During a heat wave, I've been trying to understand why dual-hose portable air conditioners and split systems are generally more efficient than single-hose units. A single-hose unit exhausts warm air outdoors, which creates negative pressure and pulls replacement air into the room through gaps and leaks. That incoming air is hot and has to be cooled.
A dual-hose unit also brings in outside air, uses it to remove heat from the condenser, and exhausts it again. Since outside air is still involved, I'm unsure why this is more efficient. Is the main difference simply that a dual-hose system avoids depressurizing the room? How does a split system improve things further?
3 Answers
A split system avoids the whole air-exchange problem. The indoor unit circulates indoor air over a cold coil, while the outdoor unit releases the removed heat into outdoor air. The indoor and outdoor air streams never mix; heat is transferred by the sealed refrigerant loop between the two units.
Portable units contain both the cold and hot sides in one cabinet, so they need hoses or some other way to reject heat. With a split system, the noisy and heat-producing components are outside, and the building envelope is not being used as an unintended air intake.
The key misunderstanding is that a dual-hose unit does not cool outside air and send it into the room. Indoor air and outdoor air remain separate. The outside hose brings air through the condenser, where it picks up heat, and the second hose sends that now-hotter air back outside. The room air circulates over the evaporator coil and stays indoors.
A single-hose unit uses some indoor air to cool its condenser, then exhausts that air outside. Because air has left the room, the room becomes slightly pressurized below outdoor pressure. Replacement air gets pulled in through cracks, doors, outlets, and other leaks. On a hot day, that incoming air adds a substantial cooling load, so the unit is constantly fighting against the air it is drawing in.
A dual-hose portable unit is better, but it is not automatically as efficient as a split system. Its condenser is still inside the room, so the compressor and other hot components leak some heat indoors. The hoses can also be poorly insulated, and an intake hose placed too close to the exhaust may draw some of the hot exhaust back in.
Still, the separate intake and exhaust keep the room close to neutral pressure. That prevents the continuous stream of hot replacement air caused by a single-hose model. A well-sealed window unit can also perform very well because its hot side is outside.
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