An installer is asked to connect an old (but still working) propane-fueled cast-iron boiler, which was salvaged from another project, to a slab-on-grade floor heating system in a new workshop.  The boiler has a rated heating capacity of 70,000 Btu, which also happens to be the output rating of the floor heating distribution system when it is supplied with 105º F water. The client keeps stressing that he wants the least expensive installation. 

The installer puts in the system shown in Figure 1. He set the boiler’s aquastat for 115º F, figuring it’s safely above the required supply temperature of the floor heating system. He installs a bypass circulator with the intent of keeping the boiler inlet temperature high enough to prevent sustained flue gas condensation.