Heat Recovery Ventilator Energy Savings Calculator

Ventilation without thermal waste

Fresh air is vital for healthy indoor environments, yet every cubic meter of conditioned air expelled from a building carries valuable heat energy with it. Traditional exhaust‑only ventilation systems discard this warmth, requiring furnaces or heat pumps to work harder to maintain comfort. A heat recovery ventilator (HRV) mitigates this loss by transferring heat from outgoing stale air to incoming fresh air through a counterflow heat exchanger. The calculator above quantifies the energy savings by comparing the theoretical heat lost through ventilation with and without recovery. By entering the building volume, desired air change rate, indoor‑outdoor temperature difference, exchanger efficiency, and energy cost, users obtain an estimate of daily energy conserved and its monetary value.

The amount of heat lost through ventilation depends on the airflow and temperature difference. Airflow is often expressed in air changes per hour (ACH), indicating how many times the entire building volume is replaced in an hour. The mass of air moved is found by multiplying volumetric flow by the density of air, approximately 1.2 kg/m³. The heat content of that mass is the product of density, specific heat capacity of air (around 1000 J/kg·K), and the temperature difference between indoor and outdoor air. The equation $Q=\rho \times \mathrm{c\_p}\times V\times \mathrm{ACH}\times \mathrm{\Delta T}/3600$ yields heat loss in watts, where the division by 3600 converts from per hour to per second. Integrating over a full day gives daily energy in kilowatt‑hours.

An HRV recovers a fraction of this energy. If the device has an efficiency $\eta$, the recovered energy is $\mathrm{E\_s}=\eta \times \mathrm{E\_l}$, where $\mathrm{E\_l}$ is the total ventilative loss. The calculator multiplies the recovered energy by the cost of heating to estimate monetary savings. Typical efficiencies range from 60% for older or poorly balanced units to over 90% for high‑performance models with enthalpy wheels or counter‑flow cores. Even at moderate efficiencies, the cumulative savings over a heating season can be substantial.

The table below lists representative HRV efficiencies and corresponding heat recovery factors. These factors indicate the proportion of heat reclaimed compared to a system with no recovery.

Consider a 250 m³ home ventilated at 0.5 ACH with an indoor‑outdoor temperature difference of 20 °C during winter. The hourly heat loss without recovery is $1.2\times 1000\times 250\times 0.5\times 20/3600=833$ watts. Over twenty‑four hours, this amounts to 20 kWh. An HRV with 70% efficiency would reclaim 14 kWh daily, translating to $2.10 per day at a cost of 0.15 $/kWh. Over a 150‑day heating season, savings exceed $300, not accounting for additional benefits such as improved comfort and humidity control.

Balanced ventilation with heat recovery helps maintain indoor humidity by tempering incoming air. In cold climates, bringing in untempered air can over‑dry interiors, leading to cracked woodwork and discomfort. HRVs mitigate this by transferring some moisture from exhaust to supply air in enthalpy models. In humid climates, energy recovery ventilators (ERVs) perform a similar function, reducing latent loads on air conditioning systems. This calculator focuses solely on sensible heat transfer but highlights the potential energy reductions from moisture control as an ancillary benefit.

Installation quality strongly influences realized efficiency. Duct runs should be short and insulated, with airflows balanced between supply and exhaust. Filters require regular cleaning to maintain airflow. Many units operate continuously at low speeds, ramping up when CO₂ or humidity sensors detect occupancy. The energy used by the HRV’s fans is relatively small compared to the recovered heat, but it should be considered in comprehensive energy audits. The calculator does not subtract fan power, so actual net savings will be slightly lower.

Limitations of this tool include the assumption of constant temperature difference and ventilation rate. Real buildings experience diurnal temperature swings, intermittent occupancy, and variable infiltration through leaks. Nonetheless, the simplified approach offers a quick way to gauge order‑of‑magnitude savings and justify investments in higher efficiency ventilation systems. For detailed design, professionals may perform hour‑by‑hour energy modeling incorporating climate data and equipment characteristics.

Economically, HRVs represent an upfront cost that must be weighed against long‑term energy savings. Payback periods vary widely depending on climate, fuel prices, and installation complexity. In cold regions with high heating costs, savings accumulate rapidly. In milder climates, the benefit may stem more from improved air quality than energy recovery alone. The calculator’s cost output can feed into payback analyses by dividing the unit price by annual savings to estimate years to break even.

Beyond cost considerations, HRVs enhance occupant health by continuously removing pollutants such as VOCs, carbon dioxide, and moisture while retaining heat. They are especially valuable in airtight homes constructed to modern energy codes or passive house standards, where natural infiltration is minimal. In such buildings, mechanical ventilation with heat recovery is not merely optional but essential to prevent indoor air quality degradation.

In conclusion, the heat recovery ventilator energy savings calculator translates ventilation parameters into tangible energy and cost benefits. By appreciating the physics of moving warm air out and bringing cold air in, homeowners can evaluate whether investing in an HRV aligns with their sustainability goals. Even if energy prices are low, the environmental advantages of reduced fuel consumption and the comfort of draft‑free fresh air make heat recovery an appealing component of high‑performance building design.