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the county where I live put in the "check valve" or backflow device. Not sure, but I know they said you need to get a thermal expansion tank or the pressure could blow up the water heater.

I spoke to a plumber, talked to other pros, and they said it wouldn't immediately make my water heater go bad. One of the plumbers even said the pressure valve may or may not work and is something the State has done to get more money by people spending cash to get valves, new water heaters, etc. One guy said it depends on the psi to my tank, but I have no idea what the psi to my house is.

I just want to know if my water heater is going to die immediately or if I might have a little time to get the money together.

Thanks. I know no one can tell me for sure, just getting advice.

edit: why the heck did this get a down-vote?

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Despite the conspiracy theories, putting in a backflow preventer (check valve) is a needed modernization to prevent contaminated water from being drawn into the city water system if pressure fails. They're highly recommended on sprinkler systems among other things to prevent ground water from being pulled into the system. We're trying to prevent disease on this one instead of needing to learn old lessons over and over again. Your plumber doesn't understand the evils of getting sewage in the drinking water. – Fiasco Labs Dec 3 '12 at 14:55
@FiascoLabs I'm a bit confused on how the sprinkler system (which isn't connected to a drain) would be drawing in sewage - (are you confusing the backflow preventer on the sewer main with the backflow preventer on the water main?- otherwise good points. – The Evil Greebo Dec 3 '12 at 20:28
Ground water is just as good as sewage at contaminating drinking water systems. And since sprinkler systems are under ground and likely to do this, you get the joy of putting in a double check valve system in most jurisdictions so if the first one fails, the second one will hopefully hold. Think dog crap... Nothing to do with sewers, thats another item altogether. – Fiasco Labs Dec 3 '12 at 21:08
And you put the backflow preventer in the house water line for the same reason you put one in the irrigation system line. The first protects others on the water system from you, the second protects you from your lawn. – Fiasco Labs Dec 3 '12 at 21:15
Johnny, I'm guessing that's your edit that I approved (otherwise, go ahead and revert it). I didn't down vote, but it makes sense since the question isn't about "how do I fix this problem" and there were lots of details missing (what type/size of tank, etc). And questions that mention disagreeing with professional advice often fall into the rant category. (See the faq for more details.) – BMitch Dec 3 '12 at 22:39
up vote 2 down vote accepted

Your tank is unlikely to explode without a lot of other plumbing issues happening first, but you will have a lot of other plumbing issues without an expansion tank. When all the fixtures are turned off and the hot water heater turns on, the pressure in your plumbing system will start to climb. An expansion tank gives the water somewhere to expand without significantly increasing the pressure. Without that, you'll notice a sudden surge in water after opening a fixture when you haven't used the water in a while. You'll also see fixtures like toilets, showers, and sinks start to leak. And before the tank explodes, the TPR valve on top should release and you'll have water leaking from the hot water heater.

Typically, water leaking is bad, but it's better than an explosion. If you find that nothing is leaking but you get a surge of water occasionally, then you might want to worry. You should take the advice and have an expansion tank installed as soon as possible. You could easily end up with a much larger repair bill if something like the washing machine hoses burst and flood your home while you're away. The expansion tank is a relatively easy install, and not a difficult DIY job if you don't mind cutting some of your pipes and adding a T to the line.

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I would also expect that if the T/P valve failed but the thermostat was working, even a catastrophic failure wouldn't cause an "explosion", since the latter would require water to be heated significantly above 100C. Water below 100C might get up to very high pressure, but if it is allowed to expand even slightly pressure will drop to atmospheric--behavior very different from superheated water (significantly above 100C) which will have to expand to many times its volume before reaching atmospheric pressure. – supercat Feb 21 '15 at 18:54

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