CH Pump sucking in air from vent pipe?

[Schmill]

I'm not sure if this is possible or not...

I have a fully pumped, vented system.
My old primary pump packed up recently and I replaced it with a new one, which seems to shift the water around a lot better, (it certainly did in a 'sink' test!).
However is it possible that the new pump is sucking air in from the vent pipe (over the CH expansion tank) in the loft?
When the pump first switches on I can hear what sounds like air rushing through the pump, and I can't believe that there is really still air IN the system now.
The Vent pipe just goes up to almost the top of the roof, then arches over, and back down to an open end just above the expansion tank. Is there any kind of valve that I could fit on the end of it that would allow it to vent still, (ie. air / water / steam out), but would not let it suck air IN through that route?

Many thanks

 

[Agile]

You cannot put any restriction in that pipe.

The feed and vent should be within 150mm of each other, normally on a horizontal section of pipe.

Any air in a system will cause rusting of the rads. That produces sludge.

Tony

 

[picasso]

Try turning the pump speed down

 

[DavidDaft]

is vent and feed the correct way round? i.e vent,cold feed then pump

 

[Schmill]

Thanks for the responses, everything is as described in the posts above.

The return is a vertical 22mm pipe, which then T's. The vent pipe continuing vertically to the the loft, the return taking the T to the horizontal. It then connects to another T section with the cold feed coming from teh expansion tank entering the top of the pipe in 15mm, and the 22mm pipe continuing for a short section, before turning downwards again and into the pump. After the pump the pipe then T's into 2, each being a zone valve controlled branch to the hot water cyclinder (indirect), and the CH respectively.

I have some Sentinel X400 going round the system at the moment to try and give it a clean, and that is due for removal soon, and I'll was planning on refilling with either another dose of X400, or else the X100 inhibitor, depending on the state of the water that comes out of the system.

Was just surprised that after a few weeks & various bleeds of the rads & pump I can still here air churning around the system occasionally.

 

[Agile]

You seem a little reluctant to describe your system in a way that I can easily understand. Without knowing how its configured we cannot help.

The vent should be connected IMMEDIATELY after the flow from the boiler!

Thats not apparently what you are saying!

Tony

 

[Schmill]

You seem a little reluctant to describe your system in a way that I can easily understand. Without knowing how its configured we cannot help.

The vent should be connected IMMEDIATELY after the flow from the boiler!

Thats not apparently what you are saying!

Tony

Hi Tony, sorry, I was not being reluctant, but perhaps just provided too much detail

Yes, the boiler is in the kitchen, and the return from it comes up through and under the floors etc to the airing cupboard.
In the cupboard the pipe travels vertically up to the loft when it hoops over and into the top of the expansion tank.
In the airing cupboard there is a T in that pipe (so that the water goes round the system, and not up the Vent pipe!). The T takes the pipe run horizontally where a 15mm pipe is T-ed into it from above, (which is the cold fill), and then the pipe run heads back down again, and into the pump, (then onto the zone valves etc).

All I was wondering was whether the pump was somehow sucking the water out of the vent pipe and then sucking air in through there when it starts. It was just a thought....

 

[Agile]

As In said the vent has to be on the FLOW from the boiler. But you are saying that it is on the return !!!

Are you sure you have identified the flow correctly?

Then comes the question about the pump, depending on the flow connection at the boiler then is the pump pumping the correct way?

Are you able to measure pipe temperatures?

Tony

 

[DP]

OP you almost certainly have plumbing problem. This would need to be corrected else the new pump will fail prematurely as well.

Sit down with an A4 sheet and draw the pipe runs. Also mark the pump and flow+return and vent and feed runs. Now, mark the pipes on the flow side of the pump with a + and on the suction side with a -. You should be able to 'see' why air is being drawn into the system. When the F+E and vent pipes are 'close coupled, pump speed and orientation is not important as vent and F+E pipes are at almost same pressure be it positive or negative. Thus no air will be drawn down the vent pipe or water expelled from the vent pipe. One exception is a blockage between the said pipes.

 

[Schmill]

Sorry about the confusion regarding the flow and return. I was referencing them relative to the pump, not to the boiler, which I am guessing is the convention then?

Ok, so pictures speak a thousand words and all that...

Untitled

• Schmill
• 21 Oct 2010
• 2

That's what I've got

All I was wondering was whether with the VENT pipe being 22mm, and the fill pipe only being 15mm, whether it was possible for the pump to be sucking air into the system via the vent pipe, and if so if there was anything I could do about it

Sorry for any confusion I might have caused

 

[DavidDaft]

It's piped up correctly,might be the f and e is blocked,have you tried draining the system with water on?(ch header tank should fill as system is drained)

 

[PEDANTICVINDICTIVEMAN]

To do it properly, change 15mm feed for a 22mm one, means drilling tank with new 22mm tank conn and new 22mm tee and pipe.

 

[johnnojob]

convert to a sealed system if your boiler permits a sealed system.

 

[Homesafe]

Maybe your pump is just pushing air that had been in the system from before when you had the old pump??

 

[Agile]

That is correctly piped.

Lets assume the flow and return on the boiler are correct.

In that case as long as there is no blockage around the vent and feed pipes where they join the horizontal ones there should be no problem.

Drain a few litres from the system and see if the level in the F&E tank drops.

Tony