Central heating installation

The calculation nobody does

A room-by-room heat loss calculation works out how much heat each room loses at design conditions – through walls, windows, floor, roof and ventilation – and therefore how much heat has to be put into it. It is not difficult and it does not take long, and it is the difference between a system that works and one that is a compromise from the first day.

Without it, radiator sizing is done by eye or by whatever fits the wall. That produces the familiar pattern: a house that is broadly warm with one room that never gets there, a boiler that everybody assumes is too small, and a series of expensive interventions that do not fix it because the problem was never the boiler.

The calculation also gives you the correct boiler output, which is almost always lower than people expect. An oversized boiler short-cycles, spends its life in the least efficient part of its range, and on a condensing appliance often never reaches return temperatures low enough to condense at all. Bigger is not safer; it is simply worse.

What goes into a system design

1. Survey and heat lossEvery room measured, construction noted, glazing assessed, and the losses calculated. Including any extension or conversion that was never accounted for.
2. Emitter selectionRadiator output matched to each room’s loss at the flow temperature the system will actually run at – which matters enormously, because radiator catalogue figures are quoted at temperatures most modern systems never reach.
3. Boiler sizingOutput matched to the total load plus hot water demand, not to what was there before.
4. Pipe sizing and layoutFlow rates calculated so every circuit gets what it needs. Undersized pipework is the invisible reason plenty of systems underperform.
5. Controls strategyZoning, thermostats, programming and – where appropriate – weather compensation, decided at design rather than bolted on.
6. System protectionCleaning to BS 7593 where existing pipework is retained, inhibitor dosed, and a magnetic filter fitted.
7. Commissioning and balancingThe step that turns a correct design into a working system, and the one most commonly skipped.

Flow temperature, and why it now matters

A radiator’s quoted output assumes a specific temperature difference between the radiator and the room. Manufacturers historically quoted at high flow temperatures, which is how systems were run in the 1980s. A modern condensing boiler is most efficient at low return temperatures – the whole point of condensing is recovering latent heat from flue gases, and that only happens when the return is cool enough.

So there is a direct trade: run the system hotter and the radiators deliver more heat but the boiler condenses less and burns more fuel. Run it cooler and the boiler is far more efficient but the radiators must be larger to deliver the same heat.

A well-designed modern system is sized for a lower flow temperature with correspondingly larger radiators. It is more efficient continuously, it is quieter, it is more comfortable because the heat is gentler and more even, and it leaves the system ready for a heat pump later if that ever becomes the right answer. A system sized at high flow temperatures is locked out of all of that.

Balancing, which is where systems are won or lost

Water, like electricity, takes the easiest path. Left to itself, most of the flow goes round the short circuit nearest the boiler and very little reaches the far end of the house. That is the true cause of most ‘one cold room’ complaints, and no amount of boiler replacement fixes it.

Balancing means adjusting the lockshield valve on every radiator so that each takes its correct share of the flow, measured by temperature drop across the radiator rather than by feel. It is methodical, it takes a couple of hours on an average house, and it makes a striking difference. It is also the first thing dropped when a job is priced tightly, which is why so few systems have ever had it done properly.

There is a fuller explanation in balancing a heating system.

Cleaning, protection and the warranty

BS 7593 sets out how a heating system should be cleaned, dosed with inhibitor and maintained. It matters because a heating system is a closed loop of steel, copper, brass and aluminium sharing the same water. Without inhibitor, that water corrodes the radiators from inside, producing magnetite – black iron oxide sludge. Magnetite settles in radiator bottoms, blocks heat exchangers and destroys pump bearings.

• Clean before the new boiler goes on, not after. A new heat exchanger connected to a dirty system is a new heat exchanger with a short life.
• Dose with inhibitor and re-check it periodically – inhibitor depletes and it is not a one-time measure.
• Fit a magnetic filter and service it. It catches what forms despite the inhibitor.
• Deal with any leak, because every top-up brings fresh oxygenated water into a system that should never see any. See leak detection.

Manufacturers will decline warranty claims on heat exchangers fouled by system debris, and they are entitled to. Skipping the clean saves a small amount at installation and risks the most expensive single component.