Turn wasted exhaust heat into measurable annual savings. Adjust efficiency, runtime, fuel price, and rebates. Print results, export files, and plan upgrades with confidence.
| Scenario | Recovered Heat (kWh/yr) | Net Savings (Year 1) | Payback (yrs) |
|---|---|---|---|
| Small workshop HRV | 4,467 | $149.30 | 16.74 |
| Office ventilation retrofit | 25,524 | $1,388.84 | 3.89 |
| Restaurant make-up air | 36,210 | $3,821.09 | 1.78 |
Heat recovery is most valuable where ventilation is steady and outdoor air is cold. For example, 1,200 CFM at a 35°F temperature difference for 1,800 hours, with 65% effectiveness, recovers about 55,000 kWh of sensible heat annually. At $1.45 per therm and 92% efficiency, that recovery can offset roughly 2,000 therms, or about $2,900 before fan and upkeep.
Savings scale almost linearly with airflow, hours, and effectiveness. Typical effectiveness ranges from 55% to 85%. Common runtime assumptions are 1,000–3,000 hours per year. Added fan power is often 80–250 watts; at $0.18/kWh and 2,000 hours, that is $28–$90 per year. Fuel price swings dominate: 20% higher prices lift savings 20%.
Recovered heat is converted into avoided purchases using your selected heating source. A 92% gas furnace requires more fuel than delivered heat, so the model divides by efficiency before converting to therms. Heat pumps use COP; with COP 3.0, every 3 kWh of thermal recovery avoids about 1 kWh of electricity. Propane and oil conversions use typical energy content, so local values can shift results a few percent.
Year‑1 net savings equals gross heating savings minus fan energy and maintenance. Payback is net upfront cost divided by year‑1 net savings. NPV discounts each year’s cashflow at your chosen rate, and escalation increases future savings using the energy price growth assumption. A discount rate compresses benefits; moving from 6% to 10% can reduce NPV on long projects.
Use measured CFM where possible, and estimate ΔT from typical winter design or average operating conditions. If ventilation varies, compute a weighted average or run separate cases. Pair the results with filter schedules and access requirements to set realistic maintenance costs and sustain performance. For capital planning, compare the modeled net cost to competing upgrades with similar service life.
Q: Which inputs move savings the most?
A: Airflow, runtime hours, temperature difference, and recovery effectiveness drive recovered heat. Fuel price and heating efficiency/COP convert that heat into avoided cost. Fan watts and maintenance reduce net savings.
Q: How do I estimate ΔT and annual hours?
A: Use a typical heating-season outdoor temperature versus indoor setpoint, or use average operating data from controls. For hours, total the ventilation schedule across the year, or use BAS trend logs if available.
Q: Does the model include moisture recovery?
A: No. It models sensible heat only using 1.08×CFM×ΔT. If you have an enthalpy wheel or humid climate impacts, treat the result as conservative and add a separate latent recovery estimate.
Q: What emission factor should I enter?
A: Use your utility or national average factor for electricity (kg CO₂ per kWh). For fuels, use supplier factors per therm or per gallon. The calculator multiplies avoided units by the factor to estimate annual avoided emissions.
Q: Why might payback show as unavailable?
A: If year‑1 net savings is zero or negative, payback is not meaningful. Check fuel price, effectiveness, hours, and fan watts. Also ensure maintenance and installed cost reflect your actual scope.
Q: How should I compare two project options?
A: Run both cases with the same discount rate and escalation. Compare NPV first, then IRR and payback. If budgets are tight, also compare net cost per kWh recovered and the sensitivity range shown.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.