Hot Water Recirculation VS Tankless Water Heaters – Not Compatible?

Typically, hot water recirculation systems with a few exceptions aren’t compatible with tankless hot water heaters.

Hot water circulating systems were designed to work with storage type

water heaters. Without the tank full of water the burners would be cycling on and off constantly trying to maintain the temperature in the piping. Since the piping is continuously losing heat the heat must be replenished continuously.

Tankless water heaters are much more complex than a traditional storage heater. The tankless heater has to put much more heat into the water much more quickly than a storage unit in order to get the water up to the desired temperature before it leaves the heater.

It sounds simple but it is not. Putting all that heat energy into the heat exchanger means that if the water stops flowing or even if the flow just drops significantly the heat exchanger can be quickly destroyed. That is why tankless heaters need so many sensors. Tankless water heaters monitor the gas supply, the exhaust, and the inlet and outlet temperatures.

The minimum flow that tankless water heaters require is typically ½ to ¾ gallon per minute. Most hot water circulating pumps will not produce enough flow through tankless heater to turn on the heater.

Let’s suppose that we get a good big pump and pump enough water to turn on the heater. With a traditional tank type storage water heater the outcome would be easy to predict. When the hot water in the tank falls below the burner set point the burners will come on and heat the water until it hits the upper set point, and then the burners will shut off. The cycle will repeat over and over.

What happens with the tankless water heater is a lot more difficult to predict. The tankless heater is trying to maintain a constant outlet temperature. It doesn’t just turn the burners on and off, it regulates or modulates the gas flames (or electric current) to try and maintain a very accurate outlet temperature. Some heaters even modulate the flow of water through the unit.

Remember, the tankless heater is measuring things like inlet water inlet temperature and exhaust temperature and taking appropriate actions to keep the heater running correctly and keeping the outlet temperature at the set point.

Tankless heaters weren’t designed to raise the water temperature only a few degrees. When the inlet water gets up near the set point, which it will rather quickly, then the heater won’t know quite what to do. The Exhaust temperature might not ever stabilize, and thus could cause headaches for the tankless electronics. Tankless heaters for the most part just weren’t designed to work with circulating pumps. All sorts of strange malfunctions could pop up with such an arrangement.

Demand hot water systems are compatible with tankless heaters

A demand system is a system that acts like a circulating pump, circulating water through the hot water piping and then through the cold pipe back to the water heater. But a demand system shuts the pump off when hot water reaches the last fixture. Therefore no hot water ever gets sent to the water heater inlet.

As far as the water heater is concerned it just thinks someone turned on a faucet. As long as your demand system pump is capable of creating enough flow to turn on the water heater it will work fine with a tankless water heater.

More about hot water recirculation.  Hot Water Recirculation – What Are the Benefits and Pitfalls?

Hot Water Flow Rates with Tankless Water Heaters and Storage Heaters

Hot Water Flow Rates with Tankless Water Heaters and Storage Heaters

The flow rate of the water in a pipe, and the diameter of the pipe determine the velocity of the water
traveling in the pipe, and thus the amount of waiting time for hot water to arrive at the fixture.

Increasing the flow rate and decreasing the pipe diameter have the same affect; increasing the velocity of the water in the pipe. Flow rates that are too high can produce noise and vibration in the piping, and can cause water hammer problems. High flow rates can cause pipe erosion especially in the elbows and T fittings.

Often building codes limit the water velocity in hot water piping to 5 feet per second. At five feet per second the hot water should reach a fixture connected to the water heater with a 100 foot pipe in about 20 seconds.

The table below gives the flow velocity in feet/second for a number of flows in g/m with typical pipe sizes.

 

 

Pressure Drop Affects Flow Rate

To push for instance, 4 gallons per minute through a 100 foot long ½” diameter Type L copper tubing would require a pressure of 10.6 psi. No problem if your source of water is more than 10.6 psi, but if you are circulating water with a hot water circulating pump then you have more of a challenge.

At 1 gallon per minute the drop would be less than 1 psi but the wait would be about 112 seconds.

At 5 gallons per minute the pressure drop would be 16.1 psi. (37.1 feet of head)

Hot Water Circulating Pumps

Here is a graph showing the pump curves for TACO hot water circulating pumps and are virtually the same as all the other brands. (1 psi = 2.3 feet of head and 1 foot of head = .434 psi.)

From the graph it is easy to see that none of the pumps will be able to create a 5 g/m flow and only two a 4g/m flow.

Hot Water Return Line

If you have a hot water circulating system then you need a return line from the furthest end of the hot water supply pipe back to the pump and water heater. If you are using a hot water demand system then the cold water piping is typically used as the return line. When calculating the wait time for the hot water the return line isn’t included, but for purposes figuring pressure drop and flow rates it will need to be included.

Tankless Water Heater Pressure Drops

Storage water heaters (Tank type heater) typically have insignificant pressure drops when water flows through them. Below is a graph showing the pressure drop through various models of Rinnai tankless water heaters. Tankless hot water heaters do have significant pressure drops.

From the graph it can be seen that the larger the heater the lower the pressure drop at any given flow rate.
To find the pressure required to push a certain flow through the piping system requires one to add the pressure drops from the piping and from the water heater.

Reading from the graph with the largest model (lowest pressure drop) we find that at 4 gallons per minute the pressure drop will be approximately 5 psi, and at 5 g/m the drop will be about 8 psi. On the other end of the scale, the smallest tankless water heater has a pressure drop at 4 g/m of about 9psi and at 5 g/m it’s close to 12 psi.

Armed with the above charts and graphs it should be fairly easy for you to get a good idea of how long the wait for hot water will be for any given plumbing layout and pump combination. Basically you want to minimize pipe diameters and lengths to minimize the total volume contained in the hot water piping to have
the most efficient hot water plumbing system.

Using a Chilipepper with a Dedicated Hot Water Return Line

Hot Water Demand Systems and Dedicated Return Lines Are Compatible

For home owners who have a residential hot water circulating system and would like to convert the system to a demand system to save energy, reduce their carbon foot print and be green as far as water and energy conservation go, here is how to do it.

Traditional residential hot water circulating or recirc systems waste a lot of energy. The cost of heating water is generally substantially higher than the cost of the water itself.

With a hot water demand system you will have a wait for the hot water, and although the wait is shorter than with a non-pump residential plumbing system, there is still a waiting period before the hot water reaches the fixture. Once that wait is over then when you turn on the tap you will have nearly instant hot water. The length of the wait depends on two things, the plumbing layout, and the power of the pump being used.

Installing a Demand System with a Dedicated Hot Water Return Line

Typically a traditional hot water return line consists of a pipe connected from the hot water supply pipe at the last fixture to the inlet of the water heater. Usually the pump is located at the water.

Replacing the existing pump with a Chilipepper hot water demand pump is very easy to do. Remove the old pump and plumb in the Chilipepper. Use hoses to connect the Chilipepper inlet to the same fitting that was hooked to the old pump's inlet and do the same with the outlet.

You can then either hard wire the buttons to the various fixtures, or if you don't want to string wire in your attic or crawl space or there are other impediments to hard wiring start buttons, use an X10 remote control system for starting the pump from any location. Insteon remote control can also be used but Insteon stuff is pretty pricey.

What to Expect - Operating Your Hot Water Demand System with a Dedicated Return Line

There are differences in the way a hot water demand system and a hot water recirculation system work. .

Since hot water is continuously circulating the loop stays full of hot water. Since a hot water circulating system wastes so much heat energy lots of home owners use timers and or temperature controls to reduce the operating hours and thus reduce the energy wastage.

A timer can be used with the circulating pump to shut if off during periods of no or little use. This does of course save oodles of energy, but is quite inconvenient if you happen to need hot water during one of those periods that the system is turned off.

Another method used to reduce the energy wastage of recirc systems is to have the pump turn on when the temperature drops below a set point and turn off when it reaches an upper set point. The problem is that if you reduce the temperature low enough to realize significant energy savings you don't end up with hot water in the loop, just warm.

With a hot water demand system, when you want hot water you must push the start button and then wait for the hot water to arrive. The hot water will of course, reach the first fixture in the loop first, and then each fixture along the loop until the pump senses the arrival of hot water.

When the Chilipepper detects a temperature increase of 3 to 12 degrees, depending on the sensitivity setting, it shuts off. If you again push the button the pump will again shut off when it sees a second temperature increase... which typically would only be a second or two because the hot water has arrived.

Once the temperature hits 96 degrees or above the pump locks out and won't restart until the water temperature in the pump drops below 96 degrees.

Due to the time it takes hot water to cool off in the piping and the pump there can be short periods of time where the occupant would like to use the demand system to get hotter water than what is in the loop but the pump won't respond because the water is just above 96 degrees. 96 degree water is lower than human body temperature and therefore will feel cool, not hot. Typically that time period would be 20 minutes or so.

That's all there is too it. Be a little more green, conserve water, save time, and install a hot water demand system in your home today!