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Context is North America (Canada). Water heaters are always equipped with a pressure/temperature safety relief valve (no argument that safety precautions are a good thing). My question is: how is it possible for the water heater to build up pressure when any expansion could simply drive the small amount of water backward into the incoming supply? Is there a check valve somewhere to prevent back flow? So far as I know, there is no such thing built into the water heater... am I wrong? Is there one built into the water meter?

  • Because the dip tube moves the inlet to the bottom of the tank, and the tank can have temperature differentials. This causes thermodynamics creating possibly greater pressures at the top of the tank that can't be effectively pushed down to release pressure back up the dip tube. – Tyson Mar 19 '17 at 2:44
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    People have been known to turn-off the main stopcock (e.g. to work on plumbing elsewhere) without thinking to turn off the heating system. I guess heater makers/retailers don't like the idea of being sued by bereaved relatives. – RedGrittyBrick Mar 19 '17 at 14:40
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Either an actual check valve or something that acts like one (such as a Pressure Reducing Valve) are fairly common on cold water supplies.

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The short answer is that when you heat cold water it will expand. Even without bringing it to a boil. It's about 2% in volume from 20°C to 70°C.

The typical installation of a boiler has a backflow preventer on the feed side. So that when you open the cold faucet you don't suddenly get hot water.

Those safety valves are to relieve pressure by dumping the contents into the surrounding area when it starts boiling to avoid a rupture. These contents are boiling hot and not something you want to deal with often. Instead there is an expansion tank to absorb the expected growth in volume without letting the pressure increase too much and possibly trigger a dump or damage the fittings.

  • The basic physics was never in question. It was about the presence of devices intentionally or incidentally preventing the movement/escape of water such that thermal expansion would become an issue. In this case, mystery might be solved. I had temporarily shut off the water main because of a leaking (cold) faucet; I didn't bother bleeding any water off because I assumed any excess pressure would escape through the leaking valve. Since the water heater remained "hot", I further assumed thermal expansion would be negligible due to the few degrees between burner cycling "on" and "off". – Anthony X Mar 21 '17 at 0:12
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Historically the most common arrangement was a tank and pump in the basement with a foot valve in the well. Even with a deep well or submerged pump you'd want a check valve to prevent the water tank from raining back into the well. Long story short, no backing up into wells.

Let's go to the other extreme and consider municipal supplies, which largely depend on gravity and towers for pressure. A 200' drop gives you a cozy 86.8 psi, which is why pressure reducing valves are common. Not to mention cities that require backflow prevention to keep the water supply uncontaminated. You really want the pressure relief valve in the city too, not just the country.

You have overlooked one other thing though. A "small amount of water" is a large amount of steam. If a plumbing problem left the water heater half-empty you'd have a boiler rather than a heater and the expansion would no longer be slow or minor.

Safety aside, there's also something to be said for designing a particular point of failure into any system. PEX, I believe, is rated for 80 PSI at 200F which makes it a relatively poor option for carrying pressurized hot water back through the "cold" supply. Even with copper pipes you'd probably rather have the relief valve open than have your cold water loop find its own relief valve, such as the icemaker. Basically the relief valve opening is much less of a mess than a random pipe or fitting on the supply side failing. On top of that the relief valve will close once the pressure decreases again, where as the broken pipe will just keep filling up your house with city water pressure.

  • I'm very familiar with well water systems such as you describe. Even in such a system, the check valve is upstream of the tank (at the pump take-off), so any thermal expansion produced by the water heater could simply push back into the accumulator. In my case, it is city supply, so no accumulator tank, and no sign of a pressure reducer, just the city meter and shutoff valve as the most upstream parts of the system in the house. – Anthony X Mar 21 '17 at 0:19

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