Radiators: Sizing, Balancing and Replacement
Complaints about cold rooms usually have one of three causes: the radiator is too small for the room, the system is unbalanced, or the radiator needs bleeding or flushing.
Sizing
Radiator output is rated in watts, and the requirement depends on room volume, external wall area, window area, insulation and desired temperature.
Online calculators give a reasonable estimate. Guessing based on the size of the old radiator perpetuates an existing mistake.
Output figures are quoted at a specified flow temperature. If the system runs cooler — which is more efficient and increasingly common — the radiator delivers less than its rated output, so sizing should allow for that.
This matters particularly if a heat pump is planned, since those run at much lower flow temperatures and generally require larger radiators.
Balancing
In an unbalanced system, water takes the path of least resistance and the radiators nearest the boiler get most of the flow. Distant radiators stay cool regardless of the valve setting.
Balancing adjusts the lockshield valve on each radiator so flow is distributed correctly, measured by the temperature difference across each one.
It costs nothing but time, is well within the reach of a competent DIYer with a thermometer, and resolves a large proportion of cold radiator complaints.
Bleeding and sludge
Air collects at the top of radiators, producing a cold top and warm bottom. Bleeding releases it and takes minutes.
Cold at the bottom with a warm top indicates sludge — corrosion debris settled in the base. That requires flushing, and a magnetic filter fitted on the return prevents recurrence.
System inhibitor should be added after any drain-down, and its absence is why sludge accumulates in the first place.
Replacing
Like-for-like replacement in the same position with matching pipe centres is straightforward and inexpensive.
Changing size or position means altering pipework, which is more involved, particularly where pipes run under a solid floor.
Thermostatic valves on every radiator except the one in the room with the main thermostat allow room-by-room control and are a cheap efficiency improvement.
Practical points
Do not place furniture hard against a radiator or fit a solid shelf directly above it, since both disrupt convection.
Reflective panels behind radiators on external walls make a modest difference and cost very little.
If several rooms are cold, balance the system before buying anything. If one room is cold and the radiator is hot throughout, the radiator is undersized.
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