Water heater Overflow Issue

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Gary Swart

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This is a frequent topic of discussion here. When a pressure regulator valve is installed, it creates a "closed system". When water is heated, it expands. In an "open system", one without a PRV, the expansion is absorbed by the city main. In a closed system, it has no place to go, so when the pressure builds too high, the T/P opens to prevent the tank from exploding. An expansion tank is placed in the system after it the PRV to hold the expanded water until the heating cycles is completed and the pressure drops to normal. Why your T/P is not tripping, I can not say. It may be that the T/P is not functioning as it should, but that is just guess on my part. Here's what happened in my own home. I installed a PRV and the next thing I knew, the T/P was opening. I replaced the T/P, same results. I replaced the replacement T/P, still the same results. I brought the problem here and was told about expansion tanks. Before I installed it, I did an experiment. I put a pressure gauge on the hot water faucet of my utility sink. When the water heater was not heating, the pressure was a normal 60 psi just where I had the PRV set. Then I opened another hot water faucet and watched the gauge. When the water heater kicked on, the temperature immediately began to rise. It quickly rose to 150 psi and the T/P valve opened. The temperature actually continued to rise several more degrees. I immediately installed the expansion tank and that ended the problem. So, why isn't your T/P opening? I can't say, but if I was there, I'd do the experiment as I described above.
 

CHH

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Just offering a little explanation as a follow-up to Gary's excellent description.

Gary added a PRV and started having the T&P opening. Other folks don't have the T&P opening after adding a PRV.

The difference can be described as the compliance of the two systems. Gary's system is "hard" that is it has little compliance or ability to expand as the water is heated. Other systems, which may have more hoses for instance, have a greater ability to expand as the water is heated, that is to say they are more compliant.

I did some experimenting with my system after adding a PRV. If a check valve isolated the hot water tank so it was connected only to the hot water piping in the house the T&P would open as the water was heated. If the check valve was removed then the T&P valve would not open during the heating cycle. The water heater needed to be hydraulicly connect to the entire pipe system in order for there to be enough compliance. The piping was pretty much all hard copper except for the washing machine hoses, three toilet tank supply lines, and the icemaker line.

Of course I added an expansion tank to reduce the stress on the system.

And for the question on when an expansion tank is required, I believe that the magic number is 80 psi but as you can see from my little example it depends entirely on the system. Put a gauge on it and watch the pressures. If it climbs very high then an expansion tank is a cheap way to reduce stress on the system.
 

Terry

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If there is a leak in the system, like a leaking toilet flapper, that will prevent expansion too.
Expansion occurs when there are no leaks.

bye the way, just loved the rocketing water heater video Frenchie.
There was a water heater in the Seattle area that went through the roof of a home and landed a block away.

Has anyone ever hiked with a bag of chips and gone to high elevation with it?
 

Mr_Pike

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This post also points out a very obvious point to me. I bring it up, not to judge the poster, but for other's reading to learn a few things. It might belong in the irrigation section, but I thought about it while reading about how this problem was discovered.

Water systems in general are taken for granted so often, that it takes a dramatic event for us to even notice them. I call it the lawn service syndrome, but that is because I work mostly in irrigation. The actual home owner rarely gets outside to inspect his or her home and grounds if they have a lawn service doing all the landscape maintenance.

A broken head, or leaking main line is easily spotted by person who mows their own lawn or would even walk the area once a week. The lawn service is in and out in 30 minutes tops, and taking the time to look at anything is cutting into their profits. Let alone taking the time to inform a homeowner.

Now this is a generalization I realize, because there are some companies that do point this stuff out, and there are some homeowners who do understand the systems at their homes who don't mow their own grass.

My point is, if you own a home, take the time to at least inspect the place every couple weeks to look for problems. Some of you (but probably not others) would be surprised at the amount of calls we get about a driveway, sidewalk, retaining wall, pool, or foundation being caved in because of water damage caused by a broken head or pipe.

One call that comes to the top of the head was from an excavator who was just paid $30,000 to dig up and re-water proof the block around the front 1/2 of a commercial building's basement because of water seeping through the walls. The buildings owner was convinced it was due to poor water proofing and high water table. We were called in to repair the damage caused by the excavator to the sprinker system. Low and behold after we reconnected the lines to the system, we found multiple broken heads and most the heads were buried under an inch or more of dirt and thatch. When I inquired about how long the system had been shut off to the building's owner, he called in the maintenance man in charge of the place, and asked him how long it had been since the system was ran, who informed us that it had been running up until the day they dug up the front of the building. After taking the owner and maintence man outside to show them the broken heads, and the path the water was taking to the building, and how some heads were buried, the maintenance guy told the owner it had been at least 2 years since he had actually witnessed the system spraying outside, but that he had just assumed it was working, because the water meter next to the clock in the utility room was spinning like the dickens when he turned the system on each spring. That guy got fired on the spot.
 

humid1

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Since my first T&P leak, I check the heater at least once a week (it's in a coat closet). When I decided to replace the valve, I googled the issue, saw the exploding water heaters and just about had a stroke. Nothing to mess with. As far as the PRV and no expansion tank, I pose these questions: My house is eight years old and plumbed with pex. Will the pex allow enough expansion to possibly allow the water to heat without tripping the valve? I also keep the thermostats (also replaced when the T&P was replaced) at 123 degrees. Will this low setting reduce the expansion? Thanks for any input. I'll try the pressure test this weekend. I think everyone should by a $10 pressure gauge and have it in their tool box. Thanks in advance.
 

CHH

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humid1 said:
Since my first T&P leak, I check the heater at least once a week (it's in a coat closet). When I decided to replace the valve, I googled the issue, saw the exploding water heaters and just about had a stroke. Nothing to mess with. As far as the PRV and no expansion tank, I pose these questions: My house is eight years old and plumbed with pex. Will the pex allow enough expansion to possibly allow the water to heat without tripping the valve? I also keep the thermostats (also replaced when the T&P was replaced) at 123 degrees. Will this low setting reduce the expansion? Thanks for any input. I'll try the pressure test this weekend. I think everyone should by a $10 pressure gauge and have it in their tool box. Thanks in advance.


First the temperature question. A quick and dirty volume increase estimate assuming starting with 50 gallons of cold water:
50 F to 120 F will have a volume increase of 0.56 gallons
50 F to 140 F will have a volume increase of 0.84 gallons

Setting the thermostat lower helps reduce expansion. Obviously those are worst case numbers since a heater rarely has to start from a totally cold system and some volume won't be heated as water expands and forces some mass to leave the tank. I'm not about to do a tank expansion calculation and then look at the water expansion as a function of tank volume and temperature tonight, or ever. That's what grad students are for and I don't have one handy and they usually get the wrong answer the first few times anyway.:)

On the PEX question. Well, the practical answer is that something is allowing the pressure to remain below the T&P opening pressure. It may be PEX expansion or it may be a toilet valve opening as Terry pointed out.
 

CHH

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Got another PM on this thread

Anon said:
Hey, you'd posted 2 constants on thermal expansion for water...I've been looking like crazy online for some kind of chart for thermal expansion in common fluids...is there any chart or list you know of?

There really aren't any constants for thermal expansion for water (or most other liquids). For small temperature ranges, say about 40 degrees or so, a constant may be used as an approximation. That shortcut is unnecessary and would require a table of constants for common temperature ranges so one would still have to consult a table.

For water, density from steam tables can be used to calculate the mass of a given volume (V1) at the initial condition and then the volume (V2) of that mass at the final condition. The difference in V1 and V2 is the thermal expansion. There are a few assumptions in that computation approach as noted in the previous post.

Most folks don't want to be bothered with steam tables so the following equation can be used for water between 32 & 190 F when reasonably near atmospheric pressure. Lawyer required verbage: Although every effort has been made to provide a sound and workmanlike equation, use this equation entirely at your own risk. Absolutely do not use this equation above 190 F or for problems above about 7,000 feet elevation since that's getting pretty darn close to boiling. Do not use it below 32 F 'cause water isn't liquid anymore and the equation doesn't fit the behavior. Use of the equation signifies agreement to the above terms and conditions. Got it?

Density in lbm/gallon = 0.00000002989*T^3 - 0.00001987345*T^2 + 0.00137332539*T + 8.31993415653

T is temperature in degrees Fahrenheit. The equation is a bit long winded so put it in a spreadsheet so you can easily double check your answers. A couple check values are 8.34 at 32 and 8.097 at 180.

If you want the full steam tables, go here for verbage and an Excel(TM) add-in:
http://www.cheresources.com/iapwsif97.shtml

Are there any questions?
 

Jimbo

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"Has anyone ever hiked with a bag of chips and gone to high elevation with it?"


On the same principle, if you take a styrofoam cup and put in in the freeflood superstructure are of a submarine, and dive >>1000'++, the cup ends up the size of a small shot glass. All the air squeezed out of the foam. And if you take a string and stretch it tight across the torpedo room...from one side of the hull to the other....on the surface, when you go deep the string will sag enough to scare the poop out of a newbie. That 2" thick HY80 hull shrinks like crazy!
 
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