I've a Ikon-t gas combi boiler that's been installed for about four years. Unfortunately it's developed a fault. When hot water is called for, either by a hot tap or the heating is switched on, the boiler hums then the gas lights up. All as normal.
However, after a few seconds the needle on the pressure gauge jerks around as high as it can go and the boiler starts to make a knocking sound. Then the safety cut-out light comes on. After that the needle on the pressure dial slowly drops back to normal.
I've checked that the tap on the filling loop is closed.
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.
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.
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.
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))
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:
formatting link
Hard water can cause scaling of the mains hot water side of a plate heat exchanger in a combi boiler - however this is separate from the heating circuit. The electronic descalers are generally thought to be about as useful a back pocket on a sock, but occasionally some folks report success with them. (phosphate dosing or ion exchange water softening being the only reliable ways of preventing scale build up on heater element surfaces in hard water areas)
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?
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.