Possible air lock in central heating system

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The heating in my house has been switch off during the summer months and the boiler checked in September. In September all the radiator warmed up without problem. Today, however, the radiators in the ground floor did not do so and there was a vibrating noise around the emersion heater. Few years ago something similar happened and the plumber released air by rotating a screw in one of the fittings shown in the photographs below. Would you be able to suggest which it could be?
 
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Can't see anything that looks like an air vent. Poor pics. Is pump running?
See FAQs to find out how to test it.
 
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Thank you very much for your reply

The pump does not seem to work anymore, and for over a year. I intend to extend the house in two years time and decided to deal with the pump then.
Consequently the shower pressure is just normal, as if there was no pump. Hot water is running normally in the bathroom and kitchen.

Three years ago I removed an airlock by using a piece of hose pipe to connect the two taps in the hand basin, and running cold water into the hot water tap. I did this while the central heating was on. I do not know If this could remove what appears to be an airlock in the pipe that feeds hot water into the radiators in the ground floor. As I said earlier, the radiators in the top floor are fine.
 
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You could try unscrewing the top nut of the valve directly above the pump to release any trapped air in the pump. Or any of the nuts on the system for that matter.
 
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Thank you very much for your reply

The pump does not seem to work anymore, and for over a year. I intend to extend the house in two years time and decided to deal with the pump then.
Consequently the shower pressure is just normal, as if there was no pump. Hot water is running normally in the bathroom and kitchen.

Three years ago I removed an airlock by using a piece of hose pipe to connect the two taps in the hand basin, and running cold water into the hot water tap. I did this while the central heating was on. I do not know If this could remove what appears to be an airlock in the pipe that feeds hot water into the radiators in the ground floor. As I said earlier, the radiators in the top floor are fine.

You seem to be confused by the purpose of the pump. The central heating pump has nothing whatsoever to do with the supply to your taps/shower. Your method of connecting a hose between the taps is used to clear airlocks in the supply pipes NOT the central heating.

You need to be clearer about your fault.
 
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Thank you for the replies. All extremely useful in clarifying the problem for me.

I have watched a couple of videos on youtube and now the difference between the two problems I have experienced in the past:
- the air lock in the hot water tap and
- the cold radiator and the vibrating noise emerging from the emersion heater cupboard.
- Since my problem now is the second one, my question is:
Are there separate supply pipes for the ground floor and the top floor radiators and is the airlock is only in the supply pipe for the ground floor?
- When I unscrew the nuts in the system should the central heating be on and the radiators opened?
Thank you in advance for your help.
 
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I think you need to establish if the pump is moving the water around the pipework.
When you turn on the hot water or heating, the pump should start, the boiler should fire, and you should start to feel warm water coming to the pump.
If there is air in the pump, or an airlock, the pump will run, but because it won't be pulling the hot water from the boiler, the boiler will get hot and cut out.
So first try to check the above.
 
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The comment from Regsmith was most helpful.

I have switched on the hot water and central heating and noticed that now the boiler does not ingnite. I checked the pump and loosen the screw from the top to remove possible airlock, then restored it to its original position. After a while the pump began to make a vibrating noise, that was reduced when I rotated the pressure dial.

While the boiler is off I will try to bleed all the radiators. Then
I have reignite the boiler and try to remove any airlock around the pump.
 
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I have watched more youtube videos to understand better central heating systems and establish checking procedures. With them I managed to identify the problem. The ball cock valve in the small tank in the loft was stuck. The tank emptied after filling only some of the radiators and the boiler was switched off.

I would be grateful if you could comment on my CHECKING PROCEDURES AND ADD ANY I MAY HAVE MISSED. Here they are:

Hot Water & Central Heating system SERVICE

a. Hot Water
If there is no hot water from hot water tap, use piece of hose pipe to connect hot and cold water taps. Open both and let cold water run into the hot water tap for some time until air lock is removed from the hot water system.

b. Central Heating system
If Central Heating has been switch off for a long period the following should be checked:
b1. Small tank in the loft serving the central heating system (feed and expansion tank)
Check if the ball cock valve in the small tank (in the loft) servicing the central heating system is working properly. It could get stuck and prevent water from filling the tank.
b2. Bleed radiators
Without switching on central heating system, in each radiator open both taps and open radiators with radiator key to remove any air present. Turn the key off as soon all air is out and drops of water emerge from the radiator.

Turn on central heating system. If radiators do not warm up there could be air locks in the system
b3. Pump
If the pump is working its vibration should be felt by touching it. To test the pump for air locks loosen the top screw (pump bleed screw) for few minutes and then return to its original position. Vibrating noises in the pump could be caused by pressure that could be released by rotating the pump pressure adjust dial on the side of the pump. The Pump connects up to the MOTORISED RISER VALVE.

b4. Motorised Riser Valve
It is connected to three-way valve: A for heating and B for hot water tank. If the valve's automatic lever is faulty there will be no heating. A provisional repair of the central heating system until the valve is replaced could be locking the lever by manually moving it to the on position for central heating. If the valve is replaced it is better to also replace the three-way valve.
b5. Air released from NUTS around the pump and valve
If applying steps b3 and b4 does not restore the central heating system's function, try loosening NUTs around the pump and valve to remove air locks.

Any of the above may cause the boiler to fail to ignite
b6. Before switching the central heating system set thermostat to 25 degrees
After boiler has ignited reduce the thermostat’s temperature to 21 degrees or less.
b7. Restore boiler’s pressure
The pressure in the boiler should be between 1 and 1.5 bars. If it is below 1, it can be restored by using a flexi hose filling loop between the two water valves below the boiler. Make sure the right hand valve is fully closed. Remove the end caps from each valve, connect to them the flexi hose, open the left valve and then the right valve and allow water through until the pressure gauge reads between 1 and 1.5. Then close the right and left valves, remove the flexi hose, fit the end caps. If the boiler presure exceeds 2 bars call the engineer.
b8. Ignite the boiler
Press the gas valve button in the boiler, and without releasing it press the ignition button. Only after a minute release the gas valve button. Try this only three times. If the boiler does not ignite call the engineer.
 
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