Usually when these are plumbed in there is a bypass valve allowing you to run your filter and system withouth the heater. There would be a valve before and after the heater (which should be open) and a valve connecting the inlet and outlets ( this valve should be closed) Make sure you're not "bypassing: most of the water flow. If this isn't the problem then try the following: This switch is designed to prove flow. If you don't have enough flow to cause enough back pressure to make switch, then it assumes you dont' have flow. First make sure your filter is clean, and your skimmer and pump baskets are clean so you're pumping maximum volume of water. If these are clean have an assistant or two hold their hands on your discharge ports in the pool. If this backpressure cause the switch to close then you might try slightly smaller return jets to your pool. You can not the pressure on your filter and compare it to the pressure setting on the switch.
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Usually when these are plumbed in there is a bypass valve allowing you to run your filter and system withouth the heater. There would be a valve before and after the heater (which should be open) and a valve connecting the inlet and outlets ( this valve should be closed) Make sure you're not "bypassing: most of the water flow. If this isn't the problem then try the following: This switch is designed to prove flow. If you don't have enough flow to cause enough back pressure to make switch, then it assumes you dont' have flow. First make sure your filter is clean, and your skimmer and pump baskets are clean so you're pumping maximum volume of water. If these are clean have an assistant or two hold their hands on your discharge ports in the pool. If this backpressure cause the switch to close then you might try slightly smaller return jets to your pool. You can not the pressure on your filter and compare it to the pressure setting on the switch.