If your boiler is responding to the thermostat but your radiators aren't getting hot, there may be a problem with your boiler's circulation. Unfortunately, the flow of water through the boiler is stopping somewhere. Luckily, there are several troubleshooting steps you can take. Check out these tips:
1. Remove air from the systems.
In some cases, water cannot circulate from your boiler to your radiators because there is air locked in the system. To remove any air bubbles, grab your air bleeder key. If you don't have one, you can typically find them at places that sell spare boiler or radiator parts.
Simply open the valve on each radiator and let the air seep out. Ideally, you should release air until you see a bit of water -- have a rag on hand to catch the water.
2. Check the valves in the piping.
In most cases, the piping connecting the boiler and the radiators may have valves spread through it. These are typically located next to the radiators. Simply, check them all to make sure that they are open. They often allow you to shut off the heat moving from one part of the house to another, and if they are closed, it may cause circulation issues.
3. Examine the coupling device.
Once you've bled air and checked valves, it's time to start looking around the pump and its associated parts. In some cases, pumps are connected directly to the impeller, but in other cases, there is a coupling device.
If your boiler has a coupling device, look at it. If you see any cracks or other noticeable signs of damage, you need to replace it. A new coupling device will tighten that seal and allow water to move around your radiator.
4. Check the impeller for shearing or wear.
If the coupling device looks alright or if your boiler doesn't have one, you need to look at the impeller. If it has visible signs or wear or cracks, replace it with a spare impeller. If it looks okay, use a voltmeter to take an amp reading. If the impeller gives off a lower voltage than what it says on its side, it has been sheared and also needs to be replaced.
If none of these steps help you figure out what is wrong, contact a professional repair person. They can help you diagnose your boiler and figure out what's wrong with it. They also often have a stock of spare boiler parts that they can use for repairs.