Choosing a Radiator for Cold Rooms: Size, Type and Running Costs

The right radiator for a cold room is one sized to the room’s actual heat loss, fitted with a thermostatic control, and matched to how you use the space. In most UK homes the cold room is cold for one of three reasons: the radiator is too small, the room loses heat through walls and windows, or the heating is switched off too often. Fix those in the right order and you will usually spend less than you would by simply buying a bigger radiator.

This guide takes you through that order: diagnose the room, size the output, choose between a plumbed radiator and an electric one, and then run it without inviting damp.

TL;DR

• Work out the room’s heat requirement before you buy. An oversized radiator wastes money, and an undersized one leaves the room cold. Our BTU per m² guide shows how.
• North-facing rooms, rooms with two external walls and rooms with large windows need extra output compared with a standard allowance.
• If the room is off the central heating system, a plug-in or wall-mounted electric panel is the simplest route. Oil-filled and convector types give steadier warmth than fan heaters, according to Which?.
• Turn a cold room down rather than off. Leaving it completely unheated raises the risk of condensation and mould.
• Draught-proofing and thermal curtains cost little and often make a bigger difference than a new radiator.

Table of Contents

• Why some rooms stay cold
• How much heat does the room need?
• Plumbed radiator or electric: which suits a cold room?
• Portable heaters: which type, and what they cost
• Fixed electric radiators for cold rooms
• Where to put the radiator
• Run it right: turn down, do not turn off
• Our view on cold rooms
• Choose your radiator at Simco Direct
• FAQ
• Sources

Why some rooms stay cold

Before you spend anything, find out why the room is cold. A radiator can only replace the heat the room loses. If the room loses a lot, even a good radiator will struggle.

The usual causes:

• External walls. A room with two outside walls, such as a corner bedroom, loses heat faster than a room with one.
• Orientation. North-facing rooms get less sun, so they start each day colder and need more output.
• Windows. Old or single glazing is a big source of loss. So are large glazed areas, as in conservatories and extensions.
• Draughts. Gaps around doors, windows, floorboards and loft hatches pull cold air in.
• Under-sized radiators. Many rooms were fitted with a radiator chosen by the size of the available wall, not by the heat the room needs.
• Poor balancing or controls. A radiator at the end of the system may never reach full temperature, and a TRV set too low will stop it heating.

A council retrofit guide recommends draught-proofing windows, doors and floorboards and adding thermal curtains as cheap ways to cut heat loss, and reflector panels behind radiators on external walls to send heat back into the room. If you can do those first, any radiator you add will work harder for you.

For a whole-house view, the Surrey Home Energy Improvement Guide suggests appointing a qualified building surveyor for a detailed heat loss survey, which can include calculations on any radiators that may need upgrading. That is worth considering if several rooms are cold, not just one.

How much heat does the room need?

Radiator output is quoted in watts or BTU/h. One watt is roughly 3.41 BTU/h, so a 1,000 W electric panel is about 3,400 BTU/h. The number you need depends on the room’s volume, its insulation and its exposure.

A quick rule of thumb is a starting point, not an answer. The usual method adds an allowance for each factor that makes the room colder:

1. Start with the room’s floor area and ceiling height.
2. Add output for north-facing aspect, for each external wall beyond the first, and for large or single-glazed windows.
3. Reduce it for well-insulated rooms, or rooms with heated rooms above and below.
4. Check the radiator’s output at the right flow temperature. Published figures are normally given at a standard temperature difference, and a system running at a lower flow temperature will deliver less.

We have written the full method, with worked figures, in our guide to calculating radiator BTU per m² for UK homes. Use it before choosing any size.

A word on oversizing. It is tempting to buy the biggest radiator that fits. With a plumbed system, that can mean the radiator reaches temperature quickly, the thermostat shuts it off, and the room swings between hot and cold. A correctly sized radiator with a TRV gives a steadier result. If you are weighing a single panel against a double, our single versus double radiator guide explains why measuring first and only upsizing when the numbers call for it is the sensible order.

A worked example

Take a 3 m by 3.5 m north-facing bedroom with two external walls and one window, with standard ceiling height. Rather than quote a single number that depends on assumptions we cannot see in your home, think about the steps. A standard allowance for a bedroom gives you a baseline. A north-facing aspect and a second external wall each push it up. Good double glazing and loft insulation pull it back down. The result is a range, and the sensible choice is the radiator at the upper end of the range if you want the room warm quickly on winter mornings, or the middle if you will run it steadily with a thermostat.

The point is that two bedrooms of identical size can need quite different radiators. Measure, calculate, then buy.

Plumbed radiator or electric: which suits a cold room?

The first decision is whether the room can use your central heating at all.

If the room is on the central heating

Swapping or upgrading the radiator is usually the most economical fix. A larger panel, an extra convector fin layer (a double panel with fins gives more output from the same footprint than a single panel) or a properly balanced system can all lift output. The Cove Horizontal Single Panel Radiator and the Merlo Radiator are examples of the kind of plumbed panel we stock. If you want a statement piece, our Drifter Horizontal designer radiators put the style on show.

Fit a good valve at the same time. A thermostatic valve lets you set a lower temperature for each room, so you can keep the rooms you use warmer. If you are replacing valves, the contemporary manual radiator valves in RAL colours are a tidy option for manual control.

If you need to check whether the radiator is the problem or the system is, ask a heating engineer to look at balancing and flow temperature. Plumbed radiator work on a boiler circuit should be done by a competent heating engineer.

If the room is off the system

Conservatories, garages converted to offices, extensions, garden rooms and loft rooms often are not worth connecting to the boiler. Here electric is the practical answer. The Energy Saving Trust groups electric heating into direct systems, such as panel heaters and wall-mounted electric radiators, and storage heaters. Direct heating is simpler to install but typically costs more to run as a main heat source because it relies on standard-rate electricity. Storage heaters charge off-peak and can be cheaper if paired with the right tariff.

That same page makes an honest point that applies to cold rooms: the cost of electric heating depends heavily on how well insulated the property is. For a smaller, well-insulated space, direct electric can be a viable, low-maintenance choice. For a large or poorly insulated space, it can get expensive, and the better answer is to insulate first.

The main options at a glance

Portable heaters: which type, and what they cost

If you are choosing a portable heater as a stop-gap, the type matters more than the brand. Which? says there is no single most sustainable type of electric heater, because all of them convert electricity to heat very efficiently. What counts is matching the heater to the need: fan heaters for immediate, short bursts, radiators or convectors for steady warmth across a room.

The running costs differ in practice because of how long you leave each on. In its tests, Which? found oil-filled radiators averaged 33p an hour, convector heaters 48p and fan heaters 60p, based on an electricity price of 26.32p/kWh. Those are test-based averages at a particular tariff, so treat them as a way to compare types, not a bill forecast for your home.

To see what a heater would cost you, use the simple formula: the heater’s power in kilowatts, multiplied by hours used, multiplied by your unit rate. A 2 kW heater on for three hours uses 6 kWh. Multiply by your own rate from your bill. If you want to see how electricity use is benchmarked elsewhere, one German comparison site publishes average household electricity consumption figures (in German, for German households, so not a UK benchmark).

One more point from the same Which? testing: oil-filled radiators hold heat in the oil even after the power cuts out, which is why they suit steady background warmth rather than a quick fix.

Fixed electric radiators for cold rooms

For a room you use regularly, a fixed electric panel is usually better than a portable heater. It is wall-mounted, has a thermostat and timer, and is out of the way.

Two features matter most in cold rooms:

• Thermostat and programmer. You want the unit to hold a set temperature, not run flat out. A built-in timer helps you heat the room only when you need it.
• Low surface temperature. In nurseries, care settings, schools and rooms used by older people, a low surface temperature (LST) unit limits how hot the casing gets. Our UK checklist for LST radiators covers the 43°C limit and what to look for.

The Dimplex LST Electronic Panel Heater is the sort of model that suits those situations. If you are deciding between panel types more generally, our piece on timers and insulation beating panel or convector choice argues that how you control the heater matters more than the label on the box.

For a cold bathroom or cloakroom, an electric ladder towel radiator warms the room and dries towels. The Creda CLR Electric Ladder Towel Radiator is one we stock. If you are unsure whether an electric or plumbed rail suits your bathroom, read electric or plumbed towel rail: which suits your bathroom, and if you plan to put it on a timer, our electrician checklist for heated towel rail timers is worth a read before you book the fitting. Electrical work in bathrooms is subject to special rules, so use a qualified electrician.

Where to put the radiator

The traditional spot is under the window, because the rising warm air counters the cold downdraught from the glass. That still works well for plumbed radiators. Some practical notes:

• Do not block it. Furniture and long curtains in front of a radiator absorb heat before it reaches the room. Tuck curtains behind or stop them above the radiator.
• External walls. If the radiator sits on an external wall, a reflector panel behind it reduces the heat lost into the masonry.
• Bedrooms. Under the window is common, but if wall space is limited, a taller, narrower radiator on a side wall can work well.
• Hallways. Narrow halls often lose out on heat because the radiator is slim. Our hallway radiator ideas show how to add output without blocking the way through.

Electric panels have a bit more flexibility but need clearance around them as the manufacturer states. Always follow the installation instructions for the model.

Run it right: turn down, do not turn off

A common money-saving instinct is to shut the radiator in a spare or cold room entirely. The advice from official sources points the other way. The government’s Clean Energy campaign guidance says to turn radiators down rather than off in cold rooms, because turning heat off completely raises the risk of damp and mould. The Energy Saving Trust adds that a low background heat is more cost-effective in the long run than putting right mould damage, and the room is also quicker to warm when you need it.

On TRVs, the Energy Saving Trust suggests treating the numbers as a relative scale. Dial the valve until you find the lowest setting that keeps the room comfortable, then leave it there. It also says central heating should have at least a programmer and a room thermostat, so check your system has them.

On temperature, the Met Office advises heating your home to at least 18°C, which matters most if you have reduced mobility, are 65 or over, or have a health condition. If anyone in your household is in those groups, do not treat a cold room as somewhere to economise.

Our view on cold rooms

We sell radiators and electric heaters, and we still tell customers to start with the room, not the product page. The most common mistake we see described is buying a radiator by its width. It fits the wall, so it goes on the wall. But the wall tells you nothing about the heat the room needs.

Our advice runs in this order. First, find the draughts and deal with them. Second, calculate the output the room needs, including the extra for north-facing and exposed rooms. Third, decide whether the room is on the system or not: plumbed if it is, electric panel if it is not. Fourth, put a control on it, a TRV for plumbed or a thermostat and timer for electric. Fifth, run it low and steady, rather than off and then flat out.

For trade buyers, such as landlords and facility managers fitting out several rooms, the same logic applies at scale. Get the output right per room, keep the specification consistent, and check the controls. We also offer bulk order pricing and support for trade customers, so it is worth asking when you are buying for more than a couple of rooms.

— Carl

Choose your radiator at Simco Direct

We stock plumbed radiators, designer radiators, electric heating and the valves and controls to go with them, with fast UK delivery and a 30-day returns policy. Browse the range on the Simco Direct homepage, or go straight to a model such as the Dimplex LST Electronic Panel Heater for a cold room off the central heating.

FAQ

What size radiator do I need for a cold room?

It depends on the room’s volume and how much heat it loses. North-facing rooms, rooms with several external walls and rooms with big windows need more output than a standard allowance. Work through the steps in our BTU per m² guide, and for a whole-house answer consider a surveyor’s heat loss survey.

Is it cheaper to leave a cold room unheated?

Not reliably. Official advice is to turn radiators down rather than off, because a completely unheated room raises the risk of condensation and mould, which costs more to fix than the energy saved. A low background temperature also means the room warms faster when you need it.

Which portable heater is cheapest to run for a cold room?

In Which? testing, oil-filled radiators averaged 33p an hour, against 48p for convectors and 60p for fan heaters, at a 26.32p/kWh price. Oil-filled types suit steady warmth, while fan heaters suit short bursts. Your own cost depends on your tariff and how long you leave it on.

Should I choose an electric radiator or a plumbed one?

If the room is on your central heating, upgrading the plumbed radiator is usually the economical route. If it is off the system, such as a conservatory or garden room, a wall-mounted electric panel with a thermostat and timer is simpler to install. Electric heating generally costs more to run as a main source of heat, so insulate first.

Sources

• Which?: Best oil filled radiators
• Which?: Best convector heaters
• Energy Saving Trust: Electric heating
• Energy Saving Trust: Heating controls
• Clean Energy campaign: Save energy in your home
• Croydon Council: Energy saving tips
• Surrey Home Energy Improvement Guide
• Met Office: Keeping your home warm in winter

Recommended

• Measure First: Compare Outputs, Fit TRVs and Only Upsize to Double Rad
• Avoid 46% Oversizing: Calculate Radiator BTU per m² for UK Homes
• Timers and Insulation Beat Panel or Convector Choice for UK Homes