So in the kitchen I have a faucet with small water heater under the sink. It is always connected to electricity but works only to support set temparature, so just from time to time. When heater starts working, faucet starts dripping for up to 10 seconds, and it is getting worse everyday. Also, when I turn on just the hot water and then turn it off, the water just keeps going for couple of seconds.

The faucet has three pipes: one for incoming cold water from the house system, one for outcoming cold water to my heater, and one for incoming hot water from my heater.

The heater has a small tank, about 1 gal, but there is no relief valve. The only places for air and water to come out are two pipes for cold and hot water.

Hot water from the heater goes directly to the faucet, so doesn't have separate faucet. The faucet itself is regular, with one handle to be turned left/right to get hot/cold water.

What do you think the problem is about? The faucet or the heater? Can I do something without plumber's help?

  • Is this a small tank water heater (e.g., 1 gal) or is it a tankless water heater? The tankless is only supposed to be heating when the water is flowing and is supposed to shut off immediately when the water is turned off. A small tank however would be heating when there was no flow. This behavior of a small tank could cause a pressure rise in the hot water supply line when there was no flow. Does your water heater have a pressure-temperature relief valve and a waste line from there going to waste? Sep 17, 2023 at 10:00
  • The heater has a small tank, about 1 gal, but there is no relief valve. The only places for air and water to come out are two pipes for cold and hot water. Should I try to open them to let the air out?
    – Beks
    Sep 17, 2023 at 11:03
  • No. To relieve pressure (if needed) just open the hot water faucet slightly, then close it. Sep 17, 2023 at 15:21
  • Some municipalities supply water to their customers through a "check valve" but AFIK other municipalities do not have these checkvalves (e.g., I don't think mine does, but I have not confirmed this. I The purpose of a check valve is to prevent back flow of water from a customer's piping into the public water supply. This seems like a prudent safety measure, but creates one problem for systems with a tank water heater. If a lot of water is used in a short time, and the water (hot and cold) is shut off, the cold water in the tank expands as it is heated. This causes an increase in pressure. Sep 17, 2023 at 15:32
  • The solution is to have a small tank called a "a pressure relief tank" or a "thermal expansion tank" in the water system. This is only needed if there is a checkvalve in the water supply system Sep 17, 2023 at 15:39

3 Answers 3


In your case I would first suspect the problem is in the faucet valve. Turn the heater off and see if the leaking pattern continues.

Does this small tank water heater have its own dedicated faucet or is the output directed to the kitchen sink faucet?

  • I had the same idea and turned off the heater for a day. The random dripping during the day actually stopped. However, the delay in water-completely-stop-running is still there for both cold (dripping for several seconds) and hot (full run for several seconds) water.
    – Beks
    Sep 19, 2023 at 7:13
  • If the valve warms up when the heater is on, thermal expansion could be shifting things enough ho cause a small leak. If so, the usual fixes for a leaking valve would be worth trying -- replace the valve seal and see if it helps, check for valve seat erosion.
    – keshlam
    Sep 19, 2023 at 12:56
  • Thanks! Will try.
    – Beks
    Sep 23, 2023 at 15:48

I believe that we have a similar setup, to keep water hot. We find that if we lower the temperature a bit, it splutters less.

When heated, the water expands and can only come off the faucet. This is by design. And the delay is a side effect of the design. The faucet turns the supply to the tank on and off, not the output from it.

Thats how ours works.

  • Hmm, so it's normal? I would be okay if it was like several drops of water every now and then, but it just getting longer and longer.
    – Beks
    Sep 19, 2023 at 7:07
  • If lowering the temperature doesnt stop it, then it may be faulty. Even then our splutters once or twice a day, for a 5-10 seconds at a time. Sep 21, 2023 at 15:20
  • Thanks for sharing!
    – Beks
    Sep 23, 2023 at 15:49

I will take a SWAG and say this is an instant hot water device. The valve allows water to go into the tank, the hot water in the tank is pushed out through the faucet. When the water is cool it expands when getting heated. This causes the tank to overfill and drip out the faucet. The fact that takes longer to turn off is indicative of the fill valve failing. I have had these in the past and they do a nice job but if you do not use them for a while the water evaporates out of the tank and it will eventually fry.

This is a commercial but it explains very well how the system hot tap water works. https://www.youtube.com/watch?v=m2PcpFmBwzI

  • Do you mean the failure of the faucet's inner valve or?
    – Beks
    Sep 19, 2023 at 7:08

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