Wood burner heat exchanger heating back to burner from tank

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Hi,
I'm sure this must have been come across before and a simple solution exists!

I have a large thermal store installed (sealed system). The recently installed log burner heats the store via a heat exchange block. Each side of the heat circuit has a pump. One on the log burner side and the other on the loop around the thermal store. A pipe stat is on the burner pipe set to 45 degree. This works fine and as the stat closes both pumps on the heat exchanger start circulating the water and the tank heats up.

Only problem is that as the log burner runs out of fuel and cools down what looks to be happening is the heat then is getting transferred from the store back to the bruner side of the heating circuit and therefore the stat on the burner stays on eventhough the log burner is effectively going out. If left to it's own devices the pumps continue transferring heat from the store to the burner circuit until the store temperature is low.

Any ideas on how to solve this? I've been thinking of an additional stat somewhere but can't get my head around how it would work.
 
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You need a second thermostat on the thermal store.
Once the thermal store is hot it switches off the pump from the log burner.
So pipe from log burner hot but thermal store cold pump switches on.
log burner hot and thermal store hot pump switches off.
log burner cold and thermal store cold pump off.


Hi,
I'm sure this must have been come across before and a simple solution exists!

I have a large thermal store installed (sealed system). The recently installed log burner heats the store via a heat exchange block. Each side of the heat circuit has a pump. One on the log burner side and the other on the loop around the thermal store. A pipe stat is on the burner pipe set to 45 degree. This works fine and as the stat closes both pumps on the heat exchanger start circulating the water and the tank heats up.

Only problem is that as the log burner runs out of fuel and cools down what looks to be happening is the heat then is getting transferred from the store back to the bruner side of the heating circuit and therefore the stat on the burner stays on eventhough the log burner is effectively going out. If left to it's own devices the pumps continue transferring heat from the store to the burner circuit until the store temperature is low.

Any ideas on how to solve this? I've been thinking of an additional stat somewhere but can't get my head around how it would work.
 
I should have mentioned that the store has 2 stats - one for the hot water and one for the heating buffer. Seems to be a juggling act to get them set to all work together i guess.

dcawkwell";p="2487355 said:
You need a second thermostat on the thermal store.
Once the thermal store is hot it switches off the pump from the log burner.
So pipe from log burner hot but thermal store cold pump switches on.
log burner hot and thermal store hot pump switches off.
log burner cold and thermal store cold pump off.
 
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I like the sound of this thanks for the link. I seems to make the most sense as the flue is the bit that definately will be cooling down when the fire is going out. I would expect the temperature would need setting quite high in order for it to give chance for the water jacket around the burner to start getting warm.

The most common method is a flue stat.
http://www.termoventiler.eu/media/upload/RRT500_E.pdf

The link explains it. Ignore dwackwell.
 
The recently installed log burner

A natural draught domestic stove I assume?

heats the store via a heat exchange block

A plate heat exchanger I'm also assuming.

The easiest way to do this with a domestic woodburner stove is gravity circulation directly between stove and store. Just natural circulation, cheap, simple, safe and nothing to go wrong and will last a life time.
When you introduce a pump then you have "cold return" issues unless other devices, (ie-valves) associated more frequently with fan assisited log batch boilers are used.
Some diyers fit the valves on the stove return which can cause more issues.
A few stove manufacturers recommend the hi-lo stat configuration with conventional set up's which work in conjunction with an operational gravity circuit and provides good protection for the boiler return.

The plate heat exchanger should be connected between the thermal store and heating system. One obvious advantage of this arrangement is the thermal store doesn't end up in "sludge bucket" territory.

So the flue stat alone might not be the solution and could create more problems than it solves.
 

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