Can't remove air from central heating after replacing pump

[chrixm]

I have what I believe is a fairly standard open vented Y-plan heating system with a header tank and cold water tank in the loft (I have attached a picture of what I have worked out). I am not a plumber, but I am confident with pipe work - normally I would call a plumber but with the current COVID-19 crisis I don't want to risk it as I have a vulnerable person in the house who is isolating. A few days ago I had no CH or HW - the boiler wouldn't fire as it was reporting I had no water flow. I think the pump was at fault (a Grundfos Alpha 2L 15-60) as the lights were flickering and it was making no sound when it was being powered (normally it is quite noisy). I managed to source a new Grundfos UPS3 15-60/65 and replaced it like for like - I turned off the boiler, isolated the old pump from either side and replaced it - the arrow was pointing in the same direction (towards the 3 way value).

I initially turned the pump to 3 for 30 minutes as was directed in the instructions as it was supposedly self venting with both the CH and HW on - this resulted in a huge amount of air being drawn into the system - I then turned it down to the lowest setting. I have been bleeding radiators and manual air vents constantly for the past two days, but the amount of air isn't reducing and the boiler won't stay on for any period of time. I suspect due to the amount of air in the system. It turns on and fires then goes off after 30 seconds.

I am sometimes able to keep the boiler working if I turn on CH only, but I can still hear a lot of gurgling and air running around. As soon as I turn on HW then I can hear a lot more air gushing through the system and the boiler stops working. I am hearing a lot of gurgling coming from the 15mm vent pipe (A in the diagram) as well as hot water being expunged from the 22mm vent pipe (B in the diagram). The outlet pipework from the header tank is also boiling which implies there is back pressure somewhere (it also occasionally releases bubbles into the header tank).

I am looking for suggestions of where the problem might be and what I can try? I can live without CH at this time of year, but I really need to get the HW working.

 

[John.g]

That 15mm vent from the cold feed seems a bit strange, also the cold feed is shown as teed into the 22mm vent, the correct configuration is, (from the boiler) vent, cold feed (no more than 150mm from the vent) and then pump but if your system has run without problems previously then that combined "main vent&cold feed" is probably OK.
Running that UPS3 (even though recommended) for 10 mins at full speed (3), 6.5M head, is IMO almost bound to introduce air into almost any vented system.
You say that the boiler short cycles on pump speed setting 1 (lowest) but that setting on a UPS3 is still fairly high at 4.5M head.
I would suggest trying a few of these methods. Change the pump setting to constant pressure mode 1 which is a 3M head and see if still getting pump over with HW+CH, HW only, and CH only, and continue to vent air. Initially, I would power off the boiler, power the pump from a external supply, open the 3 way valve manually and expel all the air via manual vents and rad vents and then power up the boiler again but keep the SP less than 70C, say 60C, you can always increase it later. If this sorts out your problem you may find that changing to proportional setting 2 (3.5M) may be the optimum setting.
It's also possible that the very high initial pump setting stirred up some sludge and introduced a partial blockage somewhere (worst case is where the 15mm cold feed joins the main vent) but you can eliminate the easy things first.

 

[king of pipes]

I would suspect a blockage in the cold feed unless you are handy with a bit of pipework? you will need a plumber, one thing you could try is back filling the system via a hose but you really need to know what your doing . Kop

 

[John.g]

A relatively easy check may be to isolate a d/stairs rad, open the vent to ensure valves are holding then remove the blank 1/2" plug at the other end of the rad, get someone to observe the feed&expansion tank, open one of the rad isolation valves to give a "big" flow into a bucket/dish until say 5 litres or so drained off, if the make up to the feed&expansion tank is at full flow then that should give a reasonable indication of no blockage.

 

[king of pipes]

Just connect a hose to a low drain point run it outside open the drain point then go up into the loft and observe the ball valve in the small feed and expansion the water should flow into the tank and exit freely, lock open the 3 port valve manually on the actuator as you do this . Kop

 

[chrixm]

From what I have been reading my first assumption was that I might have a blockage from the return or header tank which resulted in air being pulled in. I have now turned the pump down to constant pressure mode 1 and will check that I have free running water from the header tank into the system by draining water from the lowest point as that looks like the easiest thing to do first. I will then run the pump on its own without the boiler with the 3 way valve in MAN position and get rid of as much air as possible through radiators, etc.
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I have now confirmed that my header tank is emptying and then refilling when I drain the system from a low point. I have also had the pump running on its own for the last 3 hours with the 3 way valve set to MAN and have been constantly bleeding radiators but it seems that air keeps being dragged into the system - after 3 hours the boiler still isn't able to stay on.

With the pump on (constant pressure mode 1) and boiler off, I can still hear gurgling noises from the 15mm vent (A) on a cycle (every 10 seconds). If I look in the bottom of the header tank then I can see water being drawn down as well as being sent back in on a cycle, as I can see small amounts of sediment moving around the outlet pipe.

 

[king of pipes]

Is it pumping the correct way ? When it was swapped post a picture please . Kop

 

[chrixm]

Is it pumping the correct way ? When it was swapped post a picture please . Kop

I have attached some photographs below - the arrow on the pump is pointing towards the 3 way valve (which was completely replaced about 10 months ago as the old one was sticking). The only thing I changed on the pump was rotating the control panel 90 degrees so the front was up the correct way.

 

[John.g]

Is it pumping the correct way ? When it was swapped post a picture please . Kop

From what I have been reading my first assumption was that I might have a blockage from the return or header tank which resulted in air being pulled in. I have now turned the pump down to constant pressure mode 1 and will check that I have free running water from the header tank into the system by draining water from the lowest point as that looks like the easiest thing to do first. I will then run the pump on its own without the boiler with the 3 way valve in MAN position and get rid of as much air as possible through radiators, etc.
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I have now confirmed that my header tank is emptying and then refilling when I drain the system from a low point. I have also had the pump running on its own for the last 3 hours with the 3 way valve set to MAN and have been constantly bleeding radiators but it seems that air keeps being dragged into the system - after 3 hours the boiler still isn't able to stay on.

With the pump on (constant pressure mode 1) and boiler off, I can still hear gurgling noises from the 15mm vent (A) on a cycle (every 10 seconds). If I look in the bottom of the header tank then I can see water being drawn down as well as being sent back in on a cycle, as I can see small amounts of sediment moving around the outlet pipe.

As KOP above, just confirm that arrow is, and as you posted originally, pointing towards the three way valve. Also confirm that the pipework is connected in as highlighted in red and blue especially where connected into the main vent (blue highlight) also try and establish where the highlighted ?? pipe (black highlight) is going to/coming from. If system is as your schematic then blank off the 15mm vent with a cork or suchlike and try again. Finally, (last resort) you could drain down the whole system after tying up the ball cock and refill very slowly with the 3 way valve manually opened to both rads and cylinder if possible.
I can't figure out what that 15mm vent is doing in the system and presume the system was working OK for years?.

 

[chrixm]

As KOP above, just confirm that arrow is, and as you posted originally, pointing towards the three way valve. Also confirm that the pipework is connected in as highlighted in red and blue especially where connected into the main vent (blue highlight) also try and establish where the highlighted ?? pipe (black highlight) is going to/coming from. If system is as your schematic then blank off the 15mm vent with a cork or suchlike and try again. Finally, (last resort) you could drain down the whole system after tying up the ball cock and refill very slowly with the 3 way valve manually opened to both rads and cylinder if possible.
I can't figure out what that 15mm vent is doing in the system and presume the system was working OK for years?.

Thank you for the suggestions. The pictures I posted should show the pipe work around the pump. The 15mm at the end is coming from the header tank which is also connected to the 15mm vent - there is a T joint in the loft.

It has never been the best of central heating systems - I have always had to bleed it quite a lot until I flooded it with Fernox F1 as I believe my constant bleeding was creating more problems by introducing oxygen into the system.

I will try and work out where the black pipe goes tomorrow - I assume it might have been an old back boiler in the kitchen which is now capped off - hard to tell as all the pipe work is hidden. I might just cut it and add a couple of caps as I can’t think of what else would need a feed from the header tank?

I will also cap off the 15mm vent as we have the 22mm vent which should be sufficient. I am starting to suspect I might have introduced a blockage into the hot water zone return as my old pump probably has been on its way out for a while and I have disrupted a load of sludge running the new pump on 3.

 

[Chuck]

I can't figure out what that 15mm vent is doing in the system and presume the system was working OK for years?.

I suspect that in the past the hot water cylinder was closer to the boiler, probably above or next to it, and at some point it was moved to the current location by someone who didn't really know and/or care what they were doing.

 

[John.g]

Yes, hard to understand why a working system now won't work because of a simple circ pump change. I wonder if the T (blue highlight) where the boiler flow, the cold feed, and the main vent are connected is acting as a ejector drawing water from the feed tank which then has no where to go only up the vent?. Also, a rule of thumb used is that the top of the main vent should be 450mm above the F&E tank water level to prevent pump over.

Before taking any more drastic action suggest trying pump on (proportional pressure) PP setting 2 (3.5M max but will probably run at ~ 2/2.5M at normal circulation rate) and if still pump over/activity in the F&E tank, try PP1 which is only a 2M head max but I feel is really too low to give adequate circulation but both worth a try as its only a button press.