Purpose of ASHRAE 84
ASHRAE Standard 84, Method of Testing Air-to-Air Heat Exchangers, defines the laboratory procedure used to measure how well a heat recovery device actually transfers energy between two airstreams. It is the underlying test method referenced by rating standards such as AHRI 1060 and by many manufacturer published values. Where ASHRAE 90.1 says "recovery is required," ASHRAE 84 is the ruler that proves a device meets the claim.
What Gets Measured
The standard describes how to set up two controlled airstreams – typically a warm "supply-side" and a cold "exhaust-side" – and measure inlet/outlet dry-bulb and wet-bulb temperatures, volumetric flow rates, and pressures. From these it computes:
- Sensible effectiveness – based on temperature change of each stream.
- Latent effectiveness – based on humidity ratio change (where moisture transfer occurs).
- Total effectiveness – combining sensible and latent energy.
- Pressure drop on each side, which drives fan energy.
Test Setup and Instrumentation
| Element | Requirement |
|---|---|
| Flow measurement | Nozzle or orifice banks with ±2% accuracy |
| Temperature sensors | Calibrated to ±0.1°C in the test range |
| Humidity | Wet-bulb or psychrometer, corrected for velocity |
| Mixing chambers | Ensure representative average samples |
| Steady state | Readings taken after thermal equilibrium |
Winter vs. Summer Rating Points
Because a heat exchanger performs differently heating versus cooling, the standard prescribes both a winter condition (cold outdoor air, warm exhaust) and a summer condition (hot, humid outdoor air, cooler exhaust). Reporting both prevents a device from being specified on its best-case number alone.
Computing Effectiveness
For a sensible plate exchanger the sensible effectiveness εs is:
εs = (Toutdoor,in − Tsupply,out) / (Toutdoor,in − Texhaust,in)
where the temperatures are the airstream inlet and outlet conditions at rated flow. Latent effectiveness uses humidity ratio in place of temperature. The lower of the two stream values governs when flow rates differ.
Why the Test Method Matters to Buyers
- It removes ambiguity: "80% effective" means something specific only against a known method.
- It exposes leakage and bypass, which pure efficiency claims hide.
- It supports code compliance documentation and third-party certification.
Practical Takeaway
When reviewing a heat recovery product, ask for ASHRAE 84 (or equivalent AHRI 1060) tested effectiveness at your design airflow, not a generic brochure figure. Real-world performance falls with face velocity and fouling, so specify the rating point close to your operating condition and plan maintenance that preserves the tested pressure drop.
Worked Example: Computing Effectiveness
Take a plate exchanger tested per ASHRAE 84 with the following readings at 1,000 cfm per side: outdoor air in 0°C / 60% RH, supply out 14°C, exhaust in 22°C / 40% RH, exhaust out 10°C. The sensible effectiveness on the outdoor side is:
εs = (0 − 14) / (0 − 22) = 0.64 → 64%
The exhaust-side figure (22−10)/(22−0) = 0.55 gives 55%. Because the two flows are equal, the governing value is about 60% sensible. At the rated winter condition the device is therefore a "60% effective" plate — the single marketing number is now defined by a method.
Frequently Asked Questions
Why test both summer and winter? Heat exchangers perform differently heating versus cooling; one number would hide the weak season.
What about latent effectiveness? It is computed from humidity-ratio change and matters for enthalpy wheels and humid climates; sensible-only plates report "n/a" for latent.
How accurate is the method? Instrumentation tolerances in ASHRAE 84 keep effectiveness uncertainty within a few points when the lab follows the procedure.
Can I field-test instead? Yes, using the same formulas on installed streams, but laboratory ratings remove duct losses and give a repeatable basis for comparison.
Reporting the Result Correctly
A credible ASHRAE 84 report states the rating condition explicitly: airflow per side, entering dry-bulb and wet-bulb on each stream, face velocity, and whether the device was clean and dry. It then lists sensible, latent, and total effectiveness with their uncertainties, plus the pressure drop on each side. Specifiers should reject any "effectiveness" that omits the condition, because the same core can read 80% at low velocity and 65% at its real operating velocity. The report should also note the leakage class, since a leaky wheel or plate undermines both energy and indoor-air-quality goals. Keep the lab report with the commissioning file so the field-measured performance can later be checked against the same basis, and so future retrofits are specified on proven numbers rather than memory.