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Old 10-26-2010, 08:13 AM   #21
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Originally Posted by windrider View Post
Let's revisit this. First of all it say ELECTRIC. Second of all, with ELECTRIC, you never want any kind of air as this will quickly burn out the element, (5 seconds or less). Now some said you need an air gap so the hot water can expand. It will expand, and back feed the hot water outlet, and the cold water inlet. Since the outlet has a check valve to let water out, it will increase the pressure at the water spigot. Since the cold inlet has no check valve, it will increase the cold pressure also. Now this is in a perfect setup. If you filled and pressurized the system, checked the pressure, and then activated the water heater, either gas or electric, the pressure would rise. How much depends on the system, Lou can give us a formula if you know the length of your lines, the size, water capacity, temp rise, etc. (only kidding). It can be calculated, but at most I would guess 5 psi. Not enough to trip a pressure relief valve. If yours is leaking, you have a faulty valve or a faulty thermostat letting the water get too hot. Get a thermometer that will read over 200 degrees Fahrenheit, and measure the water temp. If it is over 180 degrees, replace the thermostat, if not, replace the relief valve.
There have been many posts on many forums about the "lost air gap" issue in RV hot water systems. There have been as many posts where re-establishing the airgap solved the P&T leaking problem. Since RV's are a closed system and a very small closed system at that, heating 6 gallons of water with no place to go will raise the pressure inside the system enough to open the P&T valve. Also, your mention of not wanting an airgap because it's electric is flawed too. the airgap in only about 2" in the top of the tank and the element is very near the bottom of the tank, completely submerged all the time.
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Old 10-26-2010, 08:48 AM   #22
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Originally Posted by windrider View Post
Lou can give us a formula if you know the length of your lines, the size, water capacity, temp rise, etc. (only kidding).
Thermal Expansion Equations Formulas Calculator - Volume Temperature Change

If you are feeling frisky.
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Old 10-26-2010, 09:49 AM   #23
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lets see...if i read that right...the pressure will rise with temp and that pressure will be related to how much the walls of that that contains it allows it to expand. ie no expansion, would result in infinate pressure. 100% free expansion would result in 0 pressure. install a gauge and it becomes part of the expansion.
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Old 10-26-2010, 10:12 AM   #24
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better overall picture. soon to be added will be washer dryer lines to. probably about 8 feet of tubing for hot and cold lines.

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Old 10-26-2010, 12:30 PM   #25
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now that picture looks like there could be an air space that u couldn't eaisly get rid of. replace the relief and see if it stops. looks like it could be an easy fix. you could also try opening the relief a couple times and see if something flushes out of it.

kind of reminds me of a fellow at work that went thru the world to calculate just how much of a change to get products on spec. then he'd make just a 2 deg change.
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Old 10-26-2010, 04:09 PM   #26
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Originally Posted by jimh View Post
lets see...if i read that right...the pressure will rise with temp and that pressure will be related to how much the walls of that that contains it allows it to expand. ie no expansion, would result in infinate pressure. 100% free expansion would result in 0 pressure. install a gauge and it becomes part of the expansion.
Yep.
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Old 10-27-2010, 02:20 AM   #27
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Here is an illustration and accompanying explanation/instructions that DH keeps bookmarked for some reason that reiterates Bama Rambler's last post:

http://www.zoom-rv.com/scripts/pdfretail/get.php?id=318

I think he bookmarked it as a reminder for years ago he changed out the relief valve unnecessarily because it was dripping not knowing that it's a common occurrence in an RV system as Bama Rambler has stated. All he had to do was to maintain the air gap and shouldn't have had to change the valve itself.
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