Heat recovery ventilation calculator: room air flows, HRV or ERV, savings

What heat recovery ventilation gains you in Bucharest

For the 150 m² house with 4 persons and 120 m³/h: without recovery, the air exchanged would carry 2,942 kWh of heat outside over the heating season (19.6 kWh/m²; PHI gives 20–30 kWh/m² for a normally ventilated home). The Zehnder ComfoAir Q350 HRV (HRV, 91 %) keeps 2,677 kWh and leaves only 1.8 kWh/m² (PHI: 2–7). The fans use 252 kWh a year, frost protection 67 kWh. The net saving with gas at 0.065 €/kWh and a 92 % boiler is 114 € a year.

Air flow: for the house and room by room

How many m³/h of air do I need?

The Passive House Institute method starts from the people: 30 m³/h of fresh air per person. For 4 persons that is 120 m³/h. The second criterion is the volume: at least 0.30 air changes per hour, 0.30 × 375.0 m³ = 112.5 m³/h. The nominal flow is the larger of the two, rounded up to 10: 120 m³/h, 0.32 air changes per hour. The unit's base level (92 m³/h) and boost level (156 m³/h) are the nominal divided and multiplied by 1.3, as in PHPP. Above 0.50 air changes per hour the house is over-ventilated and the air dries out in winter.

Ventilation air flow per room

Fresh air enters the living room and bedrooms (supply rooms), passes the halls and leaves through the kitchen, bathrooms, WC and store room (extract rooms). The living room gets twice a bedroom's share. For extract, the PHI starting values are kitchen 60, bathroom 40, WC and store room 20 m³/h; here they add up to 180 m³/h, more than the nominal flow, so every extract room is scaled down by the same factor, rounded to 5 m³/h, until extract matches supply exactly. The table below is the sheet of the 150 m² house; in the calculator you can change it room by room and print it.

Example: heat recovery ventilation in a 120 m² single-storey house

For a 120 m² single-storey house with 3 persons (300 m³), the nominal flow is 90 m³/h, 0.30 air changes per hour. Fresh air goes into the living room (45 m³/h) and the two bedrooms (25 and 20 m³/h); stale air is extracted from the kitchen (45), bathroom (30) and pantry (15 m³/h). The HRV sits in the attic above the hall, in the middle of the house; outdoor air and exhaust go straight up through the north roof slope, 3.2 m apart.

Where to install the HRV unit in a single-storey house

In the middle of the house, in the attic above the hall. From there the ducts to every room are short and of similar length: low pressure losses, fans that use little power and flows that are easy to balance. If the attic is cold (outside the insulation), the unit and the attic ducts are insulated; the outdoor-air and exhaust ducts, which are cold in winter, get vapour-tight insulation so they do not condense. Leave room next to the unit for changing the filters.

Where to place supply and extract vents in the rooms

Supply vents go in the living room and bedrooms, extract vents in the kitchen, bathroom and pantry. Each vent sits on the far side of its room from the door: fresh air crosses the whole room on its way to the gap under the door, and stale air is taken from the far corner of the wet room. In the kitchen the extract vent is near the hob, in the bathroom near the shower.

How air moves from room to room (transfer air)

Under the doors. The hall has no vent at all: the 90 m³/h pass through it, from the living room and bedrooms to the kitchen, bathroom and pantry. Doors get a gap of about 1 cm at the bottom or a transfer grille, so the air flows with the doors closed.

HRV intake and exhaust: where they go and how far apart

Straight up from the unit through the north roof slope, with the roof terminals 3.2 m apart (at least 3 m). The outdoor ducts carry cold air through the warm part of the house, so they are kept as short as possible: every extra metre means lost heat and a condensation risk. The spacing keeps exhaust air from being drawn back in, and on the north side the fresh air is out of the sun and cooler in summer.

Air flow per room for a 120 m² house

With the calculator's rules (30 m³/h per person, at least 0.30 air changes per hour): 90 m³/h in total, shared as in the table below. The extract starting values (kitchen 60, bathroom 40, pantry 20 m³/h) add up to 120 m³/h, above the nominal flow, so they are scaled down by the same factor to 45, 30 and 15 m³/h. The unit's base level is 69 m³/h, its boost level 117 m³/h.

HRV or ERV, dry winter air and frost protection

HRV or ERV (energy recovery ventilator)?

An HRV (plain heat exchanger) recovers heat only; an ERV (enthalpy exchanger with a membrane) also recovers moisture: 78 % for the Zehnder ComfoAir Q350 ERV, per PHI. The ERV is often sold as the “better” option that cures dry air. In Romania's climate, for a normally occupied house, the figures say otherwise: in winter the moisture it returns has to be removed with a higher flow (extra electricity and heat loss); in summer it saves the air conditioning a little condensing. For the 150 m² house in Bucharest the HRV saves 114 € a year, the ERV 90 €. An ERV makes sense where the air really gets dry: few people in a large house, or a very cold climate. The Passive House Institute says the same on its ERV certificates: in cool temperate climates, moisture recovery should only be used where the house produces little moisture.

Dry air in winter: humidity with and without moisture recovery

Cold outdoor air holds little water: in January in Bucharest, 3.6 g per kg of air, even though its relative humidity is high. Warmed to 23 °C, the same air would have only 21 % RH. The house adds water (people, cooking, showers, plants; PHI counts 330 g/h for 4 persons) and the ventilation removes it. With an HRV and 120 m³/h, the monthly balance gives 34 % RH in January at 23 °C: above the 30 % RH floor below which air feels dry. An ERV with 78 % recovery would return so much water that, at the same flow, the air would pass 80 % RH. That is why PHI requires units with more than 60 % moisture recovery to raise their flow when indoor humidity rises, holding the extract air at 55 % RH. The calculation does the same: the Zehnder ComfoAir Q350 ERV holds 55 % RH in January, but in 5 months the flow rises to 270 m³/h, with 182 kWh of extra heat loss and 112 kWh of extra fan electricity. If the air does get too dry, the PHI remedy, before a humidifier or an ERV, is to lower the flow towards 20 m³/h per person.

ERV in summer, with air conditioning

In summer the outdoor air is humid (10.4 g/kg in July in Bucharest). The air conditioning holds the house at 24 °C and 50 % RH (9.3 g/kg) and condenses the excess water: with an HRV, 1,325 litres over the season, up to 15.0 l a day. This is where an ERV helps: it keeps part of the fresh air's water out, and the air conditioning condenses 1,045 litres, 280 fewer. It is the one real gain of an ERV in an ordinary house here, and it is small next to what it costs in winter. Recovery runs all year; the bypass opens only for free cooling, when it is below 18 °C outside and above 22 °C inside.

Frost protection

In an HRV the moisture of the extract air can freeze in the exchanger when it is cold outside. PHI counts a pre-heater switching on below −1.5 °C for an HRV and below −8 °C for an ERV. Bucharest has 482 hours a year below −1.5 °C and 64 below −8 °C, so 67 kWh of electricity a year for the HRV and 14 kWh for the ERV. In Brașov, 205 kWh for the HRV. Units that protect themselves by unbalancing the flows are counted the same way, as a simplification.

The ventilation unit: efficiency, supply air, requirements

What 90 % heat recovery efficiency means (PHI method)

The heat recovery efficiency says how much of the extract air's heat reaches the fresh air. The PHI method measures it at the boundary of the unit and subtracts the fan heat and casing losses, so it usually comes out a few points below the brochure figure (EN 308 or EN 13141-7). The PHI-certified units in the list range from 79 to 94 %. With 90 %, of 2,942 kWh only 294 kWh remain to be covered.

Supply air temperature at −15 °C

At Bucharest's design temperature (−15 °C, SR 1907-1), fresh air leaves the Zehnder ComfoAir Q350 HRV at 16.9 °C with 20 °C indoors, as in the PHI test (−15 + 91 % × 35), and at 19.6 °C with 23 °C indoors. PHI asks for at least 16.5 °C so the air does not feel cold; an 80 % unit would give only 13.0 °C without a pre-heater.

Passive House requirements for the ventilation unit

Heat recovery efficiency at least 75 % (PHI method); fan power at most 0.45 Wh per m³ of air; supply air at least 16.5 °C; noise at most 25 dB(A) in living rooms and 35 dB(A) in the plant room; F7 (ePM1 50 %) filters on the outdoor air and G4 on the extract air; airtightness and balance of the unit, hygiene, frost protection and occupant control are also checked at certification.

What it costs and how long it takes to pay back

How much does heat recovery ventilation cost?

For a new 150 m² house in Romania, in October 2026, the calculation starts from 6,540 € incl. VAT: the unit 2,620 € (certified units cost about 1,700–5,300 €), ducts with manifolds, valves and silencers 2,240 €, installation with design and commissioning 1,680 €. Market guides give 4,700–6,500 € for a new house and 6,000–8,400 € in a retrofit. The gas saving, 114 € a year, pays the cost back in 57 years; with a heat pump, later. Heat recovery ventilation is not done for payback but for clean air in every room, day and night, without draughts or outside noise, without mould and without open windows in winter; the payback is here because people ask.

Heat recovery ventilation in nine Romanian cities

The 150 m² house, 120 m³/h, Zehnder ComfoAir Q350 HRV (HRV) and Zehnder ComfoAir Q350 ERV (ERV), gas heating. Cold cities keep more heat, but the frost pre-heater runs longer. In no city does the ERV save more than the HRV.

Dwelling ventilation in the Romanian standard

Standard I5-2022 and dwelling ventilation

The Romanian ventilation standard I5-2022 requires balanced (supply and extract) systems in dwellings to have heat recovery (6.1.2 (3)), a bypass or control against summer overheating, and frost to be dealt with. For extract-only ventilation it gives room flows (kitchen 75–135 m³/h depending on the number of rooms, bathroom and WC 15–30 m³/h), meant for leaky houses rather than airtight ones. It sets no minimum efficiency; the Passive House criteria (75 %, 0.45 Wh/m³, 16.5 °C) are the ones used here.