## The Cost of Hot Water Recirculation

As mentioned on the DATA COLLECTION and PLANNING section of our rainwater collection page, we noted that a considerable amount of water may be wasted when running a faucet until it flowed hot. To eliminate this waste, we installed a recirculating loop and small pump in the hot water plumbing. When the pump is running, water in the hot water line never cools off and hot water runs from hot water faucets in a few seconds rather than minutes, saving many gallons each day.

The diagrams below illustrate the difference between a conventional, and a recirculating system:

A question arises about the cost of running the system -- the length of piping and associated heat loss is nearly doubled when compared to a conventional layout.  More importantly, a constant flow of water from the hot water heater is exposed to this loss whenever the pump is running.

An estimate of costs can be made from power usage distributions: In addition to displaying distributions of power generated by our solar panels and power provided by the grid, the eGauge Solar Power Monitor makes it possible to plot a separate distribution of the power used by our household.

1.    Baseline

To establish a baseline, we first show the distribution of power used starting at about 12:30PM on Monday, 7 July 2014 and ending 24 hours later, during which time the house was unoccupied and the recirculation pump was turned off.

Three major levels of energy use are evident from this graph:
• For most of this time energy use fluctuated at a “maintenance” level somewhat below 1 kW,  representing power used by appliances such as refrigerator, freezer, security system, electronic standby power, etc,

• During the period between 3:00 – 6:00 AM the pool filter pump temporarily raised the background level another .75kW, and

• Starting at about 9:15 AM a 6 kW peak occurred when the hot water heater went on.  We assume the duration and amplitude of this peak to be characteristic of system heat loss.
Next we look at energy use distribution with the recirculating system in use. The recirculating pump can be activated in one of two modes, each of which has different power requirements:

2.    Continuous Mode

In this mode the pump runs continuously during the day, but is timed to shut off at night when little or no hot water is used, between 11:00PM and 5:45AM. The figure below graphs power used in continuous mode, starting at about 12:30PM on Saturday, 7 June 2014 and ending 24 hours later, during which time the house was occupied and the recirculation pump was running. This distribution is typical of a summertime day when major energy-dependent appliances such as the clothes dryer and air conditioning are not being used.

Superimposed on the background pattern shown in the baseline figure are:
• Highly variable increases over the maintenance level during active daylight hours when more smaller appliances are used, and

• 14 prominent peaks that correspond to the times the electric hot water heater is on. These peaks recur about once every hour and a quarter except for the 6.5-hour period between 11 PM Sunday and 5:45 AM Monday during which time the timer shut off the recirculation pump and electric power usage returned to the maintenance level. The duration of each of these peaks is about 10 minutes.
Over a 32-day period of continuous mode operation, the average number of peaks per day was 14.1. Peaks were counted only if they had the characteristic shape associated with system heat loss recovery and were not associated with surges resulting from major appliances use.

3.    On Demand Mode

The house is wired so that a 30-minute timer switch is located near each of the bathroom, kitchen and laundry faucets. A turn of the knob on one of these switches activates the recirculating pump, producing hot water in a few minutes. The delay is similar to that of a conventional non-recirculating system, but no water is wasted.

The figure below shows a typical distribution of energy use with On Demand pump activation.

Here, four peaks characteristic of system heat loss recovery are seen. This distribution is  typical of energy use with on demand pump activation, again in the absence of major appliance use.

Over a 44-day period of On Demand mode operation, the average number of peaks per day was 4.6. Here too, peaks were counted only if they had the characteristic shape associated with system heat loss recovery and were not associated with surges resulting from major appliances use.

Clearly, recirculation accounts for a measurable consumption of energy.

The water heater is a 5.5kW model, so given its 10-minute duration, each peak represents about 0.92 kWh of energy used.  For the year 2013, CPS’s average cost per kilowatt hour was \$0.11. (As an aside, owing to energy generated by our solar power system, the net cost at our house for that year was \$0.04 per kWh.)

The table below summarizes estimates of hot water recirculation costs on a daily, and on a yearly basis:

 Baseline Continuous On Demand Per Diem Per Annum Peaks kWh CPS Our Net kWh CPS Our Net 1.0 0.9 \$ 0.10 \$ 0.04 335 \$   37 \$   14 14.1 12.9 \$ 1.42 \$ 0.52 4717 \$ 519 \$ 190 4.6 4.2 \$ 0.46 \$ 0.17 1539 \$ 169 \$   62

The table suggests the following:
• There is a small CPS cost (\$ 37) from system heat losses even in the absence of recirculation.

• On Demand operation costs about 1/3 that of continuous mode.
• Continuous operation of recirculation is relatively expensive even at our net cost. This amount  (\$ 190) represents about 13% of our total net electric charges for the year 2013.
Some observations are in order:
• The recirculation pump itself uses 0.08 kW per hour when running. This translates to 1.4 kWh for the 17.5-hour day that it runs in continuous mode, or 511 kWh per year.  Annual cost to run the pump in continuous mode is thus about \$ 56 at CPS rates (\$ 20 at our net cost); in either case about 10.5% of the total. In On Demand mode, assuming that local timer switches are set at their maximum 30 minutes 4.6 times per day, the annual pumping costs are about \$ 7.40 at CPS rates (\$ 2.60 at our net cost), or about 4% of the total.

• We can make a tentative effort to generalize this analysis: The total length of the 3/4" XLPE piping in this house is estimated to be about 300 feet (insulation meets R3 minimal residential hot water pipe code requirements). This implies an annual power loss per foot of piping of about 15.7 kWh, costing \$ 1.73 at CPS rates.  There is no guarantee that these numbers will apply to other housing, since building dimensions, materials, climate and patterns of use are all determinants of energy loss and its costs. Nonetheless, it should be evident that hot water recirculation systems are not cost free.
• I do not know if heavier insulation would decrease system heat loss enough to pay for itself in any reasonable length of time.
• The number and duration of wintertime peaks may exceed those shown here, owing to increases in piping heat loss with cooler crawl space temperatures. If ambition permits, I may check this some time during winter months.

• PLEASE NOTE: I have done my best to check the accuracy of the tables and calculations on this page. Further, the conclusions and assertions made are intended as guidelines and may not apply to situations other than the setting described here. Thus, I must assert the usual disclaimer that the information presented here is distributed without warranty, and that you use it at your own risk.