bolero boy Posted October 10, 2018 Share Posted October 10, 2018 I've been browsing another forum and there is a suggestion that excess heat, with a Combi in an enclosed space, might be blowing the 240v fuse, rendering the unit inoperative on electric....but gas ok. furthermore, the operation in mixed mode is seen as being the culprit for this specific issue. while I could see that the unit might trip the overheat protection, if tightly enclosed, I can't see it being responsible for blowing fuses? my own unit (Combi 6e) is in a tight location, under the fridge/freezer, and I have used it on all modes over the past (near) two years without issue.... I have a German van, the vans being discussed are AutoSleepers that 'interact' with Sargent electrics... I'm told other brands have has this issue, but googling has on.y highlighted overheating or, perhaps, PSU failure... any views? Link to comment Share on other sites More sharing options...
Derek Uzzell Posted October 10, 2018 Share Posted October 10, 2018 A link to the forum thread where this suggestion is made would be handy. Link to comment Share on other sites More sharing options...
bolero boy Posted October 10, 2018 Author Share Posted October 10, 2018 it would indeed, Derek, but I'm not very welcome on that forum and don't want to move this discussion into their territory.... one user has put a couple of computer fans in the heater box and claims that this measure, along with never using mixed mode (it makes the heater 'too hot') has cured the fuse blowing , which happened twice I think.... my own view is that heat doesn't blow fuses, excess current does...which is why I am more suspicious of other outside electrical 'influence'.... I can see that the overheat trip might be activated, but I've not heard about blown 230v (10 amp slow burn, I think) fuses sorry to be a bit vague, and I realise this doesn't help the diagnosis.... I was more interested if others had come across this issue before, specifically any known link to mixed mode use and blown fuses. thanks. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted October 10, 2018 Share Posted October 10, 2018 Without reading the entry on “that forum” I’m not prepared to waste time crystal-ball gazing. Link to comment Share on other sites More sharing options...
bolero boy Posted October 10, 2018 Author Share Posted October 10, 2018 no worries, thanks for the reply. Link to comment Share on other sites More sharing options...
Boris Posted October 10, 2018 Share Posted October 10, 2018 "I'm not very welcome on that forum " Chris, I think I know which forum you are referring to and if I'm right, I think you may be mistaken. I use that forum too and have always found your contributions to be relevant, informed and entertaining and very useful to myself. I could see where you may have ruffled the odd feather but IMHO it was one member who can't abide not always being right. You also switched to a van with the wrong badge! (as i did) Don't think it would hurt to let Derek have the link or even post on the (undisclosed) forum. Please give it a bit more thought. You never know...... Link to comment Share on other sites More sharing options...
Derek Uzzell Posted October 11, 2018 Share Posted October 11, 2018 I’m happy to have Chris send me the link to the relevant forum discussion as a PM, and I promise to keep my gob shut about that information. There is a reasonable amount of on-line discussion about the 240V fuse of Truma Combi “E” heaters blowing https://tinyurl.com/y75mkers Truma provides clear guidance on how a Combi should be installed and it’s commonplace for that advice to be ignored by motorhome converters regarding placement of the appliance, ventilation, accessibility, air-duct length, etc. My Rapido 640F has a gas-only Combi 4 and its installation (beneath the rear fixed bed) is far from optimum. I’ve massively increased the heater’s capability to recirculate warmed air by adding extra ventilation grilles and cutting large holes in the panelling near the heater, and I’ve lengthened the shortest air-trunk (which was originally about 6” long) to improve the heater’s air-distribution ‘balance’ - but the system is still relatively poor and I can’t do anything more without gutting the vehicle’s interior. There is speculation that the voltage of a mains hook-up may cause the 240V fuse to blow (ref. ‘pixer’s posting here) https://www.motorhomefun.co.uk/forum/threads/truma-combi-not-working-on-elec.156332/ and, if a Combi “E” is installed where heat can build up and the appliance is being operated on 240V, if heat were a factor in fuse-failure it’s to be expected that failure would be more likely to occur when the heater is running in ‘mixed mode’ (ie. when running on 240V + gas) rather than on 240V alone. If an unusually high number of Auto-Sleepers motorhomes fitted with Combi “E” heaters are suffering 240V fuse-failure, one might suspect this could be due to how Auto-Sleepers installs the heater and/or the remaining electrical equipment (eg. the Sargent kit) being the cause. The usual complaint on motorhome forums is not so much that the 240V fuse fails, but that when this happens it’s such a swine to replace a component that costs just a few pence. So if there’s a simple singular cause of 240V fuse-failure and this can be identified, the problem can be corrected or worked around. There will be thousands of Truma Combi “E” heaters in use in motorhomes, but no statistics to establish how common 240V fuse-failure is. If someone, having experienced 240V fuse-failure a couple of times when their Combi has been running in 'mixed mode’ and, having fitted cooling fans and stopped using ‘mixed mode’, concludes that excess heat was the cause of the fuse failure, they may be right - but a lot of Combi “E” appliances will be running ‘hot’ and in ‘mixed mode’ and it can be assumed (as otherwise it would be generally known about) that their fuses don’t fail. Link to comment Share on other sites More sharing options...
Robinhood Posted October 11, 2018 Share Posted October 11, 2018 The fuse in question blew on my Globecar. The unit being quite tightly installed against the 'van wall, and deep under the furniture, didn't make it easy to diagnose (check) and remedy. Whilst I eventually manged to to get the cover off and remove the fuse blind, I had to resort to a mirror to install the replacement. I don't tend to use mixed-mode, so I'm not inclined to blame that for my experience. Since replacing the fuse (with one of the correct rating) there has been no re-occurrence, though my usage pattern didn't change. I was inclined to blame a faulty or marginally rated fuse, and I seem to remember there was some debate around this on a number of European forums. Link to comment Share on other sites More sharing options...
aandncaravan Posted October 11, 2018 Share Posted October 11, 2018 While I have no experience of this issue happening personally, i.e. seeing the actual failure, the one time we looked at a vehicle 'after the event' we concluded the Sargent power distribution box was not the prime issue but might contribute depending on the model and battery condition. This failure also happened in mixed mode. Our conclusion, no hard evidence to suggest it is correct, was that the Truma unit gets very hot when both Gas and electricity are selected. The 'extra' rise in temperature when gas is used and the interior temperature of the van rises MIGHT lead to the 230v element getting hotter than designed. It's Resistance then rising causing it to draw more current than ideal. The 10amp fuse already runs right at it's limit when things work against the designed operation. So to understand all that more, lets run through the scenario we examined as outlined to us, and possible effects - Owners got back to the vehicle in the evening to a cold van. Heating put on Gas and 230v for fast warm up. Van warmed up quickly and interior got very warm. Wife likes it really warm so heating not turned down despite hot interior. We think that as the van warmed up more recirculated warm air was taken in by the unit. This warm air meant the element potentially 'cooled less' so resistance rose drawing more current. (Note that in this instance the 12 battery was also past it's best so, potentially the 12v fan might have turned more slowly as it's voltage dropped from the draw of the fan, the TV that then got turned on, lights, water pump, etc. The 230v power supply/charger was a poor 10a device, in reality an 8amp unit, so once load rose it would not be able to keep up. So the reduction of real '12v fan power' plus warmer air flowing over the element may have resulted in a much hotter element where the resistance rose drawing more current, closer to 9amps. This in turn caused the fuse to slowly heat up over several minutes and fail. Remember that a 10amp fuse is a resistor that gets hot and melts. It will do that with a sudden high burst of current and fail, but may equally slowly build up to near it's limit over time which will generate heat build up inside the glass, raising temperature (and resistance still further) potentially causing it to melt at just 9amps. The element is rated at about 8amps but if mains input into the van is less than ideal and the interior in the vehicle is like a sauna, as these owners say theirs was, and 12v is under par for the fan, you can see how several things will conspire to work against the designers original work. We fitted an easy access 10amp fast blow fuse into the feed to the boiler. Not fixing the issue which, we think, requires a thermo shutdown self resetting fuse on the element. The 10amp fast blow fuse should melt sooner than the Truma fuse but no guarantee, but easy to replace. Since we did the work, I think mid 2016?, and explained what we think occurred, they have taken the trouble to switch down the heating to just 230v as soon as it gets warm, without any further issues. But they also promised to look after the new battery and being a Varta LFD, it starts off with a higher 13v and potentially higher fan speed than their old conventional 12.5v battery. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted October 11, 2018 Share Posted October 11, 2018 aandncaravan - 2018-10-11 9:42 AM ...Owners got back to the vehicle in the evening to a cold van. Heating put on Gas and 230v for fast warm up... A ‘for what it’s worth’ comment. A Combi “E” operating in ‘mixed mode’ (240V + gas) prioritises on 240V but, when gas is being used as well, the output of the heater’s gas burner is deliberately restricted to avoid overheating. Consequently a Combi-6 “E” running in ‘gas mode’ has a rated maximum heat output of 6000W, whereas in ‘mixed mode’ the maximum heat output is 5800W (1800W/240V + 4000W/gas). Similarly a Combi-4 “E” running in ‘gas mode’ has a rated maximum heat output of 4000W, whereas in ‘mixed mode’ the maximum heat output is 3800W (1800W/240V + 2000W/gas). So - in principle - if you want to heat a motorhome’s interior as rapidly as possible, use ‘gas mode’. The only Truma appliance that added together the 240V and gas heat output maxima when in ‘mixed mode’ was the C-6002EH heater that could output 7800W (1800W/240V + 6000W/gas). My Hobby motorhome had one of these heaters but I never dared try provoking it into producing 7.8kW as it was pretty terrifying (like Concorde on re-heat) when outputting 6kW on gas alone. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted October 11, 2018 Share Posted October 11, 2018 Derek Uzzell - 2018-10-11 8:18 AM I’m happy to have Chris send me the link to the relevant forum discussion as a PM, and I promise to keep my gob shut about that information.... I now know where the "relevant forum discussion” is, so no need for any follow-up from Chris. Link to comment Share on other sites More sharing options...
Alanb Posted October 11, 2018 Share Posted October 11, 2018 aandncaravan - 2018-10-11 9:42 AM .........Our conclusion, no hard evidence to suggest it is correct, was that the Truma unit gets very hot when both Gas and electricity are selected. The 'extra' rise in temperature when gas is used and the interior temperature of the van rises MIGHT lead to the 230v element getting hotter than designed. It's Resistance then rising causing it to draw more current than ideal. The 10amp fuse already runs right at it's limit when things work against the designed operation. So to understand all that more, lets run through the scenario we examined as outlined to us, and possible effects - Owners got back to the vehicle in the evening to a cold van. Heating put on Gas and 230v for fast warm up. Van warmed up quickly and interior got very warm. Wife likes it really warm so heating not turned down despite hot interior. We think that as the van warmed up more recirculated warm air was taken in by the unit. This warm air meant the element potentially 'cooled less' so resistance rose drawing more current. (Note that in this instance the 12 battery was also past it's best so, potentially the 12v fan might have turned more slowly as it's voltage dropped from the draw of the fan, the TV that then got turned on, lights, water pump, etc. The 230v power supply/charger was a poor 10a device, in reality an 8amp unit, so once load rose it would not be able to keep up. So the reduction of real '12v fan power' plus warmer air flowing over the element may have resulted in a much hotter element where the resistance rose drawing more current, closer to 9amps. This in turn caused the fuse to slowly heat up over several minutes and fail. Remember that a 10amp fuse is a resistor that gets hot and melts. It will do that with a sudden high burst of current and fail, but may equally slowly build up to near it's limit over time which will generate heat build up inside the glass, raising temperature (and resistance still further) potentially causing it to melt at just 9amps. Not fixing the issue which, we think, requires a thermo shutdown self resetting fuse on the element. The 10amp fast blow fuse should melt sooner than the Truma fuse but no guarantee, but easy to replace. Since we did the work, I think mid 2016?, and explained what we think occurred, they have taken the trouble to switch down the heating to just 230v as soon as it gets warm, without any further issues. But they also promised to look after the new battery and being a Varta LFD, it starts off with a higher 13v and potentially higher fan speed than their old conventional 12.5v battery. Firstly I am unable to accept Allan's apparent inversion of Ohm's Law, which dictates that increasing resistance at constant voltage will result in less current. Secondly Allan does offer a clue when he mentions a thermal self resetting fuse. To pose the question, is the relevant fuse for overcurrent protection, or over temperature protection, or perhaps both? If it is for overcurrent protection, it may be too closely rated. On the other hand if the fuse provides over temperature protection, it is probably reacting correctly to an elevated temperature within the heater casing. Note: Thermal fuses are designed to melt when the temperature inside an enclosure exceeds their rated value. I have seen them in transformer type power supplies, and in storage heaters where they were a bolted link of low melting point metal. Alan Link to comment Share on other sites More sharing options...
Derek Uzzell Posted October 12, 2018 Share Posted October 12, 2018 Why not ask the Truma(UK) technical staff what the design-purpose is of the fuse that occasionally fails? https://www.truma.com/uk/en/service/truma-servicecenter.html Truma “Combi E” appliances have been marketed since 2007 and, as far as I’m aware, exactly the same specification fuse has been fitted to all of them. Link to comment Share on other sites More sharing options...
Sourenna Posted March 26, 2023 Share Posted March 26, 2023 Hi forum! I have a question which may be related to what you have been discussing. I wonder if anyone could shed a light on the solution to my problem. I have a Truma Combi, I think Combi(E). The display looks dead. I found that the 12 Volt fuse at the campervan blade fuse for the boiler was blown. When I tried to change it, it blew again. I tried to do this a couple of more time and the same thing happened. I disconnected the 12 wire from the boiler, and this did not blow the blade fuse. So the problem is in the combi boiler. I tested the 12 V fuse in the Combi and it seems still intact. I have not used the camper since Oct 2022. Meanwhile I changed my batteries in December 2022 to a single lithium battery. When I did that everything including the combo boiler worked fine. I would be grateful if anyone has any ideas on what and where the problem is? Or any ideas what is the next step to diagnose the issue, please? Thank you. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted March 27, 2023 Share Posted March 27, 2023 Details of your motorhome could be useful - make, exact model and the year it was built. I assume your Truma heater has this type of display and Combi "E" versions are identifiable by them having the capability to heat air/water using 230V mains electricity. There's a YouTube video relating to trouble-shooting a 'dead' Combi display and this might be helpful for you. https://www.youtube.com/watch?v=aMfO2anWMZk If the Combi's 12V fuse is OK ( I notice you've said "I tested the 12 V fuse in the Combi and it SEEMS still intact", so you might want to replace that fuse just in case it's the culprit) I'm not sure what else you can do yourself while the display remans blank (so no diagnostic error codes are available), the heater is unresponsive, and, when a 12V supply is connected to the heater, the fuse in that supply blows. You might try contacting the technical people at Truma (UK) for suggestions as to the best way forwards. Link to comment Share on other sites More sharing options...
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