Hot water recirculation saves money, energy

PLUMBING CONTRACTOR SAVING MONEY AND conserving energy aren't always the first things on our customers' minds when asking about retrofitting or adding hot water recirculation systems it's a desire for convenience and comfort! Those two often trump energy concerns. Combine them with energy savings and you've got a sales pitch that will be tough to ignore. The most frequent complaint is the long wait

PLUMBING CONTRACTOR

SAVING MONEY AND conserving energy aren't always the first things on our customers' minds when asking about retrofitting or adding hot water recirculation systems — it's a desire for convenience and comfort! Those two often trump energy concerns. Combine them with energy savings and you've got a sales pitch that will be tough to ignore.

The most frequent complaint is the long wait for hot water. It's not uncommon for that wait to last for one to two minutes until the bather is comfortable with the adjusted delivery temperature. Toss in a family with several bathrooms and the potential exists for large volumes of water to be wasted.

Taco's calculations say it's 14,000 gal. for a family of four ( Taco-HVAC.com). Laing Thermotech ( lainginc.com) says it's between 8,000 and 20,000 gal. Grundfos (Grundfos.com) publishes a chart with the calculation based on pipe size, length of run and daily usage that shows 9,855 to 55,115 gal. of wasted water per year.

Let's assume an average of 12,000 gal. wasted per year. My water bill at home lists an average charge of $.007 per gal. That comes to $84 down the drain — literally. If I use an 82% efficient means of heating that wasted water from an average temperature of 55 ° F to 140 ° F, that will cost me $138.33 if 1 cu. ft. of natural gas costs $.01335 (my average cost for natural gas this year). Toss in 18% (wasted water portion of annual use) of our yearly sewage treatment bill for an additional $52.79. My total would come to $275.12.

There are two basic methods for recirculation of potable hot water: Gravity and mechanical. Both systems require the entire supply/return be well insulated to minimize energy losses.

Newton's Law
At my house, we don't wait for hot water because we utilize a gravity circulation system. Our master bath shower is the longest distance from the water heater (an indirect tank), yet we enjoy instant delivery because that's where the return line begins.

With gravity flow, you can use smalldiameter copper or plastic. Advantages include no mechanical devices to consume electricity; no hydraulic erosioncorrosion caused by high flow; and the constant flow minimizes any bacterial amplification risks. For more on gravity systems see www.contractormag.com/articles/column.cfm?columnid=62

Mechanical recirculation
Mechanical recirculation systems include: 24/7/365 constant circulation with a dedicated return line; dedicated return line with an aquastat to turn the pump on and off; dedicated return line with timed pump circulation; cross-connected hot-to-cold timed circulation with an aquastat to limit the cold water line's temperature; and a similar cross-connected system that can be motion-sensor, infrared or manually activated.

Good grief, that's a lot of choices!

A few of the constant or timed recirculation pumps are available in low wattage and low flow versions — as low as 33 watts and 0.2 GPM! Let's examine some costs.

We're being billed at $.092 per kWh, which is 1,000 watts. A 33W circulator running 24/7/365 in my home will cost me $26.60 per year to operate.

The one remaining factor to consider is the heat loss. At 140°F in a 70°F space, 1/2-in. copper tubing that's covered with 1/2-in. fiberglass will release 15 Btuh per lineal ft. In reality, we won't be at 140°F throughout the run using a low wattage pump with low flow rates. As we lose those Btu, the Delta-T is lowered, and that will lessen the Btu-per-foot heat loss. My gut feeling is that we would either match or be lower than the cost I calculated above of $138.33 for heating water. A match would give us a net savings of $110.19.

Let's change that to a timed cycle of two hours each day to coincide with hot water peak demand usage. A 33W pump used two hours each day will cost $2.22 per year. Couple that with energy lost via piping heat emissions of $14.62 annually and you'll see a net savings of $258.28 for this application in my home.

On-demand systems will typically utilize a larger wattage pump with much higher flow rates to quickly displace colder water for rapid on-demand hot water delivery. Corrosion-erosion can become an issue and larger return lines (if it's a dedicatedreturn system) may be required.

A minimal reduction in costs (compared to limited timed recirculation) can be realized. On-demand systems that are manually operated will continue to have a delay in hot water delivery.

Cross-connected hot-to-cold recirculation systems create the potential for warm water delivery from the cold side of the faucet. Make sure that's an acceptable condition for your customers.

The system's initial cost and fees to run an electrical outlet may seem like a deal-killer. But if you promote the return on investment rather than payback, your sales will increase. ROI is calculated by dividing the system's cost into the annual fuel savings. If you project an annual saving of $100 and the system costs $600 installed, the ROI is an attractive 16.7%, which is also a tax-free ROI. No wait for hot water, save money and conserve energy — it doesn't get much better than that.

Dave Yates owns F.W. Behler, a contracting company in York, Pa. He can be reached by phone at 717/843-4920 or by e-mail at [email protected]

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