How Is a Central Heating System Designed?

A central heating system is designed by working out how much heat each room loses on a cold day, choosing the water temperature the system will run at, then sizing the radiators, pipes, pump and controls to match. Done properly, it is a set of calculations on paper before any pipe is cut. Done badly, it is a guess based on what was there before, which is why so many London homes have one freezing room and one that is always too hot.

What Does Central Heating System Design Involve?

Design means matching the heat each room needs to what the system can deliver, room by room, at a chosen water temperature. It has five parts: heat loss, flow temperature, emitter sizing, pipework and controls, followed by commissioning.

Each step depends on the one before. The heat loss figure decides the radiator sizes. The flow temperature decides how big those radiators need to be for the same heat. The radiator outputs add up to the flow rate the pipes and pump have to carry. The layout of the rooms decides how the controls and zones are arranged. Skip the first step and every later decision is built on a guess.

Not every job needs a full design. A like-for-like boiler swap on a system that already heats every room well usually does not. A new system, an extension, a loft conversion, a switch to a heat pump, or a house that has never heated evenly all do. Where design sits within the wider project, with its steps, costs and timescales, is covered in our central heating installation guide.

Step One: A Room-by-Room Heat Loss Calculation

The heat loss calculation works out, in watts, how much heat each room loses when it is cold outside. It is the foundation of the whole design, and the step most often skipped.

For each room, the designer measures the walls, windows, doors, floor and ceiling, and notes what each is made of. Solid brick, cavity wall, single or double glazing, a suspended timber floor or a solid one: each lets heat through at a different rate. Ventilation and draughts are added on top. The calculation uses a target temperature for each room, commonly around 21°C for living rooms and a little cooler for bedrooms, and a design outdoor temperature for the location, which in London is a little below freezing. The recognised method is set out in BS EN 12831, and the CIBSE Domestic Heating Design Guide applies it to UK homes.

Older London housing makes this matter. A Victorian terrace with solid walls, a bay window and a suspended floor can lose far more heat from its front room than a modern flat of the same floor area. Rooms in extensions and loft conversions are often very different again from the rest of the house. Our guide to radiator sizing and BTU calculations walks through how the numbers for a single room are worked out.

Choosing the Design Flow Temperature

The flow temperature is how hot the water leaving the boiler or heat pump is designed to be on the coldest day. It has to be chosen before any radiator is sized, because a radiator gives out much less heat when the water is cooler.

Older systems were designed to run hot, with water around 80°C leaving the boiler and returning only about 11 degrees cooler. A condensing gas boiler only condenses, and so only reaches its best efficiency, when the water coming back to it is cool enough, below roughly 55°C. A heat pump is most efficient cooler still, often at a flow temperature of 45°C or less. Designing for a lower flow temperature means larger radiators, but lower running costs for as long as the system is in use. It also leaves the door open to a heat pump later without replacing every radiator.

If you are planning a new system today and expect to keep the house for years, designing for a lower temperature is usually the better choice, even with a gas boiler.

Sizing Radiators and Other Emitters

Each radiator, or each underfloor heating zone, is chosen so that its output at the design flow temperature matches or slightly exceeds that room's heat loss. Catalogue figures have to be corrected first, because they are quoted at a hotter water temperature than most systems now run at.

Radiator outputs are normally listed at Delta T 50, meaning water averaging 70°C in a 20°C room. At a lower design temperature, a correction factor reduces that figure, sometimes to less than half. A radiator that looks big enough on the box can fall well short in practice. Position matters too: under the window or on the coldest wall is traditional because it counters the cold air coming off the glass. Large rooms may need two radiators rather than one very long one, and bathrooms often need more heat than their size suggests. Underfloor heating is designed the same way, with pipe spacing and floor finish setting the output per square metre.

Pipe Sizing, Flow Rates and the Pump

Once the radiator outputs are known, the designer works out how much water has to flow to each one and sizes the pipes to carry it without being noisy or starving the far end of the circuit. The pump is then chosen to push that flow round the longest route.

Heat carried depends on flow rate and on the temperature drop across the radiator. A system designed for a small temperature drop, like most heat pump systems, needs a much higher flow rate, and so larger pipes, than a boiler system running with a 20 degree drop. Microbore pipe, the 8mm and 10mm copper used in many 1970s and 1980s homes, limits how much water can reach each radiator. Undersized primary pipework near the boiler can restrict the whole house. Pipes that are too small cause noise, cold radiators at the end of a run, and a pump working harder than it should. In a first-time or major installation, pipe routes are also planned around floors, joists and walls to keep disruption down.

Zoning and Controls

Controls are part of the design, not an afterthought. At minimum a wet system should have a time control, a room thermostat, TRVs on most radiators and a boiler interlock, and larger homes should be split into separately controlled zones.

The Building Regulations guidance for England expects separate zones for space heating in homes with more than 150 square metres of floor area, and zoning often makes sense in smaller homes with a very different upstairs and downstairs. Where the zones go is decided from the room layout and how the house is used: a home office used all day, bedrooms used at night, an extension with its own heat loss. Motorised valves, wiring centres and thermostats are then placed to suit. Our explainer on smart heating zoning covers how zones can be set up and controlled.

Commissioning: Balancing, Water Treatment and Handover

A design is only as good as its commissioning. The system has to be filled, cleaned, dosed with inhibitor, balanced so each radiator gets its designed share of flow, and tested to confirm every room reaches temperature.

Balancing is where the design numbers are turned into valve settings. Without it, radiators nearest the boiler take most of the flow and the furthest rooms stay cool, however carefully the radiators were sized. Water treatment follows BS 7593, which covers cleaning, flushing and inhibitor for domestic heating systems, and a magnetic filter protects the boiler or heat pump from debris. A good installer leaves you the heat loss figures, radiator schedule, control settings and a record of the water treatment, so the next engineer is not starting from scratch.

We design and install wet heating systems across London, from first-time installations to reworking a system that has never heated evenly. Gas work on the boiler itself must, by law, be carried out by a Gas Safe registered engineer. You can read more about our central heating installation service, or get in touch to arrange a survey and we will tell you whether your home needs a full design or just a few rooms put right.