Radiator sizing query. They are soo tiny!

In the process of a CH overhaul including new Rads throughout the house.

My GF's builder of a dad is helping us along and we were told to use one of the many BTU calculators to work out the rad sizes needed and to then get the ones we wanted making sure we get one bigger than the BTU recommendations.

We checked on a couple of calculator websites just to check the numbers we were getting and duly ordered the new rads to be fitted.

And example would be Bedroom. 3.4m x 2.9m x 2.5m(height). insulated cavity, 300m insulation in loft, 1 outside wall, 2m2 or UPVC double glazing.

Online calculators have that at around 1400BTU or 400W.

Thats a 600 x 400 Type 11. Pretty much half what we took out!

To say they are a bit small is an understatement!

When they'd been fitted he came round the house as couldn't believe how small they were.

He's done his own calculations and agrees with us that our numbers are basically right but its hard to get over the fact that we have taken out rads that are 600x800-1000 and replaced them with 600 x 500's. I understand that technology moves on and efficiencies of them has probably increased since the 60's when the old ones were installed but even so! It's hard to see past the fact that the old pipes are sticking out the floors a foot away either side of the old rad!

Insulation in the property has been increased so it has had cavities done and 300mm in the loft plus Double glazing.

I'm thinking that realistically if they are undersized compared to what they should be then we're just going to have to get used to a slow warm up over instant heat and shut doors.

The problem with too much theory is that you end up with needing small hot radiators rather than large warm ones.

Jackrae said:

The problem with too much theory is that you end up with needing small hot radiators rather than large warm ones.

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Exactly this. You can either run small radiators really really hot, or larger ones at a much lower temperature. The former is cheaper to install, the latter is very much cheaper to run. A condensing boiler should always be run at the lowest possible temperature, fitting it to tiny rads and running it hot will ruin the efficiency

Thanks for the help guys. Looks like a couple of the smallest ones will be being moved around the house and some larger ones will be being purchased.

At least i'm happy with normal ones and not designer expensive ones!!

Long, low ones give a better spread of heat in the room.

I'd be oversizing the radiators to allow for lower flow temperatures, just oversize them equally through the house, that way the house should still heat evenly. ie for each calculation add 50% onto the heat output on every rad.

Also when speccing the radiators, be aware that you may be looking at the radiators with a 80*c flow temperature, which if your getting a new condensing boiler now or in future you want to be able to heat the house when running it a good bit lowe than that 50/60*c flow temps or lower.

I just over size rads and let the rest sort it out. Eg trv. But if space and cost is a problem then work it out. Mears Gauge is still in the van. As mentioned old style was 80c flow with 11c difference , these days it’s designed with 70c flow and 20c difference,

Well after some reshuffling the rads are still fairly 'small' but no longer barely strong enough. Some of the old really small ones are being moved into hallways and replaced with new T22 ones. They still look tiny but on paper they are plenty strong enough to cope. Plus when the Extension gets completed and plumbed into the system I have some spares for it that should be plenty strong enough with the modern levels of insulation it'll have.

Got to wonder why after looking at a good 5 or 6 BTU calculators not a single one mentioned that after getting the number for the room you'd want to size up by quite such a margin to reduce the strain on them and the boiler.

Chunkytfg said:

Got to wonder why after looking at a good 5 or 6 BTU calculators not a single one mentioned that after getting the number for the room you'd want to size up by quite such a margin to reduce the strain on them and the boiler.

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It's not a case of reducing the strain on the rads and boiler but that the rads will give of less heat when run at a lower temperature. This is a good thing as a boiler needs to run at a lower temperature if is going to condense. (Technically, condensing does not start until the return temperature is below 55C.)

Radiators are tested with a flow of 75C and return of 65C (mean 70C), and a room temperature of 20C. This gives a DeltaT of 50C (70-20). If you want to run at different temperatures you need to apply a correction factor to take into account the difference in output. Here it is:

∆t Factor
5 - 0.050
10 0.123
15 0.209
20 0.304
25 0.406
30 0.515
35 0.629
40 0.748
45 0.872
50 1.000
55 1.132
60 1.267
65 1.406
70 1.549
75 1.694

∆t is the difference between room temperature and mean water temperature [(Flow+return)/2]. So a flow of 80C and return of 60C has the same mean temperature as a flow of 75C and return of 65C - 70C in both cases.

The factor can be applied in two ways:

1. To tell you what the output of a radiator will be when run at different temperatures. e.g flow 60, return 40, room 25. Mean water is 50, so ∆t is 25. This gives a factor of 0.406. So a nominal 1kw rad will only produce 1 x 0.406 = 406 Watts.

2. To tell you what size rad is needed to produce the required heat. So, using the figure above, if you need 1kW at those
temperatures you will have to install a rad with a nominal output of 1/0.406 or 2.46kW , say 2.5kW.

Modern systems are designed for a water temperature difference of 20C (Older ones used 10C/11C). Two reasons for the change: (1) the return would be nearer to that required for condensing, e.g 75C Flow, 55C return, with only a small reduction in output (factor for 20C room is 0.872); (2) the water would be flowing through the pipes at approximately half the rate, reducing the friction in the pipes and boiler by a factor of 4, so smaller pipes and pump would be required.

Wow thats a great reply thank you. Helps explain the performance charts i'd found much better.

Okay so an example of one room just to check i've understood it correct.

Our bedroom had a recommended BTU of 1700.

Originally I fitted a 600x500 T11 which had a BTU of just over 1700. Woefully low it seems so i've replaced that with a T22 version of the same size which has a BTU of ~3100. All these numbers are at T50.

Ideally I'd want to run the boiler at a lower temp meaning lower T factor.

With the New T22 Radiator I can afford to run the boiler and rads around T30-T35 type temps as that still brings the BTU to around 1600-1900.

Looking at the manufacturers data sheet for the rad's it seems at T30 they have it at ~1600 too so I guess my maths is correct.

Overall the house still isn't perfect but it's a lot better I think. The rooms that will actually be used regularly will have rads perfectly big enough and the rest of the house will be fine. We are a couple with no kids rattling round a 3 bed semi so lots of storage/occasional use type rooms anyway.

If you're not uplifting floors and moving pipework when changing rads, it's just far simpler and better to purchase rads that "fill the gap" between the existing pipes! Rad extensions (unless minimal) are an eyesore. You can always turn a large rad down

Thanks but mostly the rads are either in new positions completely or the floors are up to be able to make the pipework changes out of sight.

The house downstairs was mostly done with the pipework on the surface running down the walls which was already unsightly enough as it was! The new positions allow the rads to be back to back on walls or the pipework can run down the opposite wall side meaning the unsightly pipes are in room like the utility or hall way and not in the lounge.