Thermosyphon failed.

[momist]

Hi folks,

I have a very old (1960's) open vent heating system with three circuits: ground floor second floor and domestic hot water cylinder. My practice has been to switch off the heating in the summer, and allow the thermosyphon effect to give us hot water only, and there is a gate valve on the first floor heating circuit to shut that off. All radiators except main hall (near the main thermostat) and bathroom have TRVs.

Last summer, we seemed to have less hot water than normal, but I put it down to the cold summer, using more hot water, and maybe not running the boiler for the usual length of time etc. This summer, the thermosyphon is not giving hot water at all, although with the valve open the upstairs radiators do get hot. If I run the pump, there's plenty of hot water (and a very warm house!).

I've drained and treated the system with two bottles of Sentinel X400 four days ago, and the situation has not changed. I still get hot water with the pump running, but not without. Obviously, if the upstairs radiators get hot, and not the water, there is some blockage in the coil, but should this kill the thermosyphon effect entirely? And with the upper circuit turned off, would there not be more pressure through the coil?

Can anyone suggest my next course of action please?

Thanks.

 

[45yearsagasman]

Are there any valves on the cylinder flow or return, like for instance a Cyltrol valve fitted to the return on the cylinder. This may have failed or the coil may be restricted inside the cylinder. I assume the system is completely full and vented of all air.
View media item 62985

 

[momist]

Are there any valves on the cylinder flow or return, like for instance a Cyltrol valve fitted to the return on the cylinder. This may have failed or the coil may be restricted inside the cylinder. I assume the system is completely full and vented of all air.
View media item 62985

Thanks for the reply. No, there are no other valves, and I have vented all the radiators and run the pump hard to blow any remaining air around and then re-vented the radiators. There is no way to vent the coil, of course. When flushing out the system, prior to re-filling with the X400 in, it started out slow to fill, but then speeded up as I assume I had dislodged some sediment near the header tank.

What puzzles me is why the X400 has not flushed the coil clean, if it is heating the water when pumped.

Edit: P.S. Sorry about the delay in coming back - I had some lunch then got shanghaied into something else.

 

[Weedrip]

Your system may have an injector tee on the gravity return from the cylinder, this will induce a better circulation when the heating is on via the pump.

probably sediment in the coil. many chemicals will loosen the sediment but usually rely on forced circulation to move it around the system.

you may have to drain the cylinder and flush the coil to clear it properly.

good luck

 

[momist]

probably sediment in the coil. many chemicals will loosen the sediment but usually rely on forced circulation to move it around the system.

you may have to drain the cylinder and flush the coil to clear it properly.

good luck

Well, it's three months later after a lovely warm summer and the heating is back on. For any others who find this thread because they have the same problem, I cured it in the end by cutting about the pipework in order to blank off the feed and expansion pipes above the cylinder level, broke the circuit just after where it divides off for the DHW cylinder and blanked off the feed from the boiler there, and then injected mains water pressure from a hose backwards through the coil and draining through the existing drain at the bottom of the boiler. All this was to avoid having to extract the cylinder from a very tight cupboard with inaccessible connections. I flushed out a load of black sludge.

This has cured the lack of syphon, and we now have lots of hot water. We also have a system which over-pumps the system even on it's lowest setting, so the blockage must have "regulating" the pressure for a long time, ever since the system was changed from Primatic to open vented.

Now, how do I stop the over-pumping? Time for a new thread perhaps.

 

[Nige F]

[Now, how do I stop the over-pumping? Time for a new thread perhaps.

Indeed - and some pics. if you can

 

[momist]

[Now, how do I stop the over-pumping? Time for a new thread perhaps.

Indeed - and some pics. if you can

Pics? Well, here goes. HtgAccess1 and 2 show the impossible to service DHW cylinder, with the two 22mm connectors above, where I blanked off the feed and vent pipes. HtgPipes1 shows the pipe arrangement in the under stair space. The top horizontal pipe with the pipe thermometer shows the piece I cut out, to inject the mains water into the left and blanked off the return to the boiler on the right.

HoseAdapt is the concoction I used to connect to the left hand 22mm connector there, to which to attach the hose pipe from the house outside tap (with check valve!). This consists of a 22 to 15 reducer followed by an old 15mm straight coupler with mis-matched nuts, then a chrome radiator tail with a tap adaptor into a hose connector. Whew.

The album is here:
//www.diynot.com/network/momist/albums/17801

Note, the 22mm straight connectors in HtgPipes1 are onto old 3/4" pipe, using my very last two 3/4" olives. Can I get more of those? I don't see them in my builders merchant or at Screwfix.

For anyone interested, the new thread about the pump over is here: //www.diynot.com/forums/plumbing/solution-to-pumping-over-problem.377802/#2889289

Thanks for the help and guidance received.