Rattling Combi Boiler Heat Exchanger

I have a Loblich (Austrian make you probably never heard of) 35 kW wall mounted gas Combi boiler/hot water heater. The unit is three years old, the heat exchanger was replaced 2 months ago as it was leaking. Now it has a new fault and I need some ideas as Loblich say it’s the pipes and my plumber says it’s the boiler.

There is a fairly loud rattle coming from the heat exchanger. It sounds like an erratic car engine with worn tappets at idle. It’s infrequent, but here are some points I can tell you:

- it’s definitely not a lose part of the casing or anything like that, although it sounds like it. The noise comes from inside the heat exchanger.

- when the gas is burning the noise almost always goes away.

- I can continuously recreate the rattle by turning off the gas and have the central heating water circulate through the boiler, even when the water is cold. While it’s doing this, if I turn on the hot tap, the noise goes away totally while the water is running.

The water pressure is about 1 bar, I’ve tried different pressures, bled the radiators and the pump, but still the same rattle.

Any ideas would be most appreciated

Thanks in advance
Chris

I should have added that the pump is integrated in the combi unit, so is also 3 years old, there is no sign of the vibration/noise coming from the pump itself. When I bled the system there was no air coming out and there are no signs of leaks, I almost never have to top up the water pressure.

was it the secondary heat exchanger that was replaced?

Are there any gate valves either side of the pump?

The heat exchanger is one unit and the whole thing was replaced, I imagine there are separate coils inside for the hot water and the central heating.

There is a thing which looks like a gate valve on the back (center) of the pump, it’s also has a pipe which goes up to the compression tank at the back of the boiler. (is that what it’s called in English – big flat red thing behind the boiler – expansion tank?). OK, this “valve” has always been fully open when I screw it in (with a screwdriver) about ¾ of the way, the rattling stops; but I’m not sure if this is a fix or just masking a separate problem. Also hanging down from the expansion tank is a thing which looks just like a car tyre valve. I assume that this is for bleeding, but when I poke something in it to open the valve, nothing comes out, no air or water.

There are also various other gate valves outside the boiler that shut off various parts of the system. If I shut off one part then the rattling is reduced and goes away completely when everything is shut off (i.e. no real water circulation). Opening any part of the system starts the rattling again, so I don’t think that the origin of the noise is somewhere else in the pipes.

Hope you can help today is a public holiday in Austria and it was minus 18 degrees C last night, with two feet of snow. Not a good time to have heating problems!

Thanks in advance
Chris

The vessel is known as expansion vessel and it should have air in it. Expansionvessel is fitted to take up expanded volume of water when boiler is hot. If you do not have any air in it, then it will be full of water. When water heats, pressure will rise to 3 bar and excess pressure will lost through the pressure relief pipe.

If there is a valve on the expansion vessel pipe, then it should be open and seal replaced.

It might help if you could post a picture of boiler functional chart.

The expansion vessel tyre valve should release air when pressed. Release the system pressure by turning the pressure relief valve (clicks when turned) then check the air pressure in the expansion vessel. It should be between 0.5 and 0.8 bar. Then repressurise the system to 0.7 to 1 bar. You should get it checked when the boiler is serviced.

This may not stop the rattle, but it needs doing soon.