This page outlines the various piping methods for hot water heating systems.
Primary Secondary
Primary-secondary piping is a method which hydraulically separates or decouples flow between boiler and loads. Key to its functioning is that the closely-spaced tees are no more than 4 pipe diameters apart. The circulator on the primary loop must be running for any flow to reach the secondary loops. Thus, each secondary loop also requires its own circulator.
The other important thing to note is that if all loads are running, the return temperature from one load becomes the supply to the second load, assuming the boiler is perfectly sized for both loads. In larger systems, where only a small part of the boiler's flow goes to each load, the supply temperature for each load will be much closer. However, if we were to give one load full priority, such that it couldn't run when the other did (which is often done with DHW), then each load would get the full boiler temperature.
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Supply Return
Reverse Return
Combi-DHW: 2 tank piping
- The common issue with a single hot water tank is that under periods of heavy load, the homeowner runs short of domestic hot water (DHW), since most of the tank's entire output is being diverted for heating. By adding a second tank in this configuration, fresh water is still being introduced to the system, and it flows through both tanks, thus solving a stagnation problem, and leaving the homeowner with one full tank of hot water at all times. A pump and timer is required, as for all combi-DHW systems, to prevent water in the slab piping from stagnating during the off-season.