Makeup Air

Detailed Explanation

Makeup air (MUA) is the fresh outdoor air drawn into a building in equal volume to balance air exhausted or lost through infiltration. It is mandatory to keep mass balance:

Qmakeup = Qexhaust + Qexfiltration − Qinfiltration

There are two basic system topologies:

1. 100% OA (DOAS — Dedicated Outdoor Air System) — no return, all supply air is outdoor; used in clean rooms, pharmaceutical manufacturing, isolation rooms, BSL-3/4 labs and similar contamination-critical spaces.

2. Mixed air — 15-30% of supply is makeup, the rest return; standard in offices, hospitals, hotels, shopping centres and other comfort applications.

Makeup air demand arises in three ways:

• Process exhaust — fume hoods, kitchen hoods, weld smoke extractors, paint booths, lab fume cupboards. Exhaust quantity must be known in kg or m³; can typically reach 0.5-2 m³/s per unit. • IAQ requirement — minimum per-person + per-area + process CO2/VOC dilution per ASHRAE 62.1 and EN 16798. • Building pressure control — for positive pressure (5-15 Pa), makeup > exhaust; particulate ingress is blocked.

The energy load of makeup air (heating + cooling + humidification + dehumidification) accounts for 40-70% of total HVAC energy; for this reason heat recovery (heat recovery wheel, plate exchanger, ERV) and energy modulation are critical.

Why It Matters

The makeup-air quantity is the direct determinant of HVAC energy and humidification load. The difference between 100% makeup and mixed air is dramatic:

Example: 10,000 m³/h supply air, target 22°C, 50% RH (= 8.3 g/kg), winter peak (-5°C, 85% RH = 2.1 g/kg).

Makeup ratio vs humidification load: • 15% makeup: Δw = 8.3 − (0.85×8.3 + 0.15×2.1) = 0.93 g/kg → 11.2 kg/h steam • 25% makeup: Δw = 1.55 g/kg → 18.6 kg/h • 50% makeup: Δw = 3.1 g/kg → 37.2 kg/h • 75% makeup: Δw = 4.65 g/kg → 55.8 kg/h • 100% makeup: Δw = 6.2 g/kg → 74.4 kg/h

100% makeup requires 6.6× the moisture load of 15% makeup — steam capacity, water consumption, electrical energy and capex grow proportionally.

In process-exhaust spaces (kitchens, labs, paint booths, industrial halls) makeup ratio is forced by exhaust. Energy economics for these spaces:

1. ERV (Energy Recovery Ventilation) — recovers 75-85% of exhaust heat and 50-70% of moisture; humidification load drops 50-60%. 2. Demand-controlled exhaust — when the process is idle, exhaust drops to minimum, makeup with it; demand-driven dynamic. 3. Heat pump preheat — preconditions outdoor air to 5-10°C in winter before humidification.

NKT engineering does not propose capacity without comparing 100% vs mixed-air makeup; this design decision is the foundation of energy economics.

Formula

Makeup-air humidification load:

Qmoisture = mair × Δw / 1000

Qmoisture: humidification load (kg/h steam) mair: makeup-air mass flow (kg/h) Δw: target minus makeup absolute humidity (g/kg dwc)

Effective Δw in a mixed-air system:

Δweff = windoor − (xreturn × windoor + xmakeup × woutdoor) Δweff = xmakeup × (windoor − woutdoor)

xreturn: return-air ratio (0.7-0.85) xmakeup: makeup-air ratio (0.15-0.30) windoor: indoor design absolute humidity (g/kg) woutdoor: outdoor absolute humidity (g/kg)

Example calculation (Ankara winter peak): • Total supply: 50,000 m³/h • Air density: 1.2 kg/m³ • mair = 60,000 kg/h • xmakeup = 0.25 • windoor = 8.3 g/kg, woutdoor = 2.1 g/kg • Δweff = 0.25 × (8.3 − 2.1) = 1.55 g/kg • Qmoisture = 60,000 × 1.55 / 1000 = 93 kg/h

IAQ minimum makeup: Qmakeup_min = Npeople × qperson + Aarea × qarea

qperson: per-person fresh air (L/s/person, ASHRAE 62.1) qarea: per-area fresh air (L/s/m²)

Office: 2.5 L/s/person + 0.3 L/s/m² Hospital: 7.5 L/s/person + 0.9 L/s/m² Lab: 10 L/s/person + 1.8 L/s/m² + process exhaust

Engineering Note

Engineering points in makeup-air system design:

• Justification for 100% OA — required only when contamination control is critical (BSL-3/4, isolation rooms, pharmaceutical sterilisation, some clean-room classes). Otherwise mixed air + DCV (CO2 sensors) saves 30-50% energy. • ERV/HRV selection — heat recovery from exhaust to makeup; rotor type 75-85% efficiency, plate type 50-65%, sensible-only HRV vs sensible + latent ERV is an important distinction. Latent recovery cuts humidification load by 40-60%. • Frost protection — winter cold-side freezing risk in ERV; defrost cycle or preheat coil required. Otherwise capacity drops and heat-transfer surface is damaged. • Filter load and pressure drop — makeup air requires comprehensive filtration (F7 minimum + HEPA optional); 200-400 Pa SP increases fan power and impacts energy. • Freeze alarm — when preheat coil outlet drops below 4°C, a signal must be sent to the humidifier; supplying steam to a frozen coil causes major damage. • Damper quality and sealing — AMCA Class 1A; OA damper closed-position leakage <1% guaranteed. • Energy modulation — VFD-driven modulating OA fan tunes capacity to daily profile; typical savings 25-40%. • Building pressure control — manometer + control loop maintains positive pressure; exhaust fan capacity should be 5-10% below makeup (for overpressure).

NKT evaluates these criteria with pre-project energy simulation to find the capex vs opex optimum.

NKT Application Link

NKT delivers makeup-air humidification and dehumidification solutions for 100% OA DOAS systems and mixed-air AHUs:

1. High-capacity steam for 100% OA DOAS — Neptronic SKE4 (5-80 kg/h) or SKS4 (5-180 kg/h) stages; for pharmaceutical and clean-room applications. 2. Mid-capacity steam + adiabatic hybrid for mixed-air AHU — SKE4 + SKG4, with economiser-mode support. 3. ERV integration — Neptronic equipment is designed in concert with moisture recovered from ERV exhaust; humidification load drops 40-60%. 4. NKT - Climate Track makeup-ratio monitoring — T/RH/dewpoint sensors at OA, return, supply, mixed-air points; real-time energy balance reporting. 5. Demand-Controlled Ventilation (DCV) — CO2-sensor-driven dynamic makeup adjustment, NKT BMS integration.

Sample configuration: 100,000 m³/h DOAS pharmaceutical plant, winter peak 600 kg/h load. NKT selection: 8 × SKE4 80 kg/h (640 kg/h total, 7% margin), DI water system, in-duct steam manifold, ±2% RH PID, NKT - Climate Track 36-point monitoring. Annual humidification energy 1.8 GWh, 45% ERV savings.