With a flow of 40C the dT for a 20C room temp is 20 it’s the difference in temp between flow and room.  The output of a rad at dT 20 is 0.3 of its output at dT50

so why dont sites list the output based on flow temp? cause most seem to use dt.

@gary so does that mean at a dt of 50 the actual flow is 70c for a 20c room temp?

Yep exactly that 

Posted by: @adamk

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so why dont sites list the output based on flow temp? cause most seem to use dt.

Because they don't know your room temperature and it's only (average) DT that matters

Posted by: @adamk

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so does that mean at a dt of 50 the actual flow is 70c for a 20c room temp?

Not strictly I think.  It's 20 plus half the DT between flow and return so that the average dt rad to room is 50

Posted by: @gary

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Yep exactly that 

jees ive got 2 hopes of heating my bathroom then. someone suggested an standard design aluminium rad, instead of a towel rad, but im worried in a copper and steel setup ill get galvanic corrosion.

I just left the old towel rail which is now way underpowered.  I am relying on heat transfer from the adjacent hallway, which has excess capacity, together with steam when you run a bath/shower.  It works just fine - the level of thermal separation between rooms is quite modest in practice. 

Obviously this is house dependent but more than one prospective installer remarked to me that they frequently do not install a 'fully compliant' radiator in a bathroom (because of the low output of and desire for towel rails).

can i ask then if i have a heat loss for a room of 1079w and a flow of 40c@-3.1c what wattage rad do i need to hit? cause at the moment im struggling to find one big enough that would fit in the space.

The output of a rad is proportional to DT to the power 1.3, where DT is the average temperature difference between rad and room.

At a flow temperature of 40, and assuming a room temperature of 20 and a temp drop across the radiator of 5, DT is 37.5-20=17.5

Radiators are usually specified for a DT of 50.  So you will need a rad with an output at DT 50 of (50/17.5) to the power 1.3 x 1079W = 3.9x1079W=4.2kW.

That's a mighty big radiator.  What room is this in?  Consider more than one radiator or a fan radiator if the loss really is that high.

@jamespa thanks for the info but I think I must have something wrong. The heat loss for the whole house is about 7.5kw. The room is only 3.756m x 3.686m. Current rad is a P+ 1450mm 380mm.

Posted by: @adamk

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@jamespa thanks for the info but I think I must have something wrong. The heat loss for the whole house is about 7.5kw. The room is only 3.756m x 3.686m. Current rad is a P+ 1450mm 380mm.

Have you got the loss calculations, if so perhaps you could post?