Air being sucked into system

[Beowulf]

Extended 1930s Semi.
1970s heating
Gravity HW/Pumped CH (microbore).
Potterton Profile boiler
I've modified it to a "C-Plan" (valve is not blocking F&E pipe) wired to allow the HW and CH to retain their separate setpoints.
The Pump is located in the floor cavity in the room immediately above the boiler.
The system was treated with Fernox F3 and flushed about a month ago. A lot of sludge was removed. Inhibitor added -the water has stayed clean.

The old pump (15-50) failed last week, so I replaced it with a UPS 15-60. I balanced the rads and set the pump to speed 2. There was a lot of air noise near the pump, so I reduced the speed to 1. The noise was reduced.The rads are still heating up and the system is performing far better than it has since we've lived here.

Unfortunately, the radiator closest to the pump fills with air very quickly and the radiator in the downstairs hallway makes gushing/air noises.

I'm assuming that the pump is scavenging water too quickly from the boiler and drawing in air. The pump location may not be ideal and I would be happy to move it.

I unblocked the supply pipe from the F&E tank about 18months ago (when we moved in). I noticed this morning that the 'vent' pipe upstairs was warm, but the larger (28mm) flow pipe wasn't.

Any suggestions?

Cheers.

 

[45yearsagasman]

Almost always caused by a blockage or incorrectly configured pipe work involving the cold feed and vent.
Is it possible to show the pipework connections into the system?

 

[Beowulf]

Cheers. I'll take some photos later.

It is presumably a long-standing issue as there was a bleed key left in the bleed valve of the air-gathering radiator when we moved in.

 

[Beowulf]

There's not a lot of piping to see. The pipes above the boiler disappear into the extension ceiling. The flow & return take-offs are consistent for the DHW and the CH.

A potential issue is that the F&E/DHW pipes travel horizontally for a fair distance (~1 metre towards the front of the house, across a bedroom, through a toilet room, with a 'step-up' (2 90degree bends with a 15-20cm rise between them) to the level of the original part of the house, across the bathroom, then back ~1m to the cylinder cupboard and up to the F&E tank.

It's definitely not a short, vertical rise/fall from the F&E tank.

 

[warmadmax]

maybe a silly question, but did you put the replacement pump on the same way round?

 

[Beowulf]

Yes, the pump flow direction is as before -this wasn't necessarily correct, but I hvae checked that the DHW and CH flow (hot side) are both from the same boiler pipe.

The air issue is not a new one, the old pump had just become not very effective at pumping.

 

[Armo74]

Have you tried checking the cold feed where it tee's into the flow pipe from the boiler with a magnet to see if you have a blockage, if you have one the magnet should stick.

 

[Beowulf]

Have you tried checking the cold feed where it tee's into the flow pipe from the boiler with a magnet to see if you have a blockage, if you have one the magnet should stick.

I've not checked that, although I will later.

When we moved into the house the cold feed pipe was blocked between the cylinder and the F&E tank. I cleared the blockage and the system fills okay, but a blockage close to the boiler could still be a problem.

Cheers.

 

[Beowulf]

The magnet doesn't stick to any of the boiler pipes or the cold feed pipe.
-The magnet does, however, stick to the vent pipe above the DHW cylinder T, but I wasn't prepared to start chopping out pipes this evening.

The vent pipe did not appear to be sucking in air when the heating was on.

This evening I closed the cold water feed valve and drained down the system a little.

Whilst attempting to re-fill I opened the cold water feed valve. Nothing initially happened and then there was suddenly a 'glug' and the system filled.
-There may or may not have been some sludge blocking the cold feed.

I then stuck a tube down the vent pipe and then down the flow pipes from the (very awkwardly positioned just under the roof) F&E tank as far as I could and pumped as hard as could with a bicycle track pump (it's worked for clearing blockages in the past).

Who knows if it has helped...

...but hopefully the radiator near the pump won't fill with air tomorrow morning.

 

[Beowulf]

There was air pumped into the upstairs radiator again this morning.

This could be down to residual air in the system, but I'm not convinced.

Next stop, check for leaks....

 

[RoughWaters]

vent then feed should be within 6 inches BEFORE the pump. Anything else could cause to air being dragged in. On your system feed and vent could instead be from gravity primaries on cylinder. Just one possibility to look at.

 

[Beowulf]

vent then feed should be within 6 inches BEFORE the pump.

I'm not quite following.

On your system feed and vent could instead be from gravity primaries on cylinder. Just one possibility to look at.

The vent is T'd into the flow pipe of the cylinder primary coil. the feed is T'd into the the return from the cylinder. The pump is in the flow line to the radiators. The pump makes air pumping noises and the closest radiator to the pump fills with air. The others appear not to.

As above, the route between the boiler and the HW cylinder (and feed/vent) is slightly tortuous, with multiple bends. It is not a straight, vertical run.

 

[RoughWaters]

Is the rad that fill with air behind the pump. Because 'air' is molecularly smaller than water it is possible for micro leaks to take air in under negative pressure from the pump, but not to leak water when the pump is off. I would however imagine that its a more basic plumbing issue.

From what you say about feed and vent they sound like the should be fine. Tapping them in on gravity circuit feeding cyliner is fine. If the system is installed really badly it can be a **** to get the air out. I have had to stick a hose down the feed pipe and blast it in the past when ive been to systems that Stevie Wonder installed - otherwise i would have been therefor ages

 

[Beowulf]

The flow pipe comes from the boiler, rises into the floor void above it, turns to the horizontal and then reaches the pump. The rad filling with air is then about 2m from the pump's positive pressure side.

I'll check the pump joints.

 

[RoughWaters]

does the boiler sound like its got air in it? bubbling/banging?