Insulate first, then size the heating system

These are listed in the order worth acting on, which with heating systems is not the order they are usually presented in.

The order changes the specification

The size of any heating system is derived from how much heat the building loses, so reducing that loss reduces the system required. Insulating after installing means paying for capacity you no longer need and running a system outside its efficient range.

It also changes which technologies are viable, since some options only perform well in a well-insulated building. This is why energy advisers consistently recommend fabric measures before plant replacement.

Oversizing is the common fault

A boiler much larger than the building needs reaches temperature quickly and then shuts off, cycling repeatedly rather than running steadily. Cycling is less efficient, causes more wear and produces less even comfort than a smaller unit running continuously at low output. Systems are often oversized because a rule of thumb was used instead of a heat loss calculation.

Live with it a while and asking whether a proper room-by-room heat loss calculation has been done is the single most useful question to put to an installer.

Low-temperature heating changes the emitters

Heat pumps and condensing boilers both perform better at lower flow temperatures, and a lower flow temperature means each radiator emits less heat. Compensating for that requires larger radiators, more of them, or underfloor heating, which is why a straight swap often disappoints. Underfloor heating suits low-temperature sources well because the emitting surface is enormous.

A house with unchanged small radiators and a low-temperature source will feel cold, and the source will be blamed for a design error.

Controls do a lot for very little

Thermostatic radiator valves, a room thermostat in a sensible location and a programmer that matches actual occupancy typically save more than most people expect. Zoning a house so that unused floors are not heated to the same level is straightforward in most systems. Weather compensation, which varies the flow temperature with outdoor conditions, improves efficiency and comfort and is frequently available but not enabled.

In practice, a thermostat above a radiator or in a draught will misread the house and undermine everything else.

System health

Circulating water picks up corrosion products that settle in radiators and heat exchangers, reducing output and eventually blocking components. Power flushing, a magnetic filter and the correct inhibitor address this and are standard practice when a new boiler is fitted to an old system. Radiators cold at the bottom usually indicate sludge; cold at the top usually indicates air and needs bleeding.

Annual servicing of any combustion appliance is a safety requirement in many countries, and carbon monoxide alarms are inexpensive and strongly recommended.

Taste is not the argument here; how a room performs is.

Getting advice that is not selling something

Installers understandably recommend the systems they install, which is why an independent assessment before getting quotes is valuable. Grants, incentives and regulations around heating technologies change frequently and differ substantially between countries.

Anything involving gas, oil or solid fuel appliances must be installed and serviced by an appropriately registered person. Do not attempt work on combustion appliances or flues under any circumstances.

Everything above, in order of what to do first

1. The order changes the specification. The size of any heating system is derived from how much heat the building loses, so reducing that loss reduces the system required.
2. Oversizing is the common fault. A boiler much larger than the building needs reaches temperature quickly and then shuts off, cycling repeatedly rather than running steadily.
3. Low-temperature heating changes the emitters. Heat pumps and condensing boilers both perform better at lower flow temperatures, and a lower flow temperature means each radiator emits less heat.
4. Controls do a lot for very little. Thermostatic radiator valves, a room thermostat in a sensible location and a programmer that matches actual occupancy typically save more than most people expect.
5. System health. Circulating water picks up corrosion products that settle in radiators and heat exchangers, reducing output and eventually blocking components.
6. Getting advice that is not selling something. Installers understandably recommend the systems they install, which is why an independent assessment before getting quotes is valuable.

The takeaway

Reduce the heat loss first, insist on a proper heat loss calculation, and size the emitters for the flow temperature.

A room that is used will always beat a room that is finished.