When is a Heater Flow Switch Needed?
Most pool heaters, gas or heat pump, use a pressure switch to determine if there is adequate water flow and the heater should run. There are conditions when the pressure switch will sense pressure in the heater pipes but there is no flow, such as when:
- the heater is bypassed and pressure is trapped in the pipes
- the heater is installed above the water line
- the heater is installed below the waterline allowing the pressure switch to be closed from static pressure with no flow
Signs of a heater running without water flow are collapsed pipes on the heater intake or outlet.
The pressure switch has an adjustment range for heater placement. Consult the manufacturers installation manual. If the heater placement is outside the pressure switch adjustment range then the pressure switch needs to be replaced by a flow switch.
For example the chart below illustrates the adjustment range for the pressure switch in a Raypak 406A gas heater:
and Raypak has a 11 PSI Flow Switch Kit - 007142F.[1]
The Raypak pressure switch is set to 1.75 psi, which can be triggered by 4 feet of water.[2]
The MasterTemp pressure switch is set to 3 psi, which can be triggered by 6.9 feet of water.
The Pentair MasterTemp Installation Manual contains the following warning:
A flow switch for a heater should be used. SWG flow switches are calibrated for about 15 GPM and heaters typically needs 30-40 GPM.
- Raypak has a 11 PSI Flow Switch Kit - 007142F.[3]
- Hayward has the H-Series H150-H500 Flow Switch Kit or the H-Series Low NOx pool heaters, models H150 to H500.[4]
- Harwil Q-8N - Q-8N/2/2/A/VITON is an adjustable flow switch with an ON set point adjustable from a minimum of 25 gpm to a maximum of 37 gpm in a 1.5" pipe, and from 44 gpm to 65 gpm in a 2” pipe. Full specifications can be found here Harwil Q-8N Datasheet
I got lucky...now I need help adding a flow switch describes adding a Harwil flow switch to a Pentair MasterTemp heater.
It is recommended to leave the pressure switch in series when a flow switch is added.
Below is a Hayward H-Series heater with a flow switch installed at the water inlet:
