If your radiator feels lukewarm… chances are there is nothing wrong and your radiator has simply reached the desired temperature. To be on the safe side, set a higher temperature for a short space of time to determine whether the radiator starts to heat up.
Check for Trapped Air and Bleed Your Radiators
Simply use your radiator key to unscrew the bleed screw a little, just until you hear a hissing sound. Once the hissing dies away and water starts coming from your radiator, be ready to close the bleed screw. Your radiator should soon be nice and warm.
Radiators might not be getting hot due to trapped air requiring bleeding, a malfunctioning boiler or central heating pump, thermostatic radiator valves that might be stuck or set too low, or the central heating system having sludge or rust which causes blockages.
If your whole radiator is cold it could be an issue with the thermostatic valves. First, check to see if the valve on your radiator is turned to the on position, turn it to the highest temperature if required. If your radiators still don't warm up, it could be that your thermostatic valves are stuck.
Another way is to start the engine and let it run until it reaches operating temperature. Then, put your hand over the top of the radiator and feel if there's any heat coming out. If there's a lot of heat coming out, it means that the cooling system is working properly and there's no blockage.
Take off the pressure cap. Start the engine and watch the coolant to see if it begins circulating.
The commonest, and most likely, reason for a radiator being cold at the top is trapped air.
Locate the valve: Begin by finding the radiator valve, typically positioned on the side or bottom of the radiator. Examine the valve position: Observe the valve handle or wheelhead. If the thermostatic head is on position 5 it is open.
Symptoms of a faulty diverter valve often include inconsistent hot water flow and radiators not heating properly. Homeowners may notice that radiators remain cold even when the boiler is on. Inconsistent water temperatures are common, with water switching from hot to lukewarm or cold unexpectedly.
This is an indication that you have debris, a blockage, or most likely, sludge, which has accumulated in the radiator. Hot water is not able to circulate because of the build-up at the bottom of the radiator. The most efficient and safest way to deal with this problem is to seek professional help.
If your radiator is hot at the top but cold at the bottom, there may be a build-up of scale, rust, or sludge which is obstructing the flow of water. As with the middle of the radiator, if you have an open-vent system which is unpressurised and tank fed, you'll be able to use a sludge remover to flush your radiator.
Often, the reason that radiators aren't getting hot is not because of airlocks or boiler problems, but because the radiator is blocked up with a sludge comprised of rust, grime, and general silt which sits at the bottom of the unit and prevents the hot water from circulating properly.
If your radiator feels lukewarm…
chances are there is nothing wrong and your radiator has simply reached the desired temperature. To be on the safe side, set a higher temperature for a short space of time to determine whether the radiator starts to heat up.
Most radiators have two valves, one that lets the water into the radiator, and one that allows the water to be removed from the radiator. Familiarise yourself with both valves and their operation. Both valves should be turned fully open (anti-clockwise) so that the water can flow freely through the radiator.
The quickest way to balance central heating radiators is to make adjustments to the lockshield valves attached to them. If you would like one room to heat up quicker than others, the lockshield valve should be opened slightly more on the central heating radiators within this room.
If you have found that the entire radiator is cold, then the very first thing to check is to ensure that both the valves on each side of the radiator is switched on. This may seem like a basic thing to overlook, but more often or not, if a single radiator is completely cold, then this is the likely cause.
The most common reasons are typically that your engine coolant is low or there's a fault with the thermostat. First things first, make sure you've set the system to hot. And let the engine run for a few minutes to give it time to heat up the coolant.
This is typically because you have air trapped in the system and that means that the heat can't spread as it should, leaving the majority of the radiator cold. To fix this, you need to get your trusty radiator bleeding key. All you need to do is unscrew it just enough that you begin to hear the air leaking out.
A lot of the problem is due to the similarity between issues with head gaskets and other engine components, such as the following: A chipped or warped cylinder head, which could affect the head gasket, but wouldn't necessarily be the result of a blown head gasket.
If the coolant level is dropping and there is no external leak evident, then the coolant is probably leaking internally, into the engine. If the car has recently overheated then this could have caused the head gasket to fail. If it has, it could be leaking coolant into the combustion chambers.