Wow! Perhaps you should cash that in with a scrap metal merchant and use a plastic tank instead.
Has our plumber put us in danger?
- Thread starter vexation
- Start date
Wow! Perhaps you should cash that in with a scrap metal merchant and use a plastic tank instead.
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I would agree that there's some poking and hoping going on. Bear in mind that I haven't lifted a spanner to any of this. To date four different heating engineers have visited and done the work I've described here.
I can't agree that I've chosen to ignore advice though. I've just re-read this topic from the top again and can't identify any advice that's been ignored. I've been faithfully communicating all the advice offered here to the engineer(s) doing the work in my home. They are the ones actually doing the "poke and hope" - having gone over the basics with me time and time again. They say it's an odd problem for which there's no obvious cause. The location of all components in the system is (according to them, and to my own inspections) straight out of the textbook.
Norcon, the problem is that water jets out of the vent when the pump stops and air gets sucked in when it starts. This has begun to happen immediately after a boiler change, a correction of the positioning of feed and vent pipes (they were in the correct area by the pump but swapped) and a power flush, and the addition of a dirt-trap). The system does not seem to suffer from any blockages or basic plumbing errors and heats all rads and HW very effectively.
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Has it occured to you that all four of them may be less than clever?
There is clearly a problem and if there is a problem there is obviously a cause and anyone who can identify the cause will be able to offer a solution.
I am sorry to say that many of the people working in this industry are not as good as I would expect!
Tony
Here is an experimental thought. Take U shaped tube open both ends. Pour water into this U tube. Water will be at the same level in both limbs. Gently blow down one limb. Now water will be at different levels in each limb due to different pressure acting on each limb.
Now transpose this thought to your heating system. Differential pressure is created by running pump. I stress the dip in the vent pipe (your own admission that it will drink more than 500cc of water) has to be absorbed elsewhere and is likely to be the header tank.
500cc of water equal a slug of water 1.5m long in 22mm pipe. Same quantity of water in a header tank 300mmx450mm only raises water level by 3.7mm. Only way you will notice this difference in water level is to use a ruler as a dipstick and take before and after readings.
Now transpose this thought to your heating system. Differential pressure is created by running pump. I stress the dip in the vent pipe (your own admission that it will drink more than 500cc of water) has to be absorbed elsewhere and is likely to be the header tank.
500cc of water equal a slug of water 1.5m long in 22mm pipe. Same quantity of water in a header tank 300mmx450mm only raises water level by 3.7mm. Only way you will notice this difference in water level is to use a ruler as a dipstick and take before and after readings.
It's not just four engineers Tony. Once they'd started to scratch their heads (about the time I started this thread) each visitor spent as much time discussing the problem on the phone to HQ as they did indoors. This is a big firm that cover a wide area and I find it hard to believe that nobody on their books has the transferable knowledge to resolve it. It does now appear, however, that my faith may have been misplaced.
As I sit here typing this, I can hear the crackle of air bubbles squeezing past a TRV.
As I sit here typing this, I can hear the crackle of air bubbles squeezing past a TRV.
OK, now I've had another thought experiment that could help me understand things better - say my boiler downstairs fell down a deep mineshaft and the pipes magically stretched all the way down to the bottom. Now the pump can either lift water up from the bottom and circulate it back down, or it can pull down on the short run of pipe to the F+E (on the suction side). At first blush it seems like it could drop the head, but surely it can't as there's a fixed volume of non-compressible fluid in the system? (in theory)
For water to drop in the F+E it has to go somewhere. For water to rise in the F+E it has to come from somewhere. Something is reminding me about water going in two directions down the same pipe. Something is nagging me about having different diameter pipes for vent and feed.
Clearly you fail to see the point I am making. Nowhere have I said water can be compressed. Nowhere have I said water can flow in two directions within the same pipe.
What I have attempted to explain is that the dip in the vent, be it boiler is sitting at the center of the earth core, is pushed by non compressible water into the header tank. When pump stops, water rushes up the vent pipe to spill over into the header tank before settling back as it would in the U tube described in earlier post
What I have attempted to explain is that the dip in the vent, be it boiler is sitting at the center of the earth core, is pushed by non compressible water into the header tank. When pump stops, water rushes up the vent pipe to spill over into the header tank before settling back as it would in the U tube described in earlier post
Sure, it seems to make sense: when the pump stops it becomes a restriction in the flow so inertia keeps the water going up above the head in the vent . Similarly when the pump starts, inertia of the water in the bulk of the system makes it want to stay put so the suction temporarily pulls down from the head in the vent. But how can you (even temporarily) lose/gain water from the system?
We know that air must be introduced or lost (compressed) for this to happen: If a pump is connected to an otherwise sealed fluid-filled container, it will only pump the fluid out if air can get in. Sometimes this even results in the fluid and air moving in opposite directions in the pump.
I'm thinking that the air is getting into my system on account of this kind of exchange.
Common on the inlet side of the pump. Has anyone suggested that ? - sorry I haven't reread 6 pages.
Even pressurised systems will do it. You put the pressure up and it stops. So check/remake the joints that side if it's a big problem. Yo could try a leak sealer.
I can sympathise with that, I'm a moderator on another forum where the average thread is 30 pages long and people grumble like crazy if someone joins in on the end without reading everything first!
The huge potential for sucking in air through bad joints on the inlet side is appreciated. This has been the subject of some careful investigation,
The thing is, I can start and stop the pump (using the speed selector at in-between positions) and after several start/stop cycles (each one of which creates the very obvious sound of air mixing with water) a big slug of air can readily be introduced into the system. I'm well practiced with this demonstration which I give to each new engineer on the job. I do it with demand from HW only as the air that gets pulled in remains in circulation in a tighter loop (it tends to get lost in the rads when CH is in demand).
I am quite convinced that this significant quantity of air is coming in down the vent, and not the result of bad joints.
The latest modification still leaves the vent with an open path the the atmosphere - even though it's connected to the base of the feed tank. As for the "dead leg" of the old feed pipe, that's probably half full of air and acting like a spring now.
just for the fact of writing something different, had you thought of upsizing the feed, 28mm all the way back to f&e. then you'd have more chance of pulling water back down rather than air. i'd be sceptical of the feed that you have at present anyway. just another one for the hat!!
There's obviously a balancing act involved here between the bore and length of the feed and vent pipes. Somewhere (BS 5449?) there should be some formulas. I wish I could find where I read about the vent (?) having 3% the volume of the system.
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