MVHR Primer
MVHR sounds like a seriously complex piece of mechanical kit. But in reality, it’s a rather clever, largely empty box. Here’s a quick primer:
Mechanical Ventilation with Heat Recovery or MVHR is the key element in a heating system in a Passive House that delivers tempered fresh air and extracts stale or moist air to the outside. When combined with the ability to generate hot water too, it become a ‘compact unit’ equivalent in size to a large fridge freezer. They negate the need for radiators and gas boilers and are electrically operated, so that energy could be offset by renewables to provide zero carbon energy.
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Imagine you’re an air particle. You enter from the outside through a supply duct, pass through a filter and enter the MVHR plenum box through….let’s call this Door A. Opposite is your intended target Door B where supply air enters the key living spaces, and also Door C where extracted air from the rooms returns to the heat exchanger. Next to Door A is Door D, where stale air is eventually expelled to outside. To get to Door B from Door A, you mix and mingle with the air on its way from Door C to Door D. As you cross the heat exchanger you get warmed by the energy all around you, without ever directly touching. This is the ‘Heat Recovery’ part.
The fan (the Mechanical bit) pulls you past Door B to a heat coil where you are warmed to a comfortable temperature. As you’ve been pre-warmed it takes less electrical power to warm you up than if you’d arrived straight from outside. Now you enter the main Ventilation ductwork and are distributed to each supply outlet, silently and at low velocity. Eventually, you’ll be pulled back towards an extract point (kitchen/ bathroom/ store), and arrive back at Door C, mingle with the new arrivals, and exit through Door D having been filtered once more.
So, in summary – an MVHR helps deliver fresh, filtered, tempered air using electricity, draws out warm and stale air but harvests its useful energy before expelling it. Sounds like magic to me.
