I noticed the other day that my tankless hot water tank was occasionally spraying water all over the place from the Pressure Relief Valve. The only way I've found that seems to reliably trigger this is flushing the toilet, and turning the hot water in the bathroom sink off while it's still flushing. Sometimes it's only a drip, other times it's a violent spray of water that gets everywhere.

Potentially relevant information I can think of

  • Here's pictures of the offending tankless and valve

  • The tankless was put in a few months ago, and I've never noticed water leaking before a couple of days ago. It's in the laundry room though, which I do not visit on a regular basis, so perhaps this has been happening longer?

  • Tankless is set to 120F ever since it was installed, and water temperature seems to be about that.

  • I'm in NY, where the weather has recently started getting cold

  • The plumbing in this house is really old, and in some cases looks like the previous owners did DIY plumbing

  • The toilet looks equally old. I've had issues with it flushing, flushing slowly, and clogging on a regular basis. I was considering replacing it because a plumber told me they thought it may just be the calcium(?) deposits built up in the piping over the years, causing it to have a really narrow hole.

  • The pipes go from the tankless in the laundry room, through the wall to the garage to where the old hot water tank sat (PEX pipes in garage), then metal pipes go from where the old hot water tank to the floor to run through the crawlspace to the water meter on the other side of the house. The majority of the water (kitchen and bathroom) are on the other side of the 1000sqft house.

  • The crawlspace is encapsulated (as of this summer), with the furnace venting down there. It should not get unreasonably cold, although a couple months ago a section of the metal pipe in the crawlspace did spring a leak and have to be replaced.

  • I have occasionally heard the toilet making weird noises, as if water is being forced into it. This is what makes me think it's an issue with the pressure and not related to a faulty valve.

  • I just read this post about a tankless turning on while flushing. Mine does that too, so I am guessing there is a mixing valve somewhere down in the crawlspace or in the walls? I've never noticed any condensation on the toilet bowl, but only lived here since summer

I'm trying to figure out why this is happening, and what I can do to fix it. Get an expansion tank? Get a new toilet? Any thoughts or knowledge would be helpful.

  • There should be an "Expansion Tank" near your hot water heater and if not definitely put one in. You might also check the incoming water pressure; if very high (check local codes for values), put in a diaphragm restrictor. Commented Oct 30, 2019 at 17:22
  • There is not an Expansion Tank, although that was one of the solutions I am researching. I wasn't sure if it was necessary with a tankless hot water tank, but I am beginning to think that is my best solution.
    – Rachel
    Commented Oct 30, 2019 at 17:36
  • 3
    Is there a check valve or backflow preventer valve between your water supply and your house? Most houses use the mains supply network as an expansion tank to absorb pressure spikes. If your house is isolated or on a well, there is nowhere for pressure spikes to go, and pressure will go to infinity until something breaks. Your water heater's pressure relief valve is "breaking" in a controlled way. Commented Oct 30, 2019 at 19:19

2 Answers 2


Whenever a valve opens and closes, it causes a pressure spike. This is because water is incompressible, so it tends to telegraph pressure spikes a long distance.

Water has mass. When you open a valve, the water has to start to move. That causes a local low pressure area which of course is what makes the water move.

When you close a valve, a significant mass of water (in the pipe) is moving toward the open valve. The water at the valve must stop, but the water behind it is still moving toward it. So it is compressed. All the water's kinetic energy is converted to pressure, in a millisecond. The water moving toward the open valve seeks any possible outlet for the kinetic energy.

It finds the pressure relief valve in the water heater.

If it hadn't found that, it could break a pipe.

A related problem is when it finds an expansion tank on the other side of the water heater, or some comparable "place for the pressure to go". Thus the pressure is relieved by water moving through the water heater. The tankless sees this water movement, and turns on.

In a city water system, the obvious place this pressure can go is back out to the street - where it can be absorbed by the massive city water piping. This is rendered impossible if a check valve (aka backflow preventer) is added to the system.

An expansion tank is designed to create an outlet for such energy.

  • Thank you very much for that explanation, that helped clarify things in my mind. I am guessing that some kind of checkvalve is installed, although I have no idea where I would look for it. I think I will look into getting an expansion tank added.
    – Rachel
    Commented Oct 30, 2019 at 20:21
  • @Rachel The city's water meter may have a built-in check valve. Their water department should be able to confirm.
    – Greg Hill
    Commented Oct 30, 2019 at 23:26

In the question Rachel wrote:

  • The plumbing in this house is really old
  • I've had issues with [the toilet] flushing, flushing slowly, ...
  • I have occasionally heard the toilet making weird noises, as if water is being forced into it.
  • I just read this post about a tankless turning on while flushing. Mine does that too ...

Putting all the symptoms together I suspect the water main from the street may be restricted by years of corrosion or mineral build-up inside the pipe. Why would this matter?

Harper wrote an excellent answer mentioning water, mass, and kinetic energy. How much kinetic energy does a unit of water moving through a pipe have? It's mass times velocity squared. OK, what's the velocity? It's a function of the flow, ie gallons per minute, and the diameter of the pipe. For a given amount of flow the water has higher velocity when it flows in a smaller pipe.

How can this lead to the pressure relief valve operating?

When the toilet is flushed and its tank refills, water flows through the main. The added flow for running the sink faucet at the same time means the water in the main is flowing even faster. There's a column of water racing through the water main and suddenly the sink valve closes. That moving water carries a lot of energy and causes the pressure relief valve to pop just as Harper described. So, why doesn't the pressure relief valve pop when the sink valve is opened and closed while the toilet is not refilling? It's because the mass of the water in the main is the same, but its velocity is much less due to the lower flow. There's less energy involved and so a smaller pressure spike.

What about the other symptoms?

  • When you wrote the toilet will "flush slowly" did you actually mean the tank refills slowly? If the water supply is limited then tank refill takes a long time.
  • The same pressure spike effect that causes the pressure relief valve to pop might also overcome the toilet's fill valve, forcing a little more water into its tank
  • False firing of the tankless water heater. When the toilet tank fill valve opens there's a quick drop in pressure in the house -- including in the hot water pipes (water flows backward through the water heater). Then, as water begins flowing through a too-small main and system pressure builds again, there's some flow in the forward direction through the water heater while the hot water pipes re-pressurize. That can momentarily trip the heater's flow sensor just long enough to cause it to fire up and immediately shut down.

Can we test for this?

In a nearby plumbing department you can find a water pressure gauge made to screw onto a hose connection. You might have a hose bibb outdoors or at a clothes washer. Check the water pressure with nothing running and again with the toilet filling and the sink running. If the pressure drops substantially, like by 20 psi or more, then something (a restricted water main, for instance) is causing that pressure drop. You'd be able to see the pressure spike on the gauge too when the flow suddenly stops.

What to do?

Excessively high water velocity is bad because it accelerates erosion of the inside of pipes and it causes water hammer. The best solution is to get the velocity down by increasing the pipe size.

IMHO a [thermal] expansion tank really is intended to absorb the expansion that happens when water is heated -- think 40+ gallons of cold water heating in a conventional water heater tank. If a check valve prevents water from pushing back out to the street then pressure inside the house rises. An expansion tank provides a buffer to absorb that water volume without a dangerous increase in pressure.

People are obviously not often excited to replace pipe to fix a water velocity problem. Expansion tanks and water hammer arrestors are the next-best thing because they soothe the visible symptoms (noises and pressure relief valves operating), but unfortunately they can't address the erosion effects inside the pipes.

  • Thank you, perhaps I will look into getting a water pressure gauge to check on this! The toilet fills at a normal speed I think, it's just flushing that can get slow (usually from being partially-clogged I'm guessing)
    – Rachel
    Commented Oct 31, 2019 at 12:35

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