Boiler Kettling

[glynn]

This is my first post on this forum, and I'd appreciate some advice on a heating problem which has baffled me for some time.

<System Description>
I have a Potterton Statesman Utility 70/90 Oil Boiler which I installed approx 3 years ago. The previous system configuration had gravity fed HW, which I converted to fully pumped (2 x 2 port valves) at the same time as installing the new boiler, I also added TRVs to all rads (except for the room with thermostat) and added an automatic bypass valve between flow and return next to boiler, I also replaced several of the old rads which had pin holed. On removing rads / old boiler, I discovered that the existing 30 year old system was fairly sludged up, (but no problems with limescale) so I hired a pressure flushing machine to clean things up properly. The pump is located just to right of boiler on flow side, fill and vent pipes are connected into the boiler left hand flow and return ports. There are no problems with leaks, and the system is not taking in air.

<The Problem>
The problem I have is that when boiler thermostat is turned to hotter settings on cold days (above 2 on scale of 1 to 5), the boiler tends to suffer from kettling noise (as if water is boiling or air is trapped inside the boiler). The noise is worst when the house has already warmed up, and most TRVs are starting to close down.

I've tried several methods to alleviate the noise, including adding the auto bypass mentioned above.

Last night I had another look through the boiler installation manual just to check if there was anything I might have missed. One point I noticed is that for the flow connection at the top of the boiler, it advises to use an adapter, with an eccentric coupling, with the eccentric shown uppermost. At the time of installation I only used a straight 1"B.S.P to compression coupling to connect from the boiler to 22mm dia copper pipe. My question is: Is it possible that by using only a straight coupling, and not an eccentric coupling, there may be a small air pocket remaining at the very top of the boiler, which is causing this kettling noise??

 

[kevplumb]

yes

 

[oilman]

The air will usually be absorbed by the water, and your arrangement of connection is very common and does not normally cause a problem. Have you tried boiler silencer, and have you checked the thermostat operates at the correct temperature.

 

[kevplumb]

cant remember which boiler exept it was a 100,000 floorstanding and it spcifyed im mi that if you didnt use an eccentric reducer you would get this problem
ill bow to oilman on this one

 

[oilman]

I agree that eccentric fittings are better, (must start using them), but you will have seen most boilers without them. I don't know if there is a problem not using them, it's just going from what I've seen.

 

[kevplumb]

seen loads without them that was why it struck me
when you get a 250 kw boiler and there is obviously an air trap yet it works lovley dont make sense

 

[glynn]

Many thanks Gents for the advice.

I guess the lack of eccentic fittings is not the root cause of the noise, since as you say most other installations get away without them, so I'll check out the thermostat tripping temps and try the boiler silencer before I resort to changing the fittings.

One quick question about the boiler silencer liquid: Does this have any effect, other than dissolving limescale? I'm pretty sure there isn't an issue with limescale as I haven't seen evidence of this anywhere else in the system.

 

[oilman]

I'm pretty sure there isn't an issue with limescale as I haven't seen evidence of this anywhere else in the system.

If it's all in the boiler, it won't be anywhere else. You will find some in the feed pipe just before it meeds the circulation.