Residential DHW recirculation

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By Simon Blake

Waiting for hot water can be a royal pain for homeowners, not to mention wasteful of water. New products released in the past few years have made it easy for the plumber to solve the problem, but most homeowners don’t know there is a solution.

“We think it’s a huge opportunity,” remarked Simon Feddema, president of Grundfos Canada. “We think it could double the number of circulators sold in North America.”

Domestic hot water (DHW) recirculation systems have evolved to the point where they can, in some cases, be installed in less than an hour. There are now a number of different systems that can offer instant – or almost instant – hot water at the tap.

As well, new pump and control technology has changed the way some of these systems function, providing the plumber with new solutions to offer their customers.

DHW recirculation is a worthwhile service to list on the company truck.

It’s all about comfort

In Canada, water is plentiful and relatively inexpensive, so for all but the most ‘green’ homeowners, the key reason for installing a DHW recirculation system is comfort.

“It’s about having hot water delivered quicker to your faucets. It’s not going to save you money,” remarked Sean Giberson, Canadian sales manager, wholesale products, for Taco Canada, Milton, Ont.

The problem has been that while water is inexpensive the energy to heat it – electricity, gas, propane or oil – is not.

In fact recirculation systems can be energy pigs, especially those that recirculate continuously. A traditional system keeps the water in the pipes hot around the clock, using energy both to heat the water and run the pump. However, new recirculation and pump technologies have gone a long way to reducing operating costs.

The typical residential DHW recirculation system would be one of the following:

A dedicated loop system typically has a pump located near the water heater on the return line. At each fixture, a short pipe connects the loop to the hot water tap. The hot water circulates continuously and it takes only a split second for the hot water to reach the tap.

The energy costs are high, but adding a manual switch, a timer, an aquastat and/or a learning /‘smart’ pump can improve the efficiency substantially while still providing a high level of comfort.

A variation on this is the integrated loop system in which a control recirculates hot water intermittently when the water temperature drops below a setpoint.

On-demand systems

Another way to do DHW recirculation is with an on-demand control system. The simplest system involves installing a switch in the bathroom so the user can turn on the pump before they step into the shower.

Steffen Werner, director of marketing/Western sales for Wilo Canada, Calgary, remembers a recirculating DHW system in his previous home in Germany. “You turn it on, you get in the shower. You get out of the shower, you turn it off.” Today, a motion sensor can perform the same function.

The button or sensor triggers the pump, which returns sitting water to the DHW heater via the cold water line, replacing it with hot water. “It’s a much higher head and higher flow that will see hot water at that faucet in 15-20 seconds,” noted Giberson. It’s not instant, but it’s a lot better than the typical 90 seconds.

Timer systems

Installing a timer in the system can cut energy use considerably. The timer is set to provide hot water when the family requires it the most. Adding an aquastat that acts as a temperature sensor will make it even more efficient because the pump won’t run if the water is already up to temperature.

Using a variable speed pump with a timer also makes a lot of sense because it can adjust flow according to demand, added Peter Sraum, technical applications specialist for Wilo Canada. The pump adjusts flow as required. “When you have that option with a timer, I think that’s unbeatable.”

If some of these things sound complicated, they’re not. Today’s equipment is “plug and play.” In a timer system, for example, the pump plugs into the timer, which is plugged into a wall outlet.

Smart pumps offer both comfort and efficiency.

Smart pumps

There’s a new level of control possible with a smart pump, which most of the manufacturers now offer.

Grundfos recently updated its Comfort PM recirculation system with the company’s AutoAdapt smart technology, for example.

This system analyzes water use patterns in the home and adjusts the hot water supply accordingly. If the occupants use a lot of hot water first thing in the morning and then the house is vacant most of the day, the control will learn to start the recirculating system early in the morning, leave it off most of the day, and then bring the water up to temperature again just before the occupants return home.

It’s a good solution for customers that just want the best system and aren’t that concerned about cost, or the customer that wants the greenest system possible.

And manufacturers are beginning to offer equipment that can add this ‘smart’ technology to existing circulators.

A simpler solution

At the other end of the spectrum, one way to do hot water recirculation is with a crossover valve under the farthest faucet from the hot water heater. Most manufacturers have them, although the technologies differ from one manufacturer to the next.

The Taco technology, for example, uses a bi-metallic disk in the crossover valve that closes it based on a 30-degree temperature rise. The pump is installed at the tank. If the water sitting in the line overnight is at 70F, the system is open and when the circulator reaches a programmed run time it rotates the cold water back to the tank and brings warm water to the faucet. When the water reaches 100F, the valve snaps shut and the user has hot water at the tap.

Often the question is raised about having hot water in the cold line. Typically, only about two cups of warm water exists there, which is purged immediately upon operation of the cold tap, noted Giberson.

And while the standard installation is to install one crossover valve for the entire house, which will improve response time at all faucets, if the customer wants even quicker response time, a crossover valve can be installed at each faucet. That’s money in the plumber’s pocket!

A pump with a crossover valve is simple and effective.

Storage versus tankless

So far, the systems we’ve talked about are for storage tank systems. Sraum has been asked to troubleshoot a number of DHW recirculation systems over the years and many of the problems have involved tankless water heaters.

“If you’re trying to move a gallon a minute through a tankless unit, then you are looking at 10 to 15 feet of head or more depending on the length and size of the recirculation line. All of a sudden the pump that you have mounted is not good anymore, so you have to go with a bigger pump.
“In many of these cases when the first plumber leaves the smaller pump, really, it does nothing,” he added, noting that most tankless units need a minimum of 0.5 gpm to turn on.

When the disk in the Taco crossover valve, for example, opens, it only allows a quarter gpm of water through. That’s not enough to trigger the gas valve in an instantaneous DHW heater, reported Giberson. “We’ve got to make sure we can overcome the pressure drop across the heat exchanger and turn that gas valve on.”

However, there are work-arounds. A larger pump may be required. A small buffer tank will provide additional water volume. Most pump and DHW heater manufacturers have solutions and can offer advice and piping diagrams.

Water savings

Municipalities like DHW recirculation because it saves water, reducing the load on water treatment infrastructure. The average household typically wastes about 12,000 gallons (45,425 litres) of water per year running the tap until the water gets hot. Multiply that by the number of houses in the municipality and the amount of water wasted is substantial. Comfort aside, it may be only a matter of time until municipalities require DHW recirculation systems in new construction. One way or another, it’s going to be a growing part of the plumber’s business.

“The plumber needs to know there is an option. A lot of them, when they went to school, learned that you either have a dedicated recirculation line or you don’t get recirculation. They need to understand that they can do recirculation without taking a house apart,” said Giberson.

If contractors advertise recirculation systems to the public – most of whom have no idea such technology is available – it becomes a really nice “valley filler” during slow periods, he added.

“I think if the plumbers embrace it there’s one heck of a business for them!”

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