Caravan Heating Upgrades

[Brad93]

Currently caravanning.

Father in law has a repeated problem with the 1800w combi-boiler in this caravan blowing a very hard to access 10A time delayed / High Breaking Capacity fuse.

My thoughts, and the consensus on a few forums, is that a poor supply voltage, voltage drop from 25m hook up lead and other loads running at same time, causes the hot water heater to pull more than 10A (P=IV) and blow the fuse.

I believe it has blown on us today.

We were running a small convection heater at the time which probably pulls a fair few amps guessing by how hot the plug gets. (I took clamp meter out of van ). We use this to dry towels & boots in the awning.

Went to use the hot water and it started warm then went cold.

Does this all sound plausible so far?

So anyway, knowing this is a problem, i've pre-purchased a quality 20x5mm 16A fuse holder from RS Components, and some new T10A fuses - with the plan to solder two leads onto the original fuse block, and mount a fuse holder on the casing of the boiler, so fuse can be replaced easily.

However, i've had a thought. Perhaps it would be better to install a MCB?

What are peoples thoughts?

Below is attached a photo of the fuse on the PCB which is almost impossible to access without complete strip down and removal of the boiler unit.

 

[sako243]

The fuse is there to protect the PCB.

If you replace it with a 16A you may well be risking the PCB. Up sizing of fuses I'd generally advise against unless you know what's going on downstream of it. You're talking at least a potential of 60% increase of current through various bits of the PCB and that's significant in terms of track thickness. A 1oz copper PCB to handle 10A you're looking at ~8mm wide traces which don't look dissimilar to what you have there. Up it to 16A and you're looking at ~13mm tracks widths.

If you can reliably blow it then I'd be seeing if you can narrow down the cause of why it is blowing. Try a shorter lead / thicker cabling to see if voltage drops are causing issues.

 

[Dave-sparks]

My thoughts, and the consensus on a few forums, is that a poor supply voltage, voltage drop from 25m hook up lead and other loads running at same time, causes the hot water heater to pull more than 10A (P=IV) and blow the fuse.

Reduced voltage will cause a reduction in the current flowing in a heater not an increase.

If its blowing a fuse then it's more likely that there is a fault.

PCB fuseholders can also loose their springiness over time and so loose their grip on the fuse.

It is a common misconception that power remains constant, it doesn't, it is the resistance of the heating element which remains constant.

If current increased as voltage is reduced then dimmers would make lights brighter when they are turned down and reducing voltage to zero would cause infinite current flow.

 

[Brad93]

The fuse is there to protect the PCB.

If you replace it with a 16A you may well be risking the PCB. Up sizing of fuses I'd generally advise against unless you know what's going on downstream of it. You're talking at least a potential of 60% increase of current through various bits of the PCB and that's significant in terms of track thickness. A 1oz copper PCB to handle 10A you're looking at ~8mm wide traces which don't look dissimilar to what you have there. Up it to 16A and you're looking at ~13mm tracks widths.

If you can reliably blow it then I'd be seeing if you can narrow down the cause of why it is blowing. Try a shorter lead / thicker cabling to see if voltage drops are causing issues.

Sorry, i've bought a 16A fuse holder but have purchased 10A fuses. The 10A rated holders looked pants and not fit for 3A let alone 10!

At the minute it's difficult to narrow down the fault as it seems to be completely random.

Also replacement of the fuse takes 90 minutes.

 

[Brad93]

Reduced voltage will cause a reduction in the current flowing in a heater not an increase.

If its blowing a fuse then it's more likely that there is a fault.

PCB fuseholders can also loose their springiness over time and so loose their grip on the fuse.

It is a common misconception that power remains constant, it doesn't, it is the resistance of the heating element which remains constant.

If current increased as voltage is reduced then dimmers would make lights brighter when they are turned down and reducing voltage to zero would cause infinite current flow.

Thanks Dave, that makes sense.

So, as far as i know, this unit has two elements of around 60 ohms. And is rated at 1800W.

So any voltage drop would not cause a higher current draw by the heater, and neither would additional loads on the electrical circuit.

I will confirm the resistance of the two elements when i strip the unit.

Unlikely the PCB will be at fault in my opinion. It works faultlessly on gas, and has worked well since the fuse was last replaced.

 

[Dave-sparks]

Thanks Dave, that makes sense.

So, as far as i know, this unit has two elements of around 60 ohms. And is rated at 1800W.

By my maths at 230V the current should be 7.7Amps and at the UKs maximum voltage of 253V (230+10%) the current flow should be just under 8.5Amps.

Neither of which should blow a 10A fuse.

Do you have an insulation resistance tester?

 

[rich_c]

Long shot, but one more thing to think about is the ambient temperature wherever that PCB is situated - make sure it's not sat in its own heat output or the convection heater you mentioned. Tbh, even if you were roasting it, I can't see its rating dropping by more than 5%-10% though.

 

[6ply]

Thanks Dave, that makes sense.

So, as far as i know, this unit has two elements of around 60 ohms. And is rated at 1800W.

So any voltage drop would not cause a higher current draw by the heater, and neither would additional loads on the electrical circuit.

I will confirm the resistance of the two elements when i strip the unit.

Unlikely the PCB will be at fault in my opinion. It works faultlessly on gas, and has worked well since the fuse was last replaced.

The heater element is unlikely to have an Ohmic response. It's resistance is not fixed across all temperatures. If you really want to know its resistance when running you need to measure voltage drop across the heater element and current drawn then calculate R=V/I

 

[sako243]

@Brad93 can you get a picture of the whole PCB and both sides?

 

[Martin Hocking]

By my maths at 230V the current should be 7.7Amps and at the UKs maximum voltage of 253V (230+10%) the current flow should be just under 8.5Amps.

Neither of which should blow a 10A fuse.

Do you have an insulation resistance tester?

Personally I never go on holiday without an insulation resistance tester…

 

[Brad93]

@Brad93 can you get a picture of the whole PCB and both sides?

Yes i dropped caravan off today. Heating miraculously working today, we had hot water. So maybe some sort of other fault. Who Knows.

I will be going back one weekend to sort this out.

 

[Brad93]

By my maths at 230V the current should be 7.7Amps and at the UKs maximum voltage of 253V (230+10%) the current flow should be just under 8.5Amps.

Neither of which should blow a 10A fuse.

Do you have an insulation resistance tester?

That's what i made the amperage out to be also.

Yes a fluke 1507.

I'm not electrically minded/trained, but i do have to work on motors/control circuits fairly regularly.

I have a Fluke 117 DMM, Fluke 374 Clamp Meter and just picked up a 1507 Insulation tester.
I'm just a dumb welder really.

 

[Brad93]

This gives you an idea of the combi boiler.

This guy has exact same problem and it's the exact same unit.

 

[sako243]

@Brad93 you have got slow blow fuses in there?

From the little bit I can see of the circuit board in that video I'd hazard a guess it's just switching in different heating elements.

I'm guessing the wrapped wires on X2 go to the heater elements? Where do the wires on X4 go?

If there's a blower unit attached is the motor clean? It could be pulling more current if it's struggling / dirty inside.