Pumping over (when the pump stops) question - open vented system

[Stewb]

Evening all

Firstly may I open with a thanks, this forum has been a huge source of useful information for me as a DIYer. I like to research thoroughly before I undertake work and this forum has been a super valuable resource.

I have a question re pumping over on my open-vented system in a house we recently bought.

We have 16 rads and 3 towels rails spread over 3 floors, early 1980s build. Boiler is a Greenstar 40CDi installed about 10 years ago. HW via a vented cylinder. 28mm from boiler in garage to airing cupboard.

Some build-up of sludge/magnetite, it has built up in a few places and inside some original rads however heat is still good. The filter seems to do a reasonable job of catching the magnetite and levels have dropped since I drained/refilled to do some work (adding 12 TRVs and a second heating zone). One day I will get it power flushed but its ££. I've previously used X400 but it didn't seem to do much. System has 2 litres of X100 in to protect it and system colour is like light straw.

So I have now noticed that when the pump switches off there is several seconds of discharge up the vent pipe into the F&E tank. I've dropped the pump speed but it still does it. I reckon it has always done this and I've only just noticed.

The header tank collects orange gunk/growth and since I've cleaned it out more of a clear slime. It is covered and jacketed. A small amount of air builds up in the system over time always in the same tall towel rail on the first floor.

I attach a photo of plumbing. A magnet tells me there is quite a lot of magnetite on the 28mm bend and tee before the pump but none after and none where the 15mm feed tees off. I'm not sure how big an issue it is given the pipe is oversized in 28mm?

I know the obvious answer is to convert to a sealed system but I think overkill right now and given the regs on sealed systems it makes it less maintainable by me.

F&E tank is one floor up, not much head height between it and the top floor rads.

Current plan: cut off at the 22/28mm reducer on the vent pipe, clean with brushes the inside of the 28mm around the pump and tee, fit 28mm to 22mm reducer on top of tee, remove and replace 22mm copper pipe where the 15mm feed pipe joins. Do not replace feed.
In loft, replace F&E tank and a better fitting lid and jacket (current one horribly gunked), fit 22mm feed pipe from tank, tee into vent pipe below F&E tank. Ie for a combined 22mm feed and vent.

Any thoughts much appreciated.

(15mm plastic between the vent and feed is rad flow to top floor)

 

[oldbutnotdead]

You may have a large air bubble trapped somewhere between pump output and bottom of f & e tank- that could give a similar effect (bubble is compressed by pump pressure, when pump shuts off bubble expands, water has to go somewhere). That airing cupboard pipework looks interesting....

 

[Nige F]

Current plan: cut off at the 22/28mm reducer on the vent pipe, clean with brushes the inside of the 28mm around the pump and tee, fit 28mm to 22mm reducer on top of tee, remove and replace 22mm copper pipe where the 15mm feed pipe joins. Do not replace feed.
In loft, replace F&E tank and a better fitting lid and jacket (current one horribly gunked), fit 22mm feed pipe from tank, tee into vent pipe below F&E tank. Ie for a combined 22mm feed and vent.

Any thoughts much appreciated.

(15mm plastic between the vent and feed is rad flow to top floor)

Plan sounds good

 

[Stewb]

You may have a large air bubble trapped somewhere between pump output and bottom of f & e tank- that could give a similar effect (bubble is compressed by pump pressure, when pump shuts off bubble expands, water has to go somewhere).

This is interesting, thanks. I can't think where though. The pumping over comes in a couple of spurts, as though the first siphons the second spurt. Made me think of the occasions I've had too much to drink and end up hugging a toilet bowl, gasping for air inbetween!

That airing cupboard pipework looks interesting....

I know... but it works. I think converted from a gravity fed system and then changed again when the previous owner did the loft conversion. Could be tided up, would you change it? I'd rather the filter were on the return boiler leg but not worth moving I don't think.

Plan sounds good

Thank you, parts ordered, £60 all delivered ain't bad.

 

[oldbutnotdead]

That massive collection of elbows on the flow to the cylinder won't be helping anything...

 

[Stewb]

I’ve done the work in the loft and it has solved the pumping over. 22mm from the tank teed into the vent pipe 1m below the tank and put an isolation valve on the old 15mm feed pipe to have available as a vent on any future refills.

The old 15mm plastic feed pipe with its pipe inserts at joints had very limited flow so I’m not surprised that the vent was the easier flow route when pressure built on the system. The 22mm feed from the tank doesn’t have this issue.

 

[oldbutnotdead]

Happy days. Yeah effective bore of placcie is a bit tight.....

 

[Stewb]

You may have a large air bubble trapped somewhere between pump output and bottom of f & e tank- that could give a similar effect (bubble is compressed by pump pressure, when pump shuts off bubble expands, water has to go somewhere).

Dealing with a big double rad today making a lot of popping / crackling sounds and noticed it was cold on the back panel. Kept getting a small amount of air out then water. Opened opposite end where blank is a bit just got water.

Cracked it on full pump high temp all other rads TRVs closed, then tried to bleed it, just water. Then turned both lockshield and TRV off on it and bled and masses of air come out. Reckon the back panel was mostly filled with air.

Got the rubber mallet involved too, not sure if it helped other than satisfaction.

 

[oldbutnotdead]

Interesting..

 

[Stewb]

So despite the new F&E tank, replacing the attached piping and flushing, the jelly/slime/growth infected the new tank within a couple of weeks.
I've now bought Fernox F7 and added about 100ml to the tank which I hope will kill the infestation.