BEFORE FOLLOWING THIS GUIDE. YOU NEED TO BE A COMPETENT PERSON WORKING ON HIGH VOLTAGE EQUIPMENT. THIS IS A GUIDE CHANGING OUT THE FUSE ON A 240V EQUIPMENT WITH EXPOSED VOLTAGE PARTS WHICH WILL LIKELY KILL YOU OR SERIOUSLY BURN IF YOU TOUCH THEM.
We have a Truma combi boiler to heat the hot water and heating in our new Swift motorhome which has developed a fault. There is no heating or hot water when on electric but was working all OK on gas.
The system when on electric has two 240v heater elements which are rated at 900 watts each for my system, a total of 1800 watts when connected in parallel, so should be pulling around 7.8A on full power.
The swift van has a Sargent power unit which has a heater/water system isolation button and there are MCB/RCD's within the unit which need to be checked before thinking this is a Truma fault. On my fault the supply to the Truma unit was all ok.
Next check that the over heat switch has not operated which can be found near the top of the unit, a small hole on the side with a red button. Insert a pencil and push in to check the switch has not operated.
If all ok and you have 12v LED showing on the Truma control panel then the fault maybe due to the 240V fuse on the Truma control PCB blown. To get to this panel you may need to remove the Truma unit from its installation position so the side panel can be removed. Before removing any covers ensure all electrical connections have been removed.
The lower panel PCB you can see a 10A fuse which is a T rated H type fuse which means it is time delayed to rupture and is designed for surge current devices like this heater system. Heater elements can start on a low resistance when first switching on then increase current as they heat up. In the picture below the fuse shown is not the correct fuse, this was used for testing whilst the new fuses where on order.
If this fuse has blown then it may be due to a defect with the heater elements, short circuit or insulation breaking down to the bodywork or a fault with the PCB possibly a low or short across R3 which I believe is a NTC thermistor but can not confirm this.
On my unit the elements where all ok. R3 had a high resistance across it as I would expect and so I could not find any reason for the fuse blowing other than possible age of fuse, close rated fuse to operating current of unit especially when turning on in cold climate on full power, inrush current would be high. So I bought some new correctly rated type fuses from ebay and the unit powered up all OK.
When on full power the elements have 240v across each one and when on half power each element has 130 volts across it. Looking at the PCB, there is a possibility that when on half power the large diodes in the picture are put inline with the elements reduce the half AC cycle lowering the input voltage and reducing the power output. This is only my theory and maybe someone from Truma will comment and prove me wrong.
For my system the unit has been running on full power for some time with no problems and no fuse blowing. One factor also to mention, when packing stuff under the bed ensure it is not packed tight around this unit as it needs to expel heat and needs to intake air to give good air flow across the elements and the control circuit. This is something we was bad at and could also have contributed to the fuse rupturing.
Hope this is of some help. There are links below to manual and a YouTube video. Please comment below. If yo find something wrong with what I have written then again comment below so we can all gain some knowledge on this circuit.