picklefactory

Mr Skeptic Thanks for that. Regarding pressure, there isn't really much. The top up pipe feeds the system from a header tank, so there is only the pressure generated by the height of that tank (Approx 2mtrs and fed through 15mm pipe), once the system is filled and the air removed, it is purely convection that creates the flow around it, at least that is how it was explained to me. So based on that, my thought is that slackening the lower connection will only release water that will be continually replaced from the top up pipe adjacent to it, and whatever pressure the header tank gives, will just push water towards the coil and probably not allow any bubbles to move in that direction. In fact, now that I think about it, I wonder if the top up pipe is in the wrong place. Any pressure from there can go in either direction where it joins the circuit, so that could also be fighting the convection current. That top up pipe was not in that position originally, but on a separate spur that fed across into the circuit in the correct direction of flow. Hmmm, I will ask the plumber about that when he is back.

between3and26characterslon Thanks Re-additives, that is something I hadn't considered. It is common to include an additive to primarily reduce corrosion, but I wonder if it has any effect on surface tension? I haven't really shown the entire vent set up, sorry, that was a quick 3D model I knocked up for illustration. The vent shown has a thumscrew at it's top for manually venting, so water cannot escape there, but there is also a further vent pipe that branches off from that one and then travels up and into the header tank (Where the 'Top up' comes from). That is high enough that the water wouldn't rise that far. ewmon Thank also. this system is simply a replacement of what was existing, and worked fine, just with new components, so I won't be adding a pump although I agree it would be better and should also force any air out. The water circulating (Or not in my case!!!) comes directly from the boiler below, so is receiving a lot of heat. The representation of the coil is a bit of a guess, as that is hidden inside the cylinder, so I don't really know it's size, helical pitch etc, I used a little artistic license for illustration only.

Thanks swansont, I can certainly give that a try. Would heating the pipe where it exits the cylinder (Lower end of the coil) have any benefit, I'm kinda thinking if I can get the convection going the other way, it might 'push' the bubble towards the vent? I've been trying to clear the air with heating on up to now, so that would be trying to push air against any flow of water. I may have better luck with cold water. Cheers

Hello folks, first post here and hope I'm not being cheeky, but looking for a scientific answer to a domestic question. I have a domestic plumbing problem, 'Get a plumber' I hear you say'..... well, I have and he's stumped too. I'm an engineer by trade, and I'm sure there will be a scientific answer, but not my field. OK, here's the problem. I've enclosed an image of a hot water cylinder that is heated via a heat exchanger coil through which flows hot water from the boiler, ie the water in the cylinder is not directly heated itself. The water flows through the coil using convection, or so I'm told, and it is NOT pumped. Apparently the lighter hot water rises, the cooler water sinks and creates the flow around the system. Now due to a new cylinder being fitted, I believe there is air trapped within the coil restricting the flow. There is a vent at a high point on the inlet flow pipe, but my initial question is (And probably daft), will any trapped air ALWAYS rise through the water in the pipe, even against the flow, or could it get 'stuck'? I think it should rise, surely bubbles would always come to the highest point and should escape through the vent. Can anybody suggest another reason why the flow would not occur? Blockage is the obvious one, but as all the pipework is brand new, that has been ruled out. If it is possible for air to get trapped, how would I encourage it to move? Would applying heat at any point in the circuit have any effect? Apologies if this is not the sort of thing normally posted here....... but thought it worth a go. Cheers