Material Insights · Understanding the Material World

Building heat recovery: Engineering and integration

Small kit, low risk, but timing and space matter

What this tab covers Engineering and operating integration: how tightly it is coupled to the host process, how steady the waste stream is, and how hard it is to retrofit.

These are small, mostly plug-in technologies with low coupling risk; the main engineering issues are installation quality and matching supply to demand in time.

Process interdependence (coupling risk)

Favourable

Failure rarely stops the host. A drain-water unit simply stops preheating; a heat-pump water heater usually has a resistance element as backup; chiller heat recovery is a side loop that can be bypassed. The exception is ventilation: if an HRV or ERV fails in an airtight building, fresh-air supply is lost, so maintenance of fans and filters matters for health as well as energy.6

Load factor and intermittency

Mixed

Simple drain-water units only work when hot water is used and drained at the same time, as in a shower; a test house in Ontario recovered about 790 kWh a year. Chiller heat recovery works when cooling and hot-water demand coincide, which they do all day in hotels; the Mumbai hotel case saved about 80 litres of diesel a day. Ventilation recovery runs continuously but gains most in extreme seasons.28

Test-house drain-water recovery about 790 kWh/yr2 Hotel diesel saved about 80 L/day8

Footprint and retrofit complexity

Mixed

Drain-water units need a vertical drain run below the shower, so they are easiest in new build. Heat-pump water heaters need about 700 cubic feet (about 20 m3) of surrounding air and lose efficiency in cupboards. HRV/ERV retrofit needs ducts, which is disruptive in existing homes. Kitchen demand control is a retrofit of sensors and fan drives: a 5,000 cfm hood cost about USD 43,000 installed.47

HPWH air space needed about 700 cubic feet4 Kitchen demand control, 5,000 cfm hood about USD 43,000 installed7

Sources

  1. Evaluation of Residential Drain Water Heat Recovery Unit (study begun 2010) Sustainable Technologies Evaluation Program (Toronto and Region Conservation Authority), 2017
  2. Is a Heat Pump Water Heater Right for Your Home? ENERGY STAR (US EPA), 2023
  3. HVAC energy recovery ventilation (Energy Basics) US DOE Building Science Education
  4. Engineering and Construction Bulletin 2015-3: Demand control kitchen ventilation US Army Corps of Engineers (via WBDG), 2015
  5. 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