Boiler / Radiator BTU Delta - Should I be using T50 or T60?

[GeddyMortgage]

Hi all,

As part of some renovations, having some older radiators replaced with newer ones. Want to take the opportunity to maybe add a couple rads but without asking too much of the boiler.

Wanted to ask, when working out the total BTU / wattage, should I be calculating at T60 or T50? My boiler is a Worcester Bosch Greenstar 40CDI Central heating only boiler, 41KW. It was installed in 2015 (though Worcester say the model ran from 2005 through to 2016).

- Spec 1

- Spec2




thanks,

Ged

 

[muggles]

dT30 is best. Do you live in a vary large house? A 40CDi should be able to do around 40 radiators

 

[GeddyMortgage]

It’s a larger footprint than average yes. Originally a bungalow converted to 3 stories. So the square footage really adds up.

I’ve used a few calculators online, entered all the radiator sizes, and whether they’re single/double panel… with single or double convectors etc…and added on 3kw for the hot water tank.

What I’m confused about though is should I be calculating this based on t50 or t60? At T60 my total using the calculator is about 38kw so pretty close to the limit… though at t50 I’m coming in about 30kw…. Leaving me lots of headroom… so I’m not sure if I can add more rads or not….any idea?

 

[flameport]

should I be calculating this based on t50 or t60?

Neither, unless you have a time machine and intend to live 30+ years in the past. Same for the 'add 3kW for the hot water cylinder'. Old and outdated methods.

You need to establish the heat loss for the individual rooms and the building as a whole and size the radiators accordingly, and preferably for dt30 as already indicated.
If that results in radiator sizes that are so large they cannot be obtained, then it's time to add more insulation.

Using the sizes of whatever radiators might be installed now is totally invalid, as there is no guarantee they were ever the correct size, and even if they were, they would be sized for much higher flow temperatures which are not appropriate for any modern (last 15 years or so) boiler.

 

[GeddyMortgage]

The property was extended in 2008, rads appear to have all gone in around then. And boiler apparently went in around 2015. I assume that would be considered a modern condensing boiler, though again not certain on this as WB said that model of boiler began production in 2005.

Honestly I’m sorry but I just don’t follow what you’re saying. Dt30…. As far as I can see would mean I need a low heat system? So what? New boiler time? Looks like that’s used more for ground source heat pumps etc. which mine is not.

Are you saying I should be using t30 as my calculation? Or buy radiators that use t30 wattage outputs on their specifications?

This article was from 2017 and says T50 is the standard in the Uk now a days

Delta T Explained, The Importance of Delta T & Radiators

Are you looking to save on your energy bills? Understand Delta T to calculate how much heat you need to keep your radiators working efficiently in your home.

www.stelrad.com

I’ve got 24 rads right now. I just want to know if I can add more or not. Is it really this complicated?

 

[muggles]

Online radiator calculators vary wildly in their accuracy. www.heat-engineer.com is a good one.

dT50 only just puts your boiler into the very edge of its condensing heat range. dT60 means it's outside of condensing range and only about 80% efficient. dT30 will promote more condensing which will give you best gas efficiency. You don't need a new boiler although your current one sounds like it is massively oversized for your property

 

[Johntheo5]

New rads are based on 50 deg.
as a 60deg rad, it will emit 26% more heat. or a factor of 1.26, (60/50)^1.3
A 30deg rad will emit 51.5% of a 50deg rad, a factor of 0.515, (30/50)^1.3, you would need to oversize it by a factor of 1.94, (1/0.515) to emit the same as a 50deg rad.

 

[GeddyMortgage]

How can I know for sure which DT is appropriate for my boiler? I believe mine is in the 90% efficiency range. A rated…Or am I thinking about this totally wrong?

as part of the renovation work on the ground floor, about 6 rads have been took off the walls (4 X double panel, double connector, 2 X double panel single connector)

Rather than put them back on - we were looking at some more modern triple column ones. It’s a range called the Milano Windsor, on the website it shows the BTU and wattage for t30, t50 and t60… just not sure which I should be going off of.

Basically I’m looking to potentially put more than rads back on compared to what came off, just conscious of adding too many and overloading the boiler.

Rads I’m looking at - https://www.bestheating.com/milano-...mn-choice-of-classic-colours-and-sizes-113686

 

[Johntheo5]

How can I know for sure which DT is appropriate for my boiler? I believe mine is in the 90% efficiency range. A rated…Or am I thinking about this totally wrong?

as part of the renovation work on the ground floor, about 6 rads have been took off the walls (4 X double panel, double connector, 2 X double panel single connector)

Rather than put them back on - we were looking at some more modern triple column ones. It’s a range called the Milano Windsor, on the website it shows the BTU and wattage for t30, t50 and t60… just not sure which I should be going off of.

Basically I’m looking to potentially put more than rads back on compared to what came off, just conscious of adding too many and overloading the boiler.

Rads I’m looking at - https://www.bestheating.com/milano-...mn-choice-of-classic-colours-and-sizes-113686

View attachment 296293

You might be a small bit confused.
The highest boiler efficiency is achieved with the lowest (boiler) radiator return temperature. Your boiler will achive ~ 90% efficiency with a return temperature of 50C, 87% at a return of 65C and 95% at a return of 40C.
Rads are based on a 50C basis but this is the mean rad temperature minus the required room temperature (roomtemp of 20c is usually used)
So a 50deg rad will require a flowtemp of 75C and a return of 65C to give its full output (50deg rad), (75+65)/2) minus 20 = 50C with a boiler efficiency of ~ 87%. If you oversized the rad by a factor of ~ 1.9 (30deg rad) then you will get the same rad output at flowtemp of 60C and a returntemp of 40C, (60+40)/2) minus 20 = 30C with a boiler efficiency of 95%.
IF you don't/can't get a heat loss survey done and IF your present rads are heating the hrooms OK then measure them and see what type and you can determine their rated output (at 50C), then buy your new rads to give the same output based on a 30C rating.

 

[oldbutnotdead]

And don't worry about overloading your boiler. This sketch is about moving the same amount of thermal energy in a different way (so lower temperature differential between radiator and room needs a far larger radiating surface to deliver the amount of heat per hour required to overcome the room heat loss).

 

[flameport]

As far as I can see would mean I need a low heat system? So what? New boiler time? Looks like that’s used more for ground source heat pumps etc. which mine is not.

It will be a low flow temp system, which is what all condensing boilers should be used with.
While condensing boilers will still work at higher temperatures, they are significantly less efficient.

Every heating system of the last 15+ years should have been designed that way. Unfortunately plenty were not, and there are huge numbers of condensing boilers out there which will never achieve anywhere near their claimed efficiencies.

Heat pumps work at lower flow temperatures. If the millions of condensing boilers installed in the last decade or more had been properly installed as low temp systems, not only would they have been far cheaper to run for all those years, all of those buildings would now be heat pump ready.

Worcester Bosch Greenstar 40CDI

A condensing boiler.

 

[JonathanM]

Are you only replacing a few radiators in one area?

The size of radiators you need all depends on what temperature you are going to run the boiler at.

What flow temperature do you currently use?

You have to think about cold weather. When it was really cold in December, what boiler temperature were you using. Did it keep the house warm. Did it manage that easily. There's no point buying radiators for one part of the house based on a low flow temperature, if the radiators in the rest of the house need a higher flow temperature to keep the house warm.

 

[GeddyMortgage]

Are you only replacing a few radiators in one area?

The size of radiators you need all depends on what temperature you are going to run the boiler at.

What flow temperature do you currently use?

You have to think about cold weather. When it was really cold in December, what boiler temperature were you using. Did it keep the house warm. Did it manage that easily. There's no point buying radiators for one part of the house based on a low flow temperature, if the radiators in the rest of the house need a higher flow temperature to keep the house warm.

Thanks, that made sense to me (finally!)

:- Yes only replacing radiators in the ground floor hall/dining room. They were all double panel double convector types and will be done away with, likely in place of Triple column ones of a similar height/width....the ones on the 1st floor/top floor will all be staying the same (all of those are double panel single convector types)

:- Flow temperature, is the LCD temperature readout on the boiler? We've got it set to MAX and the temp readout peaks at about 84degrees.

:- Did it keep the house warm in december? Some areas, yes, some areas not so much, the hall struggled and we've just opened that up to a much larger space as per the renovations, hence why Im looking to swap out / and add radiators to that area. It had 3 in there, I'd like 5 in there.

 

[GeddyMortgage]

It will be a low flow temp system, which is what all condensing boilers should be used with.
While condensing boilers will still work at higher temperatures, they are significantly less efficient.

Every heating system of the last 15+ years should have been designed that way. Unfortunately plenty were not, and there are huge numbers of condensing boilers out there which will never achieve anywhere near their claimed efficiencies.

Heat pumps work at lower flow temperatures. If the millions of condensing boilers installed in the last decade or more had been properly installed as low temp systems, not only would they have been far cheaper to run for all those years, all of those buildings would now be heat pump ready.


A condensing boiler.


View attachment 296326

Thanks, so given mine is a condensing boiler, does that confirm I could/should really be running it on a lower temperature and adding more rads? Is the temperature im running it at dictating whether I should be using T50 or T30?

Mentioned on a seperate reply it's just set to MAX. LCD readout shows 84-85degrees when its fully heated up.

 

[GeddyMortgage]

You might be a small bit confused.
The highest boiler efficiency is achieved with the lowest (boiler) radiator return temperature. Your boiler will achive ~ 90% efficiency with a return temperature of 50C, 87% at a return of 65C and 95% at a return of 40C.
Rads are based on a 50C basis but this is the mean rad temperature minus the required room temperature (roomtemp of 20c is usually used)
So a 50deg rad will require a flowtemp of 75C and a return of 65C to give its full output (50deg rad), (75+65)/2) minus 20 = 50C with a boiler efficiency of ~ 87%. If you oversized the rad by a factor of ~ 1.9 (30deg rad) then you will get the same rad output at flowtemp of 60C and a returntemp of 40C, (60+40)/2) minus 20 = 30C with a boiler efficiency of 95%.
IF you don't/can't get a heat loss survey done and IF your present rads are heating the hrooms OK then measure them and see what type and you can determine their rated output (at 50C), then buy your new rads to give the same output based on a 30C rating.

Thanks, if mine is 90% efficiency (as per the manual) I should expect the flow temperature to be 75C and a return of 50C? And the rads should have 75C going in and returning 65C? What is the best way to measure this?

The present rads were somewhere between not great and OK through winter, but the spaces they're situated in have got larger since then.