Preventing thermosyphon

[ericmark]

Twice now had unwanted thermosyphon, first house it was OK if boiler ran from cold with no pump running, but if the pump ran, once flow started it would not stop, second house a little more complex, two pumps, and one pump could cause water to reverse flow through other circuit, and again once flowing would not stop.

I had not heard of London loop, so was looking at two methods, non return valves, or motorised valves, I went with latter, but not sure that was best option, as also needs two relays, but the advantage of motorised valve is it has a micro switch, but the wiring became complex, and my worry is as I get older, I will likely need to get some one else to repair, so want to keep it as standard as possible.

 

[oldbutnotdead]

Hmmm- how did you change the connection to F & E- that can be another classic place for unwanted thermosyphon once the bottom of the store gets warm & you definitely don't want any valves in that leg! If you put the 65 cartridge in your woodburner will run hotter (which is better from internal condensation point of view), not sure whether it'll improve the output though

 

[speccy]

Hmmm- how did you change the connection to F & E- that can be another classic place for unwanted thermosyphon once the bottom of the store gets warm & you definitely don't want any valves in that leg! If you put the 65 cartridge in your woodburner will run hotter (which is better from internal condensation point of view), not sure whether it'll improve the output though

Has a 65 cartridge already.

Ok will post before and after diagram to illustrate,.

As you can see before the FnE dropped straight into top of tank which caused alot of bubbling issues poor filling etc. This was thermosyphon only and WBS only heating the TS to supply hot water, 2 zone valves diverted boiler heating to TTS coil or CH.





After you can see the breather now vents into top of cylinder and the cold filler runs to bottom this is where i believe the overflow loop isn't tall enough it's going in almost horizontal in the FNE which is very high level wise, also split out the circuits to have a CH pump and Boiler pump. Adding stats with Hysteresis to control upper and lower operating temps ie Boiler pump kicking in when WBS isn't enough.

 

[oldbutnotdead]

Some fairly radical alterations there then.......the cold feed at the bottom of the store would definitely benefit from a London loop- if you get a syphon running there you can end up heating the header tank quite nicely. As long as the loop on the expansion pipe is more than 500mm above the water level in the header tank it shouldn't pump over (though worth a look and also worth checking the temperature on the header tank) and if you are getting a syphon up there it'll only be heating the contents of the pipe- insulate it (along with the rest of the pipework in unheated parts of the house) & it'll reduce the amount of energy being wasted

With your current setup on the radiators you're almost certainly getting syphoning- possibly not while the rads are cold but very likely once they've warmed up. I'd be putting a motorised valve in the flow to the rads for definite

 

[speccy]

Some fairly radical alterations there then.......the cold feed at the bottom of the store would definitely benefit from a London loop- if you get a syphon running there you can end up heating the header tank quite nicely. As long as the loop on the expansion pipe is more than 500mm above the water level in the header tank it shouldn't pump over (though worth a look and also worth checking the temperature on the header tank) and if you are getting a syphon up there it'll only be heating the contents of the pipe- insulate it (along with the rest of the pipework in unheated parts of the house) & it'll reduce the amount of energy being wasted

With your current setup on the radiators you're almost certainly getting syphoning- possibly not while the rads are cold but very likely once they've warmed up. I'd be putting a motorised valve in the flow to the rads for definite

Thanks Bud some wise words there I appreciate your input as you are describing exactly what I'm discovering. First heat of the tank was ok then once it was hot indeed CH was thermosyphoning like crazy. The initial design was my old plumber the new rework is mine with a bit of help from people on other forums and some reading. But yes alas I had no real concept of thermal issues posed neither did my current heating engineer im using to modify it. Hmm. Anyway I will take your advice and add a motorized valve to CH and see if I can raise the overflow loop. And loop to stop cold fill issue. Will post back hoping all these stem the heat I am currently hemorrhaging since modifying. Thanks to you and others for your input here.

Also learnt water expands something like 4% with heat and my FNE tank was setup to be 95 full when Cold.. oops. Another fix on the way. Seriously learnt alot doing this but people I paid to do this have never done it right