HRV and ERV in high-rise ventilation, explained.

A high-rise residential tower sealed against Delhi's winter smog cannot simply close its windows and stop breathing. Humans exhale carbon dioxide, cook, bathe and live; the apartment interior accumulates moisture, VOCs, CO2 and residual particulate that must be expelled and replaced with fresh air. At the same time, the outdoor air in the Indo-Gangetic plain is, for significant parts of the year, objectively unfit to breathe — AQI readings of 300 or higher are common in November through February. This is the paradox that every luxury residential project in Delhi NCR has to solve: how do you ventilate without bringing the outdoor AQI into the apartment, and how do you do it without losing all the energy spent conditioning the indoor air? The answer is the heat recovery ventilator (HRV) or energy recovery ventilator (ERV). This brief explains what these units actually do, how they differ, and why the specification matters to a buyer.

The problem, in first principles.

Indian high-rise residences typically operate with air conditioning for six to eight months of the year, heating for one to two months, and open windows for the remainder. The AC chills the indoor air to roughly 24°C against a 42°C peak summer outdoor temperature. The work done by the air conditioner is proportional to the temperature and humidity differential between indoor and outdoor air. Every cubic metre of fresh outdoor air introduced into the apartment needs to be cooled and de-humidified before it joins the conditioned volume. Introduce too much fresh air and the air conditioner works overtime. Introduce too little and CO2 builds up, VOCs accumulate, and the apartment goes stale.

A heat recovery ventilator resolves this by recovering the thermal energy in the outgoing stale air and transferring it to the incoming fresh air, without mixing the two streams. The result is fresh air delivered to the apartment at a temperature close to the indoor condition, with minimal additional load on the air conditioner. On a 42°C summer day, an HRV can deliver fresh air at 28°C instead of 42°C — an energy saving of approximately 65 to 80 per cent on the ventilation load.

HRV vs ERV — what is actually different.

Both HRV and ERV units exchange energy between two air streams. The difference is what they exchange.

An HRV (heat recovery ventilator) transfers only sensible heat — the temperature component — between the two streams. It uses a counter-flow or cross-flow plate heat exchanger, typically made of aluminum or polymer. The outgoing air cools the incoming air in summer, and warms it in winter. Moisture does not cross the exchanger. Typical sensible efficiency is 75 to 85 per cent at design flow.

An ERV (energy recovery ventilator) transfers both sensible heat and latent heat — the moisture component — between the two streams. It uses an enthalpy exchanger, either a rotating wheel impregnated with a desiccant or a polymer membrane that is water-vapour permeable but air-impermeable. In summer, the ERV pre-cools and pre-dehumidifies the incoming fresh air by transferring moisture to the outgoing air. In winter, in cold dry climates, it pre-warms and pre-humidifies the fresh air. Typical total efficiency is 65 to 80 per cent sensible and 55 to 75 per cent latent.

Which is correct for Delhi NCR?

For a location like Greater Noida West, the summer is long, hot and humid (45°C and 70 per cent relative humidity in July), and the winter is cold and dry (6°C and 40 per cent relative humidity in January). Under both conditions, the ERV outperforms the HRV because it recovers both temperature and humidity. The ERV is the correct choice for the NCR climate across the full year, and is the choice made by every competently specified luxury residential project in the region.

Fab Luxe uses ERV units with a polymer membrane enthalpy exchanger at every floor-level ventilation riser. The membrane is impermeable to bacteria, dust, VOCs and particulates, so there is no cross-contamination between the outgoing apartment air and the incoming fresh air. The membrane is replaceable on a 24-month maintenance cycle.

Sizing and air-change rates.

The ventilation rate for a residence is usually sized against ASHRAE Standard 62.1 or the Indian equivalent NBC 2016 Part 8. For a typical residential bedroom, the minimum fresh air requirement is 7.5 litres per second per person plus 0.3 litres per second per square metre of floor area. For a 4 BHK apartment of 2,690 sq ft with four occupants, the design fresh air requirement is approximately 110 litres per second, or roughly 400 cubic metres per hour, which translates to approximately 0.5 air changes per hour.

The ERV at every apartment level handles this load, delivering fresh filtered air through dedicated ducts to each bedroom and living room, and extracting stale air from the kitchen, bathrooms and utility spaces. The fresh air is filtered through an HEPA-13 stage before entering the ERV, so that the incoming air is AQI-neutral regardless of outdoor conditions.

The AQI question, integrated with ventilation.

An ERV without upstream filtration will simply deliver polluted outdoor air at a better temperature. This is not acceptable in the Delhi NCR context. The complete system specification requires a two-stage filtration upstream of the ERV: a pre-filter to remove coarse particulate, followed by an HEPA-13 barrier to remove PM2.5, PM1 and sub-micron particulate down to 0.3 microns at 99.95 per cent efficiency. An activated carbon stage after the HEPA removes VOCs, NO2 and SO2. Only after this filtration train does the air enter the ERV enthalpy exchanger.

The Fab Luxe system maintains a slight positive pressure in every apartment, so that the apartment envelope is always pushing air outward through any gaps. This prevents unfiltered outdoor air from infiltrating through façade joints, door seals or window gaskets, which would otherwise bypass the filtration train entirely. A positive-pressure indoor envelope is the signature of a well-engineered indoor environment in a polluted outdoor climate.

What the buyer should verify.

• Is the ventilation system HRV or ERV? (ERV is correct for NCR climate.)
• What is the HEPA grade of the upstream filter? (HEPA-13 minimum.)
• Is there an activated carbon stage for VOCs? (Yes, ideally.)
• Is the apartment maintained at positive pressure? (5 to 10 Pa is target.)
• Is real-time AQI sensing installed, and where is the data reported?
• What is the membrane replacement schedule, and who bears the cost?

Each of these answers on a properly specified luxury project will be written into the building services drawings and available to buyers before allotment. For the full AQI-managed system specification, see the project specs page. Related: our brief on the engineering behind AQI-managed indoor air.