Instant Hot Water System

An instant hot water system provides hot water to any fixture in the house within 3-5 seconds of turning on the faucet.  This is accomplished by circulating hot water through the feeder lines and returning it to the hot water tank using the cold water line of the last fixture in the loop. This system not only adds a great time-saving convenience but reduces wasted water and saves energy as well.

While best installed during new construction, a special sensor valve can be installed at the last fixture allowing for a retrofit installation without major plumbing rework.

Instant Hot Water Circulation Pump Kit
Watts 500800 Premier
Hot Water Recirculation Pump
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An instant hot water system either uses convection or a small circulation pump that is mounted on the outlet side of the hot water tank to keep the hot water moving.   Convection may work for a small, single hot water loop with fixtures mounted driectly above a hot water tank if it is located in the basement or crawl space.

However, homes with multiple loops or longer horizontal runs of pipe will find the small circulation pump much more effective in supplying hot water to all fixtures consistently.   Having built a three loop system, the circulation pump option comes highly recommended.

Hot water plumbing within the home is constructed in one or more loops.  The first loop may serve the kitchen and an adjacent laundry room while a second loop may feed an upstairs and downstairs bath that share a common wall.  Using separate loops for different areas of the house shortens the length of pipe needed between the hot water tank and the fixture.

Instant Hot Water Sensor Valve
Grundfos 595926 Replacement Sensor Valve for Circulator Pumps
When constructing an instant hot water system the last fixture in each loop has a sensor valve installed.  This sensor valve allows the circulating hot water to be returned to the hot water tank via the cold water line for that fixture.  If the temperature of the circulating water exceeds a preset value the sensor valve closes restricting circulation.   As the hot water cools in the pipe the valve reopens and circulation resumes.

The sensor valve also has an internal check valve so that when the cold water faucet is turned on the hot water circulation stops. Warm return water flows through the cold water faucet momentarily until the cold water supply replaces it. For this reason it may be desirable to plumb the shower as the last fixture instead of a lavatory sink. The only drawback to using the shower is the sensor valve is embedded in the wall and will be more difficult to access if maintenance is required.

The benefits of an instant hot water system save time, water and energy.  No longer do you have to wait for hot water to reach the kitchen sink or the shower head while watching all that previously heated water pour down the drain.  Once the water in the short riser pipe is expelled you have hot water.

Common questions arise such as "Won't the circulating hot water cool before returning to the storage tank?" and "Won't it take additional energy to reheat it?"  The technical answer is yes, but only slightly provided the all of the hot water pipes are well insulated.  An uninsulated instant hot water system will still work but will not be as enegy efficient as an insulated one.

"And what about the energy to run the circulation pump?"  If you have a small convection system it is a non-issue because there is no pump. However, consistent delivery of hot water to distant fixtures may prove to be unreliable.  We still recommend the circulation pump and here's why...

Let's Calculate Energy Savings

A typical 1/25 hp hot water recirculation pump on a timer will draw about 30 watts.  If the timer is set from 6:00 AM to 10:00 PM for a total of sixteen waking hours, 480 watts, or 0.48 kilowatts will be used.  At 10¢ per kilowatt the circulation pump will cost 5¢ per day or about $1.50 per month to operate.

Let's assume that the electric hot water heater is rated at 4800 watts or 4.8 kilowatts.  This means that the hot water heater will draw the same amount of energy in six minutes as it takes to run the 30 watt circulation pump for 16 hours per day.

Now ask yourself, how many times per day do the members of my family wait on the hot water faucet to deliver hot water?  Lets assume a family of four will wait 20 times per day at average of 30 seconds per time.  Knowing a typical faucet will deliver about 2.2 gallons per minute (gpm) we can calculate the amount of hot water that is wasted each month.

20 occurances per day x .5 minutes = 10 minutes per day

10 minutes x 2.2 gpm = 22 gallons per day

22 gallons x 30 days = 660 gallons per month

Keep in mind that the cold water coming out of the hot water faucet was once heated.  It just cooled off while standing in the pipe.  How can we estimate how much it cost to heat the 660 gallons of water that cooled off in the pipe?

Here's a suggestion... The next time you and your family are out of town for three days or more shut off your hot water tank before you leave.  When you return use your home energy monitor to measure how long it takes for the hot water tank to heat an entire tank of cold water.  If you do not have an energy monitor, simply test the water temperature with your hand in a nearby faucet and time how long it takes for the proper hot water temperature to return.

Let's assume it took 30 minutes to fully heat a 40 gallon hot water tank.  Knowing the rated load is 4800 watts, we can determine how much money was spent to heat the 660 gallons of water that went down the drain last month.

660 gallons / 40 gallons per tank = 16.5 tanks of water

16.5 tanks x 30 minutes per tank = 8.25 hours

8.25 hours X 4.8 Kilowatts per hour = 39.6 Kilowatt-hours (KWh)

39.6 KWh x $0.10 per KWh = $3.96

Recall that the cost to run the circulation pump was $1.50.  Net that against $3.96 and the savings is $2.46 per month or about $30 per year in energy alone not to mention what it cost to send 8,000 gallons of water down the drain last year.  Instant hot water systems do pay for themselves fairly quickly.

There is some additional heat loss of the water circulating through the insulated piping as mentioned earlier which we did not account for in this simple example.  This will vary from home to home as insulation coverage, loop distance and pipe diameters differ within an instant hot water system.

A simple Btu meter, installed in the circulation loop of the instant hot water system, can be used measure this heat loss.   Temperature is measured as hot water leaves the tank and again when it returns via the cold water line from the last fixture in the loop.  A small flow meter can be used to measure the rate of circulation from the pump which should remain relatively constant.  Be sure to plumb in brass or copper nipples for the temperature sensors and wrap with pipe insulation for more accurate readings.

Another advantage of an instant hot water system is the ability to minimize fluctuating water temperatures.  Ever been in the shower when someone turns on the lavatory faucet and the shower temperature changes?  This occurs because the feeder pipes are under sized.  When feeder pipes are properly sized in a standard hot water system, additional water is wasted while waiting for the hot water because the larger diameter pipe contains a greater volume of water.

This problem goes away when hot water is circulated through a properly sized, insulated feeder line.   In fact, there is less heat loss in a larger diameter pipe because of the smaller surface area per unit of volume.  When a 3/4" dia. feeder line serves 1/2" dia. riser lines, temperature and pressure changes are negligible at the faucet or shower head.

Instant hot water systems are best installed in new construction because all hot water piping can be insulated.  Retrofit installations can be done by insulating all accessible hot water piping although more heat loss will occur.

For additional ideas on how to reduce the energy costs of hot water systems please visit any of the following:

Hot Water Heating Systems   l    Tankless Hot Water Heaters   l    Heat Pump Water Heaters

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