Hot water boiler

Claim 1

A HOT-WATER BOILER, INCLUDING A PAIR OF CONFRONTING WALLS HAVING ALIGNED SEALING RIMS, SAID WALLS TOGETHER DEFINING A WATER-CONTAINING SPACE AND HAVING WATER-INLET AND WATER-OUTLET CONNECTIONS, EACH OF SAID WALLS HAVING INWARD AND OUTWARD EXTENDING CORRUGATIONS THEREIN PROVIDING MULTIPLE OUTSIDE VERTICALLY CONTINOUS ZIG-ZAG FLUEGAS SPACES ALTERNATING HORIZONTALLY WITH MULTIPLE INSIDE VERTICALLY CONTINUOUS ZIG-ZAG WATER SPACES, EACH SAID ZIGZAG WATER SPACE BEING APPROXIMATELY EQUAL TO EACH SAID ZIG-ZAG FLUE GAS SPACE WHEN VIEWED IN TRANSVERSE CROSSSECTION, THE CORRUGATIONS OF SAID PAIR OF WALLS HAVING A COMBINED DEPTH WHEN MEASURED ALONG A HORIZONTAL LINE FROM ONE OF SAID WALLS TO THE OTHER THAT IS SUBSTANTIALLY GREATER THAN HALF OF THE MAXIMUM HORIZONTAL EXTENT OF THE WATER-CONTAINING SPACE BETWEEN SAID WALLS WHEN MEASURED ALONG SAID LINE, SAID OUTWARD EXTENDING CORRUGATIONS OF EACH WALL BEING COPLANAR, AND MEANS ENGAGING SAID COPLANAR FORMATIONS FOR LATERALLY CLOSING OFF SAID FLUE GAS SPACES.