Why Conservatories Feel Cold in the First Place

A conservatory is, in effect, a large glass box bolted onto the back of your house. Even a modern one with argon-filled, low-emissivity glazing will lose heat far faster than a brick-and-block wall with cavity insulation. Older conservatories with polycarbonate roofs are worse still — a single-skin polycarbonate roof can have a U-value ten times poorer than an insulated ceiling.

That means the usual rule of thumb you would apply to a bedroom or living room, roughly 40 to 50 watts per square metre of floor area, will leave a conservatory badly under-heated. It also means that simply extending your existing central heating circuit into the conservatory is rarely a good idea. You end up with a room that never quite warms up, a boiler working harder than it should, and a chilly dining area you only use for six weeks a year.

The good news is that with correct sizing and, crucially, separate controls, a conservatory can be genuinely comfortable from March through to November, and usable in winter with a bit of forward planning.

Working Out the Right Radiator Size

For a conservatory, budget on 100 to 130 watts per square metre of floor area, and lean towards the higher figure if the roof is glass or polycarbonate, the room faces north, or it has more than one external glazed wall. A typical 3m x 3m conservatory therefore needs somewhere between 900W and 1,200W of output — noticeably more than the 400 to 500W you might instinctively choose.

Remember that quoted radiator outputs are based on a flow temperature of 75°C and a room temperature of 20°C. If your boiler runs at a lower temperature, for example with weather compensation or as part of a heat pump setup, the real output will be lower and you will need a physically larger radiator or a different emitter altogether.

• Type 22 double-panel, double-convector radiators give the most output for the least wall space.
• Vertical designer radiators suit narrow wall sections between windows or doors.
• Low-level trench convectors work well where glazing runs almost to the floor.
• Fan-assisted or fan convector radiators warm a room quickly and are useful if the conservatory is used intermittently.

Position matters too. Put the radiator under the largest glazed area if you can — it creates a curtain of warm air that offsets the cold downdraught from the glass. Avoid boxing it in behind furniture or curtains, which can cut effective output by a fifth or more.

Separate Controls Are Not Optional

This is the single most important point in the whole article. A conservatory should be its own heating zone, with its own thermostat or programmable room stat, and its own time settings. Conservatories heat up and cool down far more quickly than the rest of the house, and they benefit enormously from solar gain on a sunny afternoon, so tying them to the main house schedule wastes money.

If you are running the conservatory off the central heating, ask your heating engineer to fit a zone valve and a separate thermostat, and ideally a thermostatic radiator valve as a secondary limit. Set the conservatory zone to come on only when you actually use the room — a couple of hours in the evening, or a morning boost — rather than following the main house programme.

Set the target temperature a degree or two lower than the rest of the house. Conservatories are often used for dining, hobbies or as a garden room, and 18 to 20°C feels perfectly pleasant there. Pushing for 22°C in a glass-walled room is expensive and rarely achieved comfortably.

The Building Regulations Question

In England and Wales, a conservatory is generally exempt from full building regulations if it is under 30 square metres, is separated from the main dwelling by external-quality doors, and its heating has independent controls with its own temperature and timing. Get those three things right and you can heat it as a conservatory.

Take the doors out, though, and the room becomes part of the dwelling. At that point the glazing, walls and roof need to meet the insulation standards for an extension, which usually means a new roof and a serious conversation about whether the conservatory is worth keeping. Keeping the doors — and keeping them shut — is what allows a modest radiator to do a decent job.

Practical Installation Points

• Check your boiler's spare capacity before adding any load. An extra 1kW is fine on most modern combis, but older systems may struggle.
• Insulate the pipe run where it crosses a cold space such as a garage or under a suspended floor, and fit a drain point or frost protection if any part of the run is vulnerable.
• Keep the floor level where pipes pass through a solid conservatory floor; a neat sleeve and a sealed penetration avoids damp and draughts.
• Add inhibitor to the system after any alteration, particularly if the conservatory is a long way from the boiler.
• Consider thermal blinds and a roof blind as well as the radiator. Reducing heat loss is always cheaper than replacing it.

Alternatives Worth Knowing About

If running new pipework is impractical, an electric panel heater or oil-filled radiator on a smart plug can work well for occasional use, though running costs per kWh are higher than gas. Electric underfloor heating suits tiled conservatory floors but is slow to respond and best paired with a well-insulated floor. A small air source heat pump or a split system can heat and cool the space, which is genuinely useful in a south-facing conservatory that overheats in July.

Whatever you choose, size it properly, zone it separately, and set your expectations realistically. A conservatory will never be the cheapest room in the house to heat — but with the right radiator and independent controls, it can be one of the most enjoyable to sit in.