My rented flat (in the UK) has 2 expansion tanks behind the boiler, one red, one blue.

From googling around I can find lots of references to hot water expansion vessels; the purpose seems to be to allow for expansion of the water in the closed system of the heating circuits as it gets hotter.

What's the purpose of the cold water vessel? Is it just to prevent water hammer?

Update: here's a description of the lines and where they go, as requested in a comment. Please correct my terminology, I don't know what it should be but I'm keen to learn:

  • cold water seems to arrive from outside - there's a stopcock.
  • When I close the stopcock, I have no cold water anywhere in the flat.
  • Immediately after the stopcock, is what I think is a pressure reducing valve (Caleffi - 3.5 bar is printed on it).
  • After that, there are a few branches off this pipe:
    • one leads to the blue expansion tank.
    • one leads to a cylindrical tank below ('Keston Spa'). Where it joins, it's marked "MAINS CWS" (Cold Water Supply?)
    • one leads to a hose with closed valves that I believe can be temporarily opened to 'top up' the closed heating system: beyond the closed valves, I can see that it connects to the return port on the boiler (according to the manual), as well as the red expansion tank.
  • finally it seems to go off around the flat.
  • Can you trace the lines coming from the tank and tell us where they are in relation to appliances and supply? Dec 7, 2011 at 21:22
  • updated with appliance details.
    – bacar
    Dec 8, 2011 at 0:33
  • A picture would be useful, too.
    – Kevin Reid
    Dec 8, 2011 at 0:39
  • I think a photo will be incomprehensible :) but I might try to knock up a diagram for stackexchange posterity. Does my setup sound quite ordinary?
    – bacar
    Dec 8, 2011 at 0:48

1 Answer 1


An expansion tank is needed on a section of closed line that is subject to variable pressure. The only sane cause I can discern for having two tanks in close proximity relates to multiple devices with multiple check valves or an undersized expansion tank. Whether that tank is before or after the water heater is usually irrelevant. The job is to take up liquid expansion between a closed faucet and a supply line check valve.

It may be the case that your water for radiators is separated from your drinking water. Two distinct systems with a check valve to prevent radiator water from draining back could explain a second tank.

  • Pretty sure the blue tank is not connected to a closed system. It appears to be attached directly to the cold water mains.
    – bacar
    Dec 7, 2011 at 21:36
  • 1
    @bacar Right, but it would make sense to have one for the closed system and one for the system in general. Otherwise you'd only get relief on the main section if there were enough pressure to force more water into your heating system. That would constantly increase pressure in the heating system and eventually lead to a relief valve forcing open. Dec 7, 2011 at 21:39
  • I'm not sure I follow. Does "the main section" mean the section connected to the water main? Why would water get forced into the heating system? It's isolated from the mains by a couple of manually operated open/close valves.
    – bacar
    Dec 7, 2011 at 23:01
  • 2
    @bacar, I think Jeff's last paragraph answers it. One expansion tank is for the boiler and heating system lines that are closed off. The other is for your drinking water lines which feed into a hot water tank. With the pressure reducing valve and any check valves, expanding water from the hot water tank can't expand back out the water mains. Otherwise, after your turn off the shower, the pressure would build up until something (toilet, sink, TPR valve) leaks.
    – BMitch
    Dec 8, 2011 at 0:17
  • @bacar Yes, "main section" is everything but the heating system. The heating system circulates radiator water separately from your sink / shower hot water taps. That's why the tank is separate. Dec 8, 2011 at 0:27

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