A few people in recent years have asked me to look at their Central Heating systems because they didn't work or didn't work properly. The only reason I know about them is that I have read a few books and done a fair amount of plumbing to try things out. The basics are very straight-forward and there is no reason why you shouldn't troubleshoot and fix your own systems.
Firstly there is the question of whether the system has ever worked properly i.e. was it installed and commissioned properly or not. If it has worked and no longer does then assuming you haven't got anything fancy, either the boiler, the pump or the 2/3 port valves have broken or the system has become sludged up. If the system has never worked properly or you are unsure then you can carry out a few basic checks to narrow down what is wrong.
1) Do you know how the controller works? Are you definitely 'demanding' heat from the boiler?
2) Make sure the room thermostat is set high enough (if you have one, it is usually on the wall in the hallway or living room). The thermostat only affects the heating, not the hot water.
3) If you have a hot water cylinder (or copper cylinder) then make sure that the hot water thermostat on the side of the tank is set to about 70 degrees.
4) If you are not getting hot water or heating, ensure firstly that the boiler is firing up and getting warm. At the same time, check the two pipes connected to the boiler (the two largest on a combi) to see whether hot water is leaving the boiler. If the boiler is not firing up at all then either the controller/circuitry is broken or the boiler is faulty. Perhaps it has no gas or the control board is broken.
5) If the boiler is firing up (the fan goes on an it makes a bit of noise) but no hot water is leaving it then the pump is probably broken or the system has a blockage. Bear in mind that the water will take up to a few minutes to heat up to a noticeable temperature. If you have a combi boiler then the pump is built-in whereas if you have an indirect system with hot water cylinder, the pump is usually either next to the boiler, or next to the hot water cylinder. You can tell if they are working because they vibrate slightly when operating.
6) If the pump is turning but no hot water (or not much hot water) is leaving the boiler then the system might be blocked either because of sludge or possibly because all of the radiators are turned off and there is no bypass circuit or valve to let the water circulate. You can obviously test this out by ensuring that at least one radiator is fully opened on both sides and see whether this causes the water coming out of the boiler and that one radiator to heat up. If this is OK now, you will need to balance the radiators.
7) If only the hot water OR the heating on an indirect system is working (the system with the hot water cylinder) then the 2 port or 3 port valves might be jammed which can happen after a number of years. These are smallish silver boxes, usually next to the hot water cylinder and you can try these out by moving the manual lever on them and testing which pipes next to it get hot. One of the outputs will connect to the hot water cylinder and the other will feed all the radiators.
8) If only some of your radiators ever get properly hot then either the boiler is not substantial enough to feed all the radiators or the system is not balanced properly (also there could be a blockage somewhere but you can find that out after attempting to balance the system).
9) Firstly, once your few radiators have got hot, carefully touch the pipes leaving and coming back into the boiler. If some of your radiators are opened too much then they will be stealing too much hot water and the boiler return temperature will feel almost as hot as the feed (or pretty close) on a properly balanced system, the feed is usually too hot to touch and the return can be touched for a second or so. If the return temperature feels low but only a few radiators are hot then it is likely that either there is a blockage, the cold radiators are turned down too far or are off, or the boiler simply cannot provide enough heat for the radiators.
10) If the system has worked before then the boiler should be the right size in which case you might want to find if the cold radiators are turned down (there are two valves, the one you normally use to adjust it and the one that is set in position to try and balance up the system).
11) Balancing the system is easy in theory, you are trying to set the close-down valves (the ones you don't normally touch on the radiators) so that all the radiators heat up at the same rate. This means that the ones nearest the boiler feed will need to take less water than those further away to ensure the pressure at the end of the system is still high enough to move water. I usually start by opening up all the normal radiator valves (or thermostatic ones if fitted) to maximum and start up the central heating. I then have to dash around the house and turn down the ones that heat up quickly and if others are cold, I turn them up (all on the close-down valves remember). They don't have to be perfectly balanced, which you won't able to do anyway but a good check is that you should be able to feel a temperature difference between the input and output of every radiator like the boiler where the input is too hot to touch and the output not loads cooler but just about touchable. If you can get this across all the radiators by turning the close-down valves up or down then you should get a nice system!!!
