Heat recovery captures energy from exhaust air and uses it to pre-condition incoming fresh air. The principle is simple: why throw away heat (or cooling) you’ve already paid for?
But heat recovery equipment costs money. Understanding the return on investment helps make informed decisions.
How Heat Recovery Works
Air leaving a building carries thermal energy – warm in winter, cool in summer. Heat recovery systems transfer this energy to incoming air through a heat exchanger, reducing the load on heating and cooling systems.
Types of Heat Recovery:
Plate Heat Exchangers – Exhaust and supply air pass through adjacent channels separated by plates. Heat transfers through the plates. No moving parts, no cross-contamination risk. Typical efficiency 50-70%.
Rotary Heat Exchangers (Thermal Wheels) – A rotating wheel absorbs heat from exhaust air and releases it to supply air. Higher efficiency (up to 85%) but small risk of cross-contamination. Not suitable for all applications.
Run-Around Coils – Coils in exhaust and supply airstreams connected by a pumped fluid loop. Enables heat recovery when airstreams can’t be adjacent. Lower efficiency but flexible installation.
Calculating Savings
Heat recovery savings depend on several factors:
Airflow Rate
Higher ventilation rates mean more energy to recover. Buildings with high fresh air requirements benefit most.
Operating Hours
Longer operating hours mean more recovery opportunities. 24/7 facilities see better returns than standard office hours operations.
Climate
Larger temperature differences between inside and outside increase recovery potential. UK climate provides good recovery opportunities in winter.
Existing Heating Costs
Higher energy prices improve payback. As costs rise, heat recovery becomes more attractive.
Recovery Efficiency
Higher efficiency equipment recovers more energy. Premium equipment costs more but saves more.
Real-World Example
Consider an office building:
- Airflow: 5,000 litres/second
- Operating hours: 2,500 per year
- Location: Midlands
- Heating: Gas at 6p/kWh
Without heat recovery, annual heating energy for ventilation air: approximately £12,000.
With 70% efficient heat recovery: approximately £3,600.
Annual saving: £8,400
If the heat recovery system costs £25,000 installed, simple payback is under three years.
Factors Affecting Payback
Shorter Payback:
- High ventilation rates
- Long operating hours
- High energy prices
- Cold climate locations
- High-efficiency equipment
Longer Payback:
- Low ventilation rates
- Short operating hours
- Low energy prices
- Mild climates
- Basic equipment
Beyond Simple Payback
Simple payback ignores several factors that improve the true return:
Rising Energy Prices
Energy costs trend upward over time. Savings calculated at today’s prices understate future benefits.
Reduced Plant Capacity
Heat recovery reduces peak heating and cooling loads, potentially enabling smaller boilers and chillers. Capital cost savings on other equipment improve overall project economics.
Carbon Reduction
Increasingly important for corporate reporting and compliance. Heat recovery delivers measurable carbon savings.
Comfort Improvements
Pre-conditioned air improves comfort consistency. Difficult to quantify but valued by occupants.
When Heat Recovery Doesn’t Work
Heat recovery isn’t always beneficial:
Low Ventilation Rates
Minimal fresh air means minimal recovery opportunity. The equipment cost may never pay back.
Matched Supply and Exhaust Temperatures
If exhaust air is similar temperature to ambient, there’s nothing to recover.
Process Exhaust Contamination
Some exhaust streams (fume cupboards, industrial processes) can’t be used for heat recovery due to contamination risks.
Space Constraints
Heat recovery equipment needs space. If it can’t be accommodated, it’s not an option regardless of economics.
Getting Accurate Figures
Generic calculations indicate potential, but accurate assessment requires site-specific analysis considering actual airflows, operating patterns, energy costs, and equipment options.
We provide detailed assessments including energy modelling and payback calculations for specific projects. Contact i-Flow to discuss your heat recovery opportunity.
