what I preach. My situation is #3 above and I have not bothered to use automated freeze protection for 8 winters now without incident. But I do not have any plumbing less than 2".
 
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JasonLion

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May 7, 2007
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Some other factors to consider:

1) Weather reports may not be accurate. A weather report predicting lows around 25 might turn out to be wrong and temperatures might actually go below 20. This isn't common, but it certainly can happen.

2) The largest risk is when day time high temperatures do not go above freezing. This allows ice to accumulate over several days, and greatly increases the odds of pipe damage.

3) I favor "defense in depth" approaches, having several different things that can each separately prevent any damage. Plumbing damage is unusual, but it can be very expensive to fix (imagine you need to tear up the deck to get to the broken pipe). It is often worth some extra effort to eliminate even small risks because of the risk of relatively high costs to repair.

4) If it gets cold enough, running the pump won't be sufficient. It has to get seriously cold for this to be an issue, but given a deep enough cold snap that lasts several days simply running the pump won't always be enough.
 

chem geek

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Mar 28, 2007
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San Rafael, CA USA
The time it takes a pipe to freeze is dependent on several factors:

Size and dimensions of the pipe: More water in the pipe takes longer to freeze.
Pipe thermal conductivity: Copper is about 2000x more thermally conductive than is PVC. Although the difference is large, this actually has a fairly small affect on heat transfer.
Mark,

I earlier wrote this post where I did a very rough and simple (conservative) calculation for moving water taking a 1" length volume of water moving through the pipe and calculating what temperature would be needed with ideal heat transfer given the thermal conductivity. This would be worst case assuming plenty of wind. So how can it be that with copper being so much more thermally conductive and also being thinner than the PVC has a small affect on heat transfer? I did finally figure out that once a thin layer of ice forms then it becomes the limiting factor since it's thermal conductivity is 2.18 W/(m•K) compared to 401 for copper (PVC is 0.19), but while the copper can pretty much be ignored since the ice is limiting, the PVC should be noticeably more limiting in heat transfer than the ice or copper. So while with bare copper pipe with near freezing water only needs a slightly lower outside temperature to start forming ice at the surface of the pipe, this becomes self-limiting very quickly and the water moving through the pipe prevents it freezing solid for any reasonable temperature difference (unless the pipe run is extremely long).

I just find it hard to believe that under extreme conditions with wind that even with still water that the insulating effect of PVC has virtually no effect. With still water in a copper pipe, shouldn't it freeze nearly as fast as ice cubes in a freezer? Or is the point that the main limiting factor in time is the slowness of freezing, not so much the limit of heat transfer at least for copper, ice, or PVC (insulation on copper pipe might be different since that is 6 times more insulating than PVC comparing typical thickness for 2" PVC pipe vs. 3/8" polyurethane)?

Richard
 

JamesW

TFP Expert
Mar 2, 2011
20,300
Pipe heating tape/cable can be helpful assuming no power loss. Having a backup generator can be helpful if power is lost. Having a plan to drain in case of power loss is a good idea.
 

mas985

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May 3, 2007
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Pleasanton, CA
Those test results -- that is without the water moving through the pipes? This means your pipes wont freeze nearly within that time frame if the pump is running, correct?
Yes, without water running and yes, time to freeze can be much longer with the pump running but only for the plumbing that has water running through it. The intent of the table was to show how long to freeze if you cannot run the pump or do not want to run the pump.



chem geek said:
So how can it be that with copper being so much more thermally conductive and also being thinner than the PVC has a small affect on heat transfer?
The ice does help slow the heat transfer (~4x) but there is another factor. Take the un-insulated attic example. The primary heat transfer mechanism will be convection to the air and radiation to the roof above and those have much slower heat transfer characteristics than the pipe conductivity. So it doesn't matter if the pipe surface can conduct heat that quickly, convection and radiation will dictate the total heater transfer rate out of the pipe. This basically means that the temperature on each side of the pipe boundary will be about the same. Lower temperature delta for copper and a little higher for PVC.
 

chem geek

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Mar 28, 2007
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San Rafael, CA USA
OK, so if exposed outside to wind then we can expect to see the differences manifest themselves more, in particular where a bare copper pipe would freeze faster than PVC which would freeze faster than the copper pipe having polyurethane insulation. That's pretty much what I was calculating in the other post -- worst case assuming theoretical heat transfer rates.

Hence the advice of covering which cuts down the wind factor (convection) and provides a modicum of insulation and limits radiation.
 

mas985

TFP Expert
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May 3, 2007
13,701
Pleasanton, CA
Exactly! When covered, freeze time would be about 5% longer for PVC vs Copper but in a 15 mph wind with full sky exposure, PVC would take about 50% longer to freeze than copper. This is why pipe wrap works well for copper pipes but probably wouldn't help much for a PVC pipe unless it was a foam type wrap.

BTW, I had to update the table. I inadvertently used 1/2" numbers instead of 3/4". It should read correctly now.

I would also add heat tape as an effective method of freeze protection. It is probably more effective than running the pump (except maybe water features), works at much lower temps and it costs a whole lot less to run.

http://www.homedepot.com/p/Frost-King-18-ft-Automatic-Electric-Heat-Cable-Kit-HC18A/100196820
 

Leebo

Admin
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Jul 21, 2011
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Eastern Ohio
With cold weather hitting parts of the country not used to colder weather I once again have been seeing extra questions regarding freezing pipes and equipment. I’d like to bump up this thread to help ease the minds of some of our members who are facing a colder than normal Christmas.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,209
Tucson, AZ
We were down to 25F in the early morning hours here. EasyTouch freeze protect has been running since about 2AM. Air temps are still slightly below freezing (30F) but I'm more worried about my orange and lime trees than my pool (the orange has about 18 beautiful navels on it that need just a little bit more time to sweeten up). I put out my contractor lamp with a 60W bulb shining up into the tree but my plankets were all torn to shreds. I ordered some new AgFabric covers via Same-Day delivery on Amazon and hope to get my trees covered up tonight.

Oh yeah, pool plumbing is fine....
 

Hammond

Bronze Supporter
Apr 23, 2017
62
Dallas, TX
I have a freeze setting to turn on the main pump when temp,gets below freezing, but do I need to also turn on the Polaris booster pump? It's going to get really cold tonight in North Texas -supposedly down to 17
 

JoyfulNoise

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May 23, 2015
17,209
Tucson, AZ
I have a freeze setting to turn on the main pump when temp,gets below freezing, but do I need to also turn on the Polaris booster pump? It's going to get really cold tonight in North Texas -supposedly down to 17
Hmmm...hard to say but if it's got water in it, I'd be concerned about freeze damage. I typically winterize my water feature lines and drain my waterfall pump so I don't have to run it on freeze protect (it's a 3HP energy hog). I switch over to manual vacuuming during the winter since I don't get much debris in the pool. You might consider winterizing your polaris line if that's at all possible otherwise you have to run it on freeze protect and that just a big energy waste (in my opinion).
 

JamesW

TFP Expert
Mar 2, 2011
20,300
The Polaris should have water flowing through it when the main pump is on.

Check for flow at the Polaris wall fitting. As long as there is good flow, no need to run the booster.
 

mdogan

Bronze Supporter
Nov 20, 2017
96
Frisco,TX
I never used heater any before. At this moment TEMP is very low in Frisco,TX. Around 23 fells like 9...
is it too late to run heater? it is always in standby but; I mean due to very cold weather something could be freeze already inside ?
Instead of having stress of freezing planning to keep heating at last around 35...
Please advise and HAPPY NEW YEAR ALL OF YOU
 

Dealland

Member
Aug 22, 2017
12
The woodlands tx
First winter pool owner here in Houston. Outside temp is 31 F currently. I wanted to try the spa, but it took a long time to get it up above 70F. I noticed that in Spa mode the delta P on the filter was higher than in pool mode. I am thinking I am 1) stupid to try and heat the spa up 2) overdue for a filter cleaning....or both. Thanks for the help in advance. Checked chems yesterday and all is well on chemistry.

also planning to run the pool pumps overnight.
 

mknauss

Mod Squad
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May 3, 2014
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Laughlin, NV
Normally your pump will be running at a higher RPM in Spa mode versus Pool mode. That would explain the higher pressure.
 

Swampwoman

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Apr 27, 2012
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Grand Rapids, MI
I didn't want to start a new thread for this but if mods are so inclined, feel free to move it ;)

I just wanna share with folks unaccustomed to the imminent deep freeze and high winds headed for the coast that warming lights and a tented tarp or the little greenhouse I recently bought might help reduce the wind effect at night and gather heat if there are sunny breaks during the day.

Here is a link to the simple greenhouse I put on my outside air warmer heat exchanger:
https://www.amazon.com/gp/product/B000Q5VF8K/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1

They come in different and larger sizes.

In my case, I got this one because it had a screen, which I need to pull air into the dome. The warm water flowing through it still keeps it warmer inside than out.

But if I were trying to protect an outdoor equipment pad, I might use something like this with heat tape wired in a closed box and/or warming lights.

Just a thought I wanted to share, seeing the forecast and so many posts. Ymmv.

When flow got too low (my fault) last month on a -10 night the predecessor of this exchanger froze up and was ruined. Hence my experiments ;)
 

PoolguyinCT

In The Industry
Jul 21, 2014
3,077
Connecticut
Joyful - I’m looking forward to a high temp in the 20s, hopefully next week..
Anyway,
- sounds like many members are experiencing New England November weather.. freezing ambient temps, when we get early “flash” freezes of a few days &scheduling doesn’t permit a full Winterization , I drain equipment, open all valves & leave pool at normal operating level.

The water in risers drops below to pool level & ground insulates the piping branches. They we return for the whole Shabang.