House insurance for non-agrée stoves

I, like many of you, enjoy working on my house. I would love to build a poele de masse from non-french plans. But it will be completely non-certified, not installed by an artisan with a decennial, connected to a chimney built by the previous owner (so non-certified also), which makes me a bit nervous when I consider the house insurance.

My question is:

1) Is the invalidation of my house insurance inevitable, in the event of, for example, a chip pan fire, because a non-conform device exists in my house?

2) as above but the fire would be caused by a chimney fire from the 'poele de masse'?

Whatever advice you get here, you should really be asking your insurers, and their rivals, and checking what rules they apply - many insist the flue is swept twice per year by a tradesman but we insist on doing it ourselves using expensive professional equipment, after the tradesman didn't do it properly and the flue re-blocked within a month.

By "poele de masse" I assume you mean a fire insert surrounded by stone like steatite or heat-retaining tiles? My own is a Jotul in steatite installed by a pro, and so we got a tax rebate for it. I do know someone in the next village who constructed his own "masse" around a standard insert - however as he was a CERN physicist he probably had calculations to back it all up!

I must say the stone does a wonderful job, a bit like a storage radiator it heats the house long after the fire has died down or even out. Putting it in the right position is important to make the most of it, and you also profit from heat from the flue if you don't insulate that until it nears the roof. Rules about flues and insulation are possibly more important than those relating to the fireplace itself - ours pass through two concrete floors, with a circle of tiles on each surface for about a foot from the flue and wooden boards beyond that. Our ground floor ceiling is painted concrete - no plasterboard ceiling or battens, and we had to leave a minimum distance between the flue and the staircase of 600mm to rule out scorching. The exit through the wooden roof required a much larger hole in the roof timbers and an insulated pipe, finished off with Rockwell foil-coated mineral wool and mineral fibre panels supported by metal brackets and a metal chimney on the roof.

If you intend to use an existing flue your insurers may insist you line it. I think I'd have a word with the senior Pompiers in your area and with your insurers before investing in materials. Small modification in your plans could be cost effective to ensure you comply with local rules and guidance. If your insurers hear you've already consulted the Pompiers they may have more confidence, too. You also need lots of ventilation for the fire to work efficiently - a pipe letting air in from outside may be needed. If the fire doesn't get enough air it will alternate between dying down and flaring up as a vacuum develops and disappears. This can make a lot of noise and smoke, can cause flue pipes to blow apart and can even cause the fire insert to explode, so research pays in the long term.

When we bought our derelict house it contained a 3-metre wide bread oven. The oven opened into the original kitchen and had a chimney outside the oven leading through the roof. As the structure was in the centre of the house and the chimney was demolished as unsafe many years before we bought it, the oven had to go: a friend climbed in and demolished the lining, numbering each tile so that, one day, we can construct a communal wood-fired oven outside. Heaven knows what Aviva would have said if we had managed to retain the bread oven!