Loud, scary boiler noise!

[milleniumaire]

Having unblocked the central heating pipes and radiators and got them working again, today I started the process of giving my CH a good clean.

It is a sealed system and I emptied the water, cleaned out the magnaclean, added a couple of bottles of Fernox F5 cleaner via the magnaclean and a towel rail and then refilled the system. The boiler was turned back on and I wasn't surprised to hear gurgling sounds as it initially pushed air and water around. This soon passed and it started running smoothly.

About 5 to 10 minutes later the boiler started making a very load rumbling noise. I could hear it from upstairs even though the boiler is in the garage. I noticed the supply pipe was hot, but the return pipe was cold, also reflected in the temperature gauges on the pipes. Because the noise was so loud I turned the boiler off, then back on. 10 minutes later it happened again. This time however, the return pipes seemed to be a warmer and the radiators were starting to get warm. Again, the noise was so loud and disturbing I turned the boiler off and back on. This seemed to fix it and the noise didn't come back!

In the 9 years I've had the boiler, it has never made a noise like this. I wondered if it was something to do with the trapped air, or possibly the Fernox chemicals I had added. Googling suggested this type of noise results from Kettling, but it has never done this before, so why should it start now after it has been refilled.

3 hours later and everything seems to be working fine, however, next weekend I will be emptying the cleaner from the system and flushing it a couple of times before adding inhibitor. I'm now concerned the same thing might happen again.

Does anyone have any idea what might have caused this disturbing noise and should I be concerned?

 

[SJB060685]

It is most likely trapped air

 

[king of pipes]

As above what boiler brand is it please many have internal filters that can become partially blocked especially as you have had blockages in pipework , rads etc in future turn the thermostat to 60° on the boiler when restarting it and ensure the system is full a vented well . Kop

 

[milleniumaire]

It is most likely trapped air

I hope so, but it isn't like any sound of trapped air I've heard when refilling the system in the past.

As above what boiler brand is it please many have internal filters that can become partially blocked especially as you have had blockages in pipework , rads etc in future turn the thermostat to 60° on the boiler when restarting it and ensure the system is full a vented well . Kop

The boiler is a Vaillant Ecotec 637 system boiler. I did wonder if the chemical had shifted some sludge and caused it to block something (heat exchanger?), but surely this wouldn't have happened within 5 minutes of turning it back on?

Initially, although I could feel the supply pipes under the boiler were very hot, the return pipes were cold and even the closest radiator to the boiler wasn't getting warm. This seemed strange and I wonder if there was possibly an air lock in the supply or return pipe, or possibly in the motorised valve, so the boiler was heating water and this couldn't be pumped out due to the resistance caused by the air lock. If this happened, I assume the boiler might start kettling as it is continuing to heat up very hot water, which isn't going anywhere. The boiler does have a built in by-pass, but I assume in this scenario, it wouldn't help? Not sure why turning off/on the boiler would cure this though.

The other thing I did, that I haven't done before, is to manually latch open the Honeywell motorised valves while emptying the system. I then de-latched them before filling up. Again, I wondered if air may have been trapped in the supply valve or if it may not have opened properly when the boiler was turned back on and turning the boiler off/on re-sent the appropriate signals allowing it to finally open and for the hot water to circulate.

 

[DaveB]

It seems like you've given the clue. You closed the zone valves before refilling. Most likely air lock.Leave zone valves open when you refill next time.

 

[milleniumaire]

It seems like you've given the clue. You closed the zone valves before refilling. Most likely air lock.Leave zone valves open when you refill next time.

Thanks for confirming that Dave. It does appear to be the most likely cause and I shall be sure to close the zone valves after re-filling this weekend when I clean out the Fernox F5.

 

[Darren Jackson]

Yes make sure motorised valve is locked open when draining down & when filling back up again.

 

[milleniumaire]

I'm guessing what happened was that re-filling the system caused an airlock in the pipes at the zone valve on the supply (as has already been mentioned). So, when the boiler turned on the first time, it heated the water up, but this hot water wasn't able to be circulated due to the air lock restricting the flow. I assume the noise I heard was actually kettling as the boiler continued to heat already hot water, which wasn't going anywhere. This explains why the radiators weren't heating up as expected.

Each time I turned the boiler off and back on, the pump would have started up and the second time I assume there was enough pressure to clear the air lock in/around the zone valve, thereby allowing the water to be pumped around the system and so preventing the kettling from occurring again. The system has been running smoothly since.