Hydronic systems are often designed with a backup pump for both the hot-water and chilled-water loops. This not only allows uninterrupted service when one of the pumps is down for maintenance, but also— with a few simple controls—can be set up to maintain flow whenever the main pump fails. Usually there’s not a problem being sure one of the pumps is on-line, but as more controls are installed on the system, it becomes increasingly important to be sure fluid is circulating in the loop, or many expensive machines can be ruined.
The main and standby pumps are piped in parallel, with check valves to prevent flow back through the off-line pump. A flow switch in the main line remains open while the primary pump is working properly. When the pump fails, the flow stops, the switch opens, and the standby pump comes on-line.
Additional precautions are prudent to ensure that the main pump—for example, if it failed because of an intermittent electric short—does not start up after the standby pump kicks in. The resultant excessive fluid pressure downstream may damage control valves and fittings, and perhaps even damage the pump motor.
One solution is to install the same flow switch/pump switch arrangement on the main pump, so that whenever there is no flow through the standby pump, the main pump is running. This double system will enable either pump to be the primary pump, while preserving complete controls and backup. Maintenance personnel often alternate primary pumps anyway, so that both pumps wear equally, essentially doubling the useful life of the system.
Another elaboration might be to cause a remote trouble light to burn whenever the standby pump is on, to caution maintenance personnel of problems with the main pump.
__________
Excerpt from Energy Conservation Projects.