How to use the calculator
Start with the amount of indoor air the ventilation system serves, then add the ventilation rate, the indoor to outdoor temperature difference,
the HRV efficiency, and the cost of delivered heat. The form uses metric units and produces results in kWh per day and dollars per day.
If you are comparing design options, it often helps to keep the building volume and energy price fixed while you vary ACH, temperature difference,
and efficiency across a few runs.
- Enter building volume in m3. A quick estimate is floor area multiplied by average ceiling height.
- Enter air changes per hour (ACH). This should represent intentional ventilation airflow, not necessarily uncontrolled leakage.
- Enter the indoor to outdoor temperature difference in °C for the period you want to examine.
- Enter HRV efficiency as a percent. Use sensible heat recovery efficiency if that is what the product literature reports.
- Enter heating cost in $ per kWh of delivered heat, then press Calculate.
After calculation, the result area reports the daily ventilation heat loss without recovery, the daily energy the HRV recovers, and the estimated
daily cost savings. The summary table repeats those numbers in a format that is easy to scan or share.
Tip for faster scenario testing: the biggest swings usually come from three inputs. Higher ACH moves more air, larger ΔT means every cubic meter of air
carries a larger heating penalty, and higher efficiency captures more of that penalty before it reaches the heating system.
Ventilation heat loss is estimated from the sensible heat needed to warm incoming air from the outdoor temperature to the indoor temperature.
To keep the calculator simple and fast, it uses typical dry-air constants: air density ρ ≈ 1.2 kg/m3 and specific heat cp ≈ 1000 J/(kg·K).
Those values are appropriate for a quick estimate and help turn the ventilation rate into a heat flow rate.
First, the calculator finds the steady ventilation heat loss rate in watts.
Formula: Q = ρ × c_p × V × ACH × ΔT /3600
That heat loss rate is then converted to daily energy in kWh per day. Recovered energy is a fraction of the loss based on the HRV efficiency η.
Formula: E_saved = η × E_loss
Finally, the daily cost savings are estimated by multiplying recovered energy by your heating cost per delivered kWh. The result is intentionally direct:
more air moved, colder outdoor conditions, and better heat recovery all push the savings number upward. That is why cold climates and tighter homes with
mechanical ventilation often benefit most visibly from heat recovery.
Worked example
Suppose a home has a volume of 250 m3 and is ventilated at 0.5 ACH. If the indoor temperature is 20 °C and the
outdoor temperature is 0 °C, then the temperature difference is 20 °C. Using the same constants as the calculator gives a useful back-of-the-envelope estimate.
- Heat loss rate: Q ≈ (1.2 × 1000 × 250 × 0.5 × 20) / 3600 ≈ 833 W
- Daily loss: Eloss ≈ 833 × 24 / 1000 ≈ 20.0 kWh/day
- If HRV efficiency is 70 percent, Esaved ≈ 0.70 × 20.0 ≈ 14.0 kWh/day
- If heating cost is $0.15 per kWh, savings ≈ 14.0 × 0.15 ≈ $2.10 per day
The result is small enough to feel realistic on a daily basis but large enough to matter across an entire heating season. Over 150 heating days,
that simple example suggests roughly $315 of recovered heating value. Actual savings depend on weather, operating schedule, and equipment performance,
but the example makes the relationships easy to see: doubling ACH roughly doubles the loss, a colder day increases the loss in direct proportion to ΔT,
and a better exchanger captures a larger share of that total.
Typical HRV efficiency ranges
Reported HRV performance depends on airflow balance, duct layout, core design, test conditions, and frost strategy. The table below gives representative
sensible heat recovery efficiencies so you can choose a starting point when exact data is unavailable.
Typical HRV efficiency ranges
| HRV type |
Efficiency (%) |
Recovery factor |
| Simple crossflow |
60 |
0.60 |
| Counterflow core |
75 |
0.75 |
| Enthalpy wheel or high-performance exchange core |
85 |
0.85 |
| Passive house grade unit |
90+ |
0.90+ |
Limitations and practical notes
This calculator is intentionally simplified so it stays transparent and fast. That makes it ideal for comparison and early planning, but it also means you
should read the output as an estimate rather than a guaranteed bill reduction. Several practical effects are deliberately left outside the model.
- Constant conditions: it assumes the same ACH and the same temperature difference all day.
- Sensible heat only: it does not model moisture transfer or latent energy in detail.
- No fan power subtraction: the electrical energy used by HRV or ERV fans is not deducted from the recovered heat value shown.
- No detailed duct effects: duct losses, defrost cycles, leakage, and balancing issues can change real-world performance.
- Heating system efficiency matters: your effective cost per delivered kWh of heat may differ from your utility rate if you use gas, oil, pellets, or a heat pump.
Even with those simplifications, the calculator remains useful because it highlights the dominant drivers. If the savings are small even under favorable inputs,
a premium HRV may not be justified purely on heat recovery. If the savings are substantial, the tool gives you a quick screening result before you move on to a
more detailed design analysis.
Additional context: interpreting results
Good ventilation matters for health and comfort. Fresh air dilutes carbon dioxide, odors, volatile organic compounds, and excess moisture. In newer airtight homes,
that usually means relying on mechanical ventilation instead of accidental leakage. The downside is simple: if you bring in cold air without recovery, the heating
system has to do extra work every hour that ventilation runs.
HRVs reduce that penalty by tempering incoming air. The result is often better comfort near diffusers, lower peak heating demand associated with ventilation, and a
more predictable indoor environment. Installation quality matters just as much as brochure efficiency, though. Short insulated duct runs, balanced airflow, clean filters,
and proper commissioning help the unit operate close to its rated performance.
When you read the calculator output, remember that it expresses the thermal value of recovered heat. If you heat with a resistance heater, the price per delivered kWh
of heat may be close to your electric rate. If you heat with a heat pump, your effective delivered heat cost is lower because each purchased kWh of electricity can deliver
multiple kWh of heat. If you heat with combustion equipment, the delivered heat cost depends on the fuel price and the appliance efficiency.
You can still use the tool for non-electric fuels. Convert your fuel cost into a cost per delivered kWh of heat, then enter that value in the heating cost field. Doing so
makes the daily savings number more meaningful, especially when you are comparing equipment upgrades or trying to estimate a seasonal payback.
Another useful way to interpret the output is as a sensitivity test. Run one case at 0.3 ACH, another at 0.5 ACH, and another at 0.7 ACH. Then do the same with low and high
temperature differences. That exercise often reveals whether the project is mainly sensitive to climate, ventilation design, or exchanger efficiency. It also helps communicate
the value of balanced ventilation to clients, homeowners, or project teams who may not think in heat-flow terms every day.
FAQ
Is ACH the same as infiltration?
Not exactly. In this calculator, ACH is best treated as the intentional ventilation rate from fans and ducts. Infiltration is uncontrolled air leakage through the building envelope.
Real buildings experience both. If you want a conservative rough estimate of total air exchange heat loss, you can test a somewhat higher ACH to reflect the combined effect.
What temperature difference should I use?
Use a representative average for the period you care about. For a typical cold day, indoor 20 °C and outdoor 0 °C gives ΔT = 20 °C. For a longer planning view, use a seasonal
average outdoor temperature or run several scenarios such as 10, 20, and 30 °C to see how strongly the result moves.
Does HRV efficiency stay constant?
No. Published efficiency is measured under specific airflow and temperature conditions. Real performance can drop because of frost control, dirty filters, unbalanced airflow, or poor
duct insulation. If you want a cautious estimate, enter a lower value than the headline rating.
Why are the results shown per day?
Daily numbers are easy to understand and easy to scale. Multiply the outputs by the number of heating days you want to examine to estimate a seasonal total. Many users test several
season lengths, such as 120, 150, and 180 days, to see the range of likely outcomes.
Can I use this for an ERV?
Yes for a rough sensible-heat estimate. ERVs can also transfer moisture, which can matter in humid climates or shoulder seasons, but that latent component is not modeled here.
Treat the result as the sensible portion of the savings.
What about fan electricity?
Fan energy is not subtracted in this calculator. If you know the unit's average fan power, estimate its daily kWh use and subtract that from the recovered energy for a rough net value.
That step is optional for comparison work, but it improves realism when you are estimating actual operating cost impact.
Accessibility note: after you press Calculate, the results region updates inside a live status area. The Copy Summary button becomes available only after a
successful calculation.
