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Old 01-17-2021, 10:25 AM   #1
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Water Heaters and Thermal Expansion

There's been past discussions on how thermal expansion affects water pressure in an R/V.

When the water in a water heater gets hot, it expands, it then has to go somewhere. Typically, water heaters have a small air pocket at the top of the tank that compensates for this expansion. Also, many times folks add an aftermarket expansion tank to smooth out the pump surge but it also aids in thermal expansion.

On longer term stays (for me, usually about a week or 10 days) the air pocket in the water heater depletes, just from use. I can usually tell when it is about depleted as the first use of the toilet in the morning after the plumbing system has not been opened all night requires extra effort to push the pedal, signaling higher pressure.

There's been considerable discussion as to what that pressure will climb to and there have been references to Suburban saying it could reach 150 PSI.

I decided to do my own study over about a two week period while here in FL. I have a water pressure guage (used to check campground pressures on arrival) that I put on my hot, low point drain line, left it there and monitored pressures throughout.

Here are the results... (note: I'm not using an inline pressure regulator)
Nominal static water pressure: 52 PSI.

Water pressure during water use: 40 PSI.

Water pressuse WITH air pocket in water heater, after sitting for 8 hours of non use: 58 PSI. (11PM to 7 AM)

Water pressure with air pocket in water heater depleted, after sitting for 8 hours of non use 92 PSI. (11 PM to 7 AM)

Definitely an increase and at 92 PSI, it shows why poorly installed/poorly crimped/loose fittings can and do leak.

I've yet to see the 150 PSI figures but I'm going to leave my gauge on and see if anything changes.Click image for larger version

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Old 01-17-2021, 11:12 AM   #2
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Wow ,,, you must be really bored. hot water pressure in toilet? if you open taps periodicaly as hot water heater tank fills pressure is released . same on cold side during water heater fill up. there is always air in lines after draining lines. air has to go somewhere so just open each faucet to let air escape.
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Old 01-17-2021, 11:41 AM   #3
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Originally Posted by 5picker View Post
There's been past discussions on how thermal expansion affects water pressure in an R/V.

When the water in a water heater gets hot, it expands, it then has to go somewhere. Typically, water heaters have a small air pocket at the top of the tank that compensates for this expansion. Also, many times folks add an aftermarket expansion tank to smooth out the pump surge but it also aids in thermal expansion.

On longer term stays (for me, usually about a week or 10 days) the air pocket in the water heater depletes, just from use. I can usually tell when it is about depleted as the first use of the toilet in the morning after the plumbing system has not been opened all night requires extra effort to push the pedal, signaling higher pressure.

There's been considerable discussion as to what that pressure will climb to and there have been references to Suburban saying it could reach 150 PSI.

I decided to do my own study over about a two week period while here in FL. I have a water pressure guage (used to check campground pressures on arrival) that I put on my hot, low point drain line, left it there and monitored pressures throughout.

Here are the results... (note: I'm not using an inline pressure regulator)
Nominal static water pressure: 52 PSI.

Water pressure during water use: 40 PSI.

Water pressuse WITH air pocket in water heater, after sitting for 8 hours of non use: 58 PSI. (11PM to 7 AM)

Water pressure with air pocket in water heater depleted, after sitting for 8 hours of non use 92 PSI. (11 PM to 7 AM)

Definitely an increase and at 92 PSI, it shows why poorly installed/poorly crimped/loose fittings can and do leak.

I've yet to see the 150 PSI figures but I'm going to leave my gauge on and see if anything changes.Attachment 246107Attachment 246108Attachment 246109Attachment 246110

Great post. Thank you for taking the time to do this experiment and sharing your results.

One other thing to consider that you probably cant experiment with since youre down south is lowering the incoming water temperature to the water heater. You will get different results when the incoming water temperature is in the mid to upper thirties than you will when it is probably much warmer than that where you are right now. Did you happen to measure the temperature of your cold water where you are now?

Do you have any plans to camp farther north anytime soon where the incoming water will be colder and you can replicate your experiment? The comparison would be very interesting.

Thanks again,

Bruce
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Old 01-17-2021, 11:43 AM   #4
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Originally Posted by Thomas ho View Post
Wow ,,, you must be really bored. hot water pressure in toilet? if you open taps periodicaly as hot water heater tank fills pressure is released . same on cold side during water heater fill up. there is always air in lines after draining lines. air has to go somewhere so just open each faucet to let air escape.
I think you missed the point of the experiment. The thermal expansion from the water heater will affect the pressure in the entire water system — both hot and cold sides.

Bruce
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Old 01-17-2021, 11:51 AM   #5
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Originally Posted by Thomas ho View Post
Wow ,,, you must be really bored. hot water pressure in toilet? if you open taps periodicaly as hot water heater tank fills pressure is released . same on cold side during water heater fill up. there is always air in lines after draining lines. air has to go somewhere so just open each faucet to let air escape.
Not bored at all and obviously you haven't a clue how anything works.
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Old 01-17-2021, 11:56 AM   #6
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Problem solved by adding an expansion tank or pressure tank which many have done to prevent pump running to much . mine is 2 gallons and added to so pump isn't on/off/on /off . this also takes care of thermal expansion
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Old 01-17-2021, 11:57 AM   #7
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Originally Posted by nomad297 View Post
Great post. Thank you for taking the time to do this experiment and sharing your results.

One other thing to consider that you probably cant experiment with since youre down south is lowering the incoming water temperature to the water heater. You will get different results when the incoming water temperature is in the mid to upper thirties than you will when it is probably much warmer than that where you are right now. Did you happen to measure the temperature of your cold water where you are now?

Do you have any plans to camp farther north anytime soon where the incoming water will be colder and you can replicate your experiment? The comparison would be very interesting.

Thanks again,

Bruce
Bruce,
Thanks. I haven't measured the water temperature but by FL standards it has been cool here so water temps are not probably what one might expect for this geographic area. (39 this morning) Not sure I even have a thermometer along to measure but I'll be on the lookout for one for sure. I'll continue to post readings should anything become different than what I've witnessed up to now.
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Old 01-17-2021, 12:01 PM   #8
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Originally Posted by MR.M View Post
Problem solved by adding an expansion tank or pressure tank which many have done to prevent pump running to much . mine is 2 gallons and added to so pump isn't on/off/on /off . this also takes care of thermal expansion
Yep, I mentioned this in my original post. I do not currently have one although I have added them to other R/Vs I've owned.
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Old 01-17-2021, 12:12 PM   #9
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I find that most of my hot water use is in the AM, or at least before Noon. During the rest of the day I only use water for hand washing which releases any pressure build up.

Dishes are usually washed in the evening well before bedtime and again, use of the toilet and hand washing seems to keep expansion pressure down. The small amount used at those times is naturally replaced but total expansion is determined by the amount of heat that needs to be added to the tank. Since tank is already hot, not much expansion after my shower use has been reheated.

I've never taken the time to measure but based on the initial "spurt" when opening a faucet it doesn't seem to be a problem.

Also, no leaks in the almost four seasons I've owned my TT.

It would be interesting to know what pressure Forest River uses to test their plumbing systems during manufacture.
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Old 01-17-2021, 12:18 PM   #10
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Bruce,
Thanks. I haven't measured the water temperature but by FL standards it has been cool here so water temps are not probably what one might expect for this geographic area. (39 this morning) Not sure I even have a thermometer along to measure but I'll be on the lookout for one for sure. I'll continue to post readings should anything become different than what I've witnessed up to now.
Since Im a plumber, measuring water temperature is something I do almost daily, especially as fall transitions into winter and people call because they think they have a problem with their water heater since they are having to mix more hot water with the cold to get their desired showering temperatures. With the municipal water systems, I have seen cold water temperatures as low as 36 degrees in the winter and as high as 82 degrees in the summer. My customers on wells generally have cold water temperatures of right around 56 degrees all year long. At my seasonal campground, which is on a well system, the cold water temperature is always right around 50 degrees from March to November.

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Old 01-17-2021, 01:25 PM   #11
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Since I'm not a plumber and just asking....
Would a "water hammer arrester" of which there are various sizes help to relieve or cushion the water pressure? Just asking.
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Old 01-17-2021, 01:32 PM   #12
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Since I'm not a plumber and just asking....
Would a "water hammer arrester" of which there are various sizes help to relieve or cushion the water pressure? Just asking.
No. However, a thermal expansion tank can be used as a water hammer arrestor.

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Old 01-17-2021, 02:30 PM   #13
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Makes sense.

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No. However, a thermal expansion tank can be used as a water hammer arrestor.

Bruce
Makes sense. Interesting topic but for those of us with smaller units like my MicroLite 21DS limited space to install a thermal expansion tank is a problem. Assuming the expansion tank has a bladder, can it be mounted in any position?
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Old 01-17-2021, 02:35 PM   #14
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Makes sense. Interesting topic but for those of us with smaller units like my MicroLite 21DS limited space to install a thermal expansion tank is a problem. Assuming the expansion tank has a bladder, can it be mounted in any position?
They make relatively small ones for RV's.

RecPro sells one on Amazon for ~$33. It measures 5 1/8" wide by 4 1/2" high by 8" deep and has an internal volume of 0.75L.

Has an air bladder in it.
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Old 01-17-2021, 02:58 PM   #15
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Really appreciate this educational forum.

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Originally Posted by TitanMike View Post
They make relatively small ones for RV's.

RecPro sells one on Amazon for ~$33. It measures 5 1/8" wide by 4 1/2" high by 8" deep and has an internal volume of 0.75L.

Has an air bladder in it.
I see where Rec Pro sells it on their website for $32.95 with free shipping and this upright configuration should work well in my limited space. Thanks for sharing.
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Old 01-17-2021, 03:16 PM   #16
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I installed an expansion tank in my house, never gave thought to the TT. Great thread!
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Old 01-17-2021, 03:22 PM   #17
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Just a quick observation...

Quote:
Originally Posted by 5picker View Post
I decided to do my own study over about a two week period while here in FL. I have a water pressure guage (used to check campground pressures on arrival) that I put on my hot, low point drain line, left it there and monitored pressures throughout.
Just a quick observation...Bruce touched on this point but I want to highlight it.

There was no particular reason to pick the hot water line for the measuring point. When there's no water flow the pressure is the same at every point in the water system. You could have attached to cold, to a showerhead, or even the T&P valve on the water heater (open it briefly for each measurement).

It's just silly for other folks (not you, 5Picker), to complain that "There's no pressure at the shower/kitchen/lavatory sink/toilet." when it's really constricted flow.

Even sillier for them to turn a main valve down, constricting flow, in the belief that it will somehow lower the pressure.
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Old 01-17-2021, 03:33 PM   #18
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Originally Posted by TitanMike View Post
I find that most of my hot water use is in the AM, or at least before Noon. During the rest of the day I only use water for hand washing which releases any pressure build up.

Dishes are usually washed in the evening well before bedtime and again, use of the toilet and hand washing seems to keep expansion pressure down. The small amount used at those times is naturally replaced but total expansion is determined by the amount of heat that needs to be added to the tank. Since tank is already hot, not much expansion after my shower use has been reheated.

I've never taken the time to measure but based on the initial "spurt" when opening a faucet it doesn't seem to be a problem.

Also, no leaks in the almost four seasons I've owned my TT.

It would be interesting to know what pressure Forest River uses to test their plumbing systems during manufacture.
Mike, what you've experienced is common and you are right, once the faucet is opened, the pressure is pretty much normalized. Even at the 90 PSI, I didn't notice much of a 'spurt' when opening the faucet but do notice it on the toilet valve. (effort to push to open)

The issue only comes when the air space in the water heater depletes and then the system sets for a while unused. As I mentioned, for me, that takes about 2 weeks of full time use on my 6 gallon tank. Up until then, pressures are what I referenced and the high (with thermal expansion) is only about 10 PSI higher than static throughout that period.

A quick relief of water pressure (close shut-off on city water hose) and opening the T&P to drain some water and reintroduce the air space, has me good for another 10 days or two weeks before I start seeing those much higher pressures.

My post is simply to show that pressures in the plumbing, higher than what someone believes their pressure regulator is protecting their system, can certainly be a reality when the system is sitting unused for any length of time because of thermal expansion.

I suspect those who only turn their water heaters on when they want hot water might never experience much of this at all. We leave ours on electric 24/7 and supplement with LP daily during showers.
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Old 01-17-2021, 03:41 PM   #19
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Just a quick observation...Bruce touched on this point but I want to highlight it.

There was no particular reason to pick the hot water line for the measuring point. When there's no water flow the pressure is the same at every point in the water system. You could have attached to cold, to a showerhead, or even the T&P valve on the water heater (open it briefly for each measurement).

It's just silly for other folks (not you, 5Picker), to complain that "There's no pressure at the shower/kitchen/lavatory sink/toilet." when it's really constricted flow.

Even sillier for them to turn a main valve down, constricting flow, in the belief that it will somehow lower the pressure.
Correct Larry, I only picked the HOT low point drain because it didn't have anything attached to it and it was convenient. As anyone can clearly see, a hose (for washing truck/watering the DWs flowers) is attached to the cold low point drain and I didn't want to disconnect it but pressure in either would be the same.
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Old 01-17-2021, 04:45 PM   #20
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5picker, I found an easier way for you to work it out. Here you go:


Pressure increase due to thermal expansion of a trapped liquid

The calculation of pressure increase due to thermal expansion of a liquid fully filling, without any gas bubbles or pockets, a metallic enclosure, may be treated as follows.

The phenomena to be considered are:

1)thermal expansion of liquid due to the change of its bulk temperature
2)thermal expansion of the vessel or pipe, assumed in the following as having the same temperature as the fluid
3)compressibility of liquid under the increase in pressure due to the constrained volume
4)increase in volume of the vessel under the increased pressure of the fluid.

The corresponding contributions to the relative change in volume δV/V of fluid or of the containing space may be evaluated as follows:

1)αfΔT
2)αvΔT
3)βΔP
4)ΔPD/tE

where:

-αf is the volumetric coefficient of thermal expansion of the fluid
-αv is the volumetric coefficient of thermal expansion of the vessel (= three times the linear one)
-ΔT is the change in temperature
-β is the compressibility factor of the fluid, or the relative change in volume per unit change in pressure
-ΔP is the change in pressure of the fluid (it is what we seek)
-D,t and E are respectively the diameter, the thickness and the elastic modulus of the vessel (or pipe)
By equating the change in volume of the fluid to the change in volume of the vessel one gets:
αfΔT-βΔP=αvΔT+ΔPD/tE
or
ΔP=(αf-αv)ΔT/(β+D/tE)

Let's take as an example water as the entrapped fluid and carbon steel for the container, at temperatures not far from room temperature.

We have:

αf=210x10-6 C-1
αv=36x10-6 C-1
β=4x10-4 MPa-1
E=2x105 MPa

D/t will of course widely vary according to the dimensions and design pressure of the container; we may assume a variation between 100 for a light vessel or pipe and 10 for a quite heavy vessel, the value of 0 corresponding to an infinitely rigid container.

With these figures we get (per degree C of change in temperature):

ΔP=0.19 MPa =~ 2 bar for the light container
ΔP=0.39 MPa =~ 4 bar for the heavy container
ΔP=0.44 MPa =~ 4.4 bar for the rigid container



Have at it!

Bruce
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