With my system the thermal store (2000 lts) is next to the boiler and I have 4 temp gauges on the TS (to see the heat available and to gauge the amount of wood to put on the boiler) and a temp gauge on the output (top) of the boiler

Part of the function of the laddomat is that when power is off there is flow through the laddomat excluding the bypass to allow gravity flow between the boiler and the TS. However this also allows permanent gravity flow which means that my boiler matches the temp in the TS and the boiler gauge is in step with the TS gauge at the same level. If I were to use the residual heat from the boiler then the gravity flow would replace the boiler heat from the TS. But this also means that any available heat in the boiler will gravity circulate to the TS given any thermal gradient available. This circulation post burn using gravity is in the marketing blurb that came with the laddomat.

The only way to avoid this circulation would be to have a valve to close off the gravity circulation but this would remove an important safety feature.

If there is no power to the laddomat then it is open to circulation to the TS and the boiler recirculation is closed. Designed like this to allow gravity circulation in the event of a power cut and to utilise the remaining heat after a burn (documented so in my Laddomat blurb)

My boiler has an adjustment on the start up / close down setting whereby the temp to exit start up and the switch off at end of burn can be adjusted. Top temp for this is about 70 deg.. Set to this the boiler shuts off the Laddomat at 70 at end of burn and allows any residue heat to gravity circulate to the TS which in practice means that the boiler maintains TS temp. matching that of the stratification point in the TS so any heat left in the boiler will go to the TS according to thermo syphon temps.

If you take heat out of the boiler at the end of a burn this will be replaced from the TS. I don't think it would be a good idea (at all) to close off the boiler from the TS at the end of a burn to stop this. Perhaps look to increasing the insulation on the boiler to stop loss to atmosphere and so retain more heat to transfer to the TS.

My boiler is a Künzel 40kW circa 1995. Basic wood gasifying boiler with no electronics, controls being capillary thermostats and relays.

Next to the boiler (about 1m) there is the TS which is 2.4m high (2000 lts). The height of the TS allows for thermosyphon between the boiler and the TS if the Laddomat is switched off - either power cut or end of burn.

When the boiler is running the Laddomat keeps the boiler temp at 72 deg. and heats the TS from top down to 72 deg. top to bottom. When the whole TS is at 72 deg. the TS and boiler then heat up together until the high set temp on the boiler is reached whereupon the boiler fan switches off.

When drawing heat the TS temp will drop at the bottom first with the colder portion progressing up the TS. If the Laddomat is running then the laddomat will hold the boiler at 72 deg. Once the boiler senses end of burn the Laddomat switches off and allows thermosyphon between the boiler and the TS. At this point the temp gauge on the top of the boiler shows the same temp. as the horizontally equivalent place on the TS and changes with it. I don't know what the temp is at the bottom of the boiler but I suspect it will be the same as the bottom of the TS. If I am around when the wood is exhausted but the boiler temp is high enough that the end of burn is not yet sensed then I switch of the boiler to save the electricity.

Given the way heat is taken out of the my TS and how the boiler reacts I don't think that with the way my system works there would be anything to gain by my complicating things to take heat from the boiler and then switching back to the TS.

I have thought of fitting a thermostat to the flue to sense end of burn before the boiler temp drops sufficient to switch the boiler with its own circutary - its on the list of things to do, but not near the top.

I have thought of better insulating the boiler but that is on the same list as the flue thermostat.

One useful (to me) feature I fitted is to put 2 thermostats set to 65 deg., one 2/3 up the TS and the other at the top. These turn on indicator lamps in the house. (the boiler is in a room next to but not connected to the house). These lights let me know when I should light the boiler without the need to periodically go to the boiler house to check the temps.

Sorry, looks like photos won't appear.

Posted By: johnnyhI see what you're saying about the TS and boiler being "as one" but how does it siphon at the end of the burn when the top of the boiler is colder than the top of the tank ?

Heat travels from hot to cold so after a burn when the the bottom of the boiler is colder than the bottom of the TS then the colder water in the boiler will move to the bottom of the TS which 'pulls' warmer water into the top of the boiler.
Circulation after a burn will be in the opposite direction to that of during a burn

Photo reduced to 0.7 mb

Yes I can't seem to upload it. Each time I submit a post it says something like "site not secure ! " and I have to agree to accept the imminent danger of going to the site then re-enter my password even though I'm already logged in and I think the photo is getting lost somewhere in that process.

Ah, so the boiler is also losing heat from the TS ? I didn't think of that, but it can't be that much I would have thought? Maybe some sort of one-way valve might be able to stop it without reducing safety.

I have a swing-check valve of about the right size kicking around, that would be ok ?
How would I go about testing the gravity circulation ?

Posted By: johnnyhI have a swing-check valve of about the right size kicking around, that would be ok ?

Yes

Posted By: johnnyhHow would I go about testing the gravity circulation ?

During a burn measure the temps. of the pipes (flow and return) and note the boiler and TS temps then turn off the power to everything and monitor the previously noted temps. I would expect the boiler temp to rise because the flow will be slower without the Laddomat pump and the flow to the TS should also rise (measure it close to the TS) in sympathy. If it does not then I would suspect no gravity flow. In the same way I would expect the return pipe temp. to be the same at the TS and the boiler, with no flow I would expect the pipe temp at the boiler to slowly fall due to loss to air without replacement heat from the TS. The greater the heat difference the faster the flow will be.

Posted By: johnnyhJust a thought: the TS top connection is 1 to 1.5m higher than the boiler top connection, would that not stop the back-siphoning of heat into the boiler ? I mean the heat would have to go downhill quite a bit.

When the TS is hot and the boiler has been out for a bit what is the temp at the top of the boiler (or boiler temp) and what is the temp at the middle of the TS. Does this relationship change over time?

The heating season has kind of ended here so may not be lighting the boiler for a while, having said that we could still have a cold "snap" before the end of spring so if I can be prepared I may just get the chance to collect the data.