In an airtight compact building this is not a comfort upgrade — it is the system that stops the walls getting wet. It is also the one most often specified badly.
Why a small house needs it more, not less
Two people produce several litres of water vapour a day through breathing, cooking and washing. In 200 cubic metres that is diluted; in 70 it is not. The smaller the volume, the faster the humidity climbs and the sooner it reaches the coldest surface.
Opening a window works and throws the heat out with the moisture. Heat recovery keeps 80 to 90 per cent of it, which in a house with a small heat demand is most of the heating bill.
Sizing is about air changes, not unit size
The unit is chosen for the volume and the number of occupants, and in a tiny house that means the smallest unit on the market, running slowly. A unit that is too large runs at its noisiest low setting and short-cycles.
Ask for the design airflow per room: supply to living and sleeping, extract from kitchen and bathroom. A specification with no room-by-room figures has not been designed.
Ducting decides whether it works
Short runs, smooth bore, gentle bends and a sealed connection at every joint. Flexible ribbed duct pulled tight around a corner halves the flow, and that is the most common installation fault in any building.
Route the ducts on the drawing. A unit fitted into whatever space is left over ends up with the worst possible duct run and a cupboard you cannot use.
Filters and noise are the ownership experience
Two filters, twice a year, is the whole maintenance routine — and it is the routine that decides whether the system still works in year five. Buy the filters in pairs and put a reminder in the calendar.
Site the unit away from the sleeping area and on resilient mounts. It runs continuously, so anything audible becomes intolerable rather than familiar.
Summer bypass and frost protection
A summer bypass stops the unit recovering heat you do not want on a warm night. Frost protection stops the heat exchanger icing when it is well below zero outside. Both are standard on good units and absent on cheap ones.
In a Scandinavian or alpine winter the frost strategy matters: a pre-heater draws power, a defrost cycle interrupts flow. Ask which the unit uses before it is ordered.