What temperature should a thermosyphon/gravity rad start?

[ABCwarrior]

Hi Folks,
Apart from the subject question, can anyone advise what the maximum temperature a thermal store should be allowed to get? As some of you are probably aware, we have a bizarre system for our hot water+central heating combining a multifuel stove/immersions/solar panel.
The Resol unit tells us the temperature in the tank top and bottom.
The reason I ask is that our system has to incorporate a failsafe device into it, a gravity radiator/thermosyphon thing that operates when there is a power cut, the pump stops working for the central heating and the store can't shift any heat from it when the fire is on this is supposed to cut in and take over safely removing excess heat.
Well I've had our tank up to 100.1c(turned off the heating mainswitch) and this gravity fed thermosyphon thing has not kicked in at all. The system is all bled and fine, but at this temperature the store sounds like a huge kettle boiling away. The minute it got to this temp I restarted the pumps and opened up the radiators to drop the temp.

So prior to going back to the installers what should the max temp of a thermal store be? 90-95? It surely shouldn't be anywhere on the boiling point especially if it's pressurised(Is it?!?!)

 

[Onetap]

So prior to going back to the installers what should the max temp of a thermal store be? 90-95? It surely shouldn't be anywhere on the boiling point especially if it's pressurised(Is it?!?!)

It should be as hot as the water coming out of the boiler. It should be hot all the time the boiler is hot. It sounds as if you believe there is some temperature actuated valve in the circuit; I'm fairly certain that there shouldn't be anything like that, no radiator valves that can be closed, no TRVs, no anything.

Is the thermal store pressurized? Open vented, surely?????

 

[ABCwarrior]

Ta for the reply onetap- I don't know if it's open or pressurised. I do know that they said they had to fit this big radiator in the upstairs hall which is directly above the stove but on the back wall(ie on the upper floor if you went through the wall behind this radiator you'd hit the flue), and they said it was necessary in the event of power failure so all I can assume is it is like some manual thermostat that once a certain temp has been reached kicks in and redirects the flow of water from the store directly to the radiator, or else it's electric/electronic and stays in the thermal store position when there's power but in the event of power failure switches the flow to the gravity radiator. Either way it aint working!

We don't have any gas heating/combi set up, the only control we have is on the immersions which are always off, but as for the multifuel stove once it's lit we only have some control via the vents as to how much heat is transfered to the store, hence the potential for it overheating and probably bowing up!

 

[oilhead]

There may be a zone valve that is held closed whilst there is power, so just stopping the pump will mot necessarily bring the heat sink into action. Switch off all the power and see if it works.

 

[Norcon]

A control stat should be energising the ch pump/s when the store rises above a preset temperature.
This will be automatic and not need manual intervention.
Providing the electric stays on.
An ideal one is a kp78...

Industrial control reliability with an adjustable differential from 4.5c - 20c. And a sensor maximum temp of 150c. Ideal for solid fuel.
I know this is not your problem but I'm just pointing out the best choice for such control imo.


After that its the heat leak rads job, if one is fitted which should be inside the home and not dumping heat constantly to an outside space.
If its outside the heated envelope then an avta cooling valve is an option.....

Not cheap mind but very reliable since no electrics are involved and operates under all conditions.
Radiator valves should not be fitted.

For additional safety you can fit a thermal release valve to the hw coil outlet if the store has such a coil fitted ....
This is a Danfoss BVTS biomass valve manufactured from forged brass and stainless steel and is preset to open at 95c....

Honeywell, Watts, Caleffi, Syr, Regulus etc make versions also.
And utilises the hw coil as a cooling exchanger should all else fail.
Again no electrics so very reliable.

 

[ABCwarrior]

Thanks for the replies everyone. I see your point Oilhead it may have been a futile experiment I don't know if this "mainswitch" controls the whole supply to all the parts of the system(again through lack of info as to what does what from the installers- like getting a new car with no manual!).
All I can gather from it is that when we had the proper power cut it didn't work anyway and must have been at this same temperature given that it was bubbling and burbling away the same.

Ta for the detailed stuff there Norcon- I'l re-read and digest as much of that as I can as I'm sure these cowboys will be back to look at it, hopefully I can arrange to be there when they do so I can get right on their t*ts with every question I can spear them with!
I think that the KP78 or equivalent is needed anyway as the pump now runs all the time regardless of store temperature(ie the stat is f***ed)
We're having to manually switch off the mainswitch to keep enough hot water for showers but control the fire at night so it doesn't boil the store. Bit of a balancing act but the pump is a noisy b*stard and it's located in a cupboard in our room so not only would it pump the store cold overnight it keeps us awake... a system designed by bell-ends...