I'm not from the UK and am used to water being heated on-demand by a gas heater.
Having recently moved to London, the flat has electrical heating and I cannot figure out how I'm supposed to set this electric boiler system.

Here's some pictures to get an idea of what's there:
- http://i.imgur.com/6ASWnpS.jpg - the boiler and it's Control Unit (not sure what it's actually called)
- http://i.imgur.com/7OIgZu7.jpg - the Control Unit buttons and text

The control unit has a switch with positions Off and Timed. There is also a boiler power switch labelled Immersion.

The Immersion switch seems to turn the boiler on, as it starts making water noise when turned on and five minutes later I can get some hot water on the tap.

With the Immersion switch On, changing anything else on the Control Unit doesn't seem to have any effect. Again, when the Immersion switch is Off, there's no hot water, regardless of the Control Unit.

On the top side of the boiler, to the left of the Important label, is a valve which is itself labelled as T&P valve and the label next to it refers to a Temperature and Pressure release valve.
When I turn on the Immersion switch, water starts heating, but about 10 minutes later, water comes out of this T&P valve. It's not dripping, it's actually running, I can't estimate the debit, but it's probably like a half-on tap, which to me seems like a lot.

The Important label says: If the T&P valve is dripping (not running) to 1) Turn off mains water, 2) Open a tap, 3) Hold the valve open until it stops making noise, 4) Turn off tap and mains water back on.

I've tried turning on the Immersion switch, waiting 10 minutes, water would come out of the T&P valve and then I've turned on the shower. The shower was warm, but not boiling hot, and the water turned to completely cold in about 3 minutes. After this point, the T&P valve still had water running out.

After I turn off the Immersion switch, the valve will stop in a few minutes.

Does anyone know how this thing is supposed to work?

1 Answer 1


I've not used a system like you have, but I'll give you some information based on what's common for heating systems in the UK.

As you've discovered, the immersion switch immediately switches the heater on. It's usually used as an override when you want water to start heating outside the timed period. (I'd hazard a guess that having the immersion on for 10 minutes won't be long enough to heat sufficient water for a shower).

You'd usually set the heating on timed, but it will take a while to heat up the full tank of water. For example, if you get up at 07:00 and have a shower, you might need to set the timer to come on at 06:00 to give you a tank of hot water ready for when you get up.

It's common to set the timer to come on twice a day, so you get hot water in the morning and again in the evening - though you'd adjust that dependent on your usage pattern. It might take a little experimentation to find out when you need to set the timer to come on to give you the hot water you need.

If the T&P valve is flowing quite an amount of water, it sounds like something is amiss with the system.

  • I was thinking of timing it something like the twice per day program you mentioned, but there's the two problems: 1) the time seems to be completely ignored and 2) the T&P valve flows a huge amount of water after 10 minutes. Thanks for the information though.
    – Mydiyac
    Dec 8, 2014 at 14:17
  • Have you bought the flat or are you renting?
    – John
    Dec 8, 2014 at 20:17

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