It depends on the model. The Grundfos Comfort System uses the home's existing cold-water line as the return, so no separate return line is needed. Grundfos's COMFORT pumps are built for homes with a dedicated return line, and the maker limits their use with tankless heaters.
What is a return line, and why does it matter?
The Department of Energy's building science site says recirculating systems save time and deliver hot water to faucets quickly. It also says that between 400 billion and 1.3 trillion gallons of water are wasted in U.S. households per year while waiting for water to heat up. A pump can only cut that wait if the cooled water in the hot line has somewhere to go. That somewhere is the return path.
A return line is a pipe that carries water from the far end of the hot water supply line back to the water heater. With a return line, a pump can keep water moving through a loop. The Department of Energy's building science site describes a compact plumbing layout with a recirculation line and short branches to each hot water fixture, with the loop placed within 10 feet of every fixture. It says a circulating pump draws hot water through the supply portion of the loop and returns any ambient-temperature water left sitting in the loop to the water heater.
Many existing houses have no such loop. Their hot water pipe dead-ends at the fixtures. That is where a kit like the Comfort System comes in, and it is why Grundfos treats the two families as separate products.
Which Grundfos pump needs no return line?
The Grundfos Comfort System does not. Grundfos says the system uses the existing cold-water return line in the home, so no separate new return line is required. In practice, water leaves the water heater through the pump, travels through the hot-water supply line, and comes back to the water heater through the home's existing cold-water line. Grundfos says the Comfort System is intended for indoor residential potable drinking water use, and is to be considered where there is no hot-water return line available.
Two parts make this work. A circulator pump goes on the water heater, and a valve goes at the furthest faucet. Grundfos also says no electrical power is required under the sink. For the steps, see our page on how to install a Grundfos Comfort System pump.
### What does the valve do?
The valve joins the hot and cold sides at the far faucet so water can cross from the hot line into the cold line. Grundfos's test for a working valve shows how it behaves. With the cold angle stop closed and the cold faucet open, the flow of cold water should gradually decrease and stop as the bypass valve closes. When you turn on the hot faucet, hot water should begin to flow quickly.
Which Grundfos pumps do need a return line?
The COMFORT range does. Grundfos describes COMFORT pumps as efficient circulator pumps designed for hot-water recirculation in residential buildings with a dedicated return line. In its application drawing, the COMFORT pump sits in the return line (item 5, the "Dedicated return line"), which carries water back to the water heater. The booklet says the pumps are compatible with all types of water heater tanks, tankless systems, solar and heat pumps, regardless of whether the system has mixing or tempering valves installed. It also says the range suits single-loop systems, branched systems (balancing valves are recommended), open-vented systems, and pressurized closed systems with or without mixing valves.
### Does the COMFORT range work with tankless heaters?
Grundfos qualifies it. The booklet says application with tankless water heaters is limited due to a higher pressure loss and a minimum start flow rate requirement. For this type of application, it says special attention is required during design calculations. In other words, a tankless heater needs design work before you pick a pump. If you have a tankless heater and cold water in the middle of a shower, see our page on tankless heaters that give cold water mid-shower.
### How do the two compare?
| Question | Comfort System | COMFORT pump range |
|---|---|---|
| Return line | Uses the existing cold-water line | Dedicated return line |
| Where the pump goes | On the water heater | In the return line |
| Extra hardware | Bypass valve at the farthest faucet | Optional valves, sensor on some models |
| Power under the sink | None | Not applicable |
| Intended for | Homes with no hot-water return line | Homes with a dedicated return line |
The COMFORT TA variant adds an external temperature sensor. Grundfos says the TA pump must be installed in the return line and the sensor mounted on the supply line, in the range between 20 and 50 cm (8 to 20 inches) from the water heater. The sensor wire is 2.5 m (8 ft) long. Our page on Grundfos COMFORT control modes explains what the sensor does.
What happens to the cold water?
When a system returns hot water through the cold line, the cold line warms up. ENERGY STAR describes it this way for an integrated loop system: hot water is recirculated intermittently and returned to the water heater through the cold water pipes. This raises the temperature of the cold water slightly, and it returns to the usual cold temperature in a short time. Grundfos's Comfort System instructions carry a related note. After installation you may find some warm water in the cold-water line at the sink where the valve is installed, and it will dissipate in a very short time once the cold-water line is opened. Our page on why a recirculating pump warms your cold water lines covers this effect and when it points to a problem.
The COMFORT range with a dedicated return line does not use the cold line as its return path. Neither Grundfos document compares the two designs on comfort or energy use, so this page does not rank them.
### How do you tell which kind of home you have?
Grundfos does not give a test in either document. The instructions for the Comfort System simply say to consider it where there is no hot-water return line available. A plumber can trace the hot water piping from the water heater and tell you whether a return pipe exists. Our page on which kind of recirculation system you have lists signs of each type, and our page on whether a recirculation pump is worth it covers the energy side.
When should you call a plumber?
Call before you buy if you are not sure whether your house has a return line, if you have a tankless water heater, or if the hot line branches to several bathrooms. Grundfos says that if your home has a branched hot water line, more than one bypass valve may be necessary for the Comfort System. HQ Plumbing & Air is licensed and insured, gives free estimates, quotes an upfront price, and offers 24/7 service.