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.

Capacity should not be fixed without comparing 100% outdoor air with mixed-air makeup; this decision underpins the 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).

These criteria are evaluated with a pre-project energy calculation to find the balance between capex and opex.

NKT Application Link

For this application NKT – Nem Kontrol Teknolojileri supplies TFT, Neptronic and Microwell equipment. It supports unit selection with capacity calculation and technology matching based on space conditions, target temperature and RH (or dew point) and moisture load, and provides technical service and spare parts. Design, installation and commissioning remain within the scope of the facility's designer and contractor.