Shared immersion heater circuit?

[Philip Saunders]

Hello, everyone

I want to install a second unvented hot water cylinder.

The existing (indirect) dhw cylinder has a backup immersion heater on a dedicated circuit with a timer. The immersion is timed to come on overnight, but usually does nothing as the boiler keeps the water hot enough so the immersion thermostat never turns on unless the boiler goes out (not often, but not unknown either - usually due to an empty oil tank). This way, we always have hot water to wash with in the morning,

I want to install a second direct unvented cylinder in a completely different area of the house.

The existing immersion circuit is re-purposed from what was once a radial circuit run in 4mm to power an outbuilding (long demolished), and runs right past where I want the second cylinder.

Can the second cylinder share the immersion circuit? There is/will be nothing else on the circuit.

I know that immersion heaters are supposed to have dedicated circuits, but it doesn't explicitly state that they need one each.

Thanks.

 

[Wilko]

Hi - it may be possible, but first a few quick questions

• what is the installation method of the existing 4mm cable?
• what is the existing circuit breaker rating?
• what are the two immersion heater kW ratings?

 

[Philip Saunders]

Hi - it may be possible, but first a few quick questions

• what is the installation method of the existing 4mm cable?
• what is the existing circuit breaker rating?
• what are the two immersion heater kW ratings?

Hi thanks for replying

Installation method is mostly clipped to joists on the attic under 100mm of insulation. About 3 feet is in conduit through several walls
MCB is B16 (Square D)
The existing immersion is 3kw, as will be the new one.

Obviously I will have to change the breaker - based on the wiring in the rest of the house, a B32 should be OK (there are other radials in 4mm on 40a RCBO's)?

Assuming it's OK, I'll be putting each one on a DP switch so I can isolate them individually for servicing.

 

[Wilko]

As soon as you mention that the cable is under insulation there may be a problem. The 4mm cable exceeds 32A if it is clipped direct in free air, but not under insulation, sorry.

 

[Philip Saunders]

As soon as you mention that the cable is under insulation there may be a problem. The 4mm cable exceeds 32A if it is clipped direct in free air, but not under insulation, sorry.

OK. If I either re-route or more likely re-run in 6mm, can I share it or should I pull in another 4mm? There is a spare way in the CU but I was trying to avoid creating a new circuit. Running another cable will be a pig to do - access isn't great.