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Old 06-11-2018, 07:14 AM   #1
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The Truth About Water Pressure

There is so much discussion here and at all other RV websites about the amount of water pressure your RVís potable water system can handle. Iíll admit, its a good idea to try to maintain a water pressure of no more than 60psi, but there is just about nothing you can do if you have a standard tank water heater to keep the pressure below 150psi.

Have you ever noticed sometimes when you first turn on your kitchen faucet (and other faucets), that you get a blast of high flow, then it dies down to a sustained lower flow? The reason this happens is because of the thermal expansion created by your water heater heating the cold water. This will happen even if you think you have an air pocket at the top of your tank ó thereís just not enough room in that perceived ďpocketĒ to absorb the pressure created by the thermal expansion. Have you ever noticed that your temperature and pressure relief valve on your water heater drips sometimes? If you havenít noticed, it is still happening. The reason it is doing this is because the pressure of your potable water system (not just your water heater) has exceeded 150psi. Thatís right ó 150psi. Your potable water system is, for most of the time, at a pressure of 150psi.

If you have a check valve on the cold side of your water heater, the cold side may not increase to 150psi, but these check valves are very weak and can fail from the pressure created by the water heater. They can, and usually do also bleed through, so your cold side will usually also be at 150psi.

My point here is, there is way too much time put into debate on whether or not you should use a water pressure regulator for your RV, unless, of course, your water source pressure is over 150psi, which is very unlikely.

If you were wanting to combat this high pressure, there are a couple of ways to deal with it. You could install an expansion tank, but you will usually have a hard time finding space for it. Or, you could remove the check valve where your hose connects to your RV to allow the excessive pressure to push back into the water supply. However, if you have a pressure regulator installed, this usually wonít work because the regulator also works as a check valve unless you use a regulator that is designed to allow the excessive pressure to feed back through it into the water source. Plus, if you remove the check valve at your RV, you will no longer be able to use your fresh water tank and pump unless you work out a way to cap the city water connection at you RV so the water doesnít pump out of it, emptying your fresh water tank.

I just thought Iíd put this fact out there as food for thought and further debate on the water regulator and maximum water pressure issue.

Bruce
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Old 06-11-2018, 01:43 PM   #2
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I just donít believe that the hot water side is 150psi. I must be missing something because my hot water side donít ever jump out. Hot water cold water it is all the same. If the hot water tank is 150psi then everything to the faucets is 150psi.
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Old 06-11-2018, 01:55 PM   #3
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I've never seen 150 PSI on my water pressure gauge once the water gets hot.

I would think at 150 PSI in the tank, the T & P valve (pop-off) would be thinking about opening!
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Old 06-11-2018, 02:11 PM   #4
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Quote:
Originally Posted by nomad297 View Post
Iíll admit, its a good idea to try to maintain a water pressure of no more than 60psi, but there is just about nothing you can do if you have a standard tank water heater to keep the pressure below 150psi.
I appreciate your post Bruce but disagree that there is nothing you can do to keep the water pressure below 150psi. Every three or four days of camping (if I leave my water heater on) I turn off the pump, open a hot water faucet to relieve pressure in the line, then I open the pressure relief valve for a few seconds to allow hot water to escape and re-introduce a new air pocket at the top of the tank. That inturn allows for expansion of heated water in the tank resulting in reduced pressure elsewhere in the line. Key is to remember to do this every 3 or 4 days with the tank continuously heated.

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Old 06-11-2018, 02:33 PM   #5
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Believe it. This is why water dribbles out of your temperature and pressure relief valve.

Quote:
Originally Posted by spock123 View Post
If the hot water tank is 150psi then everything to the faucets is 150psi.
Exactly my point.

Bruce
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Old 06-11-2018, 02:38 PM   #6
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I've never seen 150 PSI on my water pressure gauge once the water gets hot.

I would think at 150 PSI in the tank, the T & P valve (pop-off) would be thinking about opening!
Where have you measured the pressure? Before or after where the water enters the RV? If you wanted to measure it, you would need to put a gauge on a faucet after the water enters the RV.

The T&P valve does start to relieve pressure at 150psi. It doesnít pop open and run water full-flow like a geyser. It will dribble out until the pressure is below 150 psi. Have you never seen your T&P valve dripping? This is why it drips.

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Old 06-11-2018, 02:48 PM   #7
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I appreciate your post Bruce but disagree that there is nothing you can do to keep the water pressure below 150psi. Every three or four days of camping (if I leave my water heater on) I turn off the pump, open a hot water faucet to relieve pressure in the line, then I open the pressure relief valve for a few seconds to allow hot water to escape and re-introduce a new air pocket at the top of the tank. That inturn allows for expansion of heated water in the tank resulting in reduced pressure elsewhere in the line. Key is to remember to do this every 3 or 4 days with the tank continuously heated.

Rich J.
The air that can sometimes be captured at the top of the tank is not enough to keep the pressure from building ó especially in a water system as small as those in RVs. Think about how high on the tank the temperature and pressure relief valve is ó maybe one inch from the top. The temperature rod of the T&P valve must be submersed in water, so the water heater has to be designed for the water to fill higher than that point. Thereís just not enough space above the T&P valve for a pocket of air to make a difference.

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Old 06-11-2018, 02:55 PM   #8
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Well, we disagree. My arguement is that a typical 6 gallon Suburban WH heats water up to 130 degrees. The amount of water expansion from room temperature up to 130 degrees is fairly negligible, thus the room at the top of the tank for an air pocket is sufficient, but should be renewed through continued use.



Cheers
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Old 06-11-2018, 03:00 PM   #9
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Quote:
Originally Posted by nomad297 View Post
Where have you measured the pressure? Before or after where the water enters the RV? If you wanted to measure it, you would need to put a gauge on a faucet after the water enters the RV.

The T&P valve does start to relieve pressure at 150psi. It doesn’t pop open and run water full-flow like a geyser. It will dribble out until the pressure is below 150 psi. Have you never seen your T&P valve dripping? This is why it drips.

Bruce
I have a pressure gauge (with a capped end) that screws onto a garden hose fitting. I use it to check initial pressure at the spigot.

I converted my low point drains to shutoff's with garden hose ends.
Once I know the spigot pressure, I'll install my inline pressure regulator to the spigot (when necessary) and move the gauge to one of the low point drain valves so I can then quickly glance at system pressure. When the water is hot, the pressure is usually right around 75 PSI.

I've never seen my T&P weep.
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Old 06-11-2018, 03:13 PM   #10
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Quote:
Originally Posted by 5picker View Post
I have a pressure gauge (with a capped end) that screws onto a garden hose fitting. I use it to check initial pressure at the spigot.

I converted my low point drains to shutoff's with garden hose ends.
Once I know the spigot pressure, I'll install my inline pressure regulator to the spigot (when necessary) and move the gauge to one of the low point drain valves so I can then quickly glance at system pressure. When the water is hot, the pressure is usually right around 75 PSI.

I've never seen my T&P weep.
Sure, the water can be hot and your pressure may be 75psi when you glance at it. How often have you sat under your RV and watched that gauge from the time your hot water has been depleted of hot water to the time the water reaches 130 degrees without any use in the RV? How many times have you done this with incoming water temperatures of 40-50 degrees?

Prove me wrong. Somehow.

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Old 06-11-2018, 03:18 PM   #11
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Quote:
Originally Posted by HangDiver View Post
Well, we disagree. My arguement is that a typical 6 gallon Suburban WH heats water up to 130 degrees. The amount of water expansion from room temperature up to 130 degrees is fairly negligible, thus the room at the top of the tank for an air pocket is sufficient, but should be renewed through continued use.



Cheers
Rich J.
Mark Polk may know some stuff about RVs and tell you what Suburban told him, but heís no Master plumber who specializes in hydronics and water heaters. Iíll stick to what I know instead of taking his word as gospel.

But even he brings up the issue of expansion tanks. Why would he do that if the ďdesignedĒ air pocket is all you need?

Bruce
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Old 06-11-2018, 03:43 PM   #12
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Nomad297 is completely correct. The water heater will raise the system pressure and will sometimes cause the relief valve to drip. This is most noticeable when we use lots of hot water so the water heater has been depleted and refilled with cold water, then use none while the water heater is recovering. Our Sunseeker has no check valve on the cold water side of the water heater, so when it heats up, it pressurizes the cold and hot water systems. When that happens in our Sunseeker, it's almost impossible to flush the toilet until I open the sink faucet to let some pressure off.

When the water heater pressurizes the systems to the point that the relief valve drips, it in effect is pressure testing the system. That's why I never bother with a pressure regulator. Actually, I only connect to city water when I am winterizing the RV at home. I prefer to run off my fresh water tank. That way I never overfill my holding tanks.

I'm an engineer and I worked with industrial pressure relief valves for many years. A relief valves pressure rating is at full open, not where it starts to open. At a relief valve's stamped pressure, it will pass the rated flow at full open without allowing the protected vessel to rise more than 10% above the vessel's stamped pressure rating. Typical industrial relief valves are allowed to "simmer" (i.e., drip) at 3% below the set point. Domestic relief valves are probably allowed to drip as much as 10% below the set point (i.e., 135 psig).

As a side note, the relief valve that's used on RV water heaters has far more capacity than would ever be needed. It's probably the same one that's used on large residential water heaters that have much larger burners or electric heating elements.
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Old 06-11-2018, 03:46 PM   #13
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Some have mentioned the Suburban's "air pocket" that's supposed to allow for expansion. I've done the bleed through the relief valve per their instructions many times and it doesn't seem to make any difference. Maybe I'm just not doing it right.
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Old 06-11-2018, 03:50 PM   #14
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I have never seen my T&P weep or leak. But then I have better things to do. Anyway say what you want I just donít believe my hot water side has 150psi. I need another beer
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Old 06-11-2018, 04:00 PM   #15
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I have never seen my T&P weep or leak. But then I have better things to do. Anyway say what you want I just donít believe my hot water side has 150psi. I need another beer
Youíre probably not a plumber, so you may not notice the things a plumber will notice. Iím sure I wouldnít notice some things that you would in your line of expertise, but I would believe what you told me.

How many beers do you have?

Bruce
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Old 06-11-2018, 04:04 PM   #16
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Youíre probably not a plumber, so you may not notice the things a plumber will notice. Iím sure I wouldnít notice some things that you would in your line of expertise, but I would believe what you told me.



How many beers do you have?



Bruce


You must be dumber than me, I donít believe nothing that I hear and only half of what I see. Time for another beer. Itís a slow day
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Old 06-11-2018, 04:27 PM   #17
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Nomad is correct, I to have been a plumber for 30+ years and have seen first hand how much pressure will build in a heater with out proper expansion provisions. Saw 300 plus (pegged my gauge) on a boiler mate. Storage tank was at 120F system was 60psi, ran a few gal out to get boiler to fire about 5-6 gal. (Size of most of our heaters) Boiler was still burning when I looked at my gauge then ran to shut it down. Found no relief valve on tank, and a check installed on inlet, and expansion tank on inlet side of check valve. bad, bad deal. That's what can happen when anyone can install something with no knowledge of what they are doing. That is why our sticks and bricks have codes for everything.
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Old 06-11-2018, 04:43 PM   #18
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Feel free to check my calcs:

Given:
Specific volume of water at 60 deg = 0.016035 cu ft/lb
Specific volume of water at 140 deg = 0.016293 cu ft/lb
1 gal = 8.3 lbs at 62 deg
Assume: weight of water at 62 deg is essentially the same as at 60 deg.

Therefore:
6 gal of water x 8.3 lbs/gal = 49.8 lbs.
Then:
Volume of 6 gal of water at 60 deg = 49.8 lbs x 0.016035 cu ft/lb = 0.798543 cu ft
Volume of 6 gal of water at 130 deg = 49.8 lbs x 0.016293 cu ft/lb = 0.8113914 cu ft
So, the increase in water volume from 60 deg to 130 deg is
0.8113914 cu ft - 0.798543 cu ft = 0.0128484 cu ft
Assume: water heater internal surface area = 1 sq ft.

Then
The increase in water level would be 0.0128484 cu ft/1 sq ft = 0.0128484 ft.

Convert to inches:
0.0128484 ft x 12 in/ft = 0.1541808 inches
Convert to 16ths of an inch:
0.1541808 inches x 16 “16ths”/inch = 2.4 16ths of an inch
Conclusion:
Water level in the water heater will increase a little more than 1/8 of an inch
Of course, the air in the water heater will "try" to expand more than this from 60 to 130 deg, but instead will increase in pressure.

We can do that math next.
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Old 06-11-2018, 05:10 PM   #19
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So I guess all the companies that sell water pressure regulators are selling a scam.
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Old 06-11-2018, 05:17 PM   #20
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So I guess all the companies that sell water pressure regulators are selling a scam.
Youíre really not putting much thought into this. Which beer are you on now?

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