We touched on this in another thread, but I thought I'd make a couple more points about pressure relief valves on both domestic hot water heaters and boilers. Again, both use PRVs and they look identical to the untrained eye, especially when they're installed. However, PRVs for boilers open at 30 psi while those on domestic hot water heaters lift at a higher pressure. Elkimmeg says 50 psi, which sounds right to me. If you put a 30 psi PRV on a water heater, it won't work at all because the valve will lift and stay open once filled with tap water under normal, household pressure. Put a hot water heater PRV on a boiler, however, and your boiler could reach a dangerous (i.e., explosive) pressure before the relief valve blows. So it's important to know what you're doing and to read the tags, which describe the appropriate application and pressure pre-set. You should also periodically lift the lever on all your PRVs to make sure that 1.) the valve can open and 2.) water comes out when it does. PRVs can get scaled and/or gunked up or simply fail, at which point they need to be replaced. When in doubt, swap it out. In the case of boilers, yours may well contain a mixture of glycol and water. You will know this if it has a red or green or yellow color and feels a little more viscous than water. Heating system glycol (antifreeze) is expensive, so try not to waste more than necessary. Also, if you're going to let water out of the system, make sure you have a domestic water feed hooked up (and activated) that will replace it, or find another way to replenish what's been lost. The problem with manually opening a PRV is that some of that same gunk or scale can get caught on the valve seat, allowing the valve to leak after you let go of the lever to close it back up. There's not much you can do about this other than open the valve again wide and hope whatever is causing the problem flushes out. Sometimes I bang on them lightly, partly out of frustration and partly in an attempt to dislodge the offending crud. Usually, I give up eventually and hope that the valve stops leaking on its own, and usually it does. Stick a bucket or cup or other container under the drain pipe to catch the water. Oh yes, all PRVs should be piped down to near the floor (usually a piece of 3/4-inch copper soldered to a threaded adapter and screwed into the valve's 3/4-inch drain outlet. This reduces the chances of someone being scalded by steam or hot water if they happen to be standing near the valve when it blows. Finally, I like to know what my valves have been up to. If you're running a little hot and the valve opens from time to time, that's something you should know. But if you let the water or glycol drain out onto the floor, you may not notice it. That's why I always keep a can or bucket or plastic cup under the drain pipes of my valves. If they contain fluid, you know something's up. It looks a little tacky, I know, but it's a fashion compromise I'm willing to make for the sake of safety.