Two immersion heaters with switched FCUs got very hot

[pedr0]

I've had to use my immersion heaters for the first time recently due to a problem with the gas. The switches got very hot and now one won't switch. Reading threads on this forum have raised some questions about the switches....

I've got a Gledhill tank laid on it's side with two 3kW immersion heaters. Each one is controlled by a switched FCU located near the tank. The 2.5mm feed comes into one FCU, loops into the next FCU, and then exits. I haven't yet studied the CU to figure out what they are protected by - but this was installed in the last 5 years.

Q1: Should the immersion heaters ever be run at the same time (e.g. if I wanted to heat the full tank as quickly as possible), or should I always choose one or the other?
Q2: Should I replace the broken FCU for a new FCU, or swap out both FCUs for 20A DP switches, or get one of those dual immersion heater switches (the last option being linked to Q1)?
Q3: I'd prefer to move the switches about a metre to somewhere more accessible, which would mean a longer cable from the immersion heater to the switches. The cable is currently 1.5mm stranded heat resistant cable, each one is about 1.5m long. The new configuration would require longer cable of about 2.5m to each heater- is that acceptable?

 

[JohnD]

Q1 no
Q2 dual
Q3 yes


The immersion heaters should not be on FCUs

They are rated at 16Amp and run continuously for long periods. This will make the fuse get hot, though probably not hot enough to melt, resulting in sustained overheating.

Immersion heaters should be on a dedicated radial circuit, with a DP 20A Switch. Usually on a 16A or 20A MCB. And only one immersion heater on the circuit, if you are able to turn both on at the same time. If you want two heaters on one circuit, you can get a sink/bath changeover switch.

 

[JohnW2]

Q1 no ..... Immersion heaters should be on a dedicated radial circuit, with a DP 20A Switch. Usually on a 16A or 20A MCB. And only one immersion heater on the circuit, if you are able to turn both on at the same time. If you want two heaters on one circuit, you can get a sink/bath changeover switch.

If the OP so wished, I don't see why two immersions, each with their own 20A switch) could not be on a single 32A circuit, do you? (I agree than no immersion should really be fed by an FCU - although it seems that many are!).

They are rated at 16Amp and run continuously for long periods.

What are "rated at 16A"?

Kind Regards, John

 

[JohnD]

What are "rated at 16A"?

Each heating element.

 

[Harry Bloomfield]

Each heating element.

3Kw at 240v = 12.5amps.

 

[JohnW2]

Each heating element.

Why would a 3 kW heating element be 'rated' at 16A?

Kind Regards, John

 

[sparkwright]

Why would a 3 kW heating element be 'rated' at 16A?

Kind Regards, John

Not sure why he said that, though most immersion circuits have a 15 or 16 amp protective device.

Even though these things are only 3kW, they always seem to turn a 13 amp FCU to toast - so I can see why JohnD is encouraging the 16 amp stuff, and 20 amp double pole switches.

Another point is, could two immersions actually be put on a 25 amp MCB or RCBO?

 

[JohnW2]

Not sure why he said that, though most immersion circuits have a 15 or 16 amp protective device.

Quite.

Even though these things are only 3kW, they always seem to turn a 13 amp FCU to toast - so I can see why JohnD is encouraging the 16 amp stuff, and 20 amp double pole switches.

Agreed, which is why I said that FCUs were undesirable for immersions. However, as I also said, they seem to be quite common - perhaps because FCUs tend to be cheaper than 20A switches?

Another point is, could two immersions actually be put on a 25 amp MCB or RCBO?

I almost suggested that, but then remembered that there are pedants around Even the pedantic argument would then not be valid if (as is very probably the case) the 3 kW related to 240V, since two would then take only about 23.96 A at 230V. However, if they really were "3kW at 230V" (which I very much doubt), then I'm sure someone would point out that 26.08 is greater than 25 - so I kept my typing mouth shut

Kind Regards, John

 

[pedr0]

Erm - well both have been turned on in the past week at the same time and nothing tripped at the CU. I still haven't determined which circuit they're on at the CU. But I can't see anything that's obviously 25A. Almost nothing's labelled unfortunately (see picture).

Re: the immersion heater power, they are 2.7-3kW at 230, seemingly (see picture)

 

[EFLImpudence]

Re: the immersion heater power, they are 2.7-3kW at 230, seemingly (see picture)

Can't be both.

3kW @ 240V (12.5A) is 2.76kW @ 230V (12A).

 

[JohnD]

Not sure why he said that

Nor me

 

[JohnW2]

Can't be both.

Indeed not - how very odd

3kW @ 240V (12.5A) is 2.76kW @ 230V (12A).

Quite, as I said (although I didn't round 11.98A to 12A ). However, it's now clear that, whatever the power it is, it (very unusually) relates to 230V.

Kind Regards, John

 

[Harry Bloomfield]

Erm - well both have been turned on in the past week at the same time and nothing tripped at the CU. I still haven't determined which circuit they're on at the CU. But I can't see anything that's obviously 25A. Almost nothing's labelled unfortunately (see picture).

You need to work out what covers what, as a matter of urgency and label them up.

Re: the immersion heater power, they are 2.7-3kW at 230, seemingly (see picture)

The 2.7Kw relates to the 230v nominal voltage. Likely, you home will have a 240v supply, and the element will draw 3Kw.

 

[Sureitsoff?]

I wonder if they fcu's are part of one of the ring ccts so are protected by a 32a mcb

 

[JohnW2]

I wonder if they fcu's are part of one of the ring ccts so are protected by a 32a mcb

Who knows?! ... or maybe supplied by a 32A radial circuit, just for the immersions.

Kind Regards, John