Calculating Ambient Leak Ingress: The Equation Behind Monoblock AC Thermal Draft

Most portable air conditioner reviews focus on BTU ratings and decibel levels. Far fewer address the heat that walks back into the room uninvited — the thermal penalty that monoblock AC thermal draft ingress imposes on every single-hose unit in operation. This is not a design flaw that engineers overlooked; it is the unavoidable consequence of exhausting room air outdoors and the physics that follows. What makes it tractable is that it is mathematically predictable and therefore measurable, seeable, and reducible.

This article builds the full equation for infiltration heat load step by step: the sensible component that rises with temperature difference, the latent component that rises with outdoor humidity, and the combined total that the compressor must work against before achieving any net room cooling. The figures are grounded in real European summer psychrometric conditions and are consistent with values reported in published manufacturer specifications and EU EPREL entries testing.

What is monoblock AC thermal draft ingress?

Monoblock AC thermal draft ingress is the total heat load carried into a room by infiltrating outdoor air drawn in to replace the air exhausted by a single-hose portable unit's exhaust fan. It comprises two components: sensible heat (the load associated with the temperature difference between outdoor and indoor air) and latent heat (the load associated with the higher moisture content of outdoor summer air). Both must be removed by the evaporator coil, reducing the fraction of cooling capacity available for genuine room temperature reduction.

The word monoblock (a single-chassis unit containing compressor, condenser, evaporator, and fans in one portable enclosure) is important here because the exhaust fan is internal to the room's air volume. Every cubic metre per hour the exhaust fan vents outdoors is a cubic metre per hour of infiltration potential that the room's pressure deficit converts into actual inward airflow through whatever gaps exist in the building envelope.

What is the equation for calculating infiltration heat load?

The total infiltration heat load splits into two terms. Sensible load: Q_s = ρ × V × Cp × ΔT, where ρ is air density (1.2 kg/m³), V is volumetric infiltration rate in m³/s, Cp is the specific heat of air at constant pressure (1,005 J/(kg·K)), and ΔT is the outdoor-to-indoor temperature difference in Kelvin. Latent load: Q_l = ρ × V × h_fg × Δω, where h_fg is the latent heat of vaporisation of water (approximately 2,500 kJ/kg at 25°C) and Δω is the humidity ratio difference between outdoor and indoor air in kg of water per kg of dry air.

The humidity ratio ω (also called specific humidity or mixing ratio) can be read from psychrometric tables or calculated as ω = 0.622 × p_v / (p_atm − p_v), where p_v is the partial pressure of water vapour and p_atm is atmospheric pressure (101.325 kPa). At 35°C and 60 percent relative humidity, ω equals approximately 0.0215 kg/kg (21.5 grams of water per kilogram of dry air). At 22°C and 50 percent relative humidity, ω equals approximately 0.0083 kg/kg. The difference, Δω = 0.0132 kg/kg, is the moisture the evaporator must condense and drain from every kilogram of infiltrating air.

Worked example: a single-hose unit creating 50 m³/h of infiltration airflow (0.01389 m³/s) on a 35°C, 60 percent RH afternoon against a 22°C indoor target. Air mass flow rate = 1.2 × 0.01389 = 0.01667 kg/s. Sensible load: Q_s = 0.01667 × 1,005 × 13 = 218 W. Latent load: Q_l = 0.01667 × 2,500,000 × 0.0132 = 550 W. Total infiltration load: 768 W. Against a unit rated at 2,500 W cooling capacity (approximately 8,530 BTU/h), this represents 30.7 percent of total capacity consumed by self-generated infiltration before any net room cooling begins.

How does the infiltration heat load change across European summer conditions?

The total infiltration load is highly sensitive to outdoor humidity as well as temperature, because the latent component often exceeds the sensible component at typical European summer conditions. At 30°C and 55 percent RH, infiltration at 40 m³/h carries a modest combined load. At 38°C and 65 percent RH at the same infiltration rate, the load nearly trebles, with the latent component now accounting for over 70 percent of the total. This explains why humid coastal European cities experience worse single-hose performance than dry inland locations at the same air temperature.

The 80 m³/h row represents a monoblock running near its maximum exhaust fan speed in a room where infiltration closely tracks the exhaust rate — a scenario common in older housing with unsealed sash windows, letterboxes, and gap-prone floor construction. At those conditions on a 35°C day, nearly half of the unit's capacity is consumed by the infiltration load it creates, leaving the remaining 50 percent for actual room cooling.

Why the latent component is the dominant term above 33°C

The latent heat of vaporisation of water (approximately 2,500 kJ/kg) is 2,488 times larger than the specific heat of air (1.005 kJ/(kg·K) per degree). This means that the humidity ratio difference Δω needed to make Q_l equal Q_s is only ΔT / 2,488 kg/kg — an incredibly small moisture differential. At 35°C outdoor and 22°C indoor (ΔT = 13°C), Q_l equals Q_s when Δω = 0.0052 kg/kg. Real European summer conditions routinely produce Δω of 0.010 to 0.020 kg/kg, meaning the latent load is typically two to four times the sensible load on humid summer days. An air conditioner that excels at sensible cooling but whose drain pan cannot handle the condensate rate from high infiltration will exhibit performance degradation or shutdown as the evaporator ices over.

At what infiltration rate does a monoblock unit stop providing effective cooling?

A monoblock unit's net cooling benefit — the cooling delivered to the room minus the infiltration load it generates — approaches zero when the infiltration heat load equals the unit's rated capacity. For a 2,500 W unit at 38°C and 65 percent RH, the crossover occurs at approximately 118 m³/h of infiltration. While that appears high, it is reachable in a small, very leaky room (under 15 m²) with a fully open trickle vent and an unsealed door gap — conditions not uncommon in older European housing stock.

The more practical concern is partial degradation. A unit delivering only 50 to 60 percent of its rated capacity due to infiltration load will appear to work — the temperature sensor will indicate cycling — but the room temperature will rise on hot afternoons rather than declining, because the net cooling is insufficient to overcome the total ambient and solar heat gain entering through the building fabric.

• Seal the door threshold gap first — this typically cuts infiltration by 30 to 50 percent and directly reduces both the sensible and latent ingress loads.
• Close trickle vents while the AC runs. Re-open overnight when outdoor temperatures fall below indoor set point.
• At outdoor humidity above 60 percent, expect the latent component of infiltration heat load to be the dominant term — unit drain pan and condensate pump sizing must accommodate it.
• In rooms below 12 m², even modest infiltration rates (30–40 m³/h) can exceed 20 percent of a small unit's capacity at peak summer conditions.
• After sealing, re-measure room temperature drop rate: a 20 to 30 percent improvement indicates successful infiltration reduction.

Owner reports in r/airconditioning frequently describe the phenomenon of a room that cools adequately overnight but climbs steadily in temperature through the afternoon despite the AC running continuously — consistent with a unit whose net cooling (rated capacity minus infiltration load) is marginally positive under moderate conditions but becomes negative once outdoor temperature and humidity peak simultaneously.

How does infiltration load compare across portable unit types?

The infiltration heat load equation applies only to units that exhaust room air. A dual-hose monoblock draws its condenser air through a dedicated intake duct rather than from the room, reducing (but not eliminating) the net pressure deficit and cutting infiltration to 5 to 15 percent of cooling capacity under the same conditions. A mobile split unit exhausts no room air whatsoever, so the infiltration load terms Q_s and Q_l both equal zero; the equations become irrelevant, and the full rated capacity is available for room cooling.

This structural difference explains why capacity comparisons between single-hose and mobile split units at nameplate BTU ratings are misleading. A 9,000 BTU mobile split and a 9,000 BTU single-hose unit are functionally different products at outdoor temperatures above 30°C: the split delivers roughly 8,500 BTU of net room cooling, while the single-hose unit may deliver 5,500 to 7,000 BTU depending on room sealing. The equation quantifies that gap precisely rather than leaving it as a vague impression of underperformance.

Mobile split units that eliminate monoblock AC thermal draft ingress entirely are the units most in demand during European heatwaves — and consequently the first to go out of stock. For buyers who understand the infiltration equation, getting ahead of the restock matters: it is the difference between buying the unit that solves the problem and spending another summer managing the consequences of one that does not.