Choosing internal and external design temperatures and allowing for the building exposure when assessing heat loss.
Internal design temperatures
Every heat loss starts from agreed room temperatures. Typical internal design values used in UK heating design are around:
- Living room and dining room: 21°C
- Bedrooms: 18°C
- Bathroom: 22°C
- Hall and landing: 18°C
- Kitchen: 18°C
These figures agree with the room-use values in the MCS heat-pump design table, which sets minimum design temperatures for that method. Check the applicable guidance and client brief; design values and occupants’ later thermostat settings are different things.
External design temperature
Select a cold-weather design temperature for the location using the chosen method and its altitude or site adjustments. Values vary across the UK and with the percentile used; there is no single -1 to -3°C range suitable for every site. For an element exposed to outdoor air, deltaT is the room design temperature minus this outdoor value. Other boundaries, including solid ground floors, follow their own method.
Degree of exposure
Two identical houses can lose heat at different rates depending on exposure. Wind exposure can increase infiltration and affect external surface heat transfer. Assess shelter and exposure using the design method; do not substitute a human “wind chill” temperature for the outdoor dry-bulb temperature.
Putting it together
Record the room targets, outdoor design value and exposure assumptions. Understating them can understate the duty; adding unneeded margins can increase equipment size and cost. Check the design result: neither extreme is right by default.
Party walls and other heated boundaries
An external wall loses heat to outside air, but a wall shared with a heated neighbouring dwelling, or an internal wall to a heated communal area, has warm air on both sides. The usual assumption is that both sides sit close to the same design temperature. The driving temperature difference is then small or zero, and so is the heat loss counted for that wall in the room calculation. Where the wall separates the dwelling from an unheated space, or the neighbour's temperature is not known, apply the construction's own U-value and a suitable temperature difference instead of assuming zero.
Allowing for intermittent heating
Where a dwelling cools during setback, recovery can require capacity to warm its fabric and contents within the agreed time as well as replacing ongoing losses. Apply the chosen design method and schedule, and add a percentage only where the method calls for one. MIS 3005-D, for example, states that using its colder 99.6% outdoor-temperature column removes the separate intermittent-heating uplift.