If you can hear a slight humming from the pump, it may be powered but stuck after a long period of unused. A slight tap on the pump housing sometimes is enough to get the impellor turning freely again. Or flicking the on/of switch of the pump, which did it for me. There will be more noise and a gluck gluck sound from any bubbles in the pipe once the pump starts circulating the water.
What is the temp at the boiler? there is a temp/ presure guage it should read 160-180F. If that is OK then check the circulating pump. When the thermostat is turned up you should hear a little water moving through the piping as you have adjusted the system. If you hear nithing circulating you may have a defective pump.
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Don't know your specific unit, but in general, heating systems are two-stage devices:
1: Its too cold: the motorised HW/CH valve moved to the required position, and on achieving it, the circulaion pump starts and the boiler is enabled. (The circ pump may start immediately - its not usually important.) The hot water from the boiler circulates through the heat exchanger and/or radiators until the thermostat thinks its target is warm enough, whereupon it disables the boiler, stops the pump and closes/relaxes the valve(s).
2: The boiler is off while disabled. When enabled, it heats the circulating water to a 'high' temperature and seeks to keep it there. If it reaches that temperature, the boiler will cycle on and off as necessary to maintain the set 'high' temperature. (The circulation pump must run all the time the boiler is enabled.) This may be what you're noticing. Additionally, some boilers will run the circulation pump for a little while after being disabled to reduce the temperature if the water in the pipes and boiler.
Your question doesn't say how the pump running corresponds to thee thermostat settings, but if it were stuck on, I think you'd know!
The noise you refer to is called Kettling, it is caused by the hot water in your boiler not being able to leave the boiler quick enough, if the problem developed quickly and your system is a "fully pumped " system this would generally point to your pump either has failed or is on its way out, i.e . your radiators would not heat up as the hot water is stuck in the boiler hence the banging......( if you know where the pump is you can generally feel if it is running by a small vibration ) if you have gravity hot water and pumped heating circuit then I would assume that it is noisy when just doing HW, and the banging is a problem that has gradually got worse it can indicate that the boiler is scaling up (are you in a hard water area). if this is correct there are various additives that can be put in the system to help quieten it down, I suggest you really need a heating engineer to diagnose fully.
Combi boilers are unlike regular boilers in that they have quite a few moving parts, it's vital that these are serviced annually if you want the boiler to remain reliable and to avoid expensive failures.
In your case, the diverter valve has most likely failed. Normally, when the heating is on this senses when there's a demand for hot water and temporarily diverts the primary hot water circuit to feed the domestic hot water heat exchanger. When the demand for hot water ceases the primary hot water circuit is diverted back to the central heating heat exchanger. The valve has a number of moving parts including a rubber diaphragm which can wear out and split and moving shafts which can seize.
When the valve fails it can leak and also can divert hot water to the central heating heat exchanger when hot water is used. The reason that only the highest radiators in the system are affected is probably because the failed valve does not necessarily cause the central heating pump to activate and circulate the hot water throughout the system (it does on some boilers though). If hot water is introduced into a cold heating system it will rise via convection to the highest points in the system like an old fashioned gravity circulation system which was unpumped.
Call a heating engineer out as soon as possible; I'm not going to advise how to do the repair yourself as gas appliances MUST be serviced by qualified persons only for reasons of safety. If you're lucky then the valve simply needs cleaning up and a few new diaphragms plus other minor seals and rings. If it's gone too far then the valve and possibly the manifold it's attached to will need to be replaced.
In the UK the part alone typically costs between £100 to £200 depending on whether it's just the valve or the manifold as well. The diaphragm kit typically costs up to around £15 and is routinely fitted when the boiler is annually serviced, and a new valve will already have new diaphragms fitted. For a routine service expect to pay for up to two hours labour (many can be done in one hour), but if the valve/manifold have failed it will be longer as one failure often causes other parts to need further inspection/repair.
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back boiler run from a gas fire? check the instructions to operate the gas heat to the back boiler not just the fire I presume that this a fully pumped system - air lock doubtful There maybe on/off valves on the pipes to and from the back boiler [situate close to cylinder] make sure they are on - anticlockwise turn or there could be a blockage in the outlet from header tank usually in the loft. or If there is no water in it maybe the mains cold feed it is off. Note if no water in it DO NOT USE the back boiler
there should be a pressure rellief valve fitted.
it also sounds like the heat exchanger is blocked if the hot water is not as good as it used to be and the boiler reaches temperature too quickly then there is scale in the heat exchanger and needs to be cleaned or replaced
The first thing you need to do is to put some more water into your heating system. This is usually done by allowing water from your (pressurised) domestic cold water supply to flow into the system: On older boilers, this is done with a connection (often made with a removable hose) on the pipework just outside the boiler. Newer boilers may have a built-in connection with some sort of removable link (sometimes looks like a plastic key).
Once you've got the boiler up to working pressure (typically about 1 bar - 15 psi), you should be able to bleed your upstairs radiator properly.
This may be all it takes, though the pressure excursions you're getting when you run the hot tap suggest there may be another problem. See what happens to the pressure as the boiler heats up and cools down. There really ought to be very little pressure difference between the two conditions. If you see significant changes, this usually means that the "pressure vessel" (or in hydraulic terms, system accumulaor) has lost its air charge.
The pressure vessel is a metal container inside the boiler, usually painted in red enamel. Somewhere near the top, you will find a schrader valve (like the one on a car tyre), and you can use this along with a tyre pump to replenish the air in the top of the vessel. This will reduce your pressure transients for a while, but you need to keep an eye on it. If the problem returns, you probably need to replace the pressure vessel (or clag another one into the plumbing alongside the boiler).