So some of our radiators are not heating up very much when the heating is on, but others are toasty.

The kids bedroom radiators are get hot quickly. The living area ones – vertical instead of horizontal ones – only get warm. I’ve checked that they don’t need bleeding: the warm is uniform and opening the bleed valve results in water. The pressure cylinder for the circuit is at 2 bar. The thermostatic valves are at maximum and if I take the head off the radiator doesn’t get any hotter. I only know a little about central heating: anything else I should check before phoning a plumber?

Maybe they need balancing: https://www.homebuilding.co.uk/how-to-balance-radiators/

Careful doing the above. Doing 12 degrees difference between in and out caused my lock shields to whistle as they had to be well closed to achieve that.

Plus the house took for ever to warm up. So I went back to doing it by feel.

I wrote this a few years ago, hope it helps.

NOTES ON BALANCING A TWO PIPE HEATING SYSTEM

(One pipe systems are fairly rare and generally only found in older properties).

Radiators are fitted with two valves, a control valve, commonly a thermostatic radiator valve (TRV), and a lockshield or balancing valve. The control valve turns the radiator on and off, and a TRV will modulate radiator output in response to changes in the air temperature of the room. Neither of these valves has an electrical interlink with the boiler and therefore cannot control the actual firing of the boiler.

The lockshield valve adjusts the flow rate to that radiator. As the total amount of flow is limited in each system, due to design criteria such as the size of the boiler, pump, pipework diameter, and layout, it is important to ensure that all radiators receive the correct flow to maintain their rated output. To achieve this, each lockshield valve should be adjusted to obtain an 11°C differential between the flow and return. After each adjustment, a short period should be allowed for the changes to take effect. Domestic design guides recommend an 11ºC differential between flow and return pipes, although 20 ºC is becoming more common with modern condensing boilers.

Radiators are sized according to a difference between the mean water temperature of the radiator and the air temperature (assumed to be 20 ºC). If a radiator operates at a lower mean water temperature then radiator output will be reduced accordingly. For example, if a radiator has a rated output of 1000 watts with a 50ºC differential between the mean water temperature and the room air temperature, if the mean water temperature is reduced and that differential becomes 35ºC then the radiator output will be 629 watts.

. So I went back to doing it by feel.

best way.
Close both valves, fully open the flow valve then open the return valve 1/4 turn at a time until the radiator warms evenly.