Muck in Truma Combi heaters

[Guest Derek Uzzell]

Arguably the most commonly fitted heating appliance in new motorhomes is Truma's C-series "Combi", combining warm air and hot water facilities in a single unit. It's well recognised in the trade that the Combi is vulnerable to insects entering the heater's flue. The Combi has a 'double-wall' flue comprising a central inner exhaust-gas tube within an outer combustion-air tube. A spider-web in the air-space between the outer and inner tubes can throw the heater seriously out of kilter, while flies, beetles, and the like, that crawl down the central tube to the heater's core can cause clogging of the gas-burner system. If you leave your Combi-equipped motorhome out of use for lengthy periods, you may wish to consider obtaining a Truma cowl-cover (about £7) if your heater has a sidewall-mounted flue. If your Combi heater has a roof-flue, then a tied-on plastic bag will provide useful protection. But don't forget to remove the cowl-cover or bag before the heater is next used!! Another acknowledged muck-related heater problem to which Combis seem particularly prone arises from contaminated gas. It is not uncommon for gas bottles to contain oily residues and if these get into the heater they can play havoc with solenoid valves and/or the gas burners themselves. I notice that, in France, a gas filter is now available (for professional installation only) that is fitted downstream of a bulkhead-mounted regulator. Cost is about £50 with replacement every 1000kg of gas. A much less sophisticated alternative I've encountered involves using a long gas-hose that forms a 'swan neck' between the regulator and the motorhome's rigid gas pipework (when your 'van has an on-bottle regulator), or between bottle and regulator (when you've got a high-pressure hose/fixed regulator system). The rationale is that any muck in the gas will tend to collect in the lowest part of the swan neck rather than enter the rigid pipework. Regularly removing and cleaning out the long hose is recommended to prevent build up of residues within the hose. (I've also seen it suggested that you leave full bottles of gas inverted for an hour or so, then open the valve for a few seconds to expel any muck that's sunk down, then turn the bottle upright again and leave to settle for a good while before connecting to the leisure vehicle. This is a far from ecologically friendly notion however (and not that practical), but I'm toying with the idea of offering it to the Caravan Club just to see what the reaction is - probably excommunication.) The Catch-22 of Combis is that even the most minor problem with the heater-unit itself will normally require the appliance to be removed from the vehicle for investigation/rectification. In many motorhomes this can involve a major furniture dismantling exercise and it's perhaps worth noting that the Truma warranty excludes the cost of such work.