# Truma Blown Air - Explanation

Discussion in 'Tech/Mech General' started by Go Humberto!, Sep 5, 2016.

1. ### Go Humberto!Funster

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Can someone with experience explain the logic of the Truma Blown Air system please?
The fan in mine blows low then high with a logic I'm yet to understand.

Using the image from my MH I labelled everything.
Example Dial A setting 3 = OFF
Example Dial B setting 5 = Use a combination of Gas and 230V at high wattage.
Example Dial C = room thermostat

So, when I'm "off-grid" using Gas and just 12V I set the dials to A-4 and B-3 (C = room thermostat where C-9 is maximum).

The fan is 12V and is mostly a very puny warm "draught" but then the fan will increase in power until you can feel hot air blown from every outlet.

What I haven't yet figured out is what drives the logic between puny draught and lovely streams of warm air.

I though the image below may be useful to help others, besides me, understand.

2. ### WillHFunster

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The instructions are very clear on how to operate the Truma, just not easy to understand. The fan speed is varied according to the amount of heat in the boiler, which, in turn, varies according to the difference between the thermostat setting and air temperature. If the boiler is still warming up then the fan speed is low, once heat builds up then so does fan speed until set temperature is reached.

3. ### Go Humberto!Funster

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Thanks.

That probably explains the high-speed fan when I switch it off. Presumably "purging" the heat from the system.

4. ### pappajohnFunsterLife Member

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Turn dial B up to 5 and see if that makes a difference.
In the pic the heat is set at low (3)....5 is high.
Dial A is set to heat only (on gas) and fan speed (C) is max (9)

If the room is cold it will blow at full speed and reduce to nothing once at room temp selected on the thermostat

5. ### TheonlysueFunsterLife Member

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Firstly dial B.
Position 3 is using your gas only.
Position 1 and 2 use 240v only.
Position 2 is approximately 1kw.
This will depend on what is available on hook up, ie 6, 10 or 16 amp.
position 1 is approx 2 kw. Your instructions will clarify.

Position 4 is gas and low electric, and position 5 uses gas and higher electric.
This is useful when it is very cold. The electric alone may not bring the van up to temperature, but it should maintain the temp once reached.
Dial A.
Position 1 is water only at 60 degrees. This temp kills legionaries as far as I know.
Position 2 is 40 degrees, water only.
Great for summer use.
Position 3 is off.
Position 4 is heating only, but it may warm the water up a bit.
Position 5 is heating and hot water at 60 degrees. You'll need this in winter.

• Informative x 1
6. ### Go Humberto!Funster

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Great. Combined with WillH explanation of the fan I think I have it now.

What Truma have done is use the same symbols for slightly different purposes on the 2 dials and added some confusion (in my confused opinion).

Dial A is basically "What would you like to be warmed?" - Hot Water OR Blown-Air OR Both.
Dial B is basically "What fuel would you like it warmed with? - "230V OR Gas OR Both
Dial C is basically "IF you want Blown-Air how warm would you like it to be in your Motorhome?".

Since Blown-Air is heated using Gas you must always have a flame symbol on both dials to obtain it.
The Fan (12V) will try to deliver appropriate "oomph", depending upon the heat it has available to it, and the amount of warming it needs to do inside the MH.

Cheers All.