Radiator calculator results

RedQueenie

Hiya,
I'm looking to get a new radiator for my living room. I've taken the measurements and have used several calculation sites but have gotten wildly different results ranging from 4454 - 7239BTU. The largest number came from the Stelrad website where I was thinking of buying from. Is it better to go large?
Here are the measurements I'm using
Living room
Width 4.50m
Length 4.18m
Height 2.4m
Window area 2.97m
One outside wall
Heated room above
Wooden floor on soil
The house was built in 1907ish so think its 220mm solid brick, but I'm not certain.
Not sure if it effects it but the boiler is a condenser run by oil.
Hope someone can explain why the big difference and advise.
Thank you

ComicGeek

Because the sites never ask enough questions to calculate it properly. 

Uninsulated and ventilated suspended timber floor?
Single glazing or double glazed window? How old?
Is there a fire place in the living room? Open chimney, closed wood burning stove?

An open chimney can more than double the heat demand for a room, but few sites actually ask.

RedQueenie

Ah I see.
We have double glazing, a wood burning stove and the floor is laid on soil. I assume I should go bigger then?

FreeBear

ComicGeek said:

Because the sites never ask enough questions to calculate it properly. 

An open chimney can more than double the heat demand for a room, but few sites actually ask.

Orientation of the room can make quite a difference too. Got a ~7000BTU (2kW) radiator in a front facing room. No problem getting up to temperature. Same size radiator in a slightly smaller north facing room, and struggle to heat the space effectively.

Also quite surprised by the amount of heat that can be lost up an open chimney. Came across a spreadsheet that estimated the heat loss as anything between 800W and 2kW up an open flue - North facing room has an unused fireplace which is now plugged, but still struggle to get the temperature up in there.

In answer to @R@RedQueenie - Different online calculators make various assumptions. The Stelrad one appears to go for "worst case". I used it when crunching the numbers last year, and even then, went oversized by 30-50%. Aside from the aforementioned north facing room, I'm quite happy with the radiator performance. It has enabled me to reduce the flow temperature which should convert in to higher boiler efficiency. Condensing boilers (both gas and oil) rely on lower flow/return temperatures to boost efficiency.

Albermarle

RedQueenie said:

Ah I see.
We have double glazing, a wood burning stove and the floor is laid on soil. I assume I should go bigger then?

Also if you ever convert to an air source heat pump, then bigger is better as far as radiators are concerned.

ThisIsWeird

RedQueenie said:

Ah I see.
We have double glazing, a wood burning stove and the floor is laid on soil. I assume I should go bigger then?

Not just for that reason, but to make the most efficient use of your current boiler, and in particular to future-proof for an alternative energy source.

The cooler you run a system, the more efficiently it performs - the cool returning water extracts more heat out of the flame rather than let it disappear up t'flue. Larger radiators will provide the same heat output with a lower water temp.

Don't forget, you can make a 'large' rad as 'small' as you want by simply tweaking down the lockshield valve; it won't overwhelm the room. 

What size and type of rad was there? How well did it work to heat that room?

RedQueenie

At the moment we have a small Victorian style cast iron 9 column 16 section radiator which looking online gives out roughly about  5600. Does this style of radiator make a difference vs a bog standard radiator?

ComicGeek

Wall heat loss = ((4.5 x 2.4)-2.97) x 2.1 x 25 = 411W
Window heat loss = 2.97 x 2.8 x 25 = 208W
Floor heat loss = 4.5 x 4.2 x 0.56 x 25 = 265W
Ventilation heat loss (assuming open chimney) = 4.5 x 4.2 x 2.4 x 4 x 0.33 x 25 = 1497W
Total = 2381W (or 8124 BTU)

If the stove flue can be shut off when not in use, then this could be reduced to 1632W (or 5569 BTU). If the room has a party wall with another property then it would need to be increased.

You also want to have a 15% reheat factor to allow for heating the room from cold, so 6404 BTU or 9343 BTU depending on whether you can close off the chimney effectively or not.

So likely that your current radiator at 5600 BTU is slightly undersized, but probably close enough not to notice if your chimney is closed when your stove isn't in use, and if your boiler is running at its higher temperature.

Column radiators are typically much less effective at heating than panel radiators, so you need larger radiators and more wall space.

RedQueenie

Brilliant thank you. At the moment the room is chiller than any other room. The woodburner stove can be closed but I might go closer to 8000 as I'm not convinced it closes completely.
Can I ask why you would need to increase if there is a party wall to another property? We do, but I'm also interested to know.

ComicGeek

RedQueenie said:

Brilliant thank you. At the moment the room is chiller than any other room. The woodburner stove can be closed but I might go closer to 8000 as I'm not convinced it closes completely.
Can I ask why you would need to increase if there is a party wall to another property? We do, but I'm also interested to know.

Because if it is a party wall with a ventilated cavity then the heat loss through the wall could still be high.

Even if it is a solid party wall (or insulated and sealed cavity wall) then you will still get heat loss through the wall if your neighbour is on holiday or just turns the heating off or down.

If the party wall is just one side of the room, then would probably add around 200 BTU to the calc.

I'm a big fan of the Ultraheat rads https://www.inspiredheating.co.uk/radiators/designer-radiators/ultraheat-designer-radiators - they give out a lot of heat for their size, compared to traditional column rads, so worth a look. Normally provide 15-20% extra output for their size when compared against Stelrad products, and are cheaper too.