Heating, Hot Water and Renewable Systems
This domain tests heating and hot water design, controls and the renewable options that increasingly sit alongside them.
Heat loss and sizing
Boiler and emitter sizing start from room-by-room heat loss and U-values, not from floor-area rules of thumb. Oversized boilers cycle and run inefficiently; undersized ones never satisfy the house.
Condensing and controls
A condensing boiler only condenses when the return is cool - roughly below 55 °C - so low flow temperatures, weather compensation, interlocked controls, TRVs and zoning all raise efficiency. Hot water cylinders need a thermostat, a cylinder-stat interlock and a compliant temperature and pressure relief arrangement.
Renewables
Solar thermal preheats; heat pumps need low flow temperatures, larger emitters, good insulation and often a buffer vessel. Underfloor heating suits 35–45 °C flow. Heat-pump cylinders are usually larger to give a usable store at lower temperatures.
Worked example
Heat 150 L from 10 °C to 60 °C. Energy = 150 × 4.186 × 50 = 31,395 kJ = 8.72 kWh. A 24 kW boiler at 90% efficiency delivers 21.6 kW, so reheat takes 8.72 ÷ 21.6 = 0.40 h, about 24 minutes, before standing losses. The reasoning earns the marks: convert kJ to kWh by dividing by 3600.
Traps that cost marks
Sizing by room count; setting a cylinder stat too low for legionella control; fitting a heat pump to existing small radiators; ignoring the expansion vessel when fitting an unvented cylinder.