CH venting into header

[Peaklander]

Hi,

As per title, we (well my aged parents 97 and 89), have a regular boiler (Worcester Greenstar) feeding DWH through a thermostat-controlled zone valve and central heating to the house, again through a zone valve.

There's a large header for the HW and smaller one for CH and the main flow pipework to the pump is in 28mm, with 22mm up and 15mm down from the CH header.

This installation has been in place for many years but some weeks ago a visitor noticed that 'some radiators need bleeding'.

 

A plumber did this but said that he couldn't get sufficient flow around the system, suggesting there was a faulty pump or airlock. He was unable to return for a week and so I went (at least they are local to me) and changed the pump myself for a nice new UPS3 15-50/65. Nothing instantly improved, until I bled a radiator and also turned a lockshield down. Then the rads got hot.

 

However I had already got a second plumber lined-up when the heat finally happened and he called to have a look anyway (moral support).

 

He was happy that heat was available but had a look around, didn't like the feel of the zone valves or the lack of a magnetic filter, so we agreed that I would purchase these items and he would come back to fit.

 

When he did, along with Fernox system cleaner, we could not get heat to flow - so it was just as before. The rads only got warm and the boiler kept tripping. Eventually we bled air more, including at the top of the boiler and we got heat. He looked at the header tank and showed me that hot water was coming over the vent. The level was ok so the pump was drawing from the cold feed. Maybe this has been happening for days. 

 

Since then nothing I have done has stopped this. We did a basic power flush (plumber has a pump / tank set-up with magnet that recycles the water) and made a reasonable job of closing rads one by one (with the vent pipe capped).

 

I have:

 

 - experimented with the pump speed and it's now down at 1

 

- reduced the flow target temperature (and can hear the boiler immediately modulate down)

 

- taken infra-red pics of the H pipework, where the vent and cold feed are tee-d into the flow pipe, just before the pump.

 

Plumber says there's a restriction in the radiators or flow / return circuit and he wants to move to a pressure system to try to drive it harder. (Removing the header and replacing with an expansion vessel, PRV and a cold fill point). I am hesitating to move off gravity because there are buried pipes at ground floor and I wouldn't want to find some leaks with the pressure.

 

I think that for hot water to rise into the vent rather than go into the pump, the short pipe between vent and cold fill must be restricted. However this is 28mm and it feels hot. Maybe it is conducting heat from the pipes and there's little flow through it.

 

I have tried to photograph but with the IR it's quite an art to get the correct colour temperature range and maybe too much can be read into them.

 

I'm reluctant to cut into this (with a view to replacing with a 28mm compression) because I don't know how to make two clean cuts and also not sure if the pipes will move enough to get the fitting in.

 

Soooo, to my questions...

 

1. If there is a restriction in the circuit, isn't the pump return pressure going to be lower going out and back and so it would not send water up the vent?

 

2 Or, even on speed 1., does the delivery pressure against a restriction, increase pressure backwards through the pump and cause water to flow up the vent, so the pump acts like a sort of hydraulic restriction?

 

3. Might the short pipe across from vent Tee to cold fed Tee by partially restricted and if so should I jump in there, open it up and see?

 

4. Should I do more flushing?

 

Have I missed something?

 

Thanks so much - it's a bad time of year to be doing this.

 

Pics attached

 

 

 

 

 

Edited December 28, 2024 by Peaklander

[Peaklander]

No-one? 😢

Anyone with any advice?

[marshian]

7 minutes ago, Peaklander said:

No-one? 😢

Anyone with any advice?

 

"Pump over" into the header tank is normally down to the pump having too much pressure for the circuit demand/size

 

Those Grundfos Alpha pumps have a lot of different pumping modes and they don't have a great reputation compared to the old three speed basic pumps but I can see from the first picture the old pump was running on max speed.

 

The fact that they are both fitted upside down makes my teeth itch a bit (but they are in the correct flow direction according to the arrows on the casing)

 

I tried to work out what I was looking at circuit wise and labelled up this picture

 

 

Your plumber might be right and there is a blockage somewhere but that's pretty tricky to find

 

Does the boiler behave on HW cycle ie doesn't cut out

[Mattg4321]

Had a similar problem and eventually found it was a blocked heat exchanger on my heat only boiler.
 

Bought reconditioned heat exchanger off eBay for £75 and got my friendly plumber to fit! 
 

Also sealed the system and pressurised only to just under 1 bar to reduce chances of leaks. This stopped air being drawn in and greatly reduces sludge build up too. Best thing we did. 
 

As above, does it happen when only DHW calling, as it should be flying round there? Although this can be tricky to work out as if it’s a partial blockage like mine was it appears to work on DHW as there’s little enough resistance in that circuit to get a reasonable flow

[Mattg4321]

Is the boiler a Greenstar ‘Ri’?

 

That’s what I have, so probably wouldn’t be surprising if it’s the same thing 

[Peaklander]

Thanks. On the marked up diagram it’s the flow from the boiler but otherwise OK.

Yes it’s a Greenstar Ri

I have a pic of the hex which I think shows it’s OK

The pumps are both pumping downwards.

Yes a slight pressure in a sealed system might be ok.

Maybe I should do a longer flush

I still don’t know why a restriction would cause venting. The reduced flow would be all around the circuit, unless as I say, this creates a back pressure at the pump.

Hmmm 

 

Edit: Oh and I don’t know about the DHW only. I will check but not until Wed now. 

 

Edited December 29, 2024 by Peaklander

[Mattg4321]

Pretty well known that restricted flow can cause system to pump over into feed and expansion tank. 
 

Just in case it’s the same problem and you have a friendly heating engineer who will spend a couple of hours fitting this rather than sell you a new boiler, I’ll leave this here. 3.5 years on, mine still running well. 

https://www.ebay.co.uk/itm/384909057365?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=QR3vSnEVRH6&sssrc=4429486&ssuid=088jx6SbToG&var=&widget_ver=artemis&media=COPY

 

[Peaklander]

@Mattg4321, how did you diagnose the blocked exchanger?

I don’t think that’s the problem here because a lot of heat is getting transferred to the rad circuit. 

 

[Mattg4321]

Process of elimination. 
 

It generally let enough heat through to do the DHW - especially in summer. 
 

Think it used to start playing up after boiler had been running a bit. Something moving/expanding I guess, then boiler started cycling. 
 

I cut the old heat exchanger open and it was full of c&@p

[Peaklander]

I'd like to keep this thread alive until it's fixed. I nipped to the parents' this morning. The boiler is happily supplying CH, on low flame and things are warm in all rads. There is a steady, small flow up through the vent and back down the cold feed. I didn't have time to do the suggested test with HW on and CH off but I can see that it would probably give some pointers as it eliminates all the radiator circuits. I don't know where the two returns join and I suppose there could be a restriction in the common return back to the boiler but that test seems to be a good idea.

Also I have contacted another plumbing company and a guy is meeting me on Friday morning - so more news after that I hope.

[John Carroll]

There could be a restriction where the cold feed tees into the Vent even though you are getting "recirculation", also the UPS3 is a giant of a 6.4M pump and even on fixed speed1 will still have a 4.2M head, as much if not more than the normal 6M pumps give at full speed.

You could change to constant pressure CP1 at 3.0M and see how it performs, 3 to3.5M is normally fine for most domestic systems and it may stop the pump over as it will running at over a meter lower head.

 

[John Carroll]

You say that the UPS pump inlet is from the boiler flow?, or is it from a LLH (low loss header) for the CH only, where the boiler pump is pumping into the header, the UPS is then circulating some or all of this water through the rads with their returns going back into the header, then to the boiler return?

[Peaklander]

Interesting and thanks @John Carroll

 

I don't have a pic of the header but there's only three pipes - the 22mm vent, cold water feed down in 15mm and a cold supply to the ball valve. Whilst I don't know about LLH I am pretty sure that this is a conventional header but I will do some research.

 

Now to the pump; I can't say that the CH was venting when on the old one. Plumber #1 had been (without me there) and advised changing the pump as a first job. He couldn't do it for a week and so I did it. In that week when plumber #2 came with a toolbox of moral support, he noticed the venting and we have moved from that position.

 

So that setting of CP1 you suggest is definitely an option that I can try / have in my suggestion box, when plumber #3 comes on Friday (I can't be there again until then). I know how to approach conversations with 'the trades' and it won't start with "I read on the internet"!

 

[John Carroll]

You say you have a regular boiler which  is sometines called a heat only boiler which has no internal pump so you would have a external pump like the UPS, you also say "The pumps are both pumping downwards", have you got another pump close to the boiler?, 

[Peaklander]

No, I meant that the original pump and its replacement were/are both pointing down towards the zone valves. It was in response to an earlier comment.

[John Carroll]

45 minutes ago, Peaklander said:

No, I meant that the original pump and its replacement were/are both pointing down towards the zone valves. It was in response to an earlier comment.

Ok, then one pump,  the UPS 3 on speed 1 at 4.2M  shouldn't really cause pumpover with a clean system but running at 3.0M certainly shouldn't so give it a go, it might still give sufficient circulation.

[Mattg4321]

I can’t see a low loss header in any of the pictures? 
 

I’m no plumber (electrician), but I’ve wired hundreds of boiler systems and investigated a lot of faults - although mostly electrical. Seen tens of UPS3 pumps on standard Y/S plan vented systems and never heard of one being the sole cause of pumping over. 
 

I pass the plumbing faults over to the plumbers I work alongside, but usually like to find out the cause once they’ve solved, for future reference. Pumping over very often seems to be a blockage and poor circulation. Blocked heat exchanger in boiler/blocked pipework usually where the close coupled tees where F&V pipes are, especially if this is one of those Myson aerjec things. 
 

Hopefully a plumber will see this and post soon

 

[Peaklander]

On 28/12/2024 at 17:41, Peaklander said:

Soooo, to my questions...

 

1. If there is a restriction in the circuit, isn't the pump return pressure going to be lower going out and back and so it would not send water up the vent?

That’s what I find most confusing. 

[John Carroll]

56 minutes ago, Peaklander said:

That’s what I find most confusing. 

No, any (developed) pump head is the difference between the discharge pressure and the suction pressure, the discharge pressure will depend on where the cold feed (or expansion vessel in a sealed system) is teed in, your cold feed is teed in just before the pump so assuming no blockages between there and the pump, then the pump inlet or suction head will be the height of the F&E cistern water level above the punp, so if the water level is say 2M above the pump then the pump inlet pressure  will be 2M but the pump discharge pressure will be the developed pump head + 2M, so on CC1 will be 4.2+2, 6.2M, on CC2 will be 5.2+2, 7.2M & on CC3 will be 6.4+2, 8.4M, any restriction will not change the conditions at the pump, if, for example you shut the pump discharge valve, (full restriction) the pump inlet and discharge pressures will not change, the pump speed will just ramp down to give its set (developed) pressure.

Edited January 1 by John Carroll

[John Carroll]

The system you have is well known, where, from the boiler, you have the Vent, then (no more than 150mm away) the Cold feed, then the Pump, VCP.

Your system seems to have 25MM piping at the H where the cold feed is teed in, if so then if you assume a flowrate of 15LPM, it would take a restriction (in the short piece of pipe, say 125mm), between the Vent and Cold feed) corresponding to ~ 6mm piping to cause a imbalance of 2M between the vent and  would seem very unlikely as the flowrate would then be restriced as well, but you just don't know without cutting out that whole H to check. I have seen pump over in systems with perfectly clear Hs, this can sometimes be cured by shifting the cold feed up say 100/150mm and teeing it in to the vent (and capping the top of the left top of the H, to give a combined vent and cold feed.

Edited January 2 by John Carroll

[Peaklander]

The pipework is 28mm. You can see the reducer to 22mm on the pipe to vent.

Also this system has been in place for years and the venting must be fairly recent. So it isn't a basic design / height issue.

Edited January 2 by Peaklander

[John Carroll]

It still doesn't change that "6mm" restriction calculation, have you any idea or can you measure the distance from the F&E cistern water level to the the "crook" in the vent where its turned down into the cistern.

[Peaklander]

I'm not there so it's a guess.

Ceiling is more than arms length up, so say 2.4. Header is sitting on joists so not much higher. The 150mm pipe between V & C is about 1.0 above floor. So I'd say 2.4 - 1.0 = 1.4m to header and maybe 400mm over the top of the hockey stick. So 1.8m maybe.

[Peaklander]

I still don't understand what's supposed to happen.

The pump sucks and creates pressure downstream, pushing hot water around the circuit.

This returns to the boiler and is reheated (and expands?) and comes back to the H pipework where it tries to go up to the vent but is sucked away by the pump.

 

Does the pump pull cold water from the cold feed?

If no, why doesn't it?

If yes, is that to make-up for expansion? What happens when it cools back down?

[marshian]

2 hours ago, Peaklander said:

The pipework is 28mm. You can see the reducer to 22mm on the pipe to vent.

Also this system has been in place for years and the venting must be fairly recent. So it isn't a basic design / height issue.

 

So the problem is either blockage in circuit or Pump Head exceeds system design

 

As you've recently changed the pump my money is firmly placed on the Pump "Settings" being the cause of the pump over

 

PS I was hoping @John Carroll would join this thread - he was very helpful in explaining  my system issues and as soon as the spring is here I'm moving to a combined vent and fill