Thumping sound from Central Heating

[Foo76]

Shortly after my central heating kicks in there's a loud thumping sound coming from the pipes. The sound moves around relatively quickly but generally comes from the run of pipes between the boiler and the pump. Occasionally it's accompanied by a gurgling sound, and the pump sounds as if there is air running through it.

I have an open vented system, the feed and expansion tank is well covered, seems to be clean, and is working. What is odd though is that the pipe feeding from the tank gets a lot hotter than the expansion pipe too the tank when the banging sound is happening. The pump seems to be going in the right direction!

The house has a mix of rads at different ages, some very old. I've replaced some, and flushed through others.

I've taken the floorboards up where the pipes run, they're not moving or banging into each other.

I've put cleaning agent in the system, which i will flush out on the weekend, but I'm at a bit of a loss what to try next? From previous articles I've read it sounds like there's air in the system, but how should I go about getting it out? I repeatedly bleed the radiators and pump to no avail.

Many thanks.

 

[JonCa]

I repeatedly bleed the radiators and pump to no avail.

Does :- No avail = No air comes out ???
or :- air come out but its back next time I check ???

 

[Foo76]

I'm bleeding the radiators (usually the same one or two) daily, a small amount of air is released, but the noise continues and the air builds up day after day.

 

[JonCa]

At page //www.diynot.com/forums/viewtopic.php?t=37170 see FAQ 8 note last para.
You say F&E getting hot at its outlet. If air in system then air will expand and send hot water towards F&E, should get same on expansion pipe so check that you do not have any closed valves on expansion pipe (none should be fitted anyway)

The air suggests that you are using water, the fresh water from the header tank will always carry free air which will separate out, however check at header tank when system is running to ensure that its not overflowing through the expansion pipe. If its not then its possible you may have a leak. May also (as in FAQ) be blockage (air, sediment, scale) causing problems in circulation.
For air block then :-

First of all it is likely that the downstairs rads will cause the majority of the problems so sort these out first.

Bleed all radiators again, just to be sure, and then turn off of all of the rads upstairs and downstairs bar one of the downstairs rads – this will be the first one that you are going to get working.

Switch just the heating on and confirm that the boiler comes on and stays on. After a short while you should be rewarded with some distinctive gurgling sounds from the one open radiator and heat will surge through very, very quickly. Leave it until you are certain that it is heating evenly and consistently and then open up the next downstairs radiator. When this is open, close off the first rad. Remember to bleed all of the rads in turn as the heat starts to come through. You repeat this process until all of the rads start to work. Then, open up all radiators and they will all work simultaneously.

Now turn the heating off and the hot water on. The hot water cylinder should have two 22mm pipes readily accessible. One near the bottom of the cylinder and one about a third of the way up. The uppermost one should be very hot and the lower one should be significantly cooler but still warm. If this is the case then the hot water circuit is working properly.

If this isn’t happening, make sure there is a demand from the thermostat first if still not then look for a manual air vent in the airing cupboard and open it until water comes out if you find one.

 

[mickyg]

it could just be a dodgy trv, or one fitted on the wrong end. Try turning them off one by one when its banging.

 

[Foo76]

JonCa - Thanks for the advice, really appreciate it. I put central heating cleaning fluid in the F&E tank last week so will need to drain it down tomorrow. Presumably it's best to wait until after I've done this before messing about with the rads? And/or is there anything I should be mindful of when filling the system back up to make things worse (or indeed better!)

mickyg -

it could just be a dodgy trv, or one fitted on the wrong end. Try turning them off one by one when its banging.

Sorry if this is a stupid question, but how could I tell if the TRV is on the wrong end? Do you mean that a dodgy TRV could be introducing air into the system?

 

[JonCa]

Just refill slowly so the air does not get trapped.

TRV on outlet of rad can chatter (due to water flow) as it gets close to being fully closed

 

[nairb999]

Sorry to but in on this post but I am experiencing similar air problems
no thumping but gurgling in 22mm pipe round pump area.

I have just mofified my central heating system ie changed radiator in hall
for larger unit. The hall is now much warmer and all radiators get hot so
my small balancing tweaks appear to have worked or could I have not got
it quite right.

It is interesting to note it took ages to refil system and get all the air out
of the system or maybe I did n't and have an airlock as you suggest in the
posr. What do you think?

 

[Armo74]

Hopefully you are draining and re-filling with your motorised valve(s) in the manual position. Also if you are adding cleaning chemicals via your feed and expansion tank you have drained some water out first otherwise all you will end up with is a nice clean feed and expansion tank.

 

[nairb999]

No hot water heating on this system
No inhibitor yet

 

[Foo76]

So, today I drained down and refilled the system, adding inhibitor and bleeding the rads as I went. The system fired up OK, and didn't bang like it had been, at least the first time it fired it didn't! Once the system had been off for a couple of hours I fired it up and got the same noise.

I decided to stand by the boiler when it fired, hot water almost spits out of the outlet, the pipe is red hot straight away. Its almost as if there's pockets of air coming from the boiler, I wonder if this could be the source of air in the system and therefore banging? Could it be kettling?

Really appreciate your help with this so far.