Sizing radiators properly: heat loss, not guesswork

Delta-T, and why the headline figure misleads

A radiator’s output depends on the difference between its surface temperature and the room temperature. That difference is written as delta-T, and every quoted output figure is stated at a specific one.

Historically, outputs were quoted at a high delta-T, reflecting how systems were run decades ago. Modern practice quotes at a lower figure, reflecting how efficient condensing systems actually run. The same physical radiator therefore has two very different published outputs depending on which convention is used.

The practical consequence: a radiator advertised with an impressive output figure at a high delta-T will deliver substantially less in a system running at modern low flow temperatures. This is the single most common reason a newly fitted radiator disappoints, and it is a specification error rather than a fault.

What a heat loss calculation actually does

It works out how much heat each room loses at design conditions – the coldest outside temperature the system is designed to cope with – and therefore how much has to be put in to hold the room at its target temperature.

For each room it accounts for:

• Fabric losses through walls, floor, ceiling and roof, based on the construction and its insulation.
• Glazing losses, which are frequently the largest single element in a room with big windows.
• Ventilation and infiltration – the heat needed to warm incoming air, based on air change rate.
• The target temperature for that room, which is not the same everywhere. A bathroom wants to be warmer than a bedroom.
• Adjacent spaces, because a room next to an unheated garage loses heat through that wall too.

It is not complicated and it does not take long – an hour or so of measuring for an average house, and the calculation itself is arithmetic. What it produces is a number per room, and that number is the whole basis of the specification.

From heat loss to radiator

1. Establish the room’s heat lossIn watts, at design conditions.
2. Decide the system flow temperatureThis is a design decision, not an accident. Lower is more efficient; lower also means larger radiators.
3. Calculate the delta-T at that flow temperatureBased on the flow and return temperatures and the room temperature.
4. Find radiators whose output at that delta-T meets the lossNot their headline figure at a different delta-T.
5. Check it physically fitsUnder the window if possible, within the available wall, and without fouling a door or a skirting run.
6. Add a margin for intermittent heating if appropriateA house heated from cold each day needs more output than one held at temperature continuously.
7. Check the pipework can deliver the flowA larger radiator needs more flow, and an existing 8 or 10mm microbore tail may not supply it.

The trade-off that decides everything

There is a direct exchange between flow temperature and radiator size, and it is the central decision in any heating design.

A system designed for low flow temperatures is more efficient every day it runs, more comfortable because the heat is gentler and more even, and leaves the house ready for a heat pump later without redoing the emitters. A system sized for high flow temperatures is locked out of all of that.

This is why the heat loss survey matters even if you are staying with a boiler – see oil vs heat pump for an off-grid Norfolk home.

Things that make radiators underperform

1. Radiator covers. They trap heat, restrict convection and can substantially reduce effective output. They also mislead the TRV, which is sitting inside a warm box.
2. Furniture in front, particularly a sofa across a radiator.
3. Long curtains hanging over the front, which channel heat straight up the window.
4. Dust in the convector fins. A panel radiator with fins relies on airflow through them, and years of dust reduces it measurably. Worth vacuuming through with a crevice tool.
5. Sludge in the bottom, taking a portion of the surface out of circuit – see radiators cold at the bottom.
6. Poor balancing, so it never receives its share of flow.
7. Being painted repeatedly, particularly with thick or metallic paint, which reduces emissivity.
8. A microbore tail that cannot deliver the flow a larger replacement needs.

Practical points on selection

• Under the window is still right, where possible. The coldest surface in the room is the glazing, and warm air rising in front of it counteracts the downdraught that otherwise pools cold air at floor level.
• Double panel with convector fins gives far more output per metre of wall than a single panel, at the same height and length.
• Vertical radiators solve a wall space problem and generally give poorer output per pound. Size them from the real figure.
• Towel rails have low output. A bathroom heated by a towel rail alone is frequently under-heated – it may need underfloor heating or an additional emitter.
• Cast iron is beautiful and heavy, with high thermal mass – slow to warm and slow to cool, which suits continuous heating and not intermittent.
• Check the connection centres if you are replacing like for like and want to avoid altering pipework.