Question:

When we added a mother-in–law apartment to our home, the builder ran water lines from the existing 40-gallon gas-fired hot water heater to the new bathroom (a distance of 120 feet). Then, because it took too long to get hot water to the bathroom, the builder added a 3/4 horsepower pump at the hot water heater to pump hot water to the apartment 16 hours per day. Is there a more efficient way to get hot water to the apartment?

Answer:

This is a problem as old as piped hot water—with several possible solutions, including:

• Providing a system to continuously circulate hot water to the farthest fixture.
• Providing self-regulating "pipe tracing" cable to keep water in the pipes warm between draws.
• Providing a new water heater near the new plumbing fixtures.

We'll discuss each of these potential solutions and its advantages.

Recirculation System

The builder’s solution, to add a recirculation pump, is common in large commercial buildings. In these systems, hot water is continuously recirculated from the farthest sink or other hot water load back to the water heater. Please note: it appears your recirculation pump was over-sized; a recirculation pump for residential use is typically sized at 1/20 to 1/12 HP. Here is a link from the Grundfos Company to Hot Water Recirculation - FAQs that discuss the design of hot water recirculation systems.

There are several ways to make the system more efficient. You could start by replacing the 3/4 HP pump with a 1/12 HP stainless steel or bronze pump. Next, make sure both the hot water and recirculating lines are properly insulated—codes require 1 inch of insulation (with a conductivity of 0.27 Btu/(inch/hr x sq.ft. °F.) or less, R-3.7 or more). Another way to improve the system may be to replace an older inefficient water heater with a new high-efficiency unit. Replacing a typical gas-fired water heater only 10 years old with a new high-efficiency unit can easily save over $50.00 a year.

Second Water Heater

Installing a second water heater close to the load and sized to meet the user’s hot water consumption would be another way to improve energy efficiency of the water heating system. If there are space restrictions in the apartment, by careful planning it may be possible to find the space to install a small electric storage-type water heater or perhaps an "on-demand" tankless gas-fired unit. Here are some possible places:

• In the attic with a drip pan and drain (storage type)
• Under the building in a crawl space
• On a wall outside the building (storage type or tankless)
• In an adjacent garage
• On an interior wall near the bathroom (tankless)

Pipe Tracing

A third way to improve the water heating system efficiency is to install electric heat tracing cable under the existing pipe insulation. The heat tracing provides heat to offset heat loss from the pipe. It is self-regulating and will maintain water in the pipe at about 120°F. Since a recirculation system is not required, you can shut off the existing recirculation pump and return pipe. Eliminating the recirculation piping will cut heat loss in the hot water piping in half, and eliminate the energy required to run the pump. The tracing can also be connected to a time clock to shut it down when hot water is not required, e.g. during the middle of the night.

Additional Resource