Combi Boiler knocking

Where is the needle when the boiler is cold?

Just over 1 Bar, which is in the normal range.

Is it possible that the circulation pump has failed? The water is not circulating through the heat exchanger. the water then boils making the bumps and leaps in pressure that you see. the flow thermostat takes a while to register because there is no flow. When it does it turns off the heat.

Robert

Makes sense. Is there anyway to confirm?

Flow and return pipe temperatures would indicate a flow or no flow I would imagine.

Wait. It does it when the boiler is set to just hot water and the hot tap is on. If it's just hot water and not heating, would a pump be involved?

Yes; the pump is used to circulate the primary heating water around the main heat exchanger. When in hot water mode all that changes is that a diversion valve directs this flow of water to the hot water heat exchanger rather than the radiator circuit.

Although we've not been using it, obviously, the needle has now dropped back to nearly zero.

Top it up to 1 bar again, and see what happens when you leave it for a couple of hours (unused).

Agreed with John. There should always be pressure in the system. A pressure vessel charged with air on one side of a diaphragm acts as an accumulator.

Top up - bleed - top-up.

How can you tell if the pressure drop is due to a minute water leak or a leak from the air pressure vessel ?

Mike P

After a couple of hours it seems to be staying at 1 Bar.

Ultimately, both will end up being water leaks! (there is only so much air). Also, if you run the system with no expansion capacity, the pressure will rise rapidly and cause the overpressure valve to open to blow off the excess. Once the system cools again, you will be back to zero pressure.

If you fire it up and watch the pressure gauge, what does it do? A rapid rise to higher than normal (i.e. > 3 bar) would indicate your expansion chamber has lost its air charge. If you can find the charge valve for it (it will look like a car type inflation valve (for that is indeed what it is)), a quick press on the centre pin should yield a puff of air. No air would indicate a discharged vessel in need of pumping up. Water would indicate a ruptured diaphragm inside it, meaning it needs replacing or, more likely, another one adding to the system elsewhere. (allowing some air into a rad may function as expansion space for a stopgap to get the heat back on)

A big thank you to everyone who commented. Following the instructions in the boiler's manual I managed to remove the water pump. I tried the local plumbing place and they recommended Parts Center, who ordered a replacement pump for me. A quick trip back to Part Center when I noticed the new pump pumped the opposite way, but I was assured that for that particular boiler the part had been 'enhanced'. So I fitted the new pump, filled the system, crossed my fingers and switched on. Wow, hot water and heating. :)

While I was there I overheard someone discussing adding chemical additives to the radiator water to prolong boiler and radiator life. Is there anything in this? It's not a hard water area, the kettle only has a trace of limescale, plus there is an electronic water descaler fitted to the water pipe near where it enters the house.

Rapidly increases by just under a third of a Bar then stops, falling back to the starting pressure when the pump cuts out.

The pressure seems to have stabilised now, no-longer dropping over time.

Should I still do this test even though it seems to be working normally?

That sounds ok then. If you have the pressure at about 1 bar when cold then you can expect it to go up to as much as 2 ish if its a large system (i.e. lots of rads).

Not it its working ok.

(although if you found that you had a rad full of air somewhere, then that could be masking a problem (i.e. acting as a expansion space))

Well done!

The short answer is yes, corrosion inhibitor is vital to the well being of the system.

Its not a hard water problem that you are trying to solve[1] - this is a non issue for the heating circuit since the water never changes - it will deposit a little scale, but once out of the water that's it.

The problem is corrosion. You will get some direct corrosion due to the dissolved oxygen in the water reacting with metals, and also you will get galvanic corrosion from the presence of dissimilar metals in the heating system immersed in an electrolyte (i.e. the water).

The corrosion inhibitor has an oxygen scavenger that mops up any spare O2 before it can combine with your metalwork, plus other chemicals to slow down or inhibit other corrosion processes.

Nice example here:

The pressure dropped again by about a third of a bar overnight. After consulting a diagram in the manual I removed the inner boiler cover and found that the valve is gently weeping. What spare parts do I need?