Domestic Hot Water: Recirculation Losses Before Renewables in a 40-Bed Assisted Living Facility
A 40-bed assisted living facility has two atmospheric gas storage water heaters (100 gallons each, 199,000 Btu/h input, 76% thermal efficiency) holding 140°F. A single-speed bronze recirculation pump runs 24 hours a day on a 180-foot loop: 90 feet of 1-inch copper return is bare in a 70°F mechanical room, and 90 feet is insulated in conditioned space. There is no master thermostatic mixing valve. The furthest resident room takes 95 seconds to deliver hot water, and staff report scalding complaints at the hand sink nearest the mechanical room.
Last year's gas bills attribute 3,100 therms to domestic hot water at $1.15 per therm, and electricity costs $0.14/kWh. The owner has $25,000 in a green fund and has been quoted $19,500 for a four-panel solar thermal system (estimated 45% solar fraction) or $14,000 for a 6-ton commercial heat pump water heater. Neither quote includes any distribution work or electrical upgrades.
1. Estimate the heat loss from the bare 90 feet and from the whole recirculation loop, convert it to therms at 76% efficiency, and rank the fixes by cost-effectiveness.
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Assumptions: bare 1-inch copper at 140°F in 70°F air loses about 40 Btu/h per foot; insulated, about 10-15 Btu/h per foot.
Bare section: 90 x 40 = 3,600 Btu/h. Over 8,760 hours that is 31.5 MMBtu/yr; divided by 0.76 thermal efficiency, 41.5 MMBtu of gas = about 415 therms = $477/yr. Insulating it cuts the loss to roughly 104 therms, saving about $360/yr; at $1,100-1,500 installed that is a 3-4 year payback.
Insulated 90 feet: 90 x 15 = 1,350 Btu/h = 11.8 MMBtu/yr = about 155 therms = $178/yr.
Loop total: 4,950 Btu/h, 43.4 MMBtu/yr, about 571 therms, $657/yr - roughly 18% of the 3,100-therm DHW load. The 1/12 hp pump adds about 540 kWh/yr, roughly $65.
Ranking:
- Insulate the bare 90 feet - lowest cost per therm, no operational downside.
- Control the recirculation. A plain timer will lengthen the 95-second wait residents already complain about, so specify demand-initiated push-button recirculation, or an aquastat plus a schedule matched to the morning peak.
- Re-measure for a full season before touching generation. Every therm removed here is a therm you do not have to buy equipment to supply.
2. Staff report scalding at the hand sink nearest the mechanical room. Should the storage setpoint simply be lowered from 140°F to 120°F? What control devices belong in the design?
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No - do not drop the tanks to 120°F. This is an assisted living facility, a building type ASHRAE 188 treats as housing a population at increased risk. Storage below roughly 131°F lets Legionella multiply in the tank and the 180-foot loop; you would trade a scald risk for an aerosolized pathogen risk in every shower. Keep storage at 140°F or higher and control temperature at the point of use.
Design response:
- Install a master thermostatic mixing valve (ASSE 1017) at the storage outlet, set to deliver 120°F, with the recirculation return connected downstream so the loop stays hot.
- The complaint is at the sink nearest the mechanical room, so the master valve alone may not fix it. Add a point-of-use thermostatic valve or an ASSE 1016 listed valve at that sink.
- Verify balanced hot and cold pressures at the mixing valve (within about 10-15%); an unbalanced valve hunts and delivers temperature swings that read as scalding.
- Record the setpoint and add the mixing valve to the building water management plan with annual verification and temperature logging at the furthest fixture.
The root cause is an uncontrolled 140°F distribution system, not an over-hot tank.
3. With $25,000 available, how do you decide between the $19,500 solar thermal package and the $14,000 heat pump water heater?
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Sequence decides this, not the quotes. Both were priced against the existing 3,100-therm load, which includes roughly 570 therms of avoidable pipe and loop loss. Spend about $3,000 first on insulation and recirculation control, then measure a full heating season and size generation against the residual load. A 45% solar fraction on a load already 18% smaller is a smaller array with better economics, because fixed costs are spread over fewer delivered therms.
On the residual load:
- Solar thermal: 45% of 3,100 therms = 1,395 therms, about $1,600/yr against $19,500, roughly 12 years - long, and it carries freeze, stagnation and maintenance exposure.
- Heat pump water heater: at COP near 2.8, one therm of delivered heat costs about (29.3 kWh / 2.8) x $0.14 = $1.47, versus about $1.51 for gas at 76% and $1.15/therm. Nearly a wash on energy cost, but it dehumidifies the mechanical room and improves as the grid decarbonizes.
Prerequisites before signing the HPWH: confirm adequate room air volume and a source-air temperature above roughly 50°F, and that the electrical service can carry the compressor. In a small or cold mechanical room the unit will underperform, and solar re-bid against the reduced load becomes the better answer.