Designing the Heating System You Need
Establishing the heat requirements for a satisfactory level of comfort in the home cannot be arrived at haphazardly. This does not, however, need advanced mathematical skills, for by careful and simple calculation you will be able to work out your requirements to provide comfort with reasonable economy.
Finding the comfort levels you need in the home is basically a matter of simple arithmetic. Even if you are not mathematically minded, it should not be difficult to work out your heat requirements systematically, using a simple formula.
Heating values are expressed in kilowatts per hour (kW/h). Also still used is the Btu/h, or British Thermal Unit/h. This unit of heat measurement will eventually give way fully to the kW/h measure of heat.
An apparently astronomical number of Btu’s are needed to heat a home. This is because each unit represents only roughly the heat from one lighted match.
One Btu is the degree of heat needed to raise lib of water by 1°F in temperature. Its relevance in a metric system is clearly limited.
One watt per square metre (w/m2 °C) is the heat required to raise 1kg of water by a temperature of 1°C. A watt equals 3-4 Btu. One kilowatt/h is 3412 Btu/h.
Metric and Imperial
Imperial heating terminology, though still in parallel use, is being phased out. Fittings and pipework have all been metricated. But some terminology and measurement terms in Imperial will still be encountered.
These are the principal terms: Pressure The Imperial values are given in inches water gauge (in.wg) or pounds force per square inch (lbf/psi), or lbf/in2. The metric equivalents are the millibar (mbar) for in.wg and the bar for lbf/in2.
Formerly expressed in degrees Fahrenheit (°F), Celsius (formerly called Centigrade) is now used. Example: 60°F (15°C).
30in mercury (30” Hg)-1013.25 mbar. This relates to the setting ofinput pressure and is usually carried out by the gas authority.
Therm-100 megajoules (MJ).
British Thermal Units/h is expressed in Megojoules per cubic metre (MJ/m3).
Btu/h is expressed in kW/h. This relates to consumption and not to effective output. Where information may be required to calculate running costs, megajoules per hour may be used in addition to kilowatts – kW(MJ/h).
Heat passes through different substances at different rates. This is called the heat-transference factor, or V value and is expressed in terms of watts per square metre in °C (w/m2 °C).
The lower the ‘U’ value, the better the heat-retaining qualities of a substance. This is measured by the rate in watts passing through a square metre (m2) of a substance for a fall of 1°C in temperature.
You have to establish the ‘U’ values of the materials used in constructing the home to arrive at given heating needs. Into consideration you need to take the required comfort levels and allow for a number of air changes per hour. It is not entirely a contradiction to insulate a house well and then ventilate it, for damp and stale air must be changed.
In living rooms, the number of air changes per hour should be two; for bedrooms and non-living areas, the factor is 1.5 per each hour.
The ‘U’ values you need are for windows, walls, floors and roofs. Windows possess the highest rate of heat loss, but this can be halved by double glazing.
Heat is lost in varying amounts through the house fabric, and air must be changed to retain freshness. Stale breath contains a high proportion of moisture, which is why people often complain of damp and running water on windows or other ‘cold’ surfaces.
Ensure that you are not losing excessive amounts of heat. The, in particular, should be insulated with a minimum thickness of 50mm of glass-fibre wrap or its equivalent.
Assuming a sensible level of thermal insulation and the elimination of wasteful heat losses through doors and windows, the following are suggested room temperatures and number of air changes, generally accepted as providing suitable comfort levels, allowing for temperatures outside of minus 1°C.
With these figures, conduction of heat from one room to another is ignored. Social habits of a household come strongly into finding the amount of heat needed, and thus the size of fuel bills.
In arriving at the amount of heat required for any home, factors such as the amount of hot water needed and variation in comfort levels likely to be desired have to be considered.
For example, the comfort levels of a bedroom should consider possible future use. While 13°C is acceptable for a bedroom where this may have to serve the dual function of a study, advance provision should be made for extra heat when needed.
Single-glazed windows have a factor of 5.68w/m2 °C; double glazing reduces the ‘u’ factor to 2.84; cavity walls have a factor of 1.65; solid walls 2.33; suspended floors a factor of 2.16; solid floors 3.58; tiled-and-felted roofs, 3.18, reduced by good; and a plaster ceiling a value of 1.65.
It is usually taken that comfort levels are provided by an internal temperature of 22°C in living rooms, in halls and 13°C in bedrooms. These calculations are based on an ambient (outside) temperature of 0°C, or freezing point.
This example of how to assess your needs for heat is based on a living room 3.66m x 4.57m x 2.44m high, with a 2.44m x l.22m window and a solid floor. You have to establish the area for both the inside and exterior walls, the temperature requirements of adjacent rooms, and the areas of the ceiling, floor and glass.
First, establish the volume of the room by multiplying length x breadth x height. This provides a figure of 40.83m3 (3.66 x 4.57 x 4 x 2.44).
Work out the area of the outside wall: 3.66 x 2.44 + 4.57 x 2.44 = 20.09m2. Take away the area of the glass (2.44 x 1.22 = 2.98), leaving a figure of 17.11.
Establish the area of the inside wall: 3.66 x 2.44 + 4.57 x 2.44 = 20.09.
Next, calculate the area of the floor: 4.57 x 3.66 =16.72 m2.
For final calculation establish the difference between the ambient —18° C 0°F and the desired internal temperature (22) and the temperature difference between this and the adjacent room temperature, the hall, (6).
Then multiply the number of air changes per hour x cubic capacity of the room by -37, which is the nominal ‘U’ factor for heat loss in w/m3 °C on air changes x temperature difference.
Next, multiply the area of the wall by the ‘U’ factor x the temperature difference between the inside and the outside. Follow the same procedure for the floor and other surface areas. The totals, when added, provide the units of heat in watts you need for comfort.
You will reach high totals by simple multiplication. Divide each of these by a thousand to obtain the figure in kilowatts.
Figures are arrived at as follows:
Room volume 40-83 x 2 (air changes) x -37 (air-change ‘u’ factor) x 21 (temperature) = .668
Outside wall: 2009 x 1.65 (V) value of wall x 21 = .694
Inside wall: 20.09 x 3.80 x 5 = .382
Floor (solid): 16.72 x 3.58 x 21 = 1.256
Heat loss through glass 2.98 (area) x 5.67 (‘u’ value) x 21 = 3.55
Total 3.355 kW/hr.
This is in the order of 3-1/3kW per hour.
No allowance is usual for downstairs ceilings. The final total would be correspondingly reduced by double glazing (‘u’ 2.83w/m2 °C).
These factors are based on average conditions and exposure. In excessively exposed or in sheltered conditions, you will need to make corresponding adjustments.
Sizing of radiators ensures that correct temperatures are derived at a flow temperature of 82°C on conventional small-bore systems, with a return temperature of 71°C.
The amount of radiator surface needed can be readily arrived at from manufacturers’ data. For temperatures above 82°C, exposed steel-panelled radiators are not advisable and skirting or fan convectors, thermal or other enclosed radiator units, should be used.
Skirting radiators emit 123kW at a temperature of 82°C and .180kW at 96°C. Steel-panelled radiators may be used at elevated (up to 96°C) temperatures or high temperatures (above 96°C), provided these are enclosed and cannot be directly touched.
A rule-of-thumb method of determining heat output requirements at standard temperatures is to allow 2.60m – of radiator surface for every 28.31m3 of space; for double radiators allow 3.15m2.
Another practical rule, which can be very accurate in average conditions, is to measure the room volume and, for a desired temperature of 21°C, multiply volume by 83. For a temperature of 16°C, this is multiplied by 63 and for a temperature of 13°C, multiply by 49. This gives the figure in watts. Divide this by 1,000 to obtain the kW/h rating.
10. November 2011 by admin
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