How is air getting into system?

[Robert Eadie]

Just recently, our system seems to be taking in air, and getting very noisy around 6am when the water heating starts. Less noticeable when the CH is also on.
It's a 25 year old reasonably small system with a gas boiler (replaced about 10 years ago) downstairs in the kitchen, the pump and cylinder in the upstairs airing cupboard, and the feed tank up in the loft. There is a vent pipe going 'up and over' the feed tank, which I think doubles as the feed pipe. i.e. a single 22mm pipe comes from the bottom of the feed tank, but has a T joint with feed going down and vent going up (I think). I have to admit that the vent is not at the highest position of the rest of the pipework, but what is puzzling is that the problem has only just started, interestingly when we had some work done in the airing cupboard on the secondary (DHW) system. We had a pump fitted to the hot water feed (after my wife chose a new kitchen tap which has one of those 'hoses' which pulls out of the tap for rinsing. It needed more pressure!)
I don't see how this work could have effected the primary system, but . . .
I have read that a very small hole in pipework can let in air but not let out water? Can the rather old pump be drawing in air, and if so, how can I diagnose? Or can the boiler, which is due a service, somehow be drawing in air??
Thanks for any help in getting to the bottom of this problem.

 

[centralheatking]

So you have a conventional system which has been fine for years. You have had some work done to the hot water system inc. a booster pump for the hot water supply.
Just have a look at the speed setting for your ch pump maybe some gremlins have speeded it up...if its set fast slow it down...its the little disc on the pump head ..
see what happens and report back...others on PF will look over the weekend and put their
ideas up. centralheatking

 

[Dan]

(please make sure you click like on any helpful posts)

 

[SimonG]

Do you have a central heating filter on your system and when was the last time you had any inhibitor in the system.

Maybe worth getting a cleaner in the system and run it for a couple of weeks. Then empty system, refill system, run system, empty system and then refill with some inhibitor.

 

[Robert Eadie]

So you have a conventional system which has been fine for years. You have had some work done to the hot water system inc. a booster pump for the hot water supply.
Just have a look at the speed setting for your ch pump maybe some gremlins have speeded it up...if its set fast slow it down...its the little disc on the pump head ..
see what happens and report back...others on PF will look over the weekend and put their
ideas up. centralheatking

Thanks. Yes I know about the speed setting on the Grundfoss, and it generally works fine on lowest setting. I have already tried switching up to 2, or 3, and this just makes the noises worse. So it is still running on 1. I don't think the plumber did anything with the primary system.

 

[king of pipes]

(please make sure you click like on any helpful posts)

Sorry mate could not resist

 

[Robert Eadie]

Do you have a central heating filter on your system and when was the last time you had any inhibitor in the system.

Maybe worth getting a cleaner in the system and run it for a couple of weeks. Then empty system, refill system, run system, empty system and then refill with some inhibitor.

Thanks. I'll try a cleaner and see what that does (though I don't see how that might stop air getting into the system). Anything's worth a try.

 

[king of pipes]

Are you sure the system is full ? the cold feed may be blocked so the system is only partially filled these systems can be a pita to vent out , as Simon said a good quality inhibitor is needed in the system as gases build up from the heat and corrosion process inside the steel radiators unless protected with a inhibitor, you can usually tell a bad system by the smell of the air when you vent the radiator's it is very unpleasant . Kop

 

[Ziggie]

Totally lateral thinking - does the system have a bypass valve ?

I found that changing flows through the bypass causes noise in mine.

 

[Chuck]

I don't see how this work could have effected the primary system, but . . .

I assume that the system was fully or partially drained when the work was done. If so, this may have moved sludge around in the system and its ended up somewhere that is creating a negative pressure.

When you refilled, was the correct amount of inhitor added?

New pipework will result in a certain amount of hydrogen being evolved as a passivating layer forms. This should gradually stop over the course of weeks / months.

Hold or hang a plastic cup of water so that the vent-pipe dips into it. See if the vent pipe sucks water out of the cup when the heating starts up in the morning.

I have read that a very small hole in pipework can let in air but not let out water?

This would be a long way down my list of possibilities to consider.

 

[Ziggie]

I still wonder if you have a bypass valve fitted in your system. In case you are not aware , this is fitted just after the pump and provides a route back to the boiler if the pressure is higher than a set amount. The common device is a Honeywell and has a very bright red collar.

By increasing the flow, the bypass valve is more likely to operate and this can cause noise.

 

[Robert Eadie]

I assume that the system was fully or partially drained when the work was done. If so, this may have moved sludge around in the system and its ended up somewhere that is creating a negative pressure.

When you refilled, was the correct amount of inhitor added?

New pipework will result in a certain amount of hydrogen being evolved as a passivating layer forms. This should gradually stop over the course of weeks / months.

Hold or hang a plastic cup of water so that the vent-pipe dips into it. See if the vent pipe sucks water out of the cup when the heating starts up in the morning.



This would be a long way down my list of possibilities to consider.

The work was only in the DHW system, so the primary was not drained, but thanks for your and others' suggestions. I can't find a bypass valve . .

. . but the good news is that it seems to be sorted. I went and bought myself the cleaning fluid, and inhibitor, BUT haven't yet got round to using them!! This morning I was not woken to gurgles in the system, and since then have tested with just the DHW heating (not radiators) and all was quiet! (The system may have heard me talking about flushing it out etc!!!!)
The only thing I can think of was that the CH was on last night at a rather high temperature (23) for perhaps an hour. (My dear wife has a habit of bumping the thermostat up to make the boiler fire immediately, and I didn't notice till I went upstairs to bed.) Maybe this hot water circulating has shifted some sludge, or whatever.

I will use the cleaner and inhibitor now I've got it . . . but not as urgent as it was, unless tomorrow morning I am woken again to gurgling!

Real thanks to all who have taken the trouble to contribute. These forums are invaluable, as you're not just getting the views of one plumber, which would have been the case if I'd called someone in, but lots of varied experiences. Thanks, and I'll keep you posted.