Tip & How-To about Heating & Cooling

Expansion Water Tank Boiler Expansion Tank

When water is heated the water will increase in volume. The thermal expansion tank in a boiler heating system is there to allow the extra volume of water some place to go. It also holds enough water to fill the void back up when the water cools.

The water expansion tank is a necessary item in heating systems. It also can cause problems if they do not work correctly. Some of the problems caused by a faulty expansion tank are:

  1. The boiler relief valve will be constantly dripping or blowing off if the tank is totally full of water and does not have the air cushion needed to counteract the increased volume of water as it is heated.
  2. Air can be introduced into the system if the tank does not hold sufficient volume of water to keep the system full as it cools.
  3. If fresh water is introduced into the system frequently the tank will rot out from the inside out.

One of the main things to remember is that a faulty boiler expansion tank can make other things like the relief valve look faulty when it reality it is not the problem. A bad expansion water tank or a bad pressure reducing valve can make the relief valve look bad. Or actually cause it to need replacement. Check everything out carefully before deciding what pieces need to be replaced. If go about just changing parts without a through investigation first you may be changing parts for a long time till you find the right one.
http://www.fixya.com/support/r3705970-heating_circulating_water_pump_problems

http://www.fixya.com/support/r3695967-valve_pressure_reducing_or_pressure

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1 Answer

why does the pressure rise to over 3bar when the central heating is in operation ?


3 Bar = 43.5 PSI When you heat water it always expands increasing pressure.
In a closed vessel volume can increase by 2-3%. Either expansion tanks allow for this and PSI is maintain or without an expansion outlet the PSI will increase in proportion to heat supplied.
Is this a low temp hydronic boiler operating at or <140F, a hi temp hydronic boiler @ >180F or steam boiler?

Oct 18, 2014 | Water Heaters

1 Answer

when boiler heats water the pressure increases to the point that the pressure relief valve( 30 psi) releases water from the system.What causes the pressure to increase? The system includes an expansion tank( Wilkins-model HXT-30)


You did not mention what pressure the boiler displays when not on. It should be between 12 and 15 psi. You could have a bad feed valve, bad expantion tank (tap lower 1/2 to see if hollow).

Mar 15, 2011 | Heating & Cooling

2 Answers

my worcester bosch 24i junior pressure keeps dropping if i have heating off for an hour i have to top up boiler and reset also when i have heating on the pressure jumps right up to over 4.0bar


When water is heated it expands. To deal with this all boilers are fitted with a small expansion vessel which is like a reservoir which holds the excess water.

This expansion vessel is like a balloon in a metal pot. When the water goes in the metal pot the baloon squashes a bit to make room for the extra volume of water. When it cools the balloon pushes the water back into the heating system.

What has happened is your expansion vessel no longer works. When the water heats up there is nowhere for it to go so the pressure jumps up to 4 bar. The Worcester boiler lets high pressure water escape (most boiler allow this to go through the safety valve at say 3.5 bar).

When the water cools the pressure drops very low as the volume of water contracts.

The problem is your expansion vessel - normally a little red painted metal cylinder inside the boiler.

Dec 22, 2010 | Bosch Water Heaters

2 Answers

vokera option combi boiler seems to increase in pressure until it opens the safety vave ,only when heating is on !


check to see if the expansion tank is completely full of water or isolated from the rest for the system. If full it needs replacing, if isolated (valve turned off to it) then turn it on. When a hot water heating system cannot allow for expansion then the pressure will increase when running until the saftey opens.

Nov 26, 2008 | Heating & Cooling

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