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Part II: Conservation FirstChapter 3. Seal, Insulate, and Cover

Cutting Hot-Water Waste: Recirculation, Pilot Lights, and Drain Heat Recovery

Three different ways hot water wastes energy, and the limits of each fix.

Difficulty
Intermediate
Cost
Varies
published

Hot water waste is not one problem. A recirculation loop keeps a pipe hot so the tap is instant, and it can waste as much heat as it saves you from running the tap. A standing pilot burns whether or not anyone showers. A drain-water heat exchanger grabs heat from water that is already going down the drain, and only while that water is moving. Fixing the wrong one, or fixing it in a way that stores lukewarm water, can raise a health risk while it shaves a bill.

Start with the cheap piece that is almost always worth it: insulation on the hot piping you can reach, a low-flow shower you actually like, and a look at the summer gas baseline. Solar hot water, if you still want it, comes after this page, and only in a layout the system choice allows for your climate.

Difficulty

Level
Intermediate
Time
An evening to insulate pipes, a plumber for the rest
Skills
Telling a comfort feature from an energy feature

Cost estimate

Varies

Pipe insulation is often under about $30. A demand recirculation control can be a few hundred dollars installed. A drain-water heat recovery pipe is often several hundred dollars plus plumbing. Replacing a pilot-light water heater is an appliance purchase, not a fitting.

Materials

ItemQtyNotes
Foam pipe insulationThe hot lines you can reachMatched to the pipe size. Tape the slits. Do not cover a relief-valve discharge or a flue.
A timer or a demand button for an existing loopOnly if a loop existsThe loop that runs all night is the problem. Do not add a loop to save energy.
A note of whether the water heater has a standing pilot1If it does, the efficient fix is a modern appliance, not a homemade gas-valve change.

Recirculation

A loop or a comfort pump brings hot water to the tap and sends the cooled water back, so you do not run a gallon down the drain while you wait. The pipe then loses heat all the time the pump runs. An uninsulated loop on a continuous pump can erase the water savings and add a large heat loss. Insulate the loop. Run it on a button you press when you are about to shower, or on a short timer in the hour you actually bathe. A loop that runs "just in case" from midnight to dawn is a radiator.

Do not drop the tank temperature to lukewarm so the loop feels cheaper. Water that sits in the bacterial growth range, roughly the temperatures people describe as warm but not hot, is where Legionella can grow in plumbing. Keep the tank at a normal hot setpoint and mix down at the tap with a mixing valve if the fixtures need protection from scalds. The loop should deliver that hot water, not a tepid compromise.

If you do not already have a loop, do not add one as an energy project. Insulate the trunk and accept a short wait, or add a demand system only because the wait is intolerable and you understand the pump will use some heat.

Standing pilots

Older gas water heaters and some furnaces keep a small flame lit day and night so the burner can start. A few hundred BTU per hour, all year, is a few therms a month. That is real, and it is not a reason to blow out the pilot and leave the appliance half disabled.

The safe fix is an appliance with electronic ignition, installed by someone qualified, with the venting and the combustion air the new appliance requires. Do not swap a gas valve, cap an orifice, or bypass a thermocouple from a guide. Gas valves are safety devices. A wrong one leaks or fails to shut off.

While the old appliance remains, keep the burner area clear, keep the vent connected, and keep carbon monoxide alarms. A pilot that is yellow, sooting, or blowing out is a service call, not a conservation tweak.

Drain-water heat recovery

A drain-water heat exchanger is usually a copper section in a vertical shower drain. Outgoing warm water runs down the inside. Incoming cold water runs up a wrap or a jacket and picks up heat. The preheated cold then feeds the water heater or the cold side of the shower valve, depending on the piping, which changes how much you save and how a thermostatic valve behaves.

It works during a shower, when both flows happen together. A tub that fills and later dumps its heat down the drain gives the exchanger almost nothing, because the cold water is not flowing at the same time. Savings are a share of the shower's heat, not a share of the whole water-heating bill. Longer showers and more people increase the prize. A house that bathes in tubs will be disappointed.

The stack has to be vertical for the common falling-film units, often several feet of straight drain, with access to replace it later. This is plumbing. It needs a permit where plumbing needs a permit. It must not wreck the trap, the vent, or the slope of a horizontal drain. It does not remove the need for a relief valve or a mixing valve.

Three hot-water lossesA recirculation loop that keeps a pipe hot all day, a standing pilot under a tank, and a drain-water heat exchanger on a vertical shower drain.Recirc loopRuns all dayStanding pilotBurns all yearDrain recoveryOnly while showering
A loop that runs all day, a pilot that burns all year, and a drain exchanger that only works while the shower is running.

  1. Insulate the hot pipes you can reach

    Leave relief discharges, flues, and valves operable and visible.

  2. If a loop exists, make it part-time

    A button or a short schedule, plus insulation. Do not lower the tank into a lukewarm setpoint to compensate.

  3. If a pilot burns all year, plan an appliance replacement

    With a qualified installer. Do not modify the gas valve.

  4. Consider drain recovery only for showers on a vertical stack

    Price it against the insulated pipes and the shorter showers first. Those are already yours.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.