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Question DetailsAsked on 10/9/2013

My hot water heater was keeps dripping water from the pressure release valve. We replaced it,flushed & still drips!

We have flushed this valve numerous x's! What's causing this problem? Can we fix it?

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2 Answers


It is remotely possible the first valve had gone bad just due to age, and you got a defective replacement valve, but unlikely. Another possibility - read the tag on the valve - it is possible the wrong rating valve was installed - you need one rated for hot water heaters and for the rated tank pressure you have, and it needs to be a combined pressure and temperature valve. There are at least 30 different combinations out there, and sometimes a person (including plumbers) will grab the wrong one.

Assuming you are getting full flow of hot water at the faucets (so hot water tank outlet is not plugged by a broken piece of inflow tube or such), then I would 99% guess that you have some sort of backflow preventer or pressure regulating valve on your water line. With that scenario, when the water in the tank heats up and expands during the firing cycle, if there is not enough length of hot water line in your house to absorb the expansion, because the backflow preventer or pressure regulator prevents it from pushing the water back through the cold water line and inlet line as it usually would, the only place the pressure can go is the pressure relief valve, which bleeds just a few tablespoons of water or perhaps just some wet steam to relieve the pressure.

This is a pretty common occurrence in houses that have had a backflow preventer or pressure regulator (or pressure pump) installed after the water heater was already in place and for a while all is OK, but as the heater gets older and accumulates corrosion on the bottom of the heater (inside the area where the flame contacts) eventually it creates hot spots that boil a bit when the heater is firing, so instead of just hot water you get a lot of bubbles that take up 1600 times as much volume as the water they came from - so you can see it takes only a miniscule hot spot in the bottom of the tank to create a pretty substantial amount of steam - some of which then makes it to the top of the tank as vapor, pressuring the tank up until someone opens a faucet to use hot water.

Assuming you DO have a pressure regulator, backflow preventer, pressure pump, or other pressure-affecting device on your main water line (google those terms one by one with the word "image" behind them to see what these look like), then you probably need a pressure tank to absorb the pressure. Looks like this but should NOT be on its side - should stand upright over the pipe -

and installed something like this -

If you already have such a tank, then perhaps it has lost its air pocket, or the diaphragm inside that keeps water and air separated has broken - these typically last about 10 years.

So, in summary, if you have some sort of in-line restriction on your water line and do NOT have a presure tank, you need to get one installed - typically about $150-200 installed cost. If you already have one, then you need to get it replaced (I would not recommend fixing - usually if failed once will do so again very soon) - about $100-150 cost.

Search the List (in green banner bar) for a local plumber with good ratings and reviews.

P.S. - the expansion tank has to go on the cold water line between the hot water heater inlet and any pressure control device - NOT on the overpressure valve fitting as I have seen some plumbers doing it - I don't know where, but somehow that concept has gotten into the plumbing community - it does NOT go there, and does not last in that hot environment.

Answered 6 years ago by LCD


An afterthought - check your hot water temp with a candy or oven thermometer - it should be 120-130 degrees depending on your preference and whether you have little children or invalids in the house - and should not exceed 140 at any point after you turn the water on - initially colder water will come out of the faucet, then when the hottest water from the top of the tank comes out it will be at its hottest, commonly dropping off 10-20 degrees in a few seconds as incoming water starts mixing the water in the tank.

IF your water is coming out at 180 or above you may be boiling in the tank every heat cycle - your thermostat needs to be turned down, or is broken.

Answered 6 years ago by LCD

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