Possibly 25mm Vs 22  mm pipes.  Output depends on surface area principally, but also on form factor.  A rad that is twice as tall as another will emit less than twice as much.

Plus measurement margin of course 

Output typically varies proportionally to DT^1.3 where DT is the temp diff between the average radiator temp and the room temp.  If the 50C you quote is actually DT, output will be 25%.  If the 50 you quote is the input flow temp then output will be 55%.  Both assuming room temp of 20 and flow-return dt of 5

Posted by: @jamespa

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Posted by: @adamk

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trying to choose between 2 towel rads. one is smaller has 25mm pipes 4.07L of water content and apparently chucks out 670W @50c. the next one is more expensive is taller but has 22mm pipes holds 6.2L of water and apparently outputs 493W @50c. why the descrepancy in output? surely a larger overall rad with more water content outputs more heat? i will be running it on a system designed for 40c at -3.1c outside so will likely see a lower than design temp in the room but its the biggest rad i can fit.

https://www.heatandplumb.com/acatalog/reina-eos-curved-heated-towel-rail?variation=20406

https://ukradiators.com/products/aston-stainless-steel-heated-towel-rail-h1800mm-x-w500mm-curved?_pos=5&_fid=d6849b306&_ss=c

Possibly 25mm Vs 22  mm pipes.  Output depends on surface area principally, but also on form factor.  A rad that is twice as tall as another will emit less than twice as much.

Plus measurement margin of course 

Output typically varies proportionally to DT^1.3 where DT is the temp diff between the average radiator temp and the room temp.  If the 50C you quote is actually DT, output will be 25%.  If the 50 you quote is the input flow temp then output will be 55%.  Both assuming room temp of 20 and flow-return dt of 5

both are delta T50 apparently, one is recalculated from a higher flow temp to be more realistic it says on the site.

Posted by: @adamk

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both are delta T50 apparently, one is recalculated from a higher flow temp to be more realistic it says on the site.

 

In that case output at 40C will be 25% of quoted value, ie not a lot.

Towel rails appear to be like this, options if not enough are

• add an electric element
• fit a standard radiator and a rail over (they exist)
• ignore the problem and allow heat from other rooms to make up the difference (the one I chose)
• fit a standard radiator in addition to the towel rail

Posted by: @jamespa

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Posted by: @adamk

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both are delta T50 apparently, one is recalculated from a higher flow temp to be more realistic it says on the site.

 

In that case output at 40C will be 25% of quoted value, ie not a lot.

Towel rails appear to be like this, options if not enough are

• add an electric element
• fit a standard radiator and a rail over (they exist)
• ignore the problem and allow heat from other rooms to make up the difference (the one I chose)
• fit a standard radiator in addition to the towel rail

 

im thinking of going for the taller one and letting the heat from the plant room plus the flow/return 28mm and the primary 28mm's that are in the ceiling all do there bit, as ive had a rad in a bathroom before and they tend to rust quickly. also the room is insulated on all sides including the floor and ceiling.