Material Insights · Understanding the Material World

Building heat recovery: Science and thermodynamics

Why simple exchangers recover most of a room's heat

Why simple exchangers recover most of a room's heat What this tab covers The thermodynamics: the grade (temperature or pressure) of the energy being recovered, energy versus exergy efficiency, and how harsh the waste stream is on equipment.

Buildings throw away heat in stale air, shower water, cooking exhaust and air-conditioner condensers. Each stream is lukewarm, so recovery relies on cheap passive heat exchangers or on heat pumps that upgrade it.

Grade of energy

Mixed

These are the lowest-grade streams in the recovery stage: exhaust air near room temperature, shower drains that are warm rather than hot, and condensers that reject heat slightly above outdoor temperature. The exception is the superheated refrigerant leaving a chiller compressor, which is hot enough to heat tap water; a 2003 study at a Mumbai hotel heated water from 30 C to 65 C this way on a 60 tonne-of-refrigeration chiller. A heat-pump water heater instead pulls heat from room air and upgrades it.48

Hotel chiller superheat recovery tap water 30 C to 65 C (60 TR chiller)8 Heat-pump water heater max water temperature 140-160 F (about 60-71 C)4

Exergy versus energy efficiency

Favourable

Passive exchangers recover a share of the heat with almost no added work. US DOE material says ventilation recovery units can recover about 70 to 80% of the energy in outgoing air, and one 2013 Solar Decathlon house used a heat recovery ventilator rated at 92%. Drain-water units are less effective: a University of Waterloo study reported 26 to 39% effectiveness for one common coil type depending on flow. Heat-pump water heaters reach a uniform energy factor (UEF) of 3.3 to 4.0, against 0.9 for an electric tank, meaning roughly 73 to 78% less electricity (calculated).345

ERV/HRV energy recovery about 70-80% (one competition HRV 92%)5 Drain-water unit effectiveness 25.7-39.2% (one tested unit)3 Heat-pump water heater UEF 3.3-4.0 vs 0.9 for electric tank4 Electricity saving vs electric tank about 73-78% (calculated)4

Media compatibility and corrosiveness

Mixed

Clean indoor air and potable water make most of these devices simple: drain-water units are a spiral copper tube wrapped round a vertical section of drain, with no moving parts. An energy recovery ventilator (ERV) also moves moisture between the air streams, which suits humid climates such as much of India, whereas a heat recovery ventilator (HRV) moves only heat. Kitchen exhaust is the hard case: grease coats exchanger surfaces and adds fire risk, so kitchens more often cut exhaust flow with demand-controlled ventilation than recover heat directly.16

Drain-water unit construction copper coil round a vertical drain section1

Variants compared124578

VariantWhat it recoversKey figureBest fit
HRV / ERVHeat (and moisture for ERV) from exhaust airAbout 70-80% energy recoveryAirtight homes and offices; ERV for humid climates
Drain-water heat recoveryHeat from shower drain waterAbout 790 kWh/yr in an Ontario test house; payback 2.5-7 years (DOE)Homes, hostels, gyms with frequent showers
Heat-pump water heaterHeat from room airUEF 3.3-4.0; about USD 550/yr savingReplacing electric tank heaters
AC and chiller condenser heatSuperheat or condenser heat to hot waterMumbai hotel case (2003): payback about 3 monthsHotels and hospitals with cooling and hot water demand
Kitchen exhaustExhaust heat, or less exhaust via demand controlDemand control cut exhaust airflow 23-32% at one US Army kitchenCommercial kitchens

Sources

  1. Drain-Water Heat Recovery (Energy Saver) US Department of Energy, 2024
  2. Evaluation of Residential Drain Water Heat Recovery Unit (study begun 2010) Sustainable Technologies Evaluation Program (Toronto and Region Conservation Authority), 2017
  3. Modelling the Steady-State Performance of Coiled Falling-Film Drain Water Heat Recovery Systems Using Rated Data (Manouchehri and Collins), Resources 9: 69 MDPI Resources (University of Waterloo authors), 2020
  4. Is a Heat Pump Water Heater Right for Your Home? ENERGY STAR (US EPA), 2023
  5. Technology Spotlight: Energy-Recovery Ventilation Systems (Solar Decathlon 2013 houses) US DOE Solar Decathlon, 2013
  6. HVAC energy recovery ventilation (Energy Basics) US DOE Building Science Education
  7. Engineering and Construction Bulletin 2015-3: Demand control kitchen ventilation US Army Corps of Engineers (via WBDG), 2015
  8. Benefits of superheat recovery on chillers: case study for a hotel installation (Rane and Tandale), 21st IIR International Congress of Refrigeration International Institute of Refrigeration, 2003