Do I Need A Bypass On My Pool Heater?
If you have a heater for your swimming pool then this is one of the single most expensive individual components on your pool system. Further this this it is also one of the most temperamental in terms of getting it to turn on and work when you want it to. A pool filter does not have a lot of moving parts by comparison and so filters tend to break down or require service substantially less than heaters. Being that heaters are expensive, they have a lot of moving parts by comparison to your other pieces of equipment, and heaters can break in a lot of different ways, it would make a lot of sense for you as the pool owner to take whatever steps you can to protect your investment in warmer swimming temperatures.
Protecting your pool heater is important if you want to get the longest service life possible from it. Which you definitely should. If you want to explore this topic further you can read this article about how to install a pool heater or if you have an existing heater installation you can take a look at the pool heater checklist to see if there is anything you are doing that might be damaging the heater. One topic that deserves a dedicated page is this question about whether you should be using a zoned bypass for your pool heater or not. To answer this in a meaningful way I need to touch on a few different subjects to provide some relative context:
What is a bypass? - Let's get the easy stuff out of the way. A plumbing bypass, or plumbing zones, or a zoned bypass are all ways of describing adding a side branch off of the main trunk plumbing line which is your circulation system. When something is installed "in series" (inline) this means that all of the stuff in the system (water, electricity) must pass through it. When something is installed "in parallel" (offline) then the stuff, water in this case, does not necessarily need to flow through the parallel device. You can, or you can choose not to, or with a versatile system like a three zone bypass you can have the option to send all, none or only some of the water through the peripheral device. For plumbing systems, when you hear "zone" just picture a valve. A three zone bypass on your heater could be restated as a three valve bypass for your heater. Before we get into how to install one of these and what they look like, first you need to know why this is an important consideration for your pool.
Do I need to bypass my heater? - When you care for your pool water properly you will need to regularly do something called superchlorination where you increase your free chlorine levels to at least ten times the combined chlorine value and hold them there for a sustained overnight treatment. During this period it would not be uncommon to experience free chlorine levels in excess of 10 parts per million. This is something that regularly needs to be done with chlorine pools to keep the combined chlorine levels below 1 ppm. Additionally there will very likely be times where something goes wrong with your water maintenance and you have poor quality water for any number of reasons. During both these planned events as well as unplanned events, it would be a huge advantage to your heater to be offline and not having poor or corrosive chemical laden water coursing through it. Internal corrosion, leaks and failure of pool heaters is the most common way that pool heaters fail. The conventional wisdom is to use a zoned bypass system to take the heater offline as needed to protect it from poor quality water.
How do you bypass a pool heater? - A heater bypass is not a silver bullet to solve your heater problems. This is just a tool, however it is a very useful tool when used properly. If you have poor water quality already before you take your heater offline, now you have trapped poor quality water within the heater which might very well be worse that circulating poor quality water. Taking a heater offline is best done preemptively like for a planned superchlorination. When done after a problem is discovered you want to limit how long the heater sits stagnant, and any period longer than 12 to 24 hours should consider draining or blowing out the water from inside the heater. There are a lot of different heaters and configurations out there. You should have a local pool or heater expert come out to evaluate your unique installation and provide advice about how to most effectively utilize a heater bypass for your system. It may or may not be a good idea to drain your heater when offline and it would probably depend on how bad the water is that is sitting inside it. High chlorine is very bad. Sustained high pH is bad (scale forming) and period of very low pH are bad (acidic). High chlorine combined with very low pH is probably the worst combination for your heater internals and definitely something you want to avoid.
There comes a point where you might ask yourself why your pool guy did not install a heater bypass in the first place on your pool. This is, in my experience at least, simply a function of cost. When you zone in a bypass for a pool heater you will need to have a few valves. Two at minimum and three for optimal flow control if you are using in-and-out style valves like these Jandy diverter valves. If you wanted you could create a zoned bypass with as little as one three way diverter valve such as the Jandy 4715 (1.5") or the Jandy 4717 (2"). Usually a few hundred in valves is about right, plus a few extra minutes to put it together, plus you need a little more space on the equipment pad for the valves and bypass and most equipment pads are undersized already. This is already a few hundred dollars and then add to this another consideration...pressure relief valves.
Pressure relief valves on pool heaters - Pool heaters all used to come with pressure relief valves because they used to, you know, blow up with alarming regularity. Modern pool heaters have internal safety mechanisms that have advanced greatly since the first days of pool heaters, and now only some pool heaters will have a pressure relief valve, while others do not. Some installers put them in, and some do not. Some areas require them on all pool heaters, and others do not. It is worth noting however that in many areas a pool heater installed "in series" (inline) in your plumbing system does not require a pressure relief valve, however if you decide to zone in your heater with a bypass, many areas now require that heater to have a pressure relief valve installed.
At a few hundred for the valves for the bypass, plus perhaps the better part of another hundred for the pressure relief valve itself, plus it needs to have some plumbing as well to escort any vented (boiling) water to a safe location as opposed to blasting whomever happens to be standing next to the pool equipment. At the end of the day the guy selling the $2500 heater is overflowing with business, while the $2900 heater company installing bypasses and pressure relief valves at literally no extra financial benefit to themselves hardly makes a sale. That is why most pool heaters do not have a bypass. I would definitely want a bypass for my pool heater and you should as well.
How to make a heater bypass - You can use individual valves, either two zones or three zones to make a heater bypass. Alternatively you can use a single three way diverter valve instead placed at the first pipe tee location feeding to the heater bypass. The three way diverter valve can send water straight to the pool, or it can close the pool pathway and force all water through the heater. You could even set it half way between the two to allow some flow to the heater and some to bypass it and head straight back to the pool. There is no "right way" for every application however I endorse the three in-and-out valves to make a three zone bypass for pool heaters. I do not like the potential for water to push back through the outlet side of the heater from a single diverter bypass closed on the heater side.
Maximum pool heater flow rates - Something that is more of a modern day concern due to increasing size and power of residential pool pumps is that some robust filtration pumps might exceed the maximum flow rate value for your heater. Check with your heater manufacturer or owners guide for your specific numbers, however many are in the 125 GPM range for maximum flow. This is a lot and most pool systems will not exceed this, however some of the newer 2 and 3 horsepower variable speed pumps will be able to exceed this number on some plumbing systems. If your system will move more water than the maximum that your heater can safely handle then you will require an external bypass to be installed. Most new heaters have internal bypass systems installed to prevent from sending too much flow through the heat exchanger but an external bypass may be preferable.
The pool heater checklist
Why did my pool pump fail early?
How to get the most life from your pool
Why did my pool heater break?
How to install a heater so it lasts
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