Dt30 or Dt25 Rad design

[ian.]

When installing a condensing system boiler to an existing property, do you base your radiator design on Dt30 or Dt 25

 

[John.g]

Boilers only begin to condense with a rad/boiler return temp of < 55C.

Radiators are based on " T50" basis, this is the mean radiator temperature minus the required room temperature, the mean rad temperature is the (flowtemp+returntemp)/2 and 20C is normally used as the required room temp but doesn't have to be, so "deg rating" is (flowtemp+returntemp)/2 minus 20C, if you require a T50 rad to give 100% output, then flow/return temps of 75C/65C will give this as (75+65)/2 minus 20 = 50C and 100% rad output but with only a rad dT of 10C, if you reduce the flowrate to give a rad dT of 25C then the rad becomes a, (75+50)/2 minus 20 = 42.5C, (a T42.5C rad now) but with a reduced output of (42.5/50)^1.3, = 81% output.
If you only required your rad to output say 30% (a T20 rad now) then flow/return temps of 47.5C/32.5C. rad dT of of 15C will achieve this but with very high boiler efficiency due to very low boiler return of 32.5C.

So to achieve reasonable condensing benefits, rads should be oversized by a factor of ~ 2.0 IMO.

 

[Sparkgap]

Depends on how much space you've got for rads. at dT25 your rads will have to have 2.5 times the manufacturers nominal (dT50) output and for dT30 it will be about 1.9 times to achieve the required heat input to the room.

 

[ian.]

Boilers only begin to condense with a rad/boiler return temp of < 55C.

Radiators are based on " T50" basis, this is the mean radiator temperature minus the required room temperature, the mean rad temperature is the (flowtemp+returntemp)/2 and 20C is normally used as the required room temp but doesn't have to be, so "deg rating" is (flowtemp+returntemp)/2 minus 20C, if you require a T50 rad to give 100% output, then flow/return temps of 75C/65C will give this as (75+65)/2 minus 20 = 50C and 100% rad output but with only a rad dT of 10C, if you reduce the flowrate to give a rad dT of 25C then the rad becomes a, (75+50)/2 minus 20 = 42.5C, (a T42.5C rad now) but with a reduced output of (42.5/50)^1.3, = 81% output.
If you only required your rad to output say 30% (a T20 rad now) then flow/return temps of 47.5C/32.5C. rad dT of of 15C will achieve this but with very high boiler efficiency due to very low boiler return of 32.5C.

So to achieve reasonable condensing benefits, rads should be oversized by a factor of ~ 2.0 IMO.

Thank you for such a comprehensive reply, the situation I’m in is that I’m going to get my boiler replaced with a condensing system type and have done a heat loss.

Only two of my existing radiators are ok at dt30 or dt25, but I don’t have wall space for the other room radiators at dt25 but I do for dt30.

If my flow is 55 my return will be in the condensing range if I have sized my rads and balanced them correctly for dt30.

Does that sound about right?

 

[Sparkgap]

Can I suggest if you are going for a new boiler you get one with a weather compensation option? If radiator size is an issue you could design your system for say dT40 and run at 70/50. This would not only mean the boiler is condensing most of the time anyway but the radiator size would be smaller as you would only need to oversize by about 30% to get the required output and you would also save by not heating the water so much as the compensator reduces the flow temp on warmer days.

 

[ShaunCorbs]

Thank you for such a comprehensive reply, the situation I’m in is that I’m going to get my boiler replaced with a condensing system type and have done a heat loss.

Only two of my existing radiators are ok at dt30 or dt25, but I don’t have wall space for the other room radiators at dt25 but I do for dt30.

If my flow is 55 my return will be in the condensing range if I have sized my rads and balanced them correctly for dt30.

Does that sound about right?

Yep once balanced and providing your pipe sizing is correct you will have a return around 35dc

 

[John.g]

Thank you for such a comprehensive reply, the situation I’m in is that I’m going to get my boiler replaced with a condensing system type and have done a heat loss.

Only two of my existing radiators are ok at dt30 or dt25, but I don’t have wall space for the other room radiators at dt25 but I do for dt30.

If my flow is 55 my return will be in the condensing range if I have sized my rads and balanced them correctly for dt30.

Does that sound about right?

Yes some very good savings there, all the way down to 25% rad output.

 

[ian.]

Can I suggest if you are going for a new boiler you get one with a weather compensation option? If radiator size is an issue you could design your system for say dT40 and run at 70/50. This would not only mean the boiler is condensing most of the time anyway but the radiator size would be smaller as you would only need to oversize by about 30% to get the required output and you would also save by not heating the water so much as the compensator reduces the flow temp on warmer days.

Thanks for your comments, the boiler will be an ideal vouge, I’ve fitted the external sensor in anticipation lol

 

[ian.]

Yes some very good savings there, all the way down to 25% rad output.

View attachment 82049

Thanks for the clarification, I will design for dt30 as I have the wall space (just) while still having room to live.

 

[John.g]

When you calculated the heat loss, what (minimum) outside and inside temperatures did you use or is there a "standard"?.

 

[ShaunCorbs]

-3 and 21 for most places

Up north I would advise -5 and 21 as a starting point

 

[ian.]

When you calculated the heat loss, what (minimum) outside and inside temperatures did you use or is there a "standard"?.

I used the CIBSE guide, so for me it was -3 and 21 for the living room, I made a spreadsheet based on there heat loss model to include any beneficial heating from other rooms to try and get as accurate as I could with the known bits of how the house is constructed.