6. Heat Loss - making sense of the figures
The last stage in a heat loss calculation is to add the conductive and ventilation figures together. The figures for each room can then be entered into a table which typically looks something like the example below;-
| Room | Length | Breadth | Height | Temp (ºC.) | No of o/side walls |
|
kW |
| Living Room |
5.2 |
3.5 |
2.2 |
21 |
2 |
1.5 |
2.32 |
| Kitchen/Dining |
4.0 |
3.0 |
2.2 |
22 |
2 |
1.5 |
1.68 |
| Utility Room |
2.2 |
1.8 |
2.2 |
16 |
1 |
2.0 |
.52 |
| Bathroom |
2.2 |
1.8 |
2.2 |
18 |
1 |
1.5 |
.53 |
| Bed.1 |
3.5 |
3.1 |
2.2 |
16 |
2 |
1.0 |
1.15 |
| Bed.2 |
3.5 |
3.1 |
2.2 |
16 |
2 |
1.0 |
1.15 |
| Bed.3 |
2.7 |
2.7 |
2.2 |
16 |
2 |
1.0 |
.71 |
| Hall 1 |
5.0 |
1.2 |
2.2 |
16 |
0 |
1.5 |
.57 |
| Hall 2 |
4.0 |
1.2 |
2.2 |
16 |
1 |
1.5 |
.62 |
|
Allowance for domestic hot water |
3.00 |
||||||
|
TOTAL |
12.25 | ||||||
Where central heating is being installed,
you can quickly convert the heat losses (green column) to panel
sizes, using a data sheet published by any panel manufacturer.
For instance a "Myson" data sheet will tell you that
for the 1.15kW heat loss in Bedroom 1. you need a single convector
panel 740mm high and 1,280mm long, or a double panel of the same
height 480mm long. Just to keep life interesting, panel emissions
are quoted for a standard room temperature of 21ºC. Emissions
increase at lower room temperatures so if you want to take the
difference into account you will need to apply the correcting
formula!
XXNotice
that the living-room and kitchen/dining rooms are heated to 21ºC.
whereas the bathroom is heated to 18ºC. and the hall, utility
rooms are cooler still at 16ºC. Air changes also vary according
to the function of each room. The normal allowance for bedrooms
is 1 air change per hour, whereas halls may be 2 or even higher.
An outside temperature of -1ºC. is assumed throughout. These
figures can all be found in the Code of Practice and in any standard
text book used by heating engineers.
XXHowever
there is some leaway – the design temperature for bedrooms
is sometimes reduced to 13ºC. and it is common to have kitchens
slightly cooler at 20ºC. when they are being purely as kitchens
rather than as a kitchen/dining area.
XXThe
total figure of 12.25kW may need to be 'plussed up' to allow
for heat-loss from the various pipe runs. Also, extra heat is
needed to provide a short-term boost when a system is being fired
up from cold – 10% is a fairly typical allowance.
XXAnd
there may be other things to consider that can be tricky to quantify.
In extreme conditions the wind-chill factor on exposed sites
can make both "U" values and ventilation allowances
appear somewhat adrift. There is also the issue of temperature gradient covered elsewhere on this site.
