Truma Combi(E) operation on 230V

[Derek Uzzell]

I recently asked the Truma(UK) technicians how Combi(E) heating appliances operate on 230V and their answer may be of interest to owners of motorhomes with these heaters.

 

I have attached 4 photos relating to Combi heaters. The appliance essentially lies on its side with a gas burner at one end and blown-air outlets at the other. The gas-burner heats a central heat-exchanger that passes the heat to fan-driven air passing over it (‘blown-air heating’). A 10-litre water container wraps round the heat-exchanger and water in that container is heated by conduction (‘water heating’).

 

Besides having a gas-fuelled heating capability Combi(E) appliances can heat air or water via two 230V elements (Photo 4) each having a maximum output of 900W. One element is positioned on the upper side of the heat-exchanger and the other on the lower side.

 

Either a 900W or 1800W setting can be chosen for 230V air or water heating and when the 1800W setting is selected, both elements will (obviously) operate together at their maximum output (ie. 2 x 900W). But what I wanted to confirm was what happened when the 900W setting was chosen.

 

Truma(UK) told me that – for AIR HEATING – when the 900W setting was chosen each 230V element operates at 50% of its maximum output (ie. 2 x 450W).

 

However (I’m guessing for efficiency reasons) – for WATER HEATING at the 900W setting – the lower 230V element operates at its full 900W power and the upper 230V element remains off.

[hagrid]

Interesting, I have just completed the job of changing the heating elements, one of ours had failed, the symptoms being insufficient warm air heating for the van on 240v, the water was heating up with no problem and everything worked fine on gas or gas/electric. I did try to contact Truma engineers, the nearest of which is 130 miles away but did not receive any returned calls or response to e mail so, I looked at the diagram and read all I could find but, there is little information available.

I decided to attempt the job with the knowledge that in principle the unit must be removed to access both ends so if it proved beyond my capability I could then just take the unit to an expert. I gave myself plenty of time as to access the unit involves dismantling furniture. Fortunately I found that I could complete the job by dismounting but not removing the unit from the motorhome which save a bit of time. I now know a lot more about Truma Combi 6e, I complete the job in less than 6 hours over two days and was delighted to find that everything now works as it should. :-)

 

Bob

[Alanb]

May I first commend Derek for his use of the correct term "technician", rather than the all to frequently misused word engineer, which if used correctly implies a professioally qualified individual.

 

I fully agree with Derek's assertion that only one element is used at the lower heat setting, as that allows more simple switching.

 

Two identical elements switched between parallel (full power) , and series (low power), will only provide one quarter of the maximum power when connected in series.

 

It would be theoretically possible to operate the elements in parallel with preferably opposed silicon diodes in series with each elemen,t so that each element only operated on opposite half cycles of the AC supply. The diodes would each have to dissapate several watts.

 

Alan

 

 

[Derek Uzzell]

Alanb - 2017-06-21 10:22 AM

 

...I fully agree with Derek's assertion that only one element is used at the lower heat setting, as that allows more simple switching...

 

 

If you reread my posting, you’ll see that the advice given me by Truma(UK) was that just ONE of the two 900W-power 230V elements is used with a Combi(E)’s 900W setting, but ONLY when that setting relates to WATER HEATING. When AIR HEATING using the 900W setting is selected, Truma(UK) advised that BOTH elements operate at 450W.

 

I have no hands-on experience of Combi(E) appliances (the Combi 4 in my Rapido is gas-only) but my understanding of the 230V system used with the previous C-6002EH ‘vertical’ unit (that was fitted to my Hobby motorhome) was that that both of that heater’s 230V 900W elements operated at 450W on the 900W setting irrespective of whether that setting related to blown-air or water heating. That’s really why I was interested whether Combi(E)s were the same and clearly they are not.

 

It’s perhaps worth adding that I did query initially the advice about air-heating on the 900W setting involving both elements operating at 450W (which I thought it would be) but water-heating on the 900W setting using a single element operating at 900W, as I hadn’t envisaged the latter would be the case. A further check was made and the single-element water-heating arrangement was confirmed.

[hagrid]

Alanb - 2017-06-21 10:22 AM

 

May I first commend Derek for his use of the correct term "technician", rather than the all to frequently misused word engineer, which if used correctly implies a professioally qualified individual.

 

Alan

 

Thank you so much for pointing out your perception of my error, I know not of the individual in questions professional qualifications, simply that they describing themselves as "caravan engineers", qualified and approved by Truma to work on Truma appliances.

 

Incidentally, if it is of any help to anyone, the elements are not at the top and bottom of the unit but rather at 5 o'clock and 3 o'clock, they are a tight friction fit into shaped fins on the heat exchanger, the old ones are in consequence quite difficult to remove.

[Derek Uzzell]

Bob

 

Thanks for the ‘hands-on’ feedback.

 

Truma(UK) did tell me that it was the lower 230V element (presumably the one at 5 o’clock) that operated at 900W on the 900W water-heating setting, and lt was my (wrong) assumption that one element was on the upper side of the heat-exchanger and the other on the lower side.