Cast Iron Radiator Outputs

[Annker]

I'm looking to buy cast iron radiators to install downstairs in a victorian renovation.

Cast iron radiator centre are a supplier that seem to be competitively priced, if not one of the cheapest suppliers on the market.

 

A potential catch which I am trying to resolve is the accuracy of their claimed performance (watts).

I asked AI to compile a table and it illustrates as I thought, that CIRC claim their sections put out ~25% to 30% more heat than most other brands.

 

As these sections are pretty much the same design, and I imagine are produced from same material, across all brands it seem like a claim worth verifying.

 

Has anyone ever had the similar query?

The heating system will be ASHP so I'm keen to match the respective heat loss figures for each room accurately and Obviously I don't want to end up with undersized rads. .   

 

3-Column Cast Iron Radiator Output Comparison (~745-750mm Height)

Per-Section Output @ ΔT50

Manufacturer	Model	Height	Watts/Section
Cast Iron Radiator Centre (CIRC)	Traditional Victorian 3-Col	745mm	115W
Shelbourne (CI Radiators 4u)	3-Column	745mm	86W
Paladin	Victoriana 3-Col	745mm	86W
Arroll	Neo-Classic 750	745mm	92W
Carron	Victorian 3-Col	745mm	80W

 

 

 

   

Edited February 3 by Annker

[SteamyTea]

Cast Iron has a thermal conductivity (k) of around 52 W.m^-1.K^-1.

Mild Steel is similar.

So it just comes down to the thickness really.

if they have the same mass for a similar surface area, they they will perform, near enough, the same.

(it is a bit more complicated than that as fins, and fluid to surface area ratio can make a difference, but you get the idea)

[Annker]

1 hour ago, SteamyTea said:

Cast Iron has a thermal conductivity (k) of around 52 W.m^-1.K^-1.

Mild Steel is similar.

So it just comes down to the thickness really.

if they have the same mass for a similar surface area, they they will perform, near enough, the same.

(it is a bit more complicated than that as fins, and fluid to surface area ratio can make a difference, but you get the idea)

Well that's what I'm assuming also; that is given that the designs, or material used are not significantly different, the outputs should be more or less the same.

So the question is whether 4/5 brands are understating the performance of their product or one brand is overstating their performance. The latter seems more likely.

 

AI suggested that I request their BS EN 442 test documentation to verify the claim output. I will do that tomorrow and imagine more than likely be met with silence.

 

 

[SimonD]

Here is the technical sheet for Stelrad cast iron. There 4 column are about 100W mark per section at 760 high.

28968_Stelrad-HS_Cast-Iron-Column_Web-PDF.pdf

[Annker]

20 minutes ago, SimonD said:

Here is the technical sheet for Stelrad cast iron. There 4 column are about 100W mark per section at 760 high.

28968_Stelrad-HS_Cast-Iron-Column_Web-PDF.pdf 2.65 MB · 0 downloads

Adds a bit more context that the Cast iron radiator centre are claiming that their 745mm high 3 column section has greater output than the Stelrad 760mm high 4 column section. 

It just doesn't seem likely to me.

 

The thing is that CIRC seem to be a lot less expensive than all other suppliers, even if in reality you need to size up from their performance tables.

I just want to make sure I get genuine performance figures from them.     

[Annker]

I've also noticed that CIRC state that a 4 column 760x1310 has an output of 2400W; where as their 3 column 745x1310 has an output of 2403W.

 

A smaller section with one less column having a greater(albeit slightly) output surely cannot be correct or am I missing something?  

[Gus Potter]

4 hours ago, Annker said:

I'm looking to buy cast iron radiators to install downstairs in a victorian renovation.

I had a loom at this for my own house as wanted soemthing similar.

 

4 hours ago, SteamyTea said:

Cast Iron has a thermal conductivity (k) of around 52 W.m^-1.K^-1.

Mild Steel is similar.

So it just comes down to the thickness really.

True but what I cloked was this.

 

@Anneker, I'm going to be a gent and say you are not as old as I. When I came back from Kenya to complete my secondary educuation Scotland was cold.. but the school really did have some real cast iron radiators. From memory the fins were about 150 mm thick and oval. The flow went in the top and out the bottom. The modern equivalent (replica) has the flow in the bottom and out also at the base. 

 

To make it work (modern column radiator)  you need to fit a baffle at the inlet at the base.. which basically makes it almost work like a modern radiator. Now the baffles are not perfect hence the likely discrepancy you see. Architectural radiators need to make a compromise between performance and design look.. there is no free lunch. 

 

I would if I was you chose the thing you like, over size it a bit, say by 20%,  fit a thermostatic valve. Make sure you have a good diameter flow and return pipe to it. In the round all this may cost you £100 quid more but I bet the paint on the walls is going to cost you more than that? 

 

Keep posting!

 

Gus

 

 

 

[Annker]

10 hours ago, Gus Potter said:

Now the baffles are not perfect hence the likely discrepancy you see. Architectural radiators need to make a compromise between performance and design look.. there is no free lunch. 

 

 

 

 

Hi Gus,

To my mind this is not the issue, as I am comparing one cast iron radiator section against another.

 

I called CIRC and as expected they can't give a specific reason why their section is upwards of 25% more efficient than all others, but they can say that no customer has raised this issue before.

 

Is it just my experience or do the majority of building product suppliers in the UK seem clueless about their products.

 

Anyway, as you also suggest I think if I use CIRC it is wise to oversize.

My AI assistant summarises as such:

Bottom line: With an ASHP, the risk profile is asymmetric. Undersized radiators force higher flow temperatures and reduce efficiency. Oversized radiators allow lower flow temperatures and improve efficiency. If you must err, err on the side of larger.

Sizing to Paladin/Carron figures while buying from CIRC is a sensible, low-risk strategy. Worst case: you've got slightly more efficient heating and spent a bit more on radiators. Best case: you've sized correctly.

 

  

[Gus Potter]

On 04/02/2026 at 09:33, Annker said:

I called CIRC and as expected they can't give a specific reason why their section is upwards of 25% more efficient than all others, but they can say that no customer has raised this issue before.

Ah! .. as they say "recollections may vary" . Your enquiry is perfectly justified, evidenced based.. on their figures. 

 

On 04/02/2026 at 09:33, Annker said:

Is it just my experience or do the majority of building product suppliers in the UK seem clueless about their products.

 

It's a mixed bag. It's a fascinating / frustrating subject. Why can't they just give me a straight answer?

 

I was in the past involved in bringing to the market cold formed steel portal frames. Testing and so on. Getting accreditation costs a fortune. A product supplier often has to balance the extent of testing vs how much of the product they are going to sell, the volumes, profit margin and so on. 

 

Once you can see under the bonnet ("unter der Motorhaube"?) then I think that UK suppliers are no worse / better than our European counterparts. I'm actually involved with a European supplier at the moment and they are exhibiting the same behavoir. 

 

At the end of the day if in doubt over size your radaitors. The radiator itself will cost more but the pipe work should be roughly the same size and the labour cost to install a slightly larger radiator should be the same or nearby. 

 

All the best.. but at the end of the day they have to look good, when you sit with your feet up and enjoying the fruits of your hard work. 

 

 

 

 

[JohnMo]

On 04/02/2026 at 09:33, Annker said:

err on the side of larger

Bigger is better, but only if all are the same % oversized, so don't over egg one room and neglect others. Otherwise you end up gagging back one room and still flowing a higher temp at ASHP.

[Annker]

On 08/02/2026 at 09:05, JohnMo said:

Bigger is better, but only if all are the same % oversized, so don't over egg one room and neglect others. Otherwise you end up gagging back one room and still flowing a higher temp at ASHP.

Thanks John, that makes sense.

[torre]

On 04/02/2026 at 09:33, Annker said:

why their section is upwards of 25% more efficient than all others

 

Have you checked they're using the same delta T when quoting output?

 

Any radiator can produce much higher output at a higher flow temperature, but like you, I'd be suspicious of one performing so much better than another of very similar design.

[Annker]

5 hours ago, torre said:

 

Have you checked they're using the same delta T when quoting output?

 

Any radiator can produce much higher output at a higher flow temperature, but like you, I'd be suspicious of one performing so much better than another of very similar design.

Yes, typically they all quote D50 and some then quote a few other common DT's.

But regardless I compared all against D50 

[marshian]

On 13/02/2026 at 09:23, Annker said:

Thanks John, that makes sense.

 

Just to expand on this (and give a real life example)

 

I upgraded all my upstairs rads from T11 to T22 in order to run lower flow temps from a gas boiler

 

As soon as we hit the heating season proper I had a nightmare with upstairs overheating and downstairs being not warm enough - so I throttled back the flow thro the upstairs rads - this had a big downside I effectively shrunk the avaliable circuit and had short cycling issues burning lots of energy and ending up with a house that was too damn cold all over unless I wound the flow temps up

 

I learnt a valuable lesson - since then all the rads are now upsized and sized to meet the heat loss of the individual rooms

 

I replaced the boiler with one that could modulate down to 4 kWh and could cope with low flow temps for CH and do DHWP for the cylinder recharge

 

house is super comfortable and I'm running flow temps between 25 and 35 deg C under weather compensation and can re-charge the HW cyl with 70-80 deg flow temps in 30 mins once a day.