Rads needed are much smaller than i thought

I am going to be renovating a flat soon and it needs heating installing.

I am just working out the size of the rads to use and it seems they are much smaller than i would have imagined.

Any ideas on sizes for these 2 rooms?

Bedroom = 5.2m x 4.4m x 2.7m height. one large sliding sash single glazed. One external wall. flat above and below.

Calc says need 3000BTU = double panel and fin 1000 x 600 rad is ok for this room.

Lounge = 5m x 6.5m x 2.7m height. Large sash bay plus 2 large sashes. So 10 sqm glass. 2 external walls. Flat above and below.

calc says 10500 BTU

 

Just remember that the heat loss calculations air change figure plays a significant part and it's the one parameter you'll have difficulty assessing....so your final figures are rough especially with the windows you have.
I'm assuming you've assumed minimal heatloss to the surrounding flats (they'll have their heating on surely) but you should consider the possibility of being surrounded by empty flats with no heating on.
Many years ago I had a flat and for a month one winter I had 3 empty flats adjoining (void rentals, away on holiday, up for sale etc)...I needed to supplement with electric fan heaters.
Apart from minimal cost there's no harm in over-sizing the radiators, it's more comfortable and economical running at low temperaures.

 

Ok, so what is the norm to compensate if you have an x figure BTU requirement. Do you over size by 25% or 50% for example ?

 

Go 50% bigger if you can. Radiator sizing calculators are still set up for high temperature non-condensing boilers, even though condensing boilers have been mandatory for the past 15 years. It's much cheaper to run a condensing boiler at lower temperatures - if the water returning to the boiler from the radiators exceeds 52ºC it won't condense and will revert to being a standard efficiency boiler. Bigger radiators mean you can run the whole thing cooler

 

Ok thanks

 

Bring yourself up to date and move into the 1970's

kW are a handy way to calculate heat output. Your boiler will be rated in kW, and so is your gas bill. You can picture the output of a 2kW electric heater.

Would you really rely on a 900W heater for your bedroom? In winter?

Radiator makers like BTU because nobody knows what they are, and the big numbers look impressive.

 

Ok, makes sense. Thats an easy way to work out what rad i should be thinking about. Thanks John

 

Ok so i have got some KW sizes now.

Do you reckon 1.75 KW is enough to heat 45 cubic meters in a bedroom ?

And 4.5 KW is enough to heat 92 cubic meters in the lounge kitchen area ?

 

To Muggles' point above, I did some research when replacing a number of radiators to correctly size them based on advice I'd read on here. I built a simple spreadsheet that allowed me to adjust the Delta T number by changing the flow from the boiler, such that I could see the difference between delta 50 with a flow of 80, down to delta 5 with a flow of 35. It was interesting to see the difference it made to target outputs of radiators (most radiators are specced to Delta 50, so its easy to calculate the adjustment factor as delta goes up/down.)

Using this, and aiming for a flow of 60 and therefore delta 30, I re-sized radiators accordingly and am genuinely amazed at the difference in comfort. I still have a few more radiators to swap over as we finish re-decorating a few other rooms. I can't comment scientifically about the efficiency/cost savings, however comfort is greatly improved.

I'd also suggest being wary of many of the online BTU/watt calculators, which I found to be inconsistent in many places.