What is the purpose of the hot water tank and boiler when it comes to a) heating and b) drinking water.

When I switch on my boiler (not combi) for both heating and drinking water, I see a flame lit heating something. Does this heat water that goes into the hot water tank (cylinder)?

Is this tank then used for both drinking water and heating the central heating system? Or just one of them?

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1 Answer 1


Without seeing your particular system it is impossible to tell how it is configured.

Generally, the device called a boiler is for heating water to heat the house. This is a closed system and does not supply hot water for the house.

A device called a water heater is used to heat potable water for the house that is safe to drink and all other personal needs. This is an open system and takes in cold water from the domestic water supply to heat for use.

It is possible your system does both but the two piping systems should be kept separate so the potable water supply is not contaminated. Your system may pre-heat water while the boiler is running to heat the house and store it in a tank.

To get a better answer you need to post pictures or diagrams of your system.

  • Thanks @ArchonOSX, does this photo help? Its a tank that has pipes going up to the ceiling into the loft, I assume to cold water storage? I also have a switch next to this tank which I understand uses electricity rather than gas. I guess my question is, does this tank store the same water for hot drinking water, as well as heating water? If not, where do either get stored? Excuse the christmas paper wrapping! Thanks!
    – User101
    Aug 27, 2017 at 11:05
  • I would seriously doubt the two systems are interconnected. That would violate most plumbing codes. That appears to be your domestic hot water holding tank and it is probable that it has a heating coil in it. The heating system and boiler normally don't require a holding tank although some systems have them. The heating system usually has a very small expansion tank connected to it to allow for the expansion of the water as it is heated but the tank itself has no heating coil.
    – ArchonOSX
    Aug 27, 2017 at 11:59

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