Does MHRV Heat or Cool Your Home? What It Really Does

In brief: MHRV (mechanical heat recovery ventilation, also written MVHR) is a ventilation system. It replaces stale indoor air with fresh filtered air around the clock, and uses the warmth of the outgoing air to temper the incoming air. It saves heat you would otherwise lose. It never adds heat, and it cannot cool your home below the temperature of the outside air.

What does MHRV actually do?

In brief: MHRV moves air, not heat. It pulls damp, stale air out of kitchens and bathrooms, supplies fresh air to bedrooms and living rooms, and passes the two streams through a heat exchanger so most of the warmth stays inside.

1. Extract: fans pull air from wet rooms: kitchen, bathrooms, utility.
2. Exchange: outgoing and incoming air pass each other in the heat exchanger without mixing. Warmth transfers from one to the other.
3. Filter: incoming air goes through filters that catch pollen and dust.
4. Supply: tempered fresh air flows into bedrooms and living areas.

The unit runs continuously at low speed and boosts when humidity rises, for example during a shower.

Why does air from my MHRV feel warm?

In brief: Because the heat exchanger has warmed the incoming air using heat your heating system already produced. The air arrives close to room temperature but always slightly below it, so MHRV is still a small heat loss, just a much smaller one than an open window.

Here is the calculation for a cold Dublin day. A good heat exchanger is rated around 90% efficient, and in real homes duct losses bring that down to about 76% (90% × 0.85 in-use factor).

	Temperature
Outside air	5°C
Indoor air leaving the house	20°C
Difference	15°C
Heat recovered (76.5% of 15°C)	11.5°C
Fresh air entering your rooms	16.5°C

The fresh air comes in 3.5°C cooler than the room. Your heating system covers that gap. Without heat recovery it would have to cover the full 15°C.

Can MHRV cool my house in summer?

In brief: No. On a hot day it can take the edge off incoming air, and at night its summer bypass can bring in cooler outdoor air. It can never deliver air colder than outside, and it has no refrigeration circuit.

Two things happen in summer:

• Heat recovery on a hot afternoon: outside 26°C, inside 22°C. The exchanger cools incoming air by about 3°C (76.5% of 4°C), so it arrives at roughly 22.9°C. Still warmer than your room.
• Summer bypass at night: when outside air is cooler than inside, the bypass sends fresh air around the exchanger so it is not warmed. This flushes heat out overnight. It only works when the outdoor air is actually cooler.

If your bedroom is 26°C during a heatwave and it is 24°C outside at midnight, MHRV can at best bring the room towards 24°C. Air conditioning can bring it to 20°C.

MHRV vs heat pump vs air conditioning

In brief: Three different jobs. MHRV brings in fresh air. A heat pump heats your home and water. Air conditioning cools rooms, and an air-to-air heat pump does both heating and cooling, but none of them ventilate.

	MHRV	Air-to-water heat pump	Air conditioning (air-to-air)
Main job	Fresh air, moisture removal	Space heating and hot water	Cooling, and heating if it is a heat pump model
Adds heat to the house	No	Yes	Yes, in heat mode
Cools below outdoor temperature	No	No (most models)	Yes
Brings in fresh outdoor air	Yes, continuously	No	No, it recirculates room air
Heats water for showers	No	Yes	No
SEAI grant	€1,500, only through a One Stop Shop	Up to €14,500 for a house	Up to €7,500

They work well together. In an airtight home, a heat pump provides the heat and MHRV makes sure you breathe fresh air without throwing that heat away. See air-to-water vs air-to-air heat pumps for the heating side.

What about exhaust air heat pumps?

In brief: An exhaust air heat pump is the one ventilation product that does heat your home. It combines extract ventilation with a small heat pump that takes warmth from outgoing air and puts it into your heating and hot water. SEAI grants it as a heat pump, not as ventilation.

If an estate agent or builder says the house has "heat recovery heating", ask which one it is. MHRV ventilates. An exhaust air heat pump heats.

When do you need MHRV?

In brief: When a home is airtight enough that wall vents and opening windows can no longer keep the air fresh. New Irish homes must reach an air permeability of 5 m³/(h·m²) or better, and the 2019 ventilation guidance calls for mechanical ventilation in homes tighter than 3.

• New builds: many A-rated homes built since 2019 have either MHRV or demand controlled ventilation (DCV).
• Deep retrofits: once walls, windows and attic are sealed, SEAI expects a planned ventilation system such as DCV or MHRV.
• Older draughty homes: leaks already ventilate the house. MHRV is rarely worth retrofitting until the home is airtight.

Signs your MHRV is not doing its job

• Condensation on windows in the morning: airflow too low or filters blocked.
• Stale smell or stuffy bedrooms: check the unit is on and vents are open.
• Cold draughts from vents: filters clogged, bypass stuck open, or the system was never balanced.
• Noise from vents: fan speed too high or ducts undersized.

Do not switch MHRV off to "save heat". It is designed to run all the time, and switching it off in an airtight home leads to condensation and mould.

FAQ

Does MHRV heat the house? No. It keeps most of the heat that ventilation would otherwise lose, but all the heat comes from your heating system.

Can MHRV replace air conditioning? No. MHRV cannot cool air below the outdoor temperature. In a heatwave you need air conditioning or an air-to-air heat pump for real cooling.

Is MHRV the same as MVHR? Yes. Both mean mechanical ventilation with heat recovery. Irish installers use MHRV more often.

Should I turn MHRV off in summer? No. Leave it running and let the summer bypass handle cooler nights. Turning it off stops fresh air reaching the bedrooms.

Do I need MHRV if I get a heat pump? Not always. A heat pump needs a well insulated home; whether you also need MHRV depends on how airtight the home is. Your technical assessment will tell you.

Not sure what your home needs?

Tell us about your home and we will put you in touch with Dublin installers who handle heat pumps, A/C and ventilation. We add nothing to their prices. Request quotes

Sources: Passive House Plus, "Thin air", SEAI heat pump system grant, SEAI One Stop Shop grant values, SEAI, Upgrading to an A-rated home, Technical Guidance Document F, 2019.

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