Cylinder with 2 immersion heaters

[Mick]

I have just moved into a place with all electric heating, single tariff, and have a hot water cylinder with 2 immersion heaters. The bottom one is the 'normal' heater, and the top one is the 'boost' heater. The top heater is connected to a manual wall switch and can be turned on or off. Only one heater can be on at any time, so if the boost is on and the main one comes on, the boost goes off.

We also have a timer that controls the water heating, and it only switches the bottom heater. The timer also has a 'boost' switch that also only operates the 'normal' heater and not the 'boost' heater.

Is this correct or has it been wired wrongly? I would have thought that the 'boost' switch should operate the 'boost' heater.

[BigWallop]

This is the correct way. The boost element is used to keep the boiler topped up over night. The main element is used to heat the tank when it needs full replenishment. The boost switch is there only to over ride the timer settings.

[Christian McArdle]

It seems reasonable for an Economy 7 setup. The bottom heater will heat the whole tank. This is done overnight on cheap rate. The top heater only heats half the tank and is done during the day on full price electricity when you run out of water.

With a single rate tariff, there is no reason to have the top heater. In fact, you might as well leave the main element on 24 hours a day, provided the cylinder is up to modern standards of insulation. The difference in heat loss between a fully heated tank and a half empty one will actually not be that great.

Better still, you might prefer to switch to Economy 7, particularly if you can stomach electric storage heaters. (I assume you have electric heating as well as water?)

Christian.

[robgraham]

I find this odd. In my system the boost can be put on at any time, and in addition to the main bottom one (which only comes on at night). What controls the switching off of the main heater if the boost is on?

I would imagine that the boost switch here enables you to override the timer so that you can use the main heater out of hours, as it were, so that you can maintain a full tank of hot water even during the day.

But you have a switch for the boost, although why it switches off the main heater I don't know.

Rob Graham

[Christian McArdle]

Presumably the feed is rated for a single 3kW heater (i.e. at 13A or 16A) and can't support 6kW as required by having both immersions on. An upgrade of the wiring would enable both immersions to be used simultaneously.

Christian.

[robgraham]

Yes, sounds like it. But what, I wonder, switches one heater off if the other is switched on.

Rob

[Christian McArdle]

A relay or contactor, I would imagine.

Christian.

[Wanderer]

Pardon? A twin element cylinder is intended for use on E7 or similar tariff, where the bottom element is switched on over night to give a full tank of hot water at cheap rate and the top or boost element is used at other times as a top-up.

In an ideal world, the bottom element should have a restricted circuit, and the top element an unrestricted circuit, but they are - or should be

- used on an unrestricted circuit with a Horstmann or similar controller. The t/s in the controller can be set to bring on the bottom element in the cheap rate. The boost switch on the controller gives a notional one-hour boost on the top element during peak rate times. I think the controller has some form of interlocking that stops both heaters being on at the same time, it's a long time since I poked about inside one!

What often happens is because the Horstmann Controllers can cost anything from £50 to £70, depending on where you buy them, people cut corners and fit ordinary timeswitches so you're then relying on a human being to make sure they don't get switched simultaneously.

[Keithc666767117]

You are correct in stating that both immersion heaters should have own electrical switch. I have recently left property with same design. try running a spur from current switch and connect booster to this, this should supply independant power to each immersion.