Hi all Have had a few happy weeks replacing the gas valve-boiler thermostat and one motorised valve in my 1995 open circuit central heating system. I now find the pump persists in getting air in it despite repeated system bleeding.The local Corgi man announced that my system will always drag air in.Its a conventional below the floor bungalow system with hot water tank at floor level and top up tank in the attic.The pump is mounted at the top of the boiler and the expansion pipe comes from a lower boiler tapping top up is achieved by connection to hot water circuit at the inlet side. I fail to understand where the air can gain access. Can anyone clarify
This can happen if the vent pipe is on one side of the pump and the feed/expansion pipe on the other, or simply if they are connected to the circuit sufficiently far apart that a pressure differential is created. The normal recommendation is 150mm max between the places where these two pipes join the circuit.
If you are having difficulty, one solution is the Myson Aerjec, which is a separator device that you put on the circuit and connect feed/expansion and vent pipes to it. This brings everything close together and also makes it easier for air etc to escape.
Another possibility is that the gas is actually hydrogen resulting from corrosion. You are using inhibitor, aren't you?
Pumps are meant to be self purging, but will be less likely to trap air or other gases if mounted with pipe vertically.
hi,yer corgi man is at it,if your expansion pipe is coming off th bottom tapping on your boiler thats completely wrong for a start, an will not work at all in the result of overheating, thermostat failur etc, if the system is full of water the pump should create n problems, make sure when you bleed the system the header tank in th loft is filling, if it fails to fill, the feed to the heating syste could be blocked which is quite common. meaning that your bleeding th system with no water replacing it and moving existing water around th system, failing that try an AAV (auto air valve) in the system. pace
There is a choice of tappings 2 at the top and 2 at the bottom should the expansion pipe be from the top. The boiler was installed by heating engineers-not D-I-Y me. if the system is full of water the pump should create no
The attic top up tank is filling and supplying ok. The fill enters the system at the domestic hot water cylinder which is 7 feet from the expansion pipe-both are on the supply side of the pump-could air be drawn in here. Where do I fit the automatic air bleeder.
You shouldn't normally need an automatic air bleeder, really Thomas.
I think you are using the term "expansion pipe" where it is really the vent pipe. This is the one at the boiler, yes? The expansion is dealt with through the feed/expansion pipe (which you are calling the "fill").
At any rate, if you have 2m or so of pipe between the two, then this can easily lead to air being drawn in, especially if the vent pipe is closer to the pump inlet than the feed pipe. This is because there will be a lower pressure at the point where the vent joins than that at the F/E connection point.
One thing that you could try is to reduce the pump speed. This may reduce the pressure differential to a point where air is no longer sucked in. On the other hand, it may result in heat not being transferred fast enough from the boiler and the house not warming up enough.
A better solution would be to correct the plumbing. The vent pipe and the feed/exp. pipe should be connected at a point where there is a clear path from the boiler (i.e. no motorised valves in the way), and close together. Ideally, that whould also be a high point to allow any air to escape through normal flow and rising.
It's certainly not correct to have one going to the bottom of the boiler and the other to an arbitrary point somewhere else. You have places where air can be trapped.
One solution (I had this before converting my system to sealed operation), is to take the flow pipe from the boiler, then in the airing cupboard run it through an air separator (as described in previous post), to which F/E and vent are connected; then to the pump and following that to motorised valve(s) and the rest of the system.
Other combinations are possible. The key things are:
- nothing in the way between boiler and vent
- vent and F/E pipes close together and ideally connected at a high point to allow air to escape.
ldn't normally need an automatic air bleeder, really Thomas.
It is in 22mil copper goes vertically through the roof.
The vent pipe is 2 feet max from the pump the expansion pipe is more than 6 feet from the pump and now that I think about it its on the opposite side from the vent ie the vent is on the pump feed side and expansion is on the pumped side.The expansion also has a motorised valve between it and the boiler.
Many thanks for your detailed reply Andy I can see that a bit of redesign is needed here Im adding radiators anyway so will just readjust the expansion and vent system at the same time. Does nt the closed system not give a lot of problems-- short boiler life-top up problems-not enough hot water.
This sounds a bit like a problem I had a month or so ago caused by sections without bleed facilities. Our solution after a week of fighting it was so crude I'm embarrassed to tell you:
- stop the pump:
- bleed it.
- Disconnect boiler heating to prevent overheating (those extra switches were SO useful.
- Open the bleed screw on the pump just a little. Run it with cold water in the system and the air simply 'fizzles' out.
- when air stops bubbling, turn boiler on.
Boy did that air shoot out!! Loads of it with only about a teacup of water lost. Took about 5 minutes to purge it and the system is still running fine (correction: it was when I left this morning!).
Good luck. Tell us your eventual solution or any new tricks!
Oh dear, Thomas. The vent pipe should go up and loop over the heating feed/expansion tank.
OK, so there's an even bigger reason. You are putting the whole pump head and the pressure differential through the pipe in play, so not surprising that air is entering.
That's not good either. Both the expansion and vent pipes should have a clear path to the boiler.
Good opportunity. You might also want to look at giving the system a thorough flush and clean.
Do you mean sealed CH primary? No, not at all. Take a look at Ed Sirett's FAQ on sealed systems - link in his signature. However, check if your boiler is suitable. Is it actually 1955? If so, it probably isn't anyway.
On your other comments, you may be thinking about condensing and combi boilers of which there are one or two threads in this NG.
Briefly...... Early UK designed and made condensing boilers tended to be done by adding a secondary heat exchanger to an existing design. The problem is that the condensate is mildly acidic and if the design doesn't account for it, the boiler corrodes.
Filling of sealed systems is not an issue as long as there are no leaks. More in Ed's FAQ
Combi boilers have often been fitted as a one box solution. This can be fine, but the pitfalls for the unwary are if the cold water supply is inadequate and/or the boiler power too small. Some people like mains pressure hot water. If you like that and the water flow is good, then a thermal store is a good option and can produce HW at a good rate as well. If you are tight on space and have modest to average water requirements, a combi could be interesting. If you need a lot of HW - e.g. larger house, >2 people, 2 or more bathrooms with baths and showers then some form of storage will perform better.
Its been through the roof before my ownership of 20 +years house is 1970 s bungalow brick built throughout.
At any rate, if you have 2m or so of pipe between the two, then this
It has had Fernox in it after every drain down but for several years now air has found its way into the system needing the bathroom rad bleeding every 3 weeks.>
Its actually 1995 and its not suitable for sealed use.
Problem I have with sealed systems is every person I know with one older than 6 years has problems with them.
It could be a trick that was done to try to prevent what you are now seeing. Shouldn't really be needed as long as the pipes are in the right places. On a bungalow, you (obviously) have one floor less, and this is yet another reason why the smaller head is more easily overcome by the pump.
OK, so it's poor but not severe. You may find that air is drawn in when the pump starts rather than continuously. I've had that experience in the past.
OK, so issue is academic anyway.....
The main failure mechanisms are leaks (radiator valve spindles for example) and the diaphragm of the expansion vessel.
OTOH, in the event of a leak, the amount of water is finite and expansion vessels are cheap.
Do you mean the head in the vent pipe?. Is the pump drawing down the water and hence some air in the vent pipe on start up. How would venting into the top up tank alleviate this problem-or is it a "regulation".
I think I will go with your air valve suggestion and plumb the vent and expansion into it.
I don't believe that it's a regulation as such, although the idea of the vent is to release air, or in the event of a gross boiler fault, steam. Had the vent and FE pipe been plumbed the other way, then you would have the potential for water to be pushed out of the vent pipe. If it goes into the tank, it isn't lost. Otherwise it is, and you are constantly adding fresh water and diluting the inhibitor.
I am not sure that it matters too much if the vent is left outside if you fix everything else though.
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