Before upping the wattage, a law to look at would be Ohms law!
Then I would look at the existing wiring system(wire, fuses) to see if it will safely accommodate the higher wattage.
Along with checking the safety and warantee issues, I would call my insurance company to see if they offer fire insurance---if you screw up on the wiring system evaluation you may need some!!! How close do you live to a fire plug!!!
You are likely are talking about Webasto heater. This is rare option on cars and completely unknown in US where most members are from.
I know those units from RV applications. They are very expensive and pretty unreliable. Meaning would I have to spend the money on replacement - I would rather idle the engine. Unless you sleep in the car every other night.
The thing is that here in Denmark at winter it gets pretty cold, and the CDI machine have trouble to maintain the optimal engine temp.
That’s why in Denmark the 3 kW heater I mounted as standard, as an help heater for the CDI machine, you can after og order the Benz with timer or a 5kW heater, so that you can heat the engine and cabin before starting.
I would assume the bigger heater, with bigger burner, heat exchanger and bigger flue might not be direct fit, but than again, have never seen one of them in passenger car.
Can you find MB enthusiast forum in Denmark?
Is this because of a car inspection required in your country?
If the two heaters look similar other than the numbers on the heater, could you not swap the 5kW in place of the 3kW and nobody would know the difference unless you told them? The heater is hidden when installed in the W210, isn't it? Would any inspector be able to tell that a different one is installed?
The one in the eBay auction Hirnbeiss linked to looks exactly like the gasoline Webasto heater I have in my GMC Yukon. Only the mounting bracket is different, and mine does not have the small exhaust muffler so it sounds like a little jet engine. Most of these Webasto heaters look the same unless you compare them to heaters from a much larger series like those used in large trucks.
The control electronics are integrated into the heater itself. As long as the electrical connectors plug in correctly, and the correct fuel pump for the heater is installed, it should work correctly. The fuel pump is pulsed on and off to meter the correct amount of fuel to the heater - it is important that you have the correct pump in place.
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