The system shown in Figure 1 was installed to provide domestic hot water to a high end house with several bathrooms. The installer selected the largest 119 gallon indirect water heater available from his supplier. The tank’s internal heat exchanger was rated to transfer 250,000 Btu/h when supplied with 180° F water from the boiler. He piped it in with 1 inch copper tubing, because that was the size of the piping connections on the tank. He used three identical zone circulators, one for each boiler, and one for the indirect tank. He used a larger circulator for flow to the space heating portion of the system.

Being concerned about a potential complaint over not enough hot water, he convinced the owner to double up on boiler capacity, and thus installed two boilers, each rated at 250,000 Btu/h. He also set the internal high temperature limits on each boiler for 200° F during a call for domestic water heating (because that’s as high as the control would go). He assumed that 200° F water through the coil will significantly increase the heat transfer through the tank’s coil heat exchanger.