The Radiator: Mostly Convection, and Why Temperature Is Everything

Find the radiator's catalogue rating at ΔT 50 on its label or the maker's sheet, and its dimensions. With the heating running steadily, read with the infrared thermometer: the flow pipe where it enters, the return pipe where it leaves, the top and bottom of the radiator face, and the room air a metre away with the thermometer. Work out ΔT from the flow and return readings and the room, and put it into the notebook's formula: that is what the radiator is actually giving now. Shiny pipes read low on an infrared thermometer — stick a piece of masking or electrical tape on them and read the tape. Now hold a strip of tissue a hand's width above the top of the radiator: it lifts in the rising air. Hold it the same distance in front of the face: it barely moves, but your hand still feels the warmth. That is the two halves of the heat, separated.

A radiator that is warm at the bottom and cold at the top is full of air at the top. With the heating off and the system cool, hold a cloth under the bleed valve at the top corner and open it a quarter turn with a radiator key or a flat screwdriver: air hisses out, then water spits. Close it. On a sealed system, top the pressure back up at the boiler afterwards. A system where the radiators nearest the boiler are hot and the far ones lukewarm needs **balancing**: the lockshield valve on each near radiator is closed down a little so that water is pushed on to the far ones. Balance for a temperature drop across each radiator of about 10 K — the flow and return readings from step 2.

Hot-water and steam heating with pipes and radiating surfaces grew through the nineteenth century; the cast-iron sectional radiator made it cheap enough for ordinary buildings, because one cast section could be assembled into any size. **Nelson H. Bundy's 'Bundy loop' of 1872** is one of the early, much-copied designs. No patent number is asserted here. Steel panel radiators replaced cast iron in most new work in the twentieth century, and underfloor heating — a very large, very cool radiator — is the logical end of the notebook's first table. **Honest limits.** Output collapses at low water temperature. A radiator under a window or behind furniture loses much of its convection. And a system is only as good as its balancing: an unbalanced system burns fuel to overheat the rooms nearest the boiler.