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Discuss What Delta/T Rating should I be working on for Replacement Radiators in the Central Heating Forum area at

  1. Elouise Cappa

    Elouise Cappa New Member

    Hi. Please can someone help. I'm in the process of a refurb in the house and replacing existing Stelrad horizontal radiators for new Vertical Designer type Radiators. I want to make sure I get the BTU ouput required to heat the rooms. I've done the online BTU calculator and I also have the BTU total outputs for the old radiators, but when looking online it's just a mindfield of T ratings/ Delta T50/T60/T65 etc etc against the BTU outputs indicated and some show the same radiators but give different T ratings against the BTUs eg: 10000 Btu/Tested at T65 and another says it's T50. It's all very confusing and all i want is the correct size radiator with the right amount of BTU output for the rooms. Some sites also mention flow rates and the boiler system I have. I have a New Worcestor Greenstar 40CDi Classic Regular and the system is a Condensing system. Please help !!!! URGENT
  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    I use 60 but with the new rulings I've started to use either 40 or 50

    Guide below

  3. Elouise Cappa

    Elouise Cappa New Member

    Hi. Thanks for that. How do I know what flow and return rate my system is using ? and if the Radiators on the website are showing they are tested at T50, then I'm well covered if mine is T60, but it they are tested at T65 then do I need to convert the BTU/Watss to get a T60 or T50 ? If yes, what is the conversion factors
  4. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Should say on your boiler / temperature adjustment
  5. johnduffell

    johnduffell Member

    For a condensing boiler the lower the better but at some point the radiator would be too big. Your existing system would be sized for whatever kind of boiler was originally installed, so simplest to go for a similar output to those. But if you're replacing them all then go for lower.
    My experience is our system we have replaced most of the radiators with D40 but that means that when running the boiler at the right temperature for the new rooms the old rooms don't really warm up properly. If we turned the boiler up we would have to limit the flow through the new rads to get a much bigger temp difference across them.
  6. doitmyself

    doitmyself Well-Known Member

    Delta T can have more than one meaning. If you talk to a boiler engineer it is the difference between flow and return temperatures. So a flow of 75C and return of 65C gives a DT of 20C. But to a radiator man it is the difference between mean rad temperature and room temperature - as explained by ShaunCorbs.

    Modern condensing boilers are designed to work with a (boiler) Delta T of 20C. Condensing does not occur unless the return temperature is below 55C. So the flow temperature should be no more than 75C (55C + 20C). This gives a mean water temperature of 65C. Assuming a room temperature of 20C the rad-room delta T is 45C. So you should use this when selecting your radiators. You could use a delta T of 40C with flow = 70C and return = 50C, but this will mean the radiators will be about 20% larger.

    You say that you have calculated the rads required for each room and that you need a 40kW boiler. This seems very large - particularly if you are refurbishing a house and, presumably, upgrading the insulation etc. How have you arrived at the boiler size and which online rad calculator did you use?
    • Disagree Disagree x 1
  7. Elouise Cappa

    Elouise Cappa New Member

    Hi. I already have a 40kW boiler reason is I am only refurbing the downstairs and replacing the radiators there eg: new kitchen and a knock through. No extension, but this is a big house, 3 storey and m2 for the whole downstairs is approx 125m2 with majority height in rooms of 2.6m with 2 rooms the ceiling being 4.5m !! so not small. Indication: the old radiators in the kitchen had BTU output of 11500 BTU, one room with the 4.5m ceiling averaged around 9500 BTU so working on 40C isn't an option i would freeze and the water would be cold. I used Stelrads BTU Caluculator
  8. Elouise Cappa

    Elouise Cappa New Member

    the Boiler was only installed this year. Old boiler was Glow worm Micron 100ff or 120ff Boiler with Megaflow water Heater, which is still there. Hope this helps
  9. Elouise Cappa

    Elouise Cappa New Member

    Sorry the 11500 BTU figure was spread over 3 Radiators in the Kitchen/Breakfast area. this warmed the area fine. Also new Aluminium Double Glazed windows going in, previously they were wood double glazed. House is 16 years old of Brick and Block construction... use all this info in the BTU calculator.
  10. doitmyself

    doitmyself Well-Known Member

    The old boiler was either 30kW or 35kW. So who recommended a 40kW?

    As you are only doing partial refurb., that changes things. The rads in the rest of the house will have been sized assuming a rad Delta T of 50C. (75C flow, 65C return, 20C room) If you now run them with 75C flow and 55C return their output will be reduced by 15%. You may get away with it as rads tend to be oversized.

    Make sure the installer knows that you have sized the rads assuming a 20C differential at the boiler, with 75C flow and 55C return.

    The Stelrad calculator is very good - provided you put in the correct information. :(
  11. fixitflav

    fixitflav Member

    No, a DT 10°C
    • Agree Agree x 1
  12. chris watkins

    chris watkins Plumber

    Seems to be quite a bit of misunderstanding here.

    The OP has a regular boiler which has to heat a Megaflo indirect cylinder so this must set the flow temperature any lower than 70 deg C and the domestic hot water temperature of 60 - 65 deg C will not be achieved. (Unless Weather comp is being used giving the boiler the ability to run heating at lower temps). The boiler is designed to run trying to achieve a 20 deg C drop so it is looking for a return water temp of 50 deg C which is just low enough to allow a little bit of condensing with starts at 53deg C.

    If rads are used as the heat emitters then the room temperatures are recommended to be 21 deg C for all rooms except bath / shower rooms which should be 22 deg C (D.H.D Guide & B.S en). As previously stated, we take the designed room temp from the Mean Water Temp in the rads (70+50/2 = 60MWT - 21room temp = 39) so the Dt for selecting rads is 39.
    Most rad manufacturers provide tables of outputs based on BS en 442 which requires them to be running with a Dt of 50 therefore if you don't do some sort of correction the selected rad would not provide the stated output leading to cold rooms when the outside temp gets to the designed level (normally -1 or -3 depending on whereabouts you are in the country).
    • Like Like x 1
  13. chris watkins

    chris watkins Plumber

    Radiator manufacturer often base their output figure on on Top & Bottom Same or Opposite End pipe connections which increase the output against piping the the common English way of Bottom Opposite Ends so again a correction should be provided. 39 = 0.724 x 0.96 for BOEs = a correction factor of 0.695
  14. chris watkins

    chris watkins Plumber

    The boiler sounds like it is oversized & it is highly likely that the original rads were selected based on the old design of 82 - 71 MWT - 76.5 - room temps and may well be older than the double glazing or other insulation all of which has changed the heatloss I would strongly suggest doing a heatloss calculation for each room to which a new rad is required that way you will know the required output once the correction factor has been applied. (& the boiler output requirement can be confirmed & not guessed at).
  15. kris

    kris Trusted Plumber GSR

    Many folks are being conned , well not conned more mis directed as many companies are selling there rads with the delta t of 60 which is fine if you want 10 degrees room temp but since the ruling was changed to delta t 50 there's still too many companies still using dt 60 ! But yeh they should be made to use btu output @ dt40
    • Like Like x 1