I have a Hayward CZ 250 heater and it has a flow/pressure switch which detects water flow. If I do not have water flowing through my heater, the heater will not ignite. I also put in two valves that control water flow to the heater and also used to bypass it. This is a good idea in the Spring as water usually needs to be balanced and has crud in the flow. Also, during the summer months, when you do not need the heater, there is no reason to have water running through your heater. Doing so just exposes the heat exchanger to possible damage if you run unbalance water through the heater for an extended period of time. The damage is usually due to the copper in this unit. Trust me. I have already replaced one heat exchanger and it was expensive. Two cheap valves can eliminate most of the risk.