John Metcalfe’s San Francisco renovation included the installation of two tankless water heaters and a small circulation pump in his four-story, 3,200-sq. ft. home. The water heaters, connected in series, are located on the second floor, which is a more or less central location.
His hot-water problems should be over, right? Except they’re not.
“We now wait for up to two minutes for hot water in the kitchen and upstairs shower,” he writes in a Q&A post at GreenBuildingAdvisor.
The reasons for the long wait, and the best ways to correct this problem, are the topics for this Q&A Spotlight.
Longer waits are the norm
Traditional tank heaters keep water hot all the time, leading to what are called standby losses but also making hot water available at the turn of a tap. Not so with tankless heaters, which begin heating water only when a faucet is opened and water begins to flow.
“Different brands of tankless water heaters perform differently,” writes GBA senior editor Martin Holladay, “but in general you will have to wait a little longer for hot water from a tankless water heater than from a traditional water heater with a tank.”
Holladay notes that Metcalfe’s wait for hot water would probably be shorter if he went back to a tank-style heater. “Other ways to decrease your wait include moving the water heaters closer to the bathrooms and kitchen, and decreasing the diameter of the hot water lines,” he says.
Holladay writes that Metcalfe should make sure that his circulation pump operates by a switch located in the kitchen or bathroom. This is a less wasteful option than either a continuously operating pump or a pump on a timer. Switched pumps bring hot water to the point of use without wasting water. Holladay suggests that Metcalfe consult a Fine Homebuilding article on hot water circulation pumps. The article recommends the D’Mand pump manufactured by ACT Metlund.
Jin Kazama agrees that a water heater should be located close to the fixtures it serves. “To [reduce the delay], one needs to install the tankless water heater in close proximity to the usage point,” Kazama writes. “So you should’ve installed small tankless heaters near [the] kitchen sink or a close by embranchment serving many points, and another close to the shower.”
In Kazama’s view, the only reasons for choosing a tankless heater are low water use or reducing the delay for hot water (“which you obviously didn’t implement correctly”).
Why connected in series?
When two hot water heaters are connected in series, it means one follows the other on the same water supply line, and to David Meiland that doesn’t add up.
“I’ve never seen two tankless heaters in series,” he says, “and I’ll got out on a limb and say it makes no sense to me. Each should be large enough to provide for part of the load, and they should be piped separately. So, one heater serves the first two floors, the other one the second two floors…or something like that.”
When connected in series, he adds, both heaters are trying to supply hot water for the same demand, “however small it might be.” A single unit running at a higher load all by itself would probably be more efficient, he says, and locating the heaters close to each other all but guarantees that piping runs will be longer than necessary.
“What your plumber probably should have done was install at least one tank, if not two, put them farther apart, piped them separately, installed small-diameter home runs to the sink fixtures, and installed circulation pumps to keep wait times down at the farthest fixtures.”
Nor does Richard Patterman understand the plumber’s thinking.
“The logic of two tankless heaters in series completely escapes me,” Patterman writes. “The first heater heats water from 50 degrees to 120 degrees. What does the second heater do? And a circulation pump on a tankless defeats the purpose of a tankless. The heater should only come on when you turn on a faucet. I think your plumber owes you some serious repairs!”
Tweaking the circulation system may help
“Easy solution that I used, Chillipepper on demand circulating pump that stops pumping when water temp at the pump reaches your desired setpoint,” Heidner says. The pump is only activated by a low voltage contact closure switch/relay, he adds, with the relay controlled wirelessly or when a bathroom light is turned on.
“By triggering the circulation pump to run when the bathroom light is turned on, the water is hot and ready at the sink when you want it,” he says. “Not all circulating pumps will trigger a tankless to heat. You need to exceed 1 [gallon per minute]. And you need to check the manufacturer’s warranty. Some will not cover you unless you add some kind of moderating tank.”
Heidner says he had a nearly identical problem to Metcalfe and would typically waste 2 gal. of water before it was hot enough to use.
“Now the water is almost always at temp when you want it,” he says. “Water use is very low, energy use is low (I measure both!).”
Our expert’s opinion
GBA technical director Peter Yost had this to say:
First, regardless of the domestic hot water power plant — tank or tankless — every system suffers or excels based on the efficiency of the distribution system. And just as there are ACCA standards (Manuals S and D) for forced-air, space-conditioning distribution systems, we really need standards for DHW distribution systems as well. Both types of systems deal with efficiently moving fluids around, and efficient piping is as important as efficient ducting.
In my view, either efficient piping or structured plumbing is a no-brainer in new homes; get the distribution system right and the need for a pumping or mechanical solution is either eliminated or easily achieved. Get the piping layout/pipe sizing wrong, and use hard 90s rather than soft 90s, and adding a recirculation pump is just brute force trying to overcome a design problem.
In retrofit, adding an on-demand recirc crossover system (cooled hot water returning to the plant or tank by way of the cold water line) can make a lot of sense. There is more than one company employing the ACT Metlund system (and now there is the distinctly separate Chilipepper on-demand recirc system). The keys to these systems are high-speed, efficient pumps, on-demand operation, and thermostatically controlled shut off. The cooled hot water sitting in the pipes is sent back to the tank or plant while “new” hot water arrives at or very near all points of demand.
The ACT Metlund system (including systems from Uponor and Taco, two companies that use the same base components) comes in three sizes, meant to be sized for the total system resistance or pressure drop, primarily the length of the runs. The Chilipepper has some significant differences compared to the Metlund systems, with the most significant perhaps being that Chilipepper has just one size of pump.
Metlund lists which of their models should be employed for various plants (tank or tankless) and run lengths, but does not include resistance factors such as piping diameter changes, or number and severity of elbows or bends. I asked two plumbing engineers about this, and since they both have installed retrofit Metlund systems with good results, they felt that the piping run length guidance is probably all that is needed. But they did admit that it would be helpful to actually test different distribution layouts to determine the impact of resistance factors other than length.
Another plumbing engineer expressed concern with any of the retrofit systems, given this part of the plumbing code:
“604.2 System interconnection. At the points of interconnection between the hot and cold water supply piping systems and the individual fixtures, appliances or devices, provisions shall be made to prevent flow between such piping systems.”
But he admitted that both the Metlund systems and the Chilipepper system claim code compliance on their websites and appear to be successfully installing their retrofit systems regardless.
On the Chilipepper website, they strongly encourage use of a true plumbing professional for any tankless water heating system. Tankless water heating systems are much more complicated than conventional tank systems, and therefore adding an on-demand recirc system requires a stronger, deeper background to achieve a successful installation.