Learning the hard way about diesel water heaters and how they regulate fuel Leave a comment


A friend of mine has been making a camper van heating system, covered earlier here using a Webasto Thermo Top C diesel heater at its heart.

Sadly, something happened, and both the heater electronics and its separate fuel pump stopped working.

Many trips to repair places, and the system is now running again (in the garage test rig, see photo) but at reduced heat output – 1kW instead of 5kW – measured by timing the heating of a known amount of water over a 20°C temperature rise.

Much head scratching, and finally an answer:
Despite all looking similar, all diesel pumps are not equal.
The many diesel pumps designed for use with Webasto and Eberspatcher (and other) water and air heaters are calibrated to deliver different quantities of diesel fuel per input pulse.

The pumps, which I now know are properly called metering pumps, are driven by 12V (or 24V, depending on model) pulses from the heater unit.

Each heater unit will only work properly with a pump that delivers a certain dose per pulse, as it regulates flow by pulsing the pump at a fixed rate – or more than one fixed rate if it is a multi-heat-output unit.

Through ignorance or willfully, this fact is ignored by many people selling after-market pumps, with long lists of ‘compatible’ heaters – after all, how many people are going to notice a change in heat output unless it is particularly large.

The system is open-loop, so if it has the wrong pump fitted, it gets the wrong amount of fuel – too little fuel per pulse and too little heat emerges, too much and there is a danger of over-heating.

In this case, the specified Webasto pump should deliver ~63.3μlitres/pulse.

This same pump is also specified in millilitres per hour clocked at 1Hz (228ml/hr)

Some other pumps are specified in ml per a fixed number of pulses (sometimes called ‘strokes’) – I have seen figures per 100 strokes, per 200 strokes and other numbers – sometimes the number is the number of pulses needed per minute to deliver one or other heat setting.

There are also ’22ml’ and ’16ml’ pumps, which is the volume per 1,000 pulses. These seem loosely for 1-3kW and 1-4kW air heaters.

Lastly, they can be described in pulses per 1ml

So a 63.3μlitres/pulse pump can also described as:

  • 228ml/hr at 1Hz.
  • 6.33ml/100pulses
  • 12.22/200pulses
  • 3.8ml/60pulses (1min at 1Hz)
  • 63ml (1,000 pulses)
  • ~16pulse/ml

As an example of another pump, there is an Eberspatcher unit that is is rated at 5.5 – 6.0ml for 200 strokes, half that of the required pump, so half the heat output if installed accidentally. Or a ’22ml’ pump would deliver about a third of the heat.

It has yet to be measured, but it looks like the randomly-chosen pump (from a un-branded Chinese air heater) used at the moment (just visible at the top right of the photo) is delivering much less per pulse than the Top C requires.

Something more to add to the ‘I wish I had known’ list….

As well as hours in a freezing garage, I used many sources to compile this page. Particularly recommended are:

This generously put up by boat varnish company B & D Murkin of Berkshire – diesel heaters are frequently installed in boats

The technical library of Lincolnshire-based Butler Technik, which has proved a reliable supplier of diesel heater parts

And, notice the controller with red numbers on in the upper right? – I made that to control the hot air and hot water systems, and so far it works. If anyone wants to build a similar system, I am happy to cover it here in EinW.





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