Why a Heater Bypass Valve is a Good Thing to Have
Many pools have a heater in their water flow. Water circulates through the heater core all the time the pump is running, whether the heater is on or off. If the heater springs a leak, you have to shutdown the pump, which stop waters circulation, filtering and chlorinating your pool. Having the pump stopped too long can lead to algae and a dirty pool, and more work once you get the heater repaired and the pump running again.
If you had a heater bypass valve you would just move it the bypass position and you can continue running the pump and circulating pool water normally while you get the heater repaired or replaced.
With a relatively new pool or heater you may not see a leak for many years and wish you had a heater bypass valve. However you may encounter another situation where bypassing the heater is helpful. Heater cores can be damaged by acidic low pH below 7. Maybe you misjudged your acid addition and accidentally lowered your pH too much, If you have a heater bypass you can turn the valve and stop the acidic water from flowing through the heater. Or maybe you purposely lower the pH to around 7 to treat a scale problem. You can use the heater bypass valve to take the heater offline and protect the core.
Water running through a heater all the time creates back pressure, slows the water flow, and makes the pump work harder running at higher RPMs. Especially if you have a heat pump which has smaller diameter piping then a gas heater. Some pool owners only use their heater 5% of the time, yet their water flow is restricted the 95% when the heater is not in use. Bypassing the heater during the hot summer months when it is not necessary can improve water circulation.
For some unknown reasons many Pool Builders push back on the request to install a heater bypass. It is very simple requiring a 3 way diverter valve on the heater input line, a short length of PVC to connect to a T on the heater output line, and a check valve between the T and the heater output. The added cost and plumbing is minimal.
As described above there are many situations where a heater bypass is useful. Sooner or later in the life of the pool you will likely encounter one of them. And really there are no downsides in having the heater bypass line between the heater input and output pipes.
