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In the case of a big pressure build up, could this "hypothetical pressure relief valve" piping installation indicated in black below cause my water heater to explode? What could be the consequences of having the hypothetical piping installation?

enter image description here

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    Pressure relief valves are made to prevent boom boom. Do think they are required to be installed, no matter what you think. – crip659 May 7 at 22:19
  • Why are you trying to route what ultimately is a drain against gravity? – ThreePhaseEel May 8 at 0:34
  • Note that in many jurisdictions external expansion tanks are required parts to water heaters these days in addition to a pressure relief valve. Expansion tanks help prevent your water heater from going boom more than a relief valve that goes up. – TylerH May 8 at 9:45
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A pressure relief valve does (or should) what it says: relieve pressure. That is exactly what you want - in the unlikely but non-zero event that a water heater overheats or has some other problem causing the water inside to get very hot (which leads to high pressure) or to get to much higher than normal pressure, the valve opens to relieve the pressure.

Normally I see the pipe from a pressure relief valve going down. It looks like the pipe in the picture goes sideways and then down, presumably to a drain outside.

If I understand the drawing correctly, the hypothetical part here is not "existence of the valve" but rather "pipe going straight up from the valve." That is not a good idea. Under normal circumstances, water pressure can handle any direction. But in the case of an over-temperature/over-pressure situation, you want the water to specifically go out the easiest path possible. If the pressure relief valve piping extends above other pipes, it is quite possible that even after the valve opens, the water will try to exit via other means - the pipes at the top of the water heater - which could lead to burst pipes or other problems.

One extra safety feature that may make sense is a water heater drain pan. This goes underneath your water heater to catch leaks. It is typically required when a water heater is not on the lowest level, but even on the lowest level of your house it is a good idea if there is no floor drain nearby.

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  • What is the mechanism that ensures the relief valve remains functional? I've never heard of the explosion of domestic water boilers, but it wasn't a rare occurrence for industrial boilers. – r13 May 7 at 23:19
  • given the small power input it doesn't need a high flow rate of steam or water to keep the tank in the safe zone. – Jasen May 8 at 8:47
  • what mechanism? periodic testing, like it says on the tag, If the lever can operate the valve then over pressure can too. – Jasen May 8 at 8:49
  • It is insane to me that drain pans are not required by code. They are incredibly cheap and can save you from having to replace your whole floor/subfloor if a leak happens. – TylerH May 8 at 9:43
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If you run the pipe up hill the pipe will accumulate water in the upwards part and it will leak out around the stem of the valve.

The added ppressure caused by the standing water will not be enough to cause failure of the tank, but if the stem corrodes the valve may become inoperable. (which could be disasterous)

The most likely result will just be an annoying drip.

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