Tankless water heaters are considered a green plumbing product in that they can save energy over a conventional storage water heater since they don’t have the standby heat losses associated with the storage heaters.
However, tankless water heaters are not green when it comes to saving water. A study done by the Australian government, where there is a long running severe drought going, found that tankless water heaters took an average of 10 to 20 seconds longer to get hot water to the fixture than a storage type heater.
This is because with the storage heater the water leaving the outlet is piping hot. With a tankless water heater the water first leaving the outlet is cold…the water must pass through the heat exchanger first to get heated. So it takes longer to get hot water.
Longer waits for hot water mean more water run down the drain while you wait. The ideal solution is of course a hot water demand system. The hot water demand system will speed up the hot water and you won’t run any water down the drain while you wait. And hot water demand systems do not use much energy. A typical demand system uses about $2.00 per year in electricity, and can save a typical family of four persons over 10,000 gallons of water per year.
Not all demand systems are created equal
Tankless water heaters have a heat exchanger to transfer heat from the flames for a gas tankless water heater to the water, and the same goes for electric tankless water heaters. The heat exchanger is similar to the radiator in your car, water passes through tubes and becomes heated. It takes pressure to push water through pipes and tubes. The higher the flow the more pressure is required.
Tankless water heaters have pressure drops
Below is a graph showing the pressure drop VS. flow rate for several Rinnai tankless water heater models.
From the graph one can see that to obtain a flow rate of 3 gallon per minute the pump or source of water pressure would need to produce at least 9 feet of head for the two smallest heaters. (Its hard to read on this graph, but you can find it on the Rinnai site: http://www.rinnai.us/documentation/downloads/U287-077x0101.pdf)
9 feet of head is equal to 3.9 psi.
Hot Water Demand Systems Vs Pressure drops
Many brands of hot water demand systems have models that use Taco brand recirculation pumps, Taco model 006. Metlund D’mand sytems model S-50T uses a Taco 006 pump for instance. From the Taco pump curves in the second graph below you can see that the model 006 can’t produce enough pressure to overcome the back pressure at 3 gallons per minute.
Piping pressure drops
The hot water plumbing also has pipes. The piping well of course also have a pressure drop or back pressure.
A flow rate of 2 gallons per minute through 100 feet of ½" type L copper tubing creates a pressure drop of 2.8 psi, a 3 gallon per minute flow creates a 4.1 psi pressure drop, a 4 gallon per minute flow creates a 5.5 psi pressure drop.
So if your water heater has 100 feet of pipe from the water heater to the fixture, and another 100 feet to get back to the water heater inlet, then you need to add the pressure drop from the piping to the pressure drop for the heater plus the added pressure drops created by any check valves, elbows, and other fittings. We will ignore the fitting and valve drops and just consider piping and heater drops.
With a 3 gallon per minute flow rate the 200 feet of pipe would have a back pressure or pressure drop of 8.2 psi plus the heater’s 3.9 psi equals 12.1psi or 28 feet of head.
Metlund, Armstrong, and Taco demand systems
This eliminates the Taco 008 pump which can only produce about 15 feet of head maximum. That is the pump used by the Metlund model S-70T. The most powerful Metlund pump is the Metlund S-02T which uses the Taco 011 pump.
The Taco 011 has a maximum head of about 32 feet, so there had better not be many elbows and fittings for the water to flow through. When scale starts building up in the heat exchanger things will go down hill rapidly.
This is all fine if you want to wait longer for your hot water than without the demand system, you will still save the water!
Chilipepper hot water demand system
There are no pump curves available for the Chilipepper model CP6000, but it has a much more powerful motor (1/3) horsepower compared to Metlunds most powerful S-02T which is 1/8 of a horsepower and less efficient than the direct coupled Chilipepper pump. The Chilipepper pump is capable of producing a maximum pressure of about 50 to 60 psi (115 to 138 feet of head).
The maximum flow rate of the Chilipepper is 3 gallons per minute, but since it has so much pressure capability it typically pumps near its maximum with any piping system/tankless water heater combination.
When you go to the Metlund site and navigate to the specifications and see the Metlund models rated at 8 gpm for the smallest model on up to over 20 gpm for the S-02T you will now know that the specs are a bit misleading to say the least.
That 8 gallons per minute is how much water the pump could pump if nothing was hooked to the outlet. No back pressure!
If your plumbing layout has no pipes and no water heater the Metlund system will pump the water much faster than the Chilipepper, but if you have pipes and a water heater the Chilipepper will out perform the Metlund demand systems in nearly every case.
Posts
