What Delta/T Rating should I be working on for Replacement Radiators

[fixitflav]

What is your cylinder fixitflav ? is it by chance an older copper indirect. Do you know the the kW rating is of its coil?

It's a copper indirect, foam insulated, about 20 years old. I don't know the kW rating. Whatever it was when new it will be a lot less now, I would guess not more than about 3kW. It takes about 30mins to recover, but I don't find that a problem. If it was I could turn the control-stat up higher.
I dread to think how much scale there is on the coil. I've changed 2 cylinders, this one 20 years ago, and one 20-odd years before that. In both cases there was close to a bucketful of scale.

I prefer to just use the programer to bring the hot water on 40 - 60mins before the heating comes on

I suppose that's an option, I just leave my HW on 24/7. It only calls about once/day.

Not sure I understand what you are getting at - The boiler is not in condensing mode if it is producing lots of pluming ?

That's a good point, I had tended to assume the plume was a sign it's condensing, but it could be a bad sign. It depends on where condensation to visible vapour occurs. If it's after it leaves the boiler the latent heat is lost, but if it's inside, the latent heat could still go into the circuit water, but the condensed water going out with the plume rather than into the condensate trap.

 

[chris watkins]

It's a copper indirect, foam insulated, about 20 years old. I don't know the kW rating. Whatever it was when new it will be a lot less now, I would guess not more than about 3kW. It takes about 30mins to recover, but I don't find that a problem. If it was I could turn the control-stat up higher.
I dread to think how much scale there is on the coil. I've changed 2 cylinders, this one 20 years ago, and one 20-odd years before that. In both cases there was close to a bucketful of scale.

A new cylinder (Part L complaint) would likely have a coil rated around 18 - 21 kW's, depending on its size, all designed to give a recovery time of 20mins (given that the boiler power is available) in that way the return water temperature is low so the boiler does not cycle on/off on the boiler stat & it should be in condensing mode longer.

 

[fixitflav]

That's my point if everyone stated their products in T50 or delta 50 which is supposed to be the current set standard you know exactly where you are. But they don't and it has just complicated it all. I just want to know what radiators to get

Why not just get rads, based on the current set standard T50, to give your calculated output in each case? If your boiler flow and return are 75° and 65° that will give your 50° DT. Giving some condensation, but in practice most of the year the actual flow temperature will be well below the control-stat setting 75°, so you'll get more condensation, as I was saying in #16. In any case your selected rads are likely to be the next standard size up from your calculated output, which also helps.

then do I need to convert the BTU/Watts to get a T60 or T50 ? If yes, what is the conversion factors

Conversion is 10000BTU/h = 2.93kW. (To be pedantic, BTU is energy, not power, but everybody understands the implicit per hour, so there's rarely any confusion)

 

[doitmyself]

No, a DT 10°C

😳 Thanks for pointing out my error.