Understanding Heater Effect on Flow

[randdusing]

Howdy,

After replacing the broken manifold bypass valve in my heater, I noticed my flow is lower at the same pump RPM. Is this expected? Does a working manifold bypass valve mean more water is flowing through the smaller diameter pipe of the heater core rather than just going straight back into the main plumbing? And is this one of the reasons for heater bypass valves being recommended here: Heater Bypass? - Further Reading

Thanks,
Rand

 

[JoyfulNoise]

When the internal bypass is working (and actually physically in place), there is a metal spring that pushes against the bypass door that the water pressure must overcome. So, yes, a working and present bypass should add more resistance to the water flow. People do install plumbing bypasses to improve their overall flow rates because any heater presents a lot of head loss in the manifold. Some people have noticed as much as a 30% increase in flow when a heater is fully bypassed ... YMMV on that improvement.

 

[randdusing]

Thanks for the clarification. Based on the increase in RPM to maintain the same amount of spillover flow, 30% sounds about right for my system as well. Looks like I need to install a bypass!

 

[JoyfulNoise]

Thanks for the clarification. Based on the increase in RPM to maintain the same amount of spillover flow, 30% sounds about right for my system as well. Looks like I need to install a bypass!

Most pool builders neglect this aspect of the equipment pad plumbing because they either deem it unnecessary for the typical pool owner or they don’t want the headaches of a pool owner forgetting to open a bypass up and wrecking the equipment. However, they are very useful. Also, some of the major heater manufacturers are starting to incorporate automation controls that allow the heater to activate a bypass automatically so that proper cool-down water flow can be maintained.