I have one radiator that does not work at all (although it has worked in the past). Unlike all the other radiators that come directly of the main supply and return pipe from the boiler I can see that one end of the radiator is connected to a pipe that runs directly to the bottom of the hot water tank. I cannot trace where the other pipes goes, it seems to run towards the back of the hot water tank but cannot see where it goes after this point. Because this radiator is newly installed we don't actually know which pipe is the supply and which is the return, both pipes are stone cold. Its replacing a previous one but the installer didn't make note of which end had the thermostatic valve before he removed it and redid the pipes.

My questions are:

  • Why has this radiator been attached directly to hot water cylinder

  • Will the pipe attached to the bottom of the hot water cylinder be supply or return ?

  • Where logically should other pipe attach.

  • What could stop it from working ?

1 Answer 1


Why has this radiator been attached directly to hot water cylinder

Seems like a bodge to me.

Traditional UK systems have the radiators completely separate from the hot-water (which supplies taps only). The radiator system is usually a closed loop - with some means of topping up the water - either a header tank in the loft (US: attic) or a valve next to a pressure vessel with a pressure gauge.

Simplified Diagram of Honeywell W-Plan Source: Honeywell

The hot water tank usually has four connections, two for supply and return from boiler which connect to an internal helical loop, there is no transfer of water from this loop into the tank, only a transfer of heat.

The tank is supplied with cold water at the bottom (e.g. from a separate header tank) and hot water to taps is drawn out the top.

Diagram of traditional UK boiler system Source: Potterton, modified to show pump, three-port zone-valve, vents and cold water to taps

Will the pipe attached to the bottom of the hot water cylinder be supply or return ?

Generally you remove hot water from the top and supply cold water into the bottom.

Where logically should other pipe attach.

Your arrangement seems illogical. Probably the top of the cylinder.

What could stop it from working?

  • Sediment clogging the supply pipe or radiator
  • A valve turned off, blocked or broken.
  • Lockshield valve (that "other" radiator-valve) needs adjusting?
  • Air in the radiator
  • Failure of a circulation pump (if present†)
  • Bad design or design reliant on specific characteristics of removed radiator.
  • Programmer/timer set to CH on but HW off.
  • Failure of HW thermostat or set too low for magic radiator to do its thing.
  • HW thermostat set too high or insufficient call for HW to sustain temperature gradient in tank needed to support magic for radiator?

† I believe some very old heating systems used natural convection but I've never seen one‡.
‡ I'm not a plumber.

  • I think it is a bit bodgy, essentially there is a supply and return pipe from the bolier than runs along the first floor, This has radiators pipe coming off the supply and onto the return and it also connects to the hot water tank, and then further radiators run of the supply pipe and onto the return past the tank. There is a a T-Valve ( I think that its name) that is controlled by the heating panel so that if you can have both, just central heating or just hot water and that controls the supply pipe to the hot water tank. Dec 3, 2017 at 12:04
  • There is only one pipe coming out of top of hot water tank and that connects directly to hot water pipe. So I guess the pipe I see is the return pipe and the radiator is connected to a supply pipe somewhere, there is one pipe in the airing cupboard that disappears into the floor and I dont know where it goes afterwards so perhaps it could be that one. Dec 3, 2017 at 12:07
  • If I understand you correctly I didn't realize that the supply pipe doesnt actually fill the tank with hot water but just heats an element that then heats hot water tank supplied by the cold tank Dec 3, 2017 at 12:17
  • Is the radiator return into the hot water tank just a way of dealing with excess water in the central heating system ? Dec 3, 2017 at 12:18
  • @Paul. Normally no radiator pipework is connected to the hot-water tank at all. In a traditional system, excess water is vented into the header tank in the loft. Dec 3, 2017 at 16:13

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