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.




12 Responses

  1. Paddy Falls says:

    Have exactly this problem on my Benimar. Have checked the power fuses on the van, which are all OK. I’ve tried the overheating protection reset. The button doesn’t seem to press very far, and doesn’t fix the problem. Looks like I might need to replace 230 V fuse. I don’t feel qualified to replace the 230V fuse. A couple of questions:
    1) Is there a low risk way of confirming the 230V fuse on the Combi has actually blown? I have noted that when the “W 45 H” error shows on the Combi Trauma control panel a red light on the Combi flashes a morse code of ‘..-.–.-‘ (EAAK). Have contacted Combi, but no reply from them.
    2) I’m currently in isolation in the mobile home near the A1 in the east midlands. Any ideas on where I might find a qualified person to come do the fuse change?

    • steve says:

      Hi paddy

      I assume you watch the video on changing this fuse. The hardest part is getting the unit out of the installed position. If you do not have any 240V plugged into the van then there is no risk of hurting yourself, its a matter of nuts and bolts to get to the fuse panel then pull out the fuse and put new one in.
      As for gas, as long as you connect up the gas union and use some soap water to check for any leaks. You can get a gas leak regulator to check for any leaks, or if you are not sure once all connected and fuse replaced pop into a motorhome service centre to just get a gas leak test, lot cheaper than getting them to change the fuse.
      Up to you do what you feel comfortable with, like said getting the unit out on some vans can be a pain

  2. Paddy Falls says:

    Thanks for your answer. Yes I have looked at the video, it was really helpful. I’m fortunate in having easy access to the unit. My biggest issue was trying to work out exactly what fuse I needed. The related thread below give me very good answers on that. We are new MH owners. Really pleased with how useful the forums have been. Cant say the same for Truma (unit still under warranty) I have tried ringing (‘we’ve closed due to Covid, try email’) and emailing (no response yet).

    • steve says:

      I have some spare fuses. if you want to give me your address I will post you some, alternative copy past this is ebay
      Fuse 10a 20mm HBC Antisurge/ Time delay T10A H 250v Ceramic 0215010.MXP x5pcs

      first person on search semTech are selling what I bought

      • Paddy Falls says:

        Thanks, I had already sent off for 10 fuses on EBay, costing just £3.63, before seeing your reply. Fuse replaced and clears error code. Now to see how long it lasts. I could take advice from another site about start heating on gas and then switch to EL 2. Some other site suggested run off EL 1 instead of EL 2. I’m currently connected to UK mains. In this case is there any difference between using EL 1 and EL 2?

        • steve says:

          Hi Paddy, glade you got it working ok, saved you a penny compared to dealer prices. I have ready to idea about starting on gas which I think is incorrect as the environmental temperature should not affect the fuse.
          EL-1 runs the unit on half power I think its about 800 Watts and EL-2 will be full power about 1600 watts.
          If you run on EL-1 continuously then you will have low heating and water temperature as the units only outputting 800 watts, which on a cold winter will not be adequate to heat a van up.
          I believe most of the fuses are blowing on start up. Heater elements pull more power when starting and less when they have heated up. So if you start the unit on EL-1 and then after a min when the elements have warmed up switch to EL-2 for full power.
          Hope this makes sense
          So far we have not had any more blown fuses, but only time will tell.

          • Paddy Falls says:

            Your answer does make sense. Given we are coming into warmer days, I’ll run off EL 1 and see how well that works for water and heating. We’ve been using a small convection heater to save on our gas, and that has proven to be a usable alternative.

          • steve says:

            We bought a convector heater as well, can’t beat them for heating and very reliable, shame Truma switched to this unit as we find its not great for heating at night, the unit is right under the bead, over heats the bed and noisy.
            Good luck with the van, can’t wait till this virus passes us all and we can get out driving again. Where is UK as well in Kent, looking forward to a drive to Spain in August if all goes well. Stay safe

  3. Chris says:

    Hi Steve,

    Thanks for the advice and the really useful video. I currently have the “W45H” issue on a Combi 4E in my 2018 caravan, and was able to use your video to locate the 240v fuse, remove and check it. (Fortunately there is enough room in my ‘van to work on the unit in situ 🙂 )

    Sadly the 230v fuse seems to be OK (I can’t see the fuse wire as the casing has a white coating with some stripes on, but the continuity was fine with my multimeter). I also checked the power supply, and there is 240v up to the connection between the lead from the Sargent unit, and the lead to the Combi – thanks for showing that too 🙂 Using a mirror the lead looks to be soundly connected to the Combi.

    Do you have any other suggestions for what might be causing the problem? I’ve given the reset button a couple of presses, but I wonder whether I need to do that for a few seconds, or when the error alarm is sounding. I’ve checked that all the obvious switches, MCBs etc. are in the ‘on’ position, and the 12v fuse seems fine (the green and red LEDs light up on the 12v board, and the small fan spins up when the control panel first calls for heat). The Combi works fine on gas.

    I’ve put in a call to my dealer and Truma, but given your video thought I would ask here too 🙂

    Thanks – Chris

    • steve says:

      Hi Chris.

      If your fuse is ok and you have 240v getting into the unit, then fault could be with the lower PCB board or the thermostat is open circuit. From my blog the second picture down shows the lower PCB. Check you have 240v at the bottom left connection where the supply comes in.
      Then check you have voltage on the thermostat. I have not found a schematic for this device but you should have continuity on both sides of the thermostat if unit is cold. If there is no continuity them possible the thermostat is defective. If you have continuity across the thermostat then possible there is a dry joint on the fuse carrier on back of the PCB board. If you have no continuity from incoming supply to thermostat then you have a fault with either the fuse holder or the R3 resistor

  4. Chris says:

    Hi Steve,

    Thanks for the extra information. Without wishing to sound too dim, where will I find the thermostat? Do you mean the room thermostat near the Truma control panel, or a thermostat on board the boiler?

    Sadly I can’t see an obvious way of checking the voltage where the supply cable joins the lower PCB – my multimeter’s probes don’t go far enough into the screw holes on the connection block, nor do they go far enough into the actual connection points. However, following a conversation with my dealer I checked the voltage either side of the red reset switch – that was showing at 101v, presumably as the R3 resistor was limiting the voltage immediately prior to that. My Truma control panel doesn’t show the small ‘plug’ symbol when 230V is switched on, but I’m not sure at what point that signal is generated.

    As the unit is under warranty the dealer has booked it in to have a look in a few weeks, so I’m probably going to leave it to them, but if the thermostat is easily accessible then there’s no harm in looking at that – if nothing else it will help their troubleshooting.

    Thanks again for your help so far 🙂


    • steve says:

      Hi Chris. The thermostat I am talking about is the one with the red button, this is an internal thermostat. I would think you should have 240v either side of this as the R3 resistor should only limit inrush current, one side will be a neutral so with meter on neutral here you should see 240v both side of fuse and both sides R3 then this goes to the thermostat from what I remember. Be better and safer to do this with continuity setting rather than live voltage. Please let us know on here or the youtube channel what the dealers find out. Bet they just replace the lower card and not really investigate much.

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