Radiator Installation Guide
Fitting a radiator is straightforward; fitting the right radiator in the right place and commissioning it properly is what makes the room warm. Size it to the heat loss first, then hang, connect and balance.
A radiator is only as good as the sizing and the commissioning behind it. Hang an undersized panel and the room never gets warm; skip the balancing and it either hogs the flow or gets starved by the rest of the system. The physical fitting is the easy middle bit between two decisions that actually matter.
This guide runs through a radiator install in order: sizing it to the room's heat loss, choosing the position and valves, hanging and connecting it, and then filling, venting and balancing it into the system. It assumes a wet central heating system and a competent second fix.
Key takeaways
- Size the radiator to the room's heat loss at the system's design flow temperature — not by eye or the old one's size.
- On a low-flow-temperature or heat pump system, radiators need to be larger for the same output.
- Position for good air circulation and fit a TRV on the flow and a lockshield on the return (except where a room stat controls the space).
- Fit isolation valves so the radiator can be removed later without draining the system.
- After hanging, fill, vent and then balance the radiator at the lockshield so it gets its share of the flow.
Size it to the heat loss
Start with the room's heat loss and the system's design flow temperature, and pick a radiator whose output meets it at that temperature. A radiator's quoted output assumes a specific temperature difference between the water and the room, so the same panel gives far less heat on a low-flow-temperature system than on a hot boiler system — never take the headline BTU figure at face value.
This is where a radiator BTU calculation earns its place: it converts the room's requirement into a real panel size at your actual flow temperature. On a heat pump or other low-temperature system, expect to fit noticeably larger radiators than a traditional boiler install would use for the same room.
Output depends on flow temperature
A radiator rated at 70°C flow gives only a fraction of that output at 45°C. Always size to your system's design flow temperature, not the manufacturer's headline figure.
Position and valves
Position the radiator where it heats the room evenly and isn't boxed in by furniture or long curtains that trap the heat. Traditionally radiators sit under windows to counter the cold downdraught, though on a well-insulated modern property the position is more flexible.
For valves, fit a thermostatic radiator valve (TRV) on the flow and a lockshield on the return in most rooms, so the occupant controls the room and you balance at the lockshield. Don't fit a TRV in the room that carries the main room thermostat, as the two controls fight each other. Isolation on both valves lets you remove the radiator later without draining the system.
Doing this on a job? Central heating commissioning (Benchmark) — part of TradePlanr. Free to start, no card needed.
Hanging and connecting
Mark and fix the brackets level and at the correct height, allowing clearance below for the valves and the pipework, and make sure the fixings suit the wall — a filled radiator is heavy. Hang the radiator, then connect the flow and return, fitting the valves and the pipe tails with the radiator sat squarely on its brackets.
Make the connections properly — clean, correctly jointed and not over-stressed — because a weeping radiator tail is a slow, hidden leak that corrodes the system and stains the floor. Fit an air vent (bleed) plug and a blanking plug in the top tappings.
Fill, vent and balance
With the radiator connected, open the valves, fill the system and bleed the air out of the radiator at the bleed plug until water flows. Check every joint under pressure — the flow and return tails, the bleed plug and the blanking plug — before you leave the room, and again once the system is hot and has moved.
Finally, balance the radiator into the system by adjusting the lockshield so it gets its designed share of the flow — measured by the temperature drop across it or by a balancing procedure across the whole system. A new radiator that isn't balanced can either starve the rest of the system or never get properly hot itself.
Balance, then hand over clean
TradePlanr lets you raise the quote, book the job and issue a Benchmark or commissioning certificate on completion — recording the install and the system water treatment for the customer's records.
From guidance to action
Frequently asked questions
How do I know what size radiator a room needs?
Work out the room's heat loss and select a radiator that delivers that output at your system's design flow temperature. A BTU calculation converts the requirement into a real panel size. Never size by the old radiator or by eye.
Why are heat pump radiators bigger?
Because a radiator's output depends on how much hotter the water is than the room. A heat pump runs at a low flow temperature, so each radiator gives less heat and has to be larger to meet the same room heat loss.
Which valve goes on the flow and which on the return?
In most rooms the TRV goes on the flow and the lockshield on the return, so the occupant controls the room and you balance at the lockshield. Don't fit a TRV in the room with the main room thermostat, as they conflict.
Do I need to balance a single new radiator?
Yes. Adjust its lockshield so it takes its designed share of the flow. An unbalanced new radiator can either hog flow from the rest of the system or never heat up properly itself.
Related guides
Radiator Sizing Guide (Heat Loss & BTU)
Room heat loss, BTU output and delta-T corrections — how to size radiators that actually heat the room.
Balancing a Central Heating System
How to balance radiators using lockshield valves and the temperature-drop method to get even heat across a system.
Radiator Cold at the Bottom: Causes and Fixes
Why a radiator goes cold at the bottom (sludge), how to confirm it, and how to flush, powerflush and protect the system.
