Hot Water Cylinder Valves

How to adjust hot water cylinder valves, hotwater presssure and know when the overflow pipe should drip and when it shouldn't.

Quick Fix - overflow pipe dripping

  • Its ok for the overflow pipe to drip when the water is heating.
  • Reduce the pressure - see bottom right of the picture to the right. Try about 1 anticlockwise turn of the adjustment screw at a time.
  • Replace the small washer under the cap which is shown at the top of the picture to the right. If noone has done it before you can turn over the old washer if you don't have a new one.

Health and Safety

  • Foil insulation conducts live electricity. This can happen when someone staples thru it into a live wire or when a rat chews thru the insulation around an electrical wire and later it contacts the foil. I always test and just in case clamp the foil to a good earth. When your working you could push the foil against a live wire making foil that was previously not electrically live very dangerous. Rather than reading about the four dead Australian electricians test then make your foil safe.
  • Water pipes can conduct live electricity. Use jumper leads when separating metal pipes, one each side of the "break".
  • Have a bucket with cold water ready in case of burns. Thermostats fail or can be set too high by clowns so the water can be very very hot.
  • Wear non-slip shoes on the roof. Never go on a damp corrogated iron roof.
  • Wash your hands. Hot water cylinders can run at a nice temperature to breed germs. Do NOT work on plumbing if you have hepatitus or any other contagious disease.
  • If you block off the valve or pipe that relieves the pressure to a hot water cylinder AND if the water heats above boiling point IT EXPLODES LIKE A BOMB. AS the pressure rises, the boiling point of the water rises and produces superheated steam. Click here to see the hotwater cylinder explode However this is very unlikely to happen unless you are very stupid.
  • Do not use a Neefa (or many other)relief valve with wetbacks. Vent directly on to the roof.

How it all Works

  • The pressure reduction valve reduces mains pressure (the pressure in the pipe in the road) to a pressure that the hot water cylinder can safely contain. As a starting point the cylinder fills up with cold water. As the water in the cylinder heats up it expands which increases the pressure. If the pressure is not relieved by someone turning on a tap or a leak, the pressure relief valve on the outlet side of the cylinder will open and the water flows into the overflow pipe and probably onto the roof. In these circumstances its perfectly ok for the overflow pipe to discharge water.NB some cylinders do not have a relief valve, just a pipe through the roof.
  • Once the thermostat turns off The water is no longer expanding and no water should discharge from the overflow pipe.

Adjusting pressure and fixing overflows that happen at the wrong time

  • The first thing to check is that the water comming out of the overflow pipe is actually comming out when it shouldn't - not when it should. Turn off the power to the hot water system and all taps in the house and garden. If after 15 minutes the water stops dripping out of the overflow there is no problem or an intermittent one. Or perhaps one day there was a surge in mains pressure that overwhelmed the system.(very unusual)
  • If the overflow pipe is still overflowing turn off the taps each side of the relief valve and undo the 2 screws on the cap at the top of the picture, slide out the flat steel bar, remove the thimble, pry out the washer inside with a small screwdriver and replace the washer.(or turn it over if someone hasn't already done this before)Reassemble and test.
  • The next thing to try is adjusting the pressure. Loosen the locknut at the bottom of the picture and then fit a nail through the hole in the "bolt " coming out of the bottom(in our pic) of the relief valve.Turn it anticlockwise one turn to reduce the pressure in the cylinder which will drop the height of the water in the overflow pipe about 2 feet. This will also reduce your hot water pressure(think shower) and the pressure of any cold water taps whose scource of water is a pipe UPSTREAM of the pressure reduction valve.
  • If this doesn't work the next thing is to dismantle and clean the pressure relief valve, check for a torn diaphram, replace any damaged parts (easily available) and reassemble. This is an easy job
  • If this doesn't work the next thing is to dismantle and clean the pressure reduction valve, check for a torn diaphram, replace the easily available parts and reassemble. This is also easy.
  • The reduction valves and relief valves come as a matching pair. For example Neefa valves come in 3 pressures 3.7m, 7.6m and 12.2m. Probably whats different is the spring length. Obviously if you have a 12.2m reduction valve and a 3.6m relief valve and not enough height between the two, your going to be struggling to stop the drip by adjustment.
  • Moving on to the very unusual and noting the picture to the right, if the overflow only overflows when both the hot and cold taps of a "two inlet one outlet" (see pic) taps is on, then the cold water feed is probably taken from before the pressure reduction valve. So the cold water system that is not "pressure reduced" by the pressure reduction valve and the cold water is flowing through the cold water tap and pressurising the hot water system. Did you have new 2 into 1 taps fitted recently? PIC NEEDED
  • And possible but unheard of is if your hot water thermostat has failed ON, the hot water will always be heating, hence expanding and the overflow pipe may drip at inappropriate times. Normally if this happened you'd notice your hot water getting very hot.
  • The mains pressure varies, so if you're adjusting the pressure at a busy time (eg at 6pm in the evening) wind the pressure back another half turn or otherwise you may find it will start leaking again in the middle of the night when the mains pressure is lower
  • often the dilema is to keep the shower pressure high enough with out having an overflow problem. Remember many shower mixers have an adjustment behind the face plate behind the tap. Nip down to your friendly plumbers merchant and ask if u can have a set of instructions out of the box of a new one. Most mixers are bought by plumbers who have already read them many times before
  • Its very unusual to have to replace the complete pressure reduction or pressure relief valve.


  • this article will be amended to cover tempering valves
  • Click here for more info on plumbing valves
  • In older houses the wiring is ususally not thick enough to use a 3 kilowatt element - use a 2 kw one and If your draining a hot water cylinder leave the hot tap turned on. Otherwise you can collapse the cylinder.
  • an inline strainer keeps debris out of the system
  • having the cylinder higher in the house will increase the tap pressure if using a relief valve
  • some set ups have a relief valve on the inlet