Guys,
during the summer I installed a solar hot water system, and the days are drawing in I'm begrudging the heat that is required to keep the hot water topped up now the sun has gone

I have a 250l Navitron (Newark) thermal store that has blanks for back burner connection and also an open fire in the front room....
I've started looking at woodburners that will fit into a 'standard' fireplace but I'm quite concerned at the specifications.
The smallest one I've seen quotes 5Kw to the room / 12Kw to the tank.
I'm worried that 12Kw continuous into a 250 litre tank will soon give me a tank of boiling water.
The typical usage will be weekends and high days with the gas boiler / rads used for day to day.
I'll be using it to warm the water whenever we decide to have a fire rather than as a primary heat source.
Does a typical installation rely on thermal losses to reduce the total input to the tank and keep everything running smoothly?
I thought I'd do a quick calculation to give me a view,
watt hrs = temp change C * 1.16 * 250l
12000 * 1 = temp change C * 290
temp change C = 12000 / 290
41C per hour.....
I have visions of hundreds of tanks boiling away over the winter, which can't be correct!
Any pointers as to where my thoughts are heading west?
Also, the tank is up and approximately 3m over from the fireplace, is that still in the realms of 28mm convection action or should I be looking at pumped assistance?
Cheers for any insights you can give.
Kevin