Discuss Hot water venting into header tank, this issue not covered on previous posts in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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Thanks guys.

Yes there is an immersion fitted, but it's never used.

JohnG and EvilDr, ok I could try that, you think the water is travelling too fast through the coil whereas I was thinking it was flowing too slow! I could try to use the red gate valve as a balancer and restrict the flow with that one, if that fails I will drain down and fit a different valve there.

Thanks.
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Just tried setting pump speed to 2, and adjusting the red gate valve further and further closed in increments, I'm afraid the water never stops coming out of the vent unless the gate valve is closed fully, which of course knackers the boiler.. back to pump speed 1 and all is ok again, so it doesn't seem that throttling the cylinder coil makes any difference.
so where is the immersion and cylinder on the diagram just curious
centralheatking
 
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You must currently have a positive pressure on the vent and negative on the cold feed and a similar (slightly less) resistance to that of the DHW circuit, if that makes sense?

I would alter the pipework to suit the following diagram. This is how we always piped open vented systems. You should have a neutral point in the system where the cold feed enters. If the vent connection is within the right distance and the height of the F&E tank and the vent are good, then there must be a reason why there is a pressure difference between those two points which is enough to cause circulation.

If you don't cure it by doing this, then there is obviously another cause but in my opinion you are better to rule out the possibility of it being something as simple as pipework configuration.

Note: Cold feed is inverted.


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OK - thanks, the only difference between this diagram and my installation is the inversion of the cold feed. Both my cold feed and vent attach to vertical pipe through standard 90 deg elbows, neither have a negative loop in them. Does the cold feed inversion make that much difference?
 

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If you was going to start messing with pipework though I'd just change it to a combined Feed and Vent. That'll sort the issue.
You could try that. It's not something I have ever done or thought a good idea but that's my personal view on it. I know people who do that and it works for them.

You could also change it to a sealed system provided the Boiler and system components are up to it.

It could be one of many things. De-zincification in the bottom connection of the brass tee where the cold feed enters could create a restriction enough to cause this issue. You need to start somewhere.

Maybe pull that section of pipework to bits and look inside, there could well be a restriction between the vent and cold feed connections.
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OK - thanks, the only difference between this diagram and my installation is the inversion of the cold feed. Both my cold feed and vent attach to vertical pipe through standard 90 deg elbows, neither have a negative loop in them. Does the cold feed inversion make that much difference?
I have known that to make a difference yes. I can't explain why but I can't say I have given it a great deal of thought.
Have you looked in the tee? and between the vent and feed connections?
 
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so where is the immersion and cylinder on the diagram just curious
centralheatking
See pics attached. Immersion is in the top of the cylinder.
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You could try that. It's not something I have ever done or thought a good idea but that's my personal view on it. I know people who do that and it works for them.

You could also change it to a sealed system provided the Boiler and system components are up to it.

It could be one of many things. De-zincification in the bottom connection of the brass tee where the cold feed enters could create a restriction enough to cause this issue. You need to start somewhere.

Maybe pull that section of pipework to bits and look inside, there could well be a restriction between the vent and cold feed connections.
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I have known that to make a difference yes. I can't explain why but I can't say I have given it a great deal of thought.
Have you looked in the tee? and between the vent and feed connections?
Not yet, was trying to sort without draining the system down for a third time this week, but I will get round to that, I could put an inversion in there at the same time.
 

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Thanks guys.

Yes there is an immersion fitted, but it's never used.

JohnG and EvilDr, ok I could try that, you think the water is travelling too fast through the coil whereas I was thinking it was flowing too slow! I could try to use the red gate valve as a balancer and restrict the flow with that one, if that fails I will drain down and fit a different valve there.

Thanks.
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Just tried setting pump speed to 2, and adjusting the red gate valve further and further closed in increments, I'm afraid the water never stops coming out of the vent unless the gate valve is closed fully, which of course knackers the boiler.. back to pump speed 1 and all is ok again, so it doesn't seem that throttling the cylinder coil makes any difference.
Why does shutting the balancing valve knacker the boiler if both CH&HW are selected?? as you said earlier that when CH only was selected on speed2 then everything was OK with no pump over or boiler problems as shutting off the balancing valve with both CH&HW selected is the same as having CH only selected?, excepting some problem with the 3 way valve?.

Are the rads heating up ok on speed1 IF CH only selected? and is the boiler "normal"?. If so then have you tried it on speed1 with the cylinder coil throttled in to ~ a 1/2 turn opened with both CH&HW on, if still ok then see if no pump over on speed1 with HW only selected.
 
Why does shutting the balancing valve knacker the boiler if both CH&HW are selected?? as you said earlier that when CH only was selected on speed2 then everything was OK with no pump over or boiler problems as shutting off the balancing valve with both CH&HW selected is the same as having CH only selected?, excepting some problem with the 3 way valve?.

Are the rads heating up ok on speed1 IF CH only selected? and is the boiler "normal"?. If so then have you tried it on speed1 with the cylinder coil throttled in to ~ a 1/2 turn opened with both CH&HW on, if still ok then see if no pump over on speed1 with HW only selected.
As the issue only happens when the HW is on (seems to be completely independent of CH on or off) I was seeing how closed the valve needed to be to stop the overflow with HW on only. You are correct if CH was on the boiler would be fine - but during my test I was restricting flow to the boiler.

Yes that is what I have tried now. Pump speed 1, cylinder coil throttled to one turn open. Boiler is OK, Rads are ok, no overflow through the vent. I need to try again in the morning from scratch as the house is already warm and we have a tank full of hot water, so I can't tell just yet if the heat up rate of the rads is acceptable or not! My wife will inform me of that first thing in the morning.. I'll report back soon.

Many thanks for all of this.
 
How is your system performing since change?.
Still not great really, pump speed 1 means the boiler is cutting out regularly due to temp difference between in and out, also the rads on cold days don't seem to get up to temperature quickly or stay there. I have also had a few instances once everything has gone off of a water surge up the vent into the header tank and out through the overflow into the garden.

So I have run a Sentinel x800 through the system over the weekend and today I am going to disassemble what I can around the vent and cold feed area, replace the 3 port valve (I bought a non honeywell version so we will see how that goes) and see if I can see anything new.

Will report back later today.
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New 3 way valve fitted. Most of the pipe work boiler side of the pump was blocked by about 50% including the tee for the vent, the blockage didn't go much further south than this and was also clear either side of the Y valve. Pump inlet and outlet around 30% also blocked with black residue.

Took it all apart (compression fittings only) cleaned out the pump and pipework as best as I could and refitted. After several hours of bleeding the system is 'relatively' happy, in that there is no longer any water coming out of the vent, regardless of CH/HW selection or pump speed.

There are still problems however, one of my rads (bathroom) I cannot get hot for love nor money, throttling back all other rads just creates too much of a restriction somewhere and the boiler has a hissy fit and shuts down in seconds. All other rads are getting hot fine. So there must still be an airlock somewhere, or another restriction/blockage in the CH pipework restricting flow. Or my boiler settings are just plain wrong for my house...

Same happens if I select pump speed 1 and all rads are throttled normally, boiler heats up too quick and shuts down.

One step forwards - 2 steps back!
 
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You may find that the pump ports & pump impeller are now full of crap as well, if you shut off the pump isolation valves you should be able to remove the pump head to clean out the impeller and while the head is removed you should backflush the pump ports by alternatively opening/shutting the isolation valves although you did say above that you cleaned it out as best you could.
I hope not, but there is now a possibility that the boiler HX is partially blocked as well, when you do sort it out you might consider fitting a magnetic filter before the boiler inlet, if one not already fitted.
 
You may find that the pump ports & pump impeller are now full of crap as well, if you shut off the pump isolation valves you should be able to remove the pump head to clean out the impeller and while the head is removed you should backflush the pump ports by alternatively opening/shutting the isolation valves although you did say above that you cleaned it out as best you could.
I hope not, but there is now a possibility that the boiler HX is partially blocked as well, when you do sort it out you might consider fitting a magnetic filter before the boiler inlet, if one not already fitted.
yes - the pump ports I have cleaned as best as I can. the pump impeller was new only a year ago along with a power flush. Can I do something about the boiler HX? (I have a Vaillant ecoTEC plus now) I do have a magnaclean unit fitted, I have cleaned this out a few times over the past 2 weeks and from there being a fair amount on there to begin with it's been relatively clean since then. I think I am suffering ultimately with a cascade and cumulative affect of bad jobs - I have a mishmash of 15mm and microbore installed already when I bought the house 15 years ago (the bathroom rad is on the latter) and when I first moved in the plumbers who fitted the new boiler in the garage (back boiler in the living room exploded 2 weeks in to my new residence) re-jigged the system and managed to get a couple of pipes mixed up - nonthewiser 4 years later this came home to roost with a completely blocked system which knackered the Worcester Bosch boiler completely. A lot of the microbore is under concrete flooring so I can do little about that, but I guess in the spring I can try to replace some of it. Failing that I am out of ideas.
 
If the magnetic filter was installed before or during the new boiler install then one might think that the Hx is OK.
You say that the pump impeller was renewed a year ago, was a whole new pump installed or just the pump head/impeller? if you found a reasonable build up in the pump ports then I would still remove the head and inspect/clean the impeller, its not a big job if the isol valves are holding.
 
If the magnetic filter was installed before or during the new boiler install then one might think that the Hx is OK.
You say that the pump impeller was renewed a year ago, was a whole new pump installed or just the pump head/impeller? if you found a reasonable build up in the pump ports then I would still remove the head and inspect/clean the impeller, its not a big job if the isol valves are holding.
yeah the magnaclean was fitted before my last boiler - fingers crossed that has done the job. As of now the system is sat happily on pump speed 2, 70 deg rad temp and everything hot aside from the bathroom rad. So I think the HX is ok.

Yes just the head was changed last year - I may have a go at that then tomorrow, when I removed the head it wasn't instantly clear to me how I got to the impeller - i'll have to have a look. My isolation valves are old - I may have to drain down again....
 
Your photo shows gate (wheel) valves so should be OK for isolation as you can tighten down on the glands if they (glands) leak but if the system is operating OK now on speed 2? or are you still getting problems with boiler tripping on o/heat on start up from cold. Don't know how your boiler operates but some now measure the delta T across the heat exchanger and if its greater > 30C will trip the boiler irrespective of the flow temperature.
If you have a ABV fitted ensure its index setting is correct.
 
Your photo shows gate (wheel) valves so should be OK for isolation as you can tighten down on the glands if they (glands) leak but if the system is operating OK now on speed 2? or are you still getting problems with boiler tripping on o/heat on start up from cold. Don't know how your boiler operates but some now measure the delta T across the heat exchanger and if its greater > 30C will trip the boiler irrespective of the flow temperature.
If you have a ABV fitted ensure its index setting is correct.
System operating ok on pump speed 2 aside from the one cold rad.. I'll see what it does in the morning from cold - but right now it's at steady state. I'm pretty sure I don't have an ABV fitted....

Yes that's what happens - the current temp rises too quickly in relation to the circulating water, I would say it is set to around 30 deg looking at when it trips out yes.

Some pics attached of the state of the blockage and the pump housing.
 

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I would definitely be of the opinion that if the pump head/impeller was installed during this build up that the impeller eye/vanes could be some what occluded, I include a picture of my 18 year old Salmson pump head which was still running perfectly after 18 years and is still in spotless condition used in a vented system (with the odd drop of inhibitor thrown in) so when/if you remove the pump head you should be able to see the impeller and make sure and clean out any deposits between the vanes and also the impeller eye.
Don,t know what your boiler output is but assuming a maximum deltaT of 25C then 15kw needs a circ flow of 8.6 LPM, 20kw 11.5 LPM, 25 kw 14.3 LPM and 30 kw 17.2 LPM, all at full firing.
If you have a UPS2 6M (60) pump running on speed2 which makes it a 5M pump and if you look at the pump curves or confirm that it is a ups 2 then if CH only on from cold that it might be a bit of a struggle to circulate enough water to keep the temp at 25C with anything bigger than a 20kw boiler but if you can also (first thing in the mornings) have both CH & HW on together until the boiler cuts out normally at its setpoint temperature (say 30 minutes or so) then one would expect plenty of circulation.
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A view of the impeller eye (not a great photo)
 

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I would definitely be of the opinion that if the pump head/impeller was installed during this build up that the impeller eye/vanes could be some what occluded, I include a picture of my 18 year old Salmson pump head which was still running perfectly after 18 years and is still in spotless condition used in a vented system (with the odd drop of inhibitor thrown in) so when/if you remove the pump head you should be able to see the impeller and make sure and clean out any deposits between the vanes and also the impeller eye.
Don,t know what your boiler output is but assuming a maximum deltaT of 25C then 15kw needs a circ flow of 8.6 LPM, 20kw 11.5 LPM, 25 kw 14.3 LPM and 30 kw 17.2 LPM, all at full firing.
If you have a UPS2 6M (60) pump running on speed2 which makes it a 5M pump and if you look at the pump curves or confirm that it is a ups 2 then if CH only on from cold that it might be a bit of a struggle to circulate enough water to keep the temp at 25C with anything bigger than a 20kw boiler but if you can also (first thing in the mornings) have both CH & HW on together until the boiler cuts out normally at its setpoint temperature (say 30 minutes or so) then one would expect plenty of circulation.
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A view of the impeller eye (not a great photo)
That's clean for 18 years! I will give the impeller some TLC.

This morning the house seemed to heat up ok all aside from that one rad again. The boiler trips out most often with overshoot I think - if the current temp exceeds the target by more than 5 deg it shuts down for a few mins.

The boiler is an 18KW unit, I have rated it down using a diagnostic command to 11Kw as I only have 9 radiators and I thought if it is putting less energy into the water it will not trip up so often. the temp increase should be steadier.. The pump is a UPS2.
 
OK then, a few calcs might show something. a 11 kw boiler deltaT of 20C equals to a circulation flow of 8 LPM which the ups2 will provide at a 5m head with a clean impeller and I would be surprised if the flowrate was less than this but will use it as a worst case. Now if we also assume that your rad system contents+boiler are 50 Litres (worst minimum case again) then if the boiler set point is 70C once the water contents reach 50C return then the boiler should start modulating down as the deltaT through the boiler is 20C and 50+20 = 70C. So assuming a "cold" water start up temp of 20c it should take ~ 16 mins (but probably 20 mins or so because the rads will be emitting heat) for the boiler to reach its modulation temperature of 70C. If one assumes that the rads are emitting a minimum of 7 kw then the boiler has a net input of (11-7), 4 kw, to heat the 50 litres from 70C to 75C if one assumes that the modulation is very sluggish. It would take a minimum of 4.4 minutes to achieve this and boiler cut out, one would think that even this worst case of 4.4 minutes should be ample time for the boiler to modulate down? and assuming a turn down ratio of 4:1 then the 18 kw rated boiler should have a max turn down to 4.5 kwh and continue to fire as the demand is > this at ~ 7 kw.
My daughter has a Vokera vision 20S that is not range rated, I watched it a few years ago when it fired up from cold to only one zone with only 3 rads on it and it came up to its setpoint temp of 70C in a "frightening" time of < 5 minutes but only overshot the setpoint by 1C before it reached its modulated requirement., as far as I can recall it actually started modulating shortly before it reached the set point.

Apoligies for all the sums but its the only way I can figure these things out so does this point to a (still) circulation problem or a fouled Hx or a sluggish boiler modulation problem?, its a new boiler so one would think unlikely control problem but one never knows.

Re the cold rad, have you tried venting it with boiler+pump off and with the vent completely removed. Also if you know which pipe is the flow, you could try venting with the boiler/pump on and flow v/v open with return closed and vent completely removed. But no doubt you probably have tried all these knacks,
 
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OK then, a few calcs might show something. a 11 kw boiler deltaT of 20C equals to a circulation flow of 8 LPM which the ups2 will provide at a 5m head with a clean impeller and I would be surprised if the flowrate was less than this but will use it as a worst case. Now if we also assume that your rad system contents+boiler are 50 Litres (worst minimum case again) then if the boiler set point is 70C once the water contents reach 50C return then the boiler should start modulating down as the deltaT through the boiler is 20C and 50+20 = 70C. So assuming a "cold" water start up temp of 20c it should take ~ 16 mins (but probably 20 mins or so because the rads will be emitting heat) for the boiler to reach its modulation temperature of 70C. If one assumes that the rads are emitting a minimum of 7 kw then the boiler has a net input of (11-7), 5 kw, to heat the 50 litres from 70C to 75C if one assumes that the modulation is very sluggish. It would take a minimum of 3.5 minutes to achieve this and boiler cut out, one would think that even this worst case of 3.5 minutes should be ample time for the boiler to modulate down? and assuming a turn down ratio of 4:1 then the 18 kw rated boiler should have a max turn down to 4.5 kwh and continue to fire as the demand is > this at ~ 7 kw.
My daughter has a Vokera vision 20S that is not range rated, I watched it a few years ago when it fired up from cold to only one zone with only 3 rads on it and it came up to its setpoint temp of 70C in a "frightening" time of < 5 minutes but only overshot the setpoint by 1C before it reached its modulated requirement., as far as I can recall it actually started modulating shortly before it reached the set point.

Apoligies for all the sums but its the only way I can figure these things out so does this point to a (still) circulation problem or a fouled Hx or a sluggish boiler modulation problem?, its a new boiler so one would think unlikely control problem but one never knows.

Re the cold rad, have you tried venting it with boiler+pump off and with the vent completely removed. Also if you know which pipe is the flow, you could try venting with the boiler/pump on and flow v/v open with return closed and vent completely removed. But no doubt you probably have tried all these knacks,
Thanks John, for all of your efforts with this, they are appreciated.

The sums make a fair amount of sense, there is a modulation that starts on first heat up just before it reaches set point, but as I mentioned earlier, often the temp overshoots by 5 deg and triggers a 'cut out and wait function' If the 5 deg overshoot doesn't happen then the modulation seems to work ok. This happens on pump speed 2 for example whereas with pump speed 1 the boiler was always overshooting by >5 deg on any heat cycle so it didn't settle to being able to modulate.

So I still think it's a circulation problem.. giving the boiler the benefit of the doubt.

The cold rad bleeds perfectly well under gravity, and drains down ok. There is no air in it at all and both valves are fully open to try to provide a flow path of least resistance. The feed pipe to the rad gets as hot as the rest of the system very quickly, but the rad stays cold just north of the inlet valve. Which suggested to me a dodgy valve or a blockage at the rad inlet, but eventually (probably an hour later) the rad then does heat up very slowly - so there MUST be SOME circulation there somehow. I will try siphoning off some water from the bleed screw with the inlet open and the outlet closed to see if I get any better flow and heat ingress.
 
Yes, probably well worth inspecting the pump impeller to rule it in/out for ever.
One quite accurate way of measuring the flow rate even though a PITA is to measure the pump power in watts (assuming you have the smart ups2) if you have one of those cheap energy monitors you can put (temporarily) a 3 pin plug on the pump lead and just plug it into a extension lead and then run the boiler as normal for a few minutes. If the power is > 35w on speed 2 change to speed 1 and if this power is > 23w change to PP3 mode and write down the power required. If the power is < 35W on speed2 or < 23W on speed1 you can just read the flow off the pump curves in the manual, if you have to run on PP3 for a few minutes just post the power output and I can calculate the head/flow. Some of these pumps now including one version of the UPS2 alpha 3 actually show the flowrate, my own one, a Wilo Yonos Pico 6m displays the power and its very easy to read the flowrate off its pump curves.
 
Still very odd - I can easily draw water out of the dodgy rad with either inlet or outlet closed and the boiler and pump running. Still the rad doesn't get hot. So it must be serviceable through gravity from the header tank, but not via the CH pump... can't get my head around that yet, water just doesn't want to flow in that part of the circuit, and yet the inlet pipe gets proper hot..

I have a smart energy meter (not a smart ups2) and noted the following power consumption for each pump speed:-

1 = 26W
2 = 37W
3 = 50W
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