Position of cold feed and vent

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Problem
The problem being discussed here is the “correct” position of the vent and the cold feed. The other information, about this system and its problems, is background reading. Still, I'd also welcome any suggestions about dealing with those problems.

If CH, but not HW, is on, the boiler heats up fast, cuts out fast and starts up again many minutes later. The cutting out tends to be accompanied by air swooshing through the pump. If HW is on, the cycling is more leisurely and not accompanied by air.

Air seems to be entering the system, at least when the pump turns off. This is so for any pump setting but possibly less so for lower pump settings.

System
Pumped open vent system. However:
- The (22 mm) vent enters the boiler at the top left hand side.
- The (22 mm) flow leaves the boiler at the top right hand side.
- The pump is on the flow.
- The (22 mm) return enters the boiler at the bottom left hand side.
- The (15 mm) cold feed joins the return.

About six heating engineers have looked at the system. Their opinions on the position of the vent, on the position of the cold feed, and on combining the cold feed with the vent near the F&E tank range from “That’s probably causing problems” to “That’s the best place for it”. In this forum, too, there seem to be differences of opinion about at least combining the cold feed with the vent.

I think that the cold feed used to join the flow but was then replaced (to remove scale from it) and joined to the return.

Suggested diagnosis and cure
There are restrictions in the pipework: even after power flushing magnets stay attached to short stretches of the flow (between the pump and the CH motorised valve) and the return (before the join with the cold feed, but not at or in the cold feed). These restrictions (and possibly faults with the calling by the motorised valves) are likely to be partly responsible for the boiler cutting out.

Air could be entering the system if water see-saws into and out of the F&E tank: the water level in the vent would fall when the pump is running and water would be pulled in from the F&E tank when the pump turns off. However, sometimes the water in the F&E tank is warm, so there may also be venting.

There is evidence that the CH motorised valve does not always send live to the boiler when it is open. This might account for the time taken by the boiler to turn on after it turns off.

I don’t live in the house and for various other reasons I can’t conveniently carry out many tests (such as checking that air is sucked in at the top of the vent, or seeing exactly when the water in the F&E tank is warm). However, I intend at the weekend to have the restricted sections of the pipework replaced for the benefit of the tenants. I also intend at least to have the cold feed moved to the flow, to reduce see-sawing.

Questions
Should the vent be moved so that it is between the boiler and the pump?
Should the cold feed be moved so that it is between the vent and the pump?
Should the vent and cold feed be combined?
Why is there supposed to be at most 150mm between the vent and the cold feed?
Why is there supposed to be at least 450mm (perhaps half an atmosphere) between the top of the vent and the water level in the F&E tank?
 
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If magnets still stick, it is unlikely the power flushing was done correctly.
In any case, f&e pipes in this setup will not be flush unless you cut them out and connect them seperately.

Position of f&e pipes ideally 100 mm apart and in direct pipe to the pump, on INLET side of pump.
On either side of the boiler would not be my preferred option, but doubt that it will cause problems if the rest is ok.

If easy to do, replace f&e pipes and stick them in the right place.

450 mm water column is 0.045 bar, not half a bar. In general, the higher the better, this is to minimise air being sucked in and water being sprayed into the tank.
This is why the f&e should be close together, the closer, the less pressure difference
 
Thanks, bengasman
I don't know what made me write 'half an atmosphere' and be out by a factor of ten.
As for power flushing, I thought that deposits could be too hard to be removed (except perhaps by strongly acidic solutions, which might not be appreciated by old radiators and pipes).
 
Thanks, bengasman


As for power flushing, I thought that deposits could be too hard to be removed (except perhaps by strongly acidic solutions, which might not be appreciated by old radiators and pipes).

old boilers with big lumps of cast iron inside simply love acid; it is the new one that can't stand it.

the biggest advantage of powerflushing is that it allows the use of phosphoric acid, which is what is most commonly used.
it dissolves limescale and corrosion nicely and does little to copper and steel.

as for the factor 10 mistake, don't worry, it will get better the moment you stop dying your hair blond.
 
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Hi, bengasman
Position of f&e pipes ideally 100 mm apart and in direct pipe to the pump, on INLET side of pump.
On either side of the boiler would not be my preferred option, but doubt that it will cause problems if the rest is ok.
As being on either side of the boiler (about 400 mm) is all right, what is the purpose of the 150 mm often quoted as the best maximum distance apart?
it will get better the moment you stop dying your hair blond.
My hair is going white through trying to reconcile the opinions of the heating engineers that have seen the system.
 
as far as i know the distance between feed and vent pipe is a bit arbitrary.
the whole point is to have as little resistance as possible between them.

if you have a boiler in between, you have a temperature difference as well as a pressure difference that COULD disturb things

there is no law or regulation that says it MUST be a certain distance, and you can even combine the feed and vent pipe in certain cases.

it is all a bit along the lines of should you put a rad under the window or not.

that is why i said, replace the pipes if it is not too hard.

have you checked for yourself that the f&e are on the inlet side of the pump?

as for the hair, it ain't easy to get a good engineer to come out. lot of guys out there that know little more than the basics, as seemed to have been proven by the powerflushers.
the good news is that your problem is essentially easy, and if you follow the whole thing step by step through, it will be solved, and it won't be rocket science. just a process of elimination.
start at the beginning, and look for faults. every fault you find, fix it, whether you think it has something to do with it or not.
 
Keeping the feed and vent close together is supposed to minimise any pressure difference between the connections, so it is less likely to set up a circulation between the two, either drawing air down the vent, or pumping over into the F/E tank.

If there is any restriction where the tees are connected, they may start to behave like a venturi, generating localised low pressure areas inside the tee, and starting the pumping over or drawing air into the flow.
 
In John Reginald's book "Central Heating Fault Finding & Repair" he has the optimum as:

Boiler >Feed >Vent >Pump - all on the flow
 
Thanks, guys. This all very useful. I've now decided that moving the looks slightly tricky. If the vent stays where it is (instead of moving from the left hand side of the boiler to the right hand side, where the flow is), should the feed still move to the flow? (It's currently on the return, just before that enters the boiler.) I guess the answer is "yes, if doing it is simple".

Thanks for the reference to Reginald's book, which I'll look for.

Though the book says Boiler> Feed> Vent> Pump, http://www.fernox.com/?cccpage=air_in_CHS&sub=2 (which discusses see-sawing), for example, says Boiler> Vent> Feed> Pump. I expect that, as others said, the intention is to keep the pressures (and the temperatures) in the vent and the feed as similar as possible. Making the diameters of the vent and the feed the same would make the velocities in them as similar as possible, but this mightn't matter and doesn't seem to be suggested generally.


I must also try to think where a Venturi effect could be drawing air in.
[/quote]
 
I've now looked at Reginald's book. I think it says Boiler> Vent> Feed> Pump, just like other sources. However, it also says that this is for high resistance boilers and that the boiler must have an overheat cut-out (besides the boiler thermostat, I think).

The book says that a low resistance boiler (floor-standing, with a cast iron heat exchanger) works well with the vent on the flow (before the pump) and the feed on the return (after the HW and CH). My boiler is such a boiler and has no overheat cut-out. Have you any comments?

This is also illustrated in http://randall.danfoss.com/pcmpdf/introducing controls.pdf, which also remarks that the vent and feed should only be combined with the approval of the boiler maker. I've heard claims that some boiler makers favour combining them.
 
you need to cut and clear/replace the feed and vent and probly have the system flushed properly. id dont think the guys you had in knew what they were up too. sorry.
 
I've now looked at Reginald's book. I think it says Boiler> Vent> Feed> Pump, just like other sources. However, it also says that this is for high resistance boilers and that the boiler must have an overheat cut-out (besides the boiler thermostat, I think).

.

I understand.

The book has 5 positions with diagrams. Position 5 has boiler>feed>vent>pump and this is suggested as the optimum...

UNLESS

"the boiler manufacturer permits (or recommends) a close coupled vent and feed arrangement" and then it is vent before feed... it does go on to say that this is the best layout for modern boilers (and gap between feed and pump must be less than 150mm).
 
The book has 5 positions with diagrams. Position 5 has boiler>feed>vent>pump and this is suggested as the optimum...

sorry but htis must have been written in the fifites or by bigburner.

this combo will allow air to be drawn into the system.

boiler vent feed pump.


pump sucks behind it, pushes in fron t of it. air vent provieds a pressure break and pressure is 'neautral between feed and vent(dont understand that bit myself but ill blindly believe it). this means that when the pump kicks in and sucks on the water behind it if it sucks on anything it'll be the header tank and thus only draw a small amount of water. if the vent was behind then there is a chance that it would introduce air to the system and trigger the rust/sludge problem.
the only othe combo i'd find acceptable is combined feed and vent but this must only be used with 22mm pipe and boiler must have an overheat stat.
 

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