Notifications

Clear all

help sizing rads based on room by room heat loss

Page 1 / 2 Next

Thermostats, Volumisers, Buffer Tanks, TRVs, Pumps, Piping, Valves, Radiators & Controls

Last Post by JamesPa 7 months ago

17 Posts

3 Users

2 Reactions

1,399 Views

RSS

AdamK

(@adamk)

Reputable Member Member

Joined: 9 months ago

Posts: 178

Topic starter 28/03/2025 11:14 am

scratching my head a bit as to what size rad i need for a given room based on its heat loss. the system is designed to 40c flow @-3.1c outside. lots of sites list rad outputs based on delta t50, is this the flow temp? so if i can find out the output at dt40 for instance is that the output i need to match to this chart?

Quote

Gary

(@gary)

Prominent Member Member

Joined: 3 years ago

Posts: 350

28/03/2025 11:33 am

With a flow of 40C the dT for a 20C room temp is 20 it’s the difference in temp between flow and room. The output of a rad at dT 20 is 0.3 of its output at dT50

ReplyQuote

AdamK

(@adamk)

Reputable Member Member

Joined: 9 months ago

Posts: 178

Topic starter 28/03/2025 11:58 am

so why dont sites list the output based on flow temp? cause most seem to use dt.

ReplyQuote

AdamK

(@adamk)

Reputable Member Member

Joined: 9 months ago

Posts: 178

Topic starter 28/03/2025 12:14 pm

@gary so does that mean at a dt of 50 the actual flow is 70c for a 20c room temp?

ReplyQuote

Gary

(@gary)

Prominent Member Member

Joined: 3 years ago

Posts: 350

28/03/2025 12:53 pm

Yep exactly that

ReplyQuote

JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 3398

28/03/2025 12:55 pm

Posted by: @adamk

↑

so why dont sites list the output based on flow temp? cause most seem to use dt.

Because they don't know your room temperature and it's only (average) DT that matters

Posted by: @adamk

↑

so does that mean at a dt of 50 the actual flow is 70c for a 20c room temp?

Not strictly I think. It's 20 plus half the DT between flow and return so that the average dt rad to room is 50

This post was modified 8 months ago 2 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

ReplyQuote

AdamK

(@adamk)

Reputable Member Member

Joined: 9 months ago

Posts: 178

Topic starter 28/03/2025 1:04 pm

Posted by: @gary

↑

Yep exactly that

jees ive got 2 hopes of heating my bathroom then. someone suggested an standard design aluminium rad, instead of a towel rad, but im worried in a copper and steel setup ill get galvanic corrosion.

ReplyQuote

JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 3398

30/03/2025 8:28 am

I just left the old towel rail which is now way underpowered. I am relying on heat transfer from the adjacent hallway, which has excess capacity, together with steam when you run a bath/shower. It works just fine - the level of thermal separation between rooms is quite modest in practice.

Obviously this is house dependent but more than one prospective installer remarked to me that they frequently do not install a 'fully compliant' radiator in a bathroom (because of the low output of and desire for towel rails).

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

ReplyQuote

AdamK

(@adamk)

Reputable Member Member

Joined: 9 months ago

Posts: 178

Topic starter 16/04/2025 4:48 pm

can i ask then if i have a heat loss for a room of 1079w and a flow of 40c@-3.1c what wattage rad do i need to hit? cause at the moment im struggling to find one big enough that would fit in the space.

ReplyQuote

JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 3398

16/04/2025 6:27 pm

The output of a rad is proportional to DT to the power 1.3, where DT is the average temperature difference between rad and room.

At a flow temperature of 40, and assuming a room temperature of 20 and a temp drop across the radiator of 5, DT is 37.5-20=17.5

Radiators are usually specified for a DT of 50. So you will need a rad with an output at DT 50 of (50/17.5) to the power 1.3 x 1079W = 3.9x1079W=4.2kW.

That's a mighty big radiator. What room is this in? Consider more than one radiator or a fan radiator if the loss really is that high.

This post was modified 7 months ago by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

ReplyQuote

AdamK

(@adamk)

Reputable Member Member

Joined: 9 months ago

Posts: 178

Topic starter 16/04/2025 7:51 pm

@jamespa thanks for the info but I think I must have something wrong. The heat loss for the whole house is about 7.5kw. The room is only 3.756m x 3.686m. Current rad is a P+ 1450mm 380mm.

ReplyQuote

JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 3398

16/04/2025 9:40 pm

Posted by: @adamk

↑

@jamespa thanks for the info but I think I must have something wrong. The heat loss for the whole house is about 7.5kw. The room is only 3.756m x 3.686m. Current rad is a P+ 1450mm 380mm.

Have you got the loss calculations, if so perhaps you could post?

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

ReplyQuote