Hot Water Recirculating Loops: A Tricky Green Feature To Get Right 

Hot water recirculating loops are having a moment.

They’re popular with plumbers because they reduce callbacks from homeowners who don’t want to run their taps and wait for hot water.

Recirculating loops are a water-saving feature, especially if coupled with low-flow plumbing fixtures on showerheads, toilets and faucets. But if not properly done, they can waste a lot of energy. 

• A typical “trunk and branch” hot water system has piping that comes from the water heater in a “tree” configuration, with a main trunk and smaller branches that go to each fixture. When you turn on the hot water tap, you have to wait for the cool water to flush out of the piping before you get hot water. These can be made more efficient by locating the water heater close to the fixtures so that the piping isn’t very long — this arrangement is often referred to as a “compact” plumbing system.
• “Manifold” systems have a “home run” of pipe to each hot water fixture. These home runs can be smaller and more direct, so it doesn’t take as long to flush the cool water out of the pipe.
• A recirculating system is piped differently, using a loop that travels throughout the house and gets close to all of the fixtures that use hot water. There are very short branch pipes that connect each fixture to the loop. A pump is used to circulate hot water through the loop, so it arrives at each fixture very quickly.

If the house is smaller and the water heater can be centrally located, you don’t. Sometimes in a bigger house, you can have two water heaters, both of which are relatively close to their fixtures. We also encourage clients to think about whether they need fast hot water at every fixture. They may be happy with a water heater that’s located close to the master bathroom, but be willing to wait at an infrequently used guest bathroom or a laundry room. Using a manifold system is another option for stretching out the distance from the water heater without a long wait.

When you understand how recirculating systems work, it’s pretty obvious what can go wrong from an energy standpoint. First, an inefficient pump that runs all the time can use a lot of energy. Second, the pipe loop filled with hot water can transfer a lot of heat to the house, especially if it’s not insulated. You’re essentially heating your house with your hot water — which is inefficient in the winter and fights with your air conditioning in the summer. Insulating the pipes helps, but some heating still occurs because pipe insulation isn’t very thick. These effects can add up to a potentially big energy penalty. 

If we look at all of the above strategies and feel that the plumbing fixtures in a home are just too far apart and can’t reasonably be served by compact or manifold plumbing systems, it’s worth considering recirculation. But you have to commit to doing it right, which isn’t going to be free. 

1. Insulate all of the loop piping. This is actually a code requirement in North Carolina. Unfortunately, a lot of plumbers aren’t aware of it because it’s in the energy code, not the plumbing code. It’s especially important to insulate the last few feet of pipe right at the water heater, which are often installed later and missed. Making sure to fully insulate bends and tees matters. Insulating the branches also helps.
2. Don’t install a crazy amount of loop piping. If you can do less loop, you get less heating that you don’t want. Lay out your loop to get close to those important rooms (master bath, kitchen, etc.) and maybe you don’t need to get so close to infrequently used rooms.
3. Use a control so that the pump doesn’t have to run all the time. This saves energy from the pump itself and also lets the loop cool down some to reduce the house heating effect. The best controls are either occupancy sensors or buttons that the homeowner can press when they walk into a bathroom to activate the pump when someone is likely to use hot water. A less efficient (but easier and less expensive) option is to use temperature-based control. These controllers activate the pump only when the loop cools down.
4. Don’t only rely on timers. Timers seem like a good idea, but they’re not a stand-alone solution and you get no credit in the Home Energy Rating System (HERS) for a timer. I think timers can be helpful when installed with temperature-based control. They can be used, for instance to turn the system off entirely overnight when hot water is unlikely to be needed. However, they’re not very effective during the day. People use hot water at fairly random times, and they tend to turn timers off when they aren’t getting hot water. If you have a loop that’s not on it will take much longer to get hot water at the fixture than it would with a non-loop system, so occupants likely won’t be satisfied if they want hot water when the timer is off.
5. Don’t allow your plumber to install a “future loop.” The idea behind the “future loop” is that they’re going to install the piping in a loop and only add a pump if you complain about how long it takes to get hot water. There are several problems with this:

• You’re going to complain. It takes longer to get hot water if the plumbing is installed in a loop.
• It’s almost always impossible to get access to insulate all the loop piping later. We don’t install anything else in homes for the future that we know wouldn’t meet today’s building code. If you have a loop, it needs to be insulated.
• If you don’t connect the return portion of the loop, you’ve created a “dead leg” in your hot-water piping system. A dead leg is a length of piping that is connected to your active hot-water piping but doesn’t go anywhere. Water just collects there at room temperature or slightly above. Bacteria (including the kind that causes Legionnaires’ disease) loves this range of temperatures and is not very easy to trace as the cause of someone getting sick.

So absolutely, we want you to practice water conservation. We’re also totally cool with the idea that you might also get fast hot water. Just don’t do it at the expense of energy conservation. You can have both!

Amy Musser and Matthew Vande are the owners of VandeMusser Design, PLLC. They provide HERS services, green-design consultation, and home-energy audits to homeowners in Western North Carolina. Amy is a licensed mechanical engineer and Matt is a licensed architect.

You can also view this article as it was originally published on pages 68-69 of the 2020-2021 edition of the directory.