any boiler/heating engineers on here???

dac69er

dac69er

Super Moderator
I have an issue with air in my boiler system, i will start from the beginning trying to keep it brief.

system is an standard vented system, basic boiler with 2 motorized valves, an external pump and a tank in the loft etc. last known date of any work being done was 1996 when 5l of fernox was added.

system has been fine for a couple of years since moving in. this year i started getting kettling as the boiler reached full temp before shutting off.

i have cleaned the system twice and done multiple flushes. i have been very thorough.

kettling issue has gone. radiators are alot hotter. water in rads is nice and clean. i added 2l of inhibitor when filling up.

for the last 3 weeks i keep having to vent air from 2 of the upstairs rads. if i leave it more than a few days i get a bubbling sound as the system starts.

i have done a flame test on what is vented from the radiator bleeds and it is definitely air and not hydrogen.

expansion tank seems to function as it should. however, i do get a slight suck in and blow back from the vent back to the tank when the pump starts. there is an airjec unit fitted, but no other high level bleed points.

i never had any air issues before doing the cleaning and refilling of the system.


it seems a pretty common problem after a system clean, a bloke at work has a similar issue.


any ideas? i thought of trying leak sealer, but i cant find any leaks. im thinking its the pump as its pretty vintage and doesnt sound 100% (grundfos).
im an engineer, but have limited experience with heating systems so someone who works on them all day probably has a better idea than me???
 
N

nistarbo

Member +
I am not a heating engineer either but would it be worth turning all rads off and leaving the 2 troubled ones open and increasing pressure to them, ensuring all air is out then opening one by one and bleeding as you open working your way down. Not sure if it would help but just a thought.
I take it your two with air are furthest away from pump?
 
dac69er

dac69er

Super Moderator
I've tried that. I get all the air out, but it's somehow 're entering the system.

I'm not 100% sure of the pipe route, but I think it may be the 1st 2 rads after the pump.
 
dac69er

dac69er

Super Moderator
right. fitted a new pump and still no difference :(

i get an air bubble sound when the pump starts. this happens on all speed settings. i currently use speed 1, the old pump was set on speed 2. speed 1 on the old pump wouldnt circulate well enough and the boiler would kettle.
 
dac69er

dac69er

Super Moderator
It has a high level vent attached to a De-airator in the return to the pump. No manual bleed points unfortunately.

All radiators get hot so wouldn't have thought it's an air lock. System works fine, just slowly gets air in over a few days.
 
glanzanut

glanzanut

Lifer
Hi mate im a heating engineer.
Could be incorrect positioning of the pump causing it to draw air down the vent. if the tank isnt dropping then id try turning the pump speed down to setting one, if its drawing down the vent then slowing it could help prove this.
 
Last edited:
dac69er

dac69er

Super Moderator
glanzanut said:
Hi mate im a heating engineer.
Could be incorrect positioning of the pump causing it to draw air down the vent. if the tank isnt dropping then id try turning the pump speed down to setting one, if its drawing down the vent then slowing it could help prove this.
Click to expand...

cheers mate. stat on the boiler itself went the other week so had to replace that. now the boiler is going on and off like it should.

i have just spent some time balancing the system to get an even temperature drop across the rads (12 ish degree drop). pump speed is now set at 1. boiler water temp setpoint is approx 60 degrees, return temp is about 55 degrees. cant get any air out of the rads anymore, so looking good.

will have to wait and see how it goes and report back if its still being a pain.
 
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