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Dedicated Hot Return Line

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Dedicated Hot Return Line 2018-08-14T12:26:33+00:00

Why Replace A Hot Water Circulating Pump With A Hot Water Demand System?

If you already have a dedicated hot water return line you can use your Chilipepper hot water demand system with it instead of using the cold water line as the return.Homes that are plumbed with a hot water circulating system have a dedicated hot water return line that connects the last fixture in the loop with the water heater.  The return line connects to either the water heater inlet or the drain valve at the bottom of the heater.

Normally there will be a check valve between the end of the return line and the connection with the water heater to prevent water from being drawn backwards through the return line.


Why Replace A Hot Water Circulating Pump With A Hot Water Demand System?

Traditional residential hot water circulating or recirc systems eliminate the waste associated with running the hot water while waiting for it to arrive, but they also waste a tremendous amount of energy.  The cost to heat water is far higher than the cost of the water you are heating, and running a hot water pump continuously will make the water heater run more often and longer to replace the heat energy being lost from the hot water piping.


On the plus side there is no waiting for hot water, it is pretty much instant hot water when you turn on the tap; You must of course, purge the cooled off hot water from the hoses under the sink and perhaps a couple of feet of pipe, but within two or three seconds you have piping hot water.With a hot water demand system you still have a wait for the hot water.  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.


How Long Will the Wait Be With a Demand System

Pumping at 3 gallons per minute means the water velocity in the pipe for a 3/4″ diameter pipe would be about 2 feet per second.  With 1/2 inch pipe it’s about 4 feet per second.

1/2″ dia. Type L copper pipe has a pressure drop due to friction of 0.062 psi per foot with a flow rate of 3 gallons per minute.  This means in order to pump 3 gallons per minute through a 100 foot long 1/2″ dia. Type L pipe the pump will have to produce a pressure of  at least 6.2 psi.


The Chilipepper demand pump is the most powerful pump on the market and will pump at about 3 gallons per minute in most residential plumbing layouts. The Chilipepper will only pump continuously for 3 minutes and will then shut down to avoid over heating the motor.  However, a gallon of water will fill about 45 feet of 3/4″ pipe or about 60 feet of 1/2″ pipe.  So the nine gallons pumped during that 3 minutes would fill 405 feet of 3/4 inch pipe or 540 feet of 1/2″ diameter pipe. Few residential plumbing layouts have anywhere near that much piping.

Here is a graph showing some typical pump curves for TACO pumps. Metlund D’mand systems use TACO pumps for their hot water demand systems.

Metlund S-70T D






Way down in the bottom left hand corner, the second curve, the one labeled 006, is the curve for the TACO 006 pump, the one used in the smallest Metlund System, the Metlund D’mand model S-50T.


The pump “dead heads” at about 8 ft of head which is about 3.5 psi.  That means the the TACO 006 does not have enough pressure to produce a flow of 3 gallons per minute through a 1/2 inch pipe.  Metlund, incidentally, states the capacity of the pump in their literature as 8 gallons per minute or something like that… somewhat misleading.With tankless water heaters the pressure drop through the heat exchanger is significant and must be added to the piping pressure drop, probably 2 or 3 psi.


Installing a Demand System With a Dedicated Hot Water Return Line

Typically a traditional hot water return line in a residential system consists of a 1/2″ pipe connecting from the supply pipe at the last fixture to the inlet of the water heater.  This forms a big loop of piping so the hot water can be continuously circulated from the water heater through the loop and back to the water heater.  Usually the pump is located at the water heater along with a check valve to prevent the pulling of cooler water from the bottom of the tank into the recirc line when someone turns on a hot water tap. 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

Obviously there are major differences in the way a demand hot water system and a hot water circulating system work.

The hot water circulating or recirc system is very simple. The most basic form is a continuously running pump circulating a small amount of water around the hot water piping loop.  Since the water is continuously circulating it doesn’t take much of a flow to keep the loop full of hot water.  However because a hot water recirculating system wastes so much heat energy many home owners have used timers and or temperature controls.

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 water, install hot water demand system in your home today!