Two streams, one box Every recovery device brings exhaust and supply into one casing. The question is whether they can mix. Leakage is the fraction of one stream that escapes into the other through seals, the matrix or carryover. How the technologies compare Plate (sensible): streams separated by solid walls; leakage is only through perimeter […]
Seasonal Efficiency: Beyond Nameplate Ratings
Rated vs real A core may be rated at 75% effectiveness, but the energy saved over a year is lower, because recovery is most valuable in extreme weather and can even hurt in mild weather (it reheats supply you wanted cool). The seasonal efficiency weights effectiveness by the hours and temperature lifts actually seen. What […]
U-Value: The Building-Side Recovery Metric
Definition The U-value is the rate of heat transfer through a building element per square metre per degree of temperature difference, in W/m2.K. A low-U wall or window loses little heat; a high-U one loses a lot. U-value vs the exchanger k-value Do not confuse them. The exchanger overall conductance (UxA in the epsilon-NTU method) […]
Dew-Point vs Temperature Effectiveness in ERV
Temperature effectiveness (sensible) Temperature effectiveness compares supply temperature change to the maximum possible: epsilon_T = (T_supply_out – T_supply_in) / (T_exhaust_in – T_supply_in) This is the number a sensible plate reports, and it is what most generic “efficiency” labels mean. Dew-point effectiveness (total / enthalpy) For an enthalpy wheel or membrane, moisture matters, so effectiveness is […]
Fan Static Pressure Class: Matching the Fan to the Recovery Core
Static vs total pressure Total pressure is the full energy of the moving air; static pressure is the usable pushing force after velocity energy is set aside. Fans are classed by the static pressure they can develop against a system – and the recovery core is one of the biggest resistances in that system. Core […]
Sensible vs Latent Heat in Air-to-Air Recovery
Two kinds of heat in every airstream Any heated or cooled air carries energy in two forms. Sensible heat changes the dry-bulb temperature of the air without changing its moisture content. Latent heat is the energy tied up in water vapour: it moves when moisture condenses or evaporates, and it can change without any temperature […]
Psychrometrics Basics: Reading Air Before You Recover Its Heat
Air is a mixture, not a single number Sizing any heat recovery system starts with the state of the air. Four properties define it: Dry-bulb temperature (DB): the ordinary thermometer reading. Wet-bulb temperature (WB): thermometer with a wet wick; tracks the cooling effect of evaporation. Humidity ratio (W): mass of water vapour per mass of […]
Psychrometric Processes in Heat Recovery: What Happens on the Chart
Four basic processes On the psychrometric chart, ventilation air moves along predictable lines: Heating: horizontal line to the right (dry-bulb up, moisture constant). Sensible cooling: horizontal line to the left (dry-bulb down, moisture constant). Humidification: line upward (more moisture). Dehumidification: line downward (condensation, latent heat released). Sensible recovery = a move along the constant-humidity line […]
The epsilon-NTU Method: Predicting Heat Exchanger Performance
Effectiveness, the practical metric Effectiveness (epsilon) is the ratio of actual heat transferred to the maximum thermodynamically possible: epsilon = actual_heat_transfer / (Cmin x (Thot_in – Tcold_in)) It is the number you see quoted as “70% efficient” for a recovery core, and it is directly related to the efficiency metrics reported for air-to-air units. NTU: […]
Dew Point and Frost Point: The Temperatures That Decide Core Survival
Dew point: where water appears The dew point is the temperature at which air, cooled at constant pressure and humidity, becomes saturated and water begins to condense. If a recovery core surface drops below the exhaust dew point, condensate forms on it – normal in cooling duty, a design input for drainage. Frost point: where […]