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Who's right ? (Read 23016 times)
thescruff
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Re: Who's right ?
Reply #17 - Jan 9th, 2018, 12:41pm
 
That looks 1st class, one should assume he knows what he's doing.

Ask the guy where the air is coming from and what he can do. Most of the air should be out by now.

Check the header tank when the heating is on, is the ball valve dribbling or pumping over the vent.
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londonman
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Re: Who's right ?
Reply #18 - Jan 9th, 2018, 4:33pm
 
Yes, I thought so to.

On another forum I was asked some very cogent questions,  and some that I couldn't answer.  For example, why would air 'block' the flow ?  So I did a bit of Googling to find out why and came across this...

In a well designed low pressure water pipe system, air should clear naturally even if you run out of water. As the pipe system fills again, the air should naturally rise to the top and be expelled. Air locks happen when the pipe system is not well designed.

Air bubbles rise to the top because air is much lighter than water. In a correctly installed low pressure pipe there is a natural "fall" towards the taps. Friction between the water and the pipe walls provides a resistance to the flow. The fall in the pipe is needed to overcome the resistance.

If the fall in the pipe is continuous, any trapped air going the other way in the pipe will be following a natural rise. Given a little time it should bubble back into the cold tank or, if it's a hot water pipe, bubble back towards the cylinder and up the open vent over the cold tank.

Resistance increases with the length of the pipe and long horizontal sections can be a problem because they have no fall.  Short horizontal sections of pipe are not normally a problem.


And therein lies the problem, I think.  There are two pairs of 22mm pipes that run 'horizontally' down the length of the house.  One pair were the flow and return for the radiators and the other air the flow and return to the HW cylinder.  I don't recall exactly but I have a vague recollection that they may well have risen slightly from the old pump end.

Now with the new modified layout we have a bit of a rollercoaster.  I wonder if this is the cause ?

...

Having aid that the cure has always been to bleed the return side of things and we've not touched that but the fact remains that for part of the radiator circuit the flow is in the opposite direction to what it was and most likely downhill.

The article then goes on...although I admit I don't follow it 100%

Pipes which rise towards the taps are a big problem. However, we can ignore the last short section of pipe connecting to the tap which nearly always rises vertically. Any air going to the top of that section is simply passed out through the tap. If a low pressure pipe rises on its way towards the taps, any air going the other way would have to go downhill. Because air is lighter than water it gets trapped at the top of the rise in the pipe. This air pocket adds a lot of resistance to the flow of water. If there is not sufficient overall fall along the length of the pipe to overcome that extra resistance, the pipe becomes air locked.
When an air lock forms, the flow of water from the taps may reduce dramatically and can sometimes stop completely. If the air lock is in a cold supply pipe it may also stop the loo filling.
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thescruff
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Re: Who's right ?
Reply #19 - Jan 9th, 2018, 6:47pm
 
where the pipes rise there should be air vents to take the air out at high levels.

You'd need to see it to decide where is best.
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londonman
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Re: Who's right ?
Reply #20 - Jan 9th, 2018, 9:08pm
 
Mmm...intriguing.  Thing is the rad that I bleed to fix the problem is the one furthest away from the pump ie 'downhill'.
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thescruff
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Re: Who's right ?
Reply #21 - Jan 10th, 2018, 8:46pm
 
If the air gets to the rads then it is likely to collect the air and you may find one rad collects more than the others. that is normal although once the air is out it should stay out.

Have you got an inhibitor in the system.
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londonman
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Re: Who's right ?
Reply #22 - Jan 11th, 2018, 1:00pm
 
thescruff wrote on Jan 10th, 2018, 8:46pm:
If the air gets to the rads then it is likely to collect the air and you may find one rad collects more than the others. that is normal although once the air is out it should stay out.

Have you got an inhibitor in the system.


Yes I do but given the amount of bleeding I will be topping it up.
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thescruff
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Re: Who's right ?
Reply #23 - Jan 12th, 2018, 12:29pm
 
give or take 2-3 weeks all the air should be out unless you have a system fault, which is why I'd like you to check the header tank as per my earlier posts.
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londonman
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Re: Who's right ?
Reply #24 - Jan 13th, 2018, 11:46am
 
thescruff wrote on Jan 9th, 2018, 1:24am:
Back too one of my earlier post.

You have a header tank for the heating, hold a jar of water under the vent, turn the pump on and off a few times, does it drink the water or blow bubbles.

Can you post a pic of the neutral point.



OK.

Switch the pump off and air bubbles come out.

Pump back on and the water gets sucked into the vent pipe.

I'm not sure what I was expecting....part of me was thinking nothing hence the 'neutral point'.

EDIT:  OK, think I understand what's happening and it is as expected.  We've got a body of water circulating by virtue of the pump running. When the pump stops, the momentum of that water pushed up the vent pipe a bit ...pushing out air in the downspout of the vent pipe (the bit hanging over the header tank).

When the pump starts up again there is a momentary low pressure at the neutral point as the pump starts sucking and the inertia of the water in the CH circuit takes a tad to get moving again.  So that sucks water into the vent pipe.

Ergo the pump is running the right way round.

Do I get a gold star ? Grin
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thescruff
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Re: Who's right ?
Reply #25 - Jan 13th, 2018, 10:06pm
 
No gold star only for effort and finding a problem.

The neutral point isn't neutral as nothing should happen.

The Plumber needs to come back and sort it.

If the engineer wants help get him to improve on the sketch.
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thescruff
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Re: Who's right ?
Reply #26 - Jan 13th, 2018, 10:16pm
 
Separate post to avoid confusion.

A heating pump doesn't pump, it circulates at a high velocity, You would find near as a 1bar pressure differential across the inlet and outlet of the pump.

The bubbles when the pump stops is because the pump is sucking air in when it's running
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londonman
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Re: Who's right ?
Reply #27 - Jan 13th, 2018, 10:22pm
 
thescruff wrote on Jan 13th, 2018, 10:16pm:
Separate post to avoid confusion.

......
The bubbles when the pump stops is because the pump is sucking air in when it's running


Can you please clarify this one for me as I'm struggling to understand this.  Are you saying that there is constant air being drawn into the system and escaping via the vent pipe while the pump is running ?
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Re: Who's right ?
Reply #28 - Jan 13th, 2018, 10:30pm
 
Sort of, you have a problem with air in the system, now you know where it's coming from.

Maybe the pump is at the wrong speed, the connections are wrong, the tank and or vent is too low, but that should make any difference if the neutral point is correct.

To comment further one needs information, and preferably a good as fitted drawing, or seeing the job which is unlikely.

If it wasn't new one would say the cold feed is blocked.
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« Last Edit: Jan 13th, 2018, 10:34pm by thescruff »  
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Re: Who's right ?
Reply #29 - Jan 13th, 2018, 10:58pm
 
Image 4   

A little fag package sketch I knocked out for you.

Cold feed and vent should be as close as possible. When the pump starts and shortfall should take water from the cold feed and NOT the vent, doing the later highlights a problem, normally a blocked or semi-blocked cold feed.
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Re: Who's right ?
Reply #30 - Jan 13th, 2018, 11:09pm
 
Image 5   

Assuming the flow/pump is the direction marked, then the problem as said is likely in the circle, eg blocked/restricted cold feed.
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Re: Who's right ?
Reply #31 - Jan 14th, 2018, 2:32am
 
Does it make any difference if the cold feed is 15mm and the vent 22mm ?
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Re: Who's right ?
Reply #32 - Jan 14th, 2018, 8:57am
 
No that is the normal combination, bigger systems could be 22/28.
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Re: Who's right ?
Reply #33 - Jan 21st, 2018, 12:04pm
 
Just thought I'd give an update and a drawing of the system.

...House ch layout

Where it says 'other rads' these are upstairs ones.  Flow from heatstore is hidden by the heatstore return pipe.

The radiator marked 'bleed' rad is approximately in the right position and is the one (a) highest in the circuit and (b) furthest rad upstairs from the pump.  My plumber called me up yesterday to see how things were going as he has a free day tomorrow. We agreed that I'd try switching off the pump to see what happened.  Answer....an airlock when I started it up again.

I have a permanently connected drain cock in place of the bleed valve on the 'bleed' rad.  So connect up a hose and open the lockshield.  Some air bubbles and then nothing.  No more air.  No water.  Remember the system has been working fine and that radiator was hot and so has water in it.  So there's not enough head to push out any water/air in the pipes..  Unless ....unless  :idea:   you switch the pump off when you bleed, yes ?  In that case doesn't the pump act as a block to the flow...which explains why nothing comes out ?  But then surely the head in the tank is still acting to push the water out as it's acting back through the flow from the heat store, through the heat store and back up the return to that bleed rad...so it should vent.  You have to stick suction on the end of the hose to get it flowing again,whereupon air and water comes out of the hose/lockshield route.

This time was also different in that when I closed the lockshield and opened the flow on the rad, I got a lot of air out.  Very confusing
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