Central heating/radiator problems

Found 1st Dec 2014
I have a problem with my central heating system - it pauses and then gurgles/adds water to the tank.

I'm on the ground floor, and there are only four radiators, I have done a lot of searching about information about this, and they all seem to say there is air in the system.

But I have bled every radiator and currently the main one in the lounge is switched off at both ends as the lockshield value is broken and needs replacing.

I have also drained and flushed each radiator getting rid of the sludge and added rust inhibitor.

Initially after re-filling and switching back on the system was fairly quiet and I did have to bleed one of the radiators multiple times.
Now it is back to being as noisy as before - all the radiators work and put out a lot of heat, but what I want to know is how to fix the gurgling? If it is air in the system - how did it get there and how can I remove it?
Community Updates
AskMovie & SeriesRadiator
You have to bleed ALL the rads and in the correct order (whilst ensuring the feeder tank remains topped - depending on the system).
What's the correct order?

None of them have any air at the moment.
Air can be drawn in from somewhere e.g an open vent above the f+e tank or you may have a leak on the pipework.

Do you have a tank to feed your heating in the loft or do you have a pressure gauge on the boiler or on a small cylinder in a cupboard somewhere? If you have a pressure gauge then what is the reading?

Noisy radiators are usually full of air, if your bleeding them and then doing it again within a short time ( a few days or week) then chances are you've got a small leak, if you've removed sludge a bit of pipework has probably pinholed.

If there is definitely no air then run your radiators up to temperature then placing your hand at the top - in the middle, run your hand towards the bottom. If it gets colder then it means there is still dirt in there and will need cleaned properly.
Are you sure its a sealed system and not open vent ? also is your pipe copper, plastic or plastic with barrier.
too late
Thanks, no loft as it's a ground floor flat, pipes are copper from the 60's.

There is a small water tank near the ceiling in the kitchen above the boiler, no pressure guage.

When turned up, all the radiators are good (no cold spots). Apart from the lounge but I can't open the lockshield valve.
make sure that small tank has water in it, when you bleed radiators then the tank should refill. Make sure the ball valve is moving freely. If that's all working good then chances are (remember this is just an educated guess) you've got a small leak under the floor somewhere.

also being such a small size system check your pump settings and make sure its set to 1. its the switch/lever on the side and you have a choice of 1 2 or 3. Anything higher could cause problems
p.s your pump could be anywhere in the house but as you've cleaned out rads and added chemicals im guessing that you have half a clue and may know where it is
Yeah, the small tank has water and re-fills when I drain the rads, the pump is under the boiler - it doesn't heat the system properly on the lowest setting, it was on three when I first looked - on two at the moment, so does sounds like there is a problem if it doesn't heat the rads properly on setting one...

I will test it on the lowest setting tonight with the thermostat cranked up.
Ok, so lowered the pump to one last night and still the same - I get heat this time so I guess the initial clean on the rads helped with that.

I have had the carpets replaced and there was only one damp spot on the floorboards, but that was no-where near the pipes or rads, so if there is a leak it must be pretty small.

I guess a powerflush would expose a leak - but that's not cheap.
Hire the machine and then watch a few youtube videos, you'll save a fortune and waste a day at the same time lol

My two pence worth. If the lock shield valve is stuck closed, you need to get it replaced. Because it is stuck shut, the radiator may still have a small amount of air in it, even if you have bled the radiator, due to the way that air will eventually rise to the highest point, this will cause air to be present in the pipes somewhere.
As you are in a flat and the header tank above the boiler is only a relatively low height compared to the rest of the system, there is very little pressure difference in the system to move the air to a point in the system where it can be bled off. A gravity fed system like yours will achieve a "head" pressure of about 1 bar for every 10 meters the header expansion tank is above the lowest point of the system.
When the water is heated, it expands (which is why you have the header tank, which should have 22mm pipe rising over the top of the tank and exiting into the tank, to catch any expanded water) and the trapped air will cause the pipes to bang occasionally. Gurgling can also be caused by cavitation in the circulator (pump), where air is going through the pump.

In short,drain the system, replace the lock shield and refill the system. Opening the vales on one radiator at a time (while keeping others closed) and running the circulator at speed 3 will put all the flow through one radiator at a time and help to remove air to a point where it can bled off. Once all radiators are bled, close the lock shield valve slightly as this will help it to stop getting stuck closed in the future. Make sure to turn the circulatory back to 1 or 2 as needed.

Good luck.

Thanks Russ, yep, as you say there is a 22mm pipe that goes in to the header tank.

The top of the lockshield valve is crushed and the nut that keeps it from leaking when open has split, but I can still open the valve, but as they don't have replacement parts for a valve I'll have to replace it.

There is a tap next to the boiler which I assume will switch off the supply to the header tank, but it's in the most awkward of spaces, about three inches available either side - wall on left and boiler on the right and also in a cupboard on the floor!
I have been unable to budge it with my limited tools.

I'll give it another go over the weekend.

Thanks for the advice!
Dear bigfootpete

New lockshiled valve £2.66 toolstation.com/sho…063
10" adjustable spanner £4.70 toolstation.com/sho…583

Add something else like some boiler additive at £6.42


to make £10 and you get free next day delivery.

Make sure you order the right size of valve depending on the pipe that feeds the valve, Toolstation do 10mm (large micro bore) or 15mm (standard pipe work). 8mm micro bore is also available, but this size valve will need to purchased from screw fix or a plumbers merchant. Measure the size of the outside diameter of the pipe and order the valve of this size. Remember once you have fully opened the lock shield, turn it back a little towards the closed position about 1/4 turn. This will help prevent is siezing in the future.

Technically the radiators need to be balanced, which basically means adjusting the lock shield valve on each radiator to give a temperature drop of around 12c (20f) between inlet and outlet.
More info here diydata.com/pro…php

Good luck.

Thanks Russ! I finally managed to get around to replacing the valves on the lounge radiator and it seems to have fixed the problem! You da man! The tap that supplied the tank was corroded so needed lots of wd40 to free it up, also twisted my adjustable wrench handles...
Well done Pete!

To hopefully prevent any of the lock shield valves from seizing, make sure you open the valve fully, then turn closed by 1/8 turn which should discourage any sludge from building up.
Disregard this if the you have balanced the radiators as this will mean you have already closed the lock shield valve a bit from it's fullest open position.
If you haven't balanced them, don't worry too much.

Put some inhibitor in the system too, to keep it running well.

Yup did all that
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