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Discuss Radiator Output (BTU/KW) Calculation Help ( in the Central Heating Forum area at

  1. oc75

    oc75 New Member

    Hi, I am in the midst of choosing radiators for a new build.

    There are various calculator tools available online, giving quite a range in results on what I require for each room. The tools do not specify whether they are based on T30, T50 or whatever.

    I am considering the Milano Windsor range from, so I have used their online tool and looked for radiators to meet the output required.

    On each radiator has a BTU/KW listed in the product summary (example at link below)

    However when you open up the further details for each rad it has BTU/KW's listed for T30 & T50 which are much lower.

    Which BTU/KW figure should I be going by?

    What calculator tool is reliable?

    Are the radiator suppliers/outlet BTU/KW ratings reliable?

    Milano Windsor Designer Radiators | ยป
  2. oz-plumber

    oz-plumber Plumber

    All good heating calculators allow you to put the room dimension, external walls, internal walls, window sizes, type of windows, ceiling heights, floor coverings, insulation factors and most importantly flow and return temperatures and desired room temperature.
    They will automatically adjust the heat output of a radiator to suit your needs from the flow & return temperatures and room temperature.

    You may have to pay for a suitable calculator.
    It might be worth paying someone to design the system for you to suit your requirements. Anbd a system that will allow the boiler to condensate and operate at it's optimal efficiency.

    Money well spent if you ge the right designer
    • Agree Agree x 1
  3. Last Plumber

    Last Plumber Trusted Plumber GSR

    The T30 & T50 you refer to are the Delta T figures. That is the temperature difference between the radiators mean surface temperature and the room temperature. i.e. Living rooms = 21 degrees C (70F), Dining rooms, hallways, bedrooms etc = 18 degrees C (65F),
    Bathrooms a tad warmer, maybe 22 degrees C (71/72F).

    If you have a condensing Boiler you will want to run it fairly cool by Heating standards in order for it to run at optimum efficiency. You normally want the return to be below 53 degrees Celsius.

    I usually set my Customers Boilers to 60 Degrees Flow temp and look for a return 11 degrees less. It works for me!
    If in doubt about your system and emitters (rads in this case) outputs and performance, I would advise you to get a Heating Engineer to supply and install them for you.
    • Like Like x 1
  4. oc75

    oc75 New Member

    Yes, all of the calculators I have used have asked for room dimensions, ceilings, flooring, insulation, glazing, but none of the tools ask anything about temperatures.

    Is there an assumed temperature they are using?

    What about the suppliers? do those in the UK use the same DELTA when posting the output BTU/KW advertised for each radiator?

    'Last plumber' - so if you work on the basis that the boiler is set to 60C and return of less 11C, does that mean you look for radiator output based on T50??
  5. doitmyself

    doitmyself Well-Known Member

    T50 means that the difference between room temperature and mean radiator temperature is 50C. So, if room temperature is 20C the mean radiator temperature is 70C. This could be a flow of 75C and return of 65C, or a flow of 80C and return of 60C.

    In either case the return is above 55C (has to be below this for condensing to occur)), which is where the suggested deltaT of 40C comes into play as it gives a mean rad temperature of 60C (65C/55C or 70C/50C).

    A differential of 20C at the radiator (70C/50C) is more usual now as it means less friction loss is the pipes and heat exchanger, so a smaller pump can be used.
    • Optimistic Optimistic x 1
  6. Last Plumber

    Last Plumber Trusted Plumber GSR

    No. Somewhere between 30 and 40 dependent on the room/situation. This is of course when I am designing/installing from new.

    There's a lot to take into account when designing Heating systems and it is by no means an exact science, that's why it's always better to ask a hands on, experienced person to do it.

    If you do want to do it yourself I would recommend giving the Plumbers Merchant you intend to use all the details of the rooms and ask if they will size it for you. Some will do that, especially if you're buying the rads from them.
  7. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!


  8. oc75

    oc75 New Member have kindly confirmed that their BTU rating are based on T65.

    Does anyone know what a typical operating system T is now for an oil fed boiler on a new build??
  9. chris watkins

    chris watkins Plumber

    Manufacturers (most) should be testing their radiators outputs based on Delta T =50 as required by BS EN442 so that there is a level playing field when comparing them.

    The important point oc75 is that you need to adjust the out put of the rads if you are using a different temperatures.
    As other have said some calculators do this for you but if they are just working out the heatloss you must take account of this otherwise they will not provide the outputs stated.
    Typically we would now design to 70Flow, 50Return so a MWT of 60, less 21 (or 22) room temp giving a D/T of 39. (N.Gas)
    If you installed a BS EN442 rad & made no correction it would be under sized.
    Rather than changing each rads output based on the D/T we are using it is often easer to adjust the heatloss figure so that we can compare the different sizes & designs. We can do this by dividing it by a correction factor.
    D/T 39 = 0.724 (38 = 0.7)
    So lets say your heatloss has worked out to be 2000 Watts for a living room, if we apply the correction factor 2000 / 0.724 = 2,763 Watts selecting a BS EN442 Rad based on 2763 Watts but feeding with different temperatures its resulting output would = 2000 Watts.

    Who'ed of thought selecting rads was so involved? thats why pro's charge a bit more & do it right, while others just guess.

    P.S. The above operating D/T's are based on N.gas boilers not oil.
    • Agree Agree x 1
    Last edited: Mar 25, 2019
  10. doitmyself

    doitmyself Well-Known Member

    • Agree Agree x 1
  11. oc75

    oc75 New Member

    Do any of you use the BTU calculator. I've selected rads on its estimation but am but nervous that they will not have enough output.
  12. Last Plumber

    Last Plumber Trusted Plumber GSR

    I use Mear calculators for my work. I have looked at internet calcs but they don't sit well with me. To many conflicting results for my liking.
  13. Sparkgap

    Sparkgap Active Member

    Standard temps for full temp heating systems now tend to be 80/60C (hence the T50 figures on the rad outputs) if it's a typical system using the boiler to heat a hot water cylinder and panel rads.
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