Calculating radiator sizes. Does this look right

[browfish]

I am trying to calculate the size of radiator required for each room so that i can size a new boiler. we are renovating a bungalow and extending it so the existing system will virtually all go. Below is an example of the calculation i have done for one room. Does this look right? I am a little concerned that including 3kw for water, my boiler only comes out to be around 18kw for a 5 bedroom bungalow/house. Am i using the right U values as sources on the net seem to vary.

Lounge
External walls 21.9sqm x Temp difference (22C)x U value (0.92, masonary cavity filled) = 443W
Window 4.5sqm x 22C x 5 (PVC double glazed window) = 495W
Internal walls 18.5sqm x 3C x 1.7 (internal stud wall) = 95W
Floor 23sqm x 11C (solid floor uninsulated, average ground temp 10 degC) x 0.71 = 180W
Ceiling 23sqm x 22C (loft space) x 0.34 (joists and insulation) =171W
Ventilation 1.5 air changes x 55m3 x 22C x0.33(vent factor) = 600W
Total 1984W
The U value of the solid floor was worked out using the formula U = 0.05 + 1.65(P/A) - 0.6(P/A)**2
Therefore the floor U value is different for each room dependant on the perimeter to area ratios.

 

[D_Hailsham]

Use the Boiler Size Calculator to find out what size boiler you need. Just be aware that this includes 2kW as standard for the HW cylinder.

If your house is well insulated 15kW could well be all you need.

Some of your U-values seem on the high side

Cavity filled walls - Max 0.75
DG UPV windows - Max 2.6

Floor: You have to deal with the complete floor as an entity.

Download Stelrad Stars and you can have fun working everything out.

 

[browfish]

Thanks for that
I have just run that boiler size calculator and come out with the following figures

The WINDOW heat loss is 2.62 kW

The WALL heat loss is 2.41 kW

The ROOF heat loss is 10.92 kW

+ water 3 KW

Total 18.95KW. This isnt too dissimilar to my own calculations

Why does it then suggest a boiler size of 26.94KW

 

[D_Hailsham]

Total 18.95KW. This isn't too dissimilar to my own calculations. Why does it then suggest a boiler size of 26.94KW

18.95kW does not include the loss though the floor or ventilation losses.

Are you sure entered the correct info for the number of walls front to back? A detached bungalow will have two, a semi-d bungalow only one.

It's also worthy playing with the calculator and seeing how improvements in insulation reduce the size of boiler required.

What size is the existing boiler - if it's still there?

 

[browfish]

I have calculated the U value for the floor as a whole and it comes to almost exactly the same as the sum of the individual areas i have works out.

If i have used excessively high U values, how come my calcs come to 17kw, including the ventilation and floor losses, the same as the boiler sizing website total but 9kw less than the boiler it suggests.

 

[D_Hailsham]

If i have used excessively high U values, how come my calcs come to 17kw, including the ventilation and floor losses, the same as the boiler sizing website total but 9kw less than the boiler it suggests.

It's due to the "Location Factor", which will be either 25 or 27, depending on whether you say you live in the SE or the SW.

The Location Factor covers three things:

1. 5% allowance for losses in the pipes
2. 10% heating up allowance - means the house will heat up faster from cold
3. The different winter temperatures experienced in the various regions.

The location factor can be viewed as a " temperature". The calculator first works out the loss per degree and then multiples it by this "temperature" to get the boiler size.

The calculated heat requirement for your house is 26.94-3= 23.94kW

Assuming you live in the SE (factor=27), the loss per degree is 23.94/27 = 0.89kW.

Multiplying this by 22 (21C - (-1C) gives 19.5kW, almost the same as your room by room calculation.

One thing I did notice is that you have assumed an average underfloor temperature of 10C. You can't do this; you have to assume that the ground will go down to the same temperature as everywhere else. This would almost double the loss though the floor.

Whole House Boiler Sizing explanation

As for the floor losses, if you have used the same U value for each room as for the total floor area, the answer will obviously be the same. But U values of ground floors are not constant, they vary according to the ratio of area to perimeter - see table 6 of the above link.

 

[browfish]

I am in southampton (south central) so have taken the outside temperature to be -1 deg and then used the relevant required temperature for each room eg 18deg for bedroom 21deg for lounge etc. eg temp difference acros rooms fo 19 and 22 deg respectively. Does this not take account of anny location factor?
Is my -1deg a bit lights? The 25 and 27 you mention i guess are talking more like -4 and -6 degrees outside

I had forgotten to add 5% for pipe losses and 10% for heating up. This takes my calculations up to a 19.5kw boiler

I have read reports and threads on line that the temperature below the floor slab will remain pretty constant as teh ground acts as a huge heat sink. They even go as afar as to suggest that solid floor insulation is a waste of money and this money is better spent insulatiing other areas. This suggested taking a floor temperatur of 10deg or higher.
The floor can take a while to heat up though to start .

I have calculated individual U values for each area of floor using the same equation. The sum of these heat losses comes to pretty much the same as treating the floor as a whole

 

[D_Hailsham]

I am in southampton (south central) so have taken the outside temperature to be -1 deg and then used the relevant required temperature for each room eg 18deg for bedroom 21deg for lounge etc. eg temp difference across rooms of 19 and 22 deg respectively. Does this not take account of anny location factor?
Is my -1deg a bit lights? The 25 and 27 you mention i guess are talking more like -4 and -6 degrees outside

The location factor includes the 5% and 10%. So 25 means a temperature rise of 25/1.15 = 21.73 or 22C (I assume they have rounded off). If the room temperature is 21, this means an outside temperature of -1C. 27 is equivalent to an outside temperature of -2 or -3.

The boiler calculator also assumes that all rooms are at 21C, whereas you have lower temperatures for the bedrooms.

I had forgotten to add 5% for pipe losses and 10% for heating up. This takes my calculations up to a 19.5kw boiler

You originally said your calculations came out as a 18kW boiler (including 3kW hot water), i.e 15kW heating. Multiplying this by 1.15 (10% + 5%) gives 17.25kW =3kW = 20.5kW.

If you recalculate using the same temperature for all rooms you will probably get near to the 26kW suggested by the boiler calculator.

It's not a good idea to oversize too much, but under-sizing is even worse. Whichever boiler you eventually choose, make sure that it modulates as low as possible compared to you calculated requirement. So, if you need a 21kW boiler, an 8-24kW boiler is better than a 12-30kW.

I have read reports and threads on line that the temperature below the floor slab will remain pretty constant as the ground acts as a huge heat sink. They even go as afar as to suggest that solid floor insulation is a waste of money and this money is better spent insulating other areas. This suggested taking a floor temperature of 10deg or higher.
The floor can take a while to heat up though to start .

That's interesting; can you post a link the source?

I have calculated individual U values for each area of floor using the same equation. The sum of these heat losses comes to pretty much the same as treating the floor as a whole

This is pure coincidence.