What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Same word, different thingMVHR
also mechanical ventilation with heat recovery, heat recovery ventilation, MEV, System 4
MEV is a different system entirely — continuous mechanical EXTRACT with no supply and no recovery — and the two are routinely confused because the acronyms differ by one letter.
Australia
Close, not identicalHRV
also heat recovery ventilation, ERV, balanced ventilation, mechanical ventilation
Far less common than in cold climates, since the heat being recovered is worth less; where it appears in a humid zone the ERV variant is usually the right one.
United Statesyours
Same word, different thingHRV
also ERV, heat recovery ventilator, energy recovery ventilator, balanced ventilation
HRV and ERV are separate products: an ERV transfers MOISTURE as well as heat. Which is correct depends on the climate zone, and the wrong one makes a house drier or damper than intended.
Canada
Close, not identicalHRV
also heat recovery ventilator, ERV, air exchanger, HVI rating
Effectively standard in new housing, and rated by HVI at two temperatures because a unit's efficiency and its frost behaviour are different questions.
The governing standard, by market
United Kingdom
Approved Document F / BS 5720
Ventilation requirements including continuous mechanical supply and extract rates, and the code of practice for mechanical ventilation of buildings.
United States
ASHRAE 62.2 / HVI 920
Ventilation and acceptable indoor air quality in residential buildings, and the product rating procedure for heat and energy recovery ventilators.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- What is the difference between an HRV and an ERV?
- Moisture. Both pass the outgoing and incoming air streams either side of a heat exchanger so that heat crosses without the air mixing. An HRV's core transfers SENSIBLE heat only — temperature. An ERV's core is made of a material that also transfers water vapour, so some of the humidity crosses with the heat. In a humid summer that is an advantage: the outgoing conditioned air dries the incoming muggy air before it reaches the cooling coil, which cuts the latent load substantially. In a cold, dry winter it is a different trade: the ERV returns moisture to the house that an HRV would have thrown away, which keeps indoor humidity up — comfortable, and a problem if the building already struggles with condensation. So it is a climate decision and a building decision rather than a quality ladder, and specifying an ERV because it sounds like the upgraded model is the common error.
- Why does a heat recovery unit need a frost strategy?
- Because the air it is cooling is the air people have been breathing into. The extract stream leaves the house warm and humid; inside the exchanger it is cooled by the incoming outdoor air, and once its temperature drops below its dew point it deposits water on the core. If the incoming air is below freezing, that water becomes ice, the ice blocks the extract side, the flow falls and the unit's efficiency collapses — in the coldest week, when it is needed most. Units therefore carry a defrost strategy: a pre-heater on the incoming air, a periodic recirculation cycle, or throttling the supply fan to let the warm stream melt the core. Each costs something — energy, ventilation rate, or balance — and the fact that this is a designed behaviour rather than a fault is why a system that seems to stop working in a cold snap is usually doing exactly what it was set up to do.
- Why is commissioning the ductwork as important as the unit?
- Because the recovery figure on the box is a property of the core, and everything a household actually experiences is a property of the installation. The unit only recovers heat if the two air streams are BALANCED: if extract exceeds supply the house is depressurised and pulls unconditioned air in through every gap, and if supply exceeds extract it is pressurised and pushes humid air into the construction. Balancing means measuring the flow at each terminal and adjusting, not setting a speed and assuming. The ducts themselves decide the rest: flexible duct pulled tight around a bend can lose most of the flow to that branch, insufficiently insulated duct in a cold loft condenses on its outside and drips through a ceiling, and a run that was shortened on site changes the balance of every other branch. That is why a commissioning sheet with measured flows is the deliverable, and why a system installed without one is a box with no evidence it does anything.