The quick answer is that it simply directs the flow of water to prevent it from flowing through the heater. Adjustable valves are used to either bypass the heater or to flow through it allowing the heater to operate.
A pool heater contains copper elements. More specifically, the "heat exchanger." This is the point where the water is actually heated and then returned back to the pool. Although in recent years there has been an increase in production of titanium heat exchangers, most existing pool heaters and heat pumps still have a copper exchanger.
Unbalanced water chemistry, most notably, pH and alkalinity, can create an adverse and unwanted effect on a copper heat exchanger: corrosion. When a heat exchanger becomes corroded, copper will start to release into the pool water causing copper stains.
A pool heater can be costly to operate if used to continuously heat the pool. On average, the heater on a residential pool is used occasionally but doesn't run daily. It's not always necessary to change the bypass valve settings every time the heater is used or each time the pool is finished being heated. It's ok for water to flow through the heater on a continuous cycle unless for the following reasons:
Most copper stains are caused from a heater that has a corroded exchanger and is causing copper to enter the water flow. The copper will start to adhere to the floor and walls of the pool finish. When this happens, stains will start to form. This can be a blue or teal colored etched look. The stains can also appear as various sized black spots. In a very severe situation it's possible for the entire surface of the pool to become stained and discolored.
Typically, ascorbic acid is used in conjunction with a metal sequestering agent and other chemical adjustments to remove the stains. Iron stains are usually yellow in color and can be treated similarly.
When running the heater, a bypass valve can be positioned so that not all flow is directed into it. This is known as a partial bypass. The heater can still operate when enough water flows through it. High pressure can occur when trying to force all of the water through the heater. Smaller pool pumps with lower horsepower may require the bypass to allow full flow through the heater and no partial bypass.
Question: Can a pool heater be used on a salt system pool?
Answer: Yes. A heater or heat pump, whether it's gas or electric can be used to heat a pool with a salt generator.
© 2018 Rob Hampton
Tom on May 29, 2019:
Thanks Rob, your information is very helpful!
Rob Hampton (author) from Port Richey, Florida on May 29, 2019:
Tom. Yes, a check valve is the right idea. In that particular photo, I agree it should have been plumbed differently as to not allow ANY backflow to the heater. (I didn't do that plumbing job lol) Good eye!
Tom on May 29, 2019:
Hi, in the bottom right picture on the top, how do you keep the water from back-flowing towards the heater? I understand how you bypass from the top pipe (the "in") but once you've bypassed, wouldn't that water want to flow back towards the heater? I am trying to install a bypass and was considering a check valve between my heater 'out' and the point where the bypassed water connects. Thanks!
Rob Hampton (author) from Port Richey, Florida on February 14, 2019:
Bill Roberts.. to determine sufficient flow through the heater, this can be done using the filter pressure gauge. With a clean filter, bypass the heater completely and take note of the pressure. Then, change valve so that ALL flow is directed through the heater. If the pressure changes drastically then there is flow restriction. (it will change slightly with any heater) the pressure will be higher if filter is plumbed BEFORE the heater, or lower if after the heater. Remember though, during normal heater operation that the valve should be positioned to allow flow through both the heater and bypass. This allows for better flow with enough flow with enough water going through heater.
Bill roberts on February 13, 2019:
New master temp installed, attached spa had almost no flow over waterfall. Called Pentair asked if this could be caused by restriction in new heater compared to previous mini max. Said No ,turn up pump speed. This seemed counter productive as I was running pool 12 hrs. A day to reduce electric cost.
Moved bypass valve about half way between heater and feed to pool. Will this hurt my heater, how do I know if there sufficient flow through heater?
Louise Powles from Norfolk, England on May 06, 2018:
I wish I had a swimming pool. I love swimming. My mother has one though where she lives in Spain. She's really lucky, she swims every day.