Home / Duct Cleaning / Guide
Duct cleaning - building ventilation
A building constantly throws away air it has spent energy heating or cooling, and pulls in fresh air it then has to condition from scratch. Energy recovery ventilation reclaims much of that wasted energy - a real efficiency gain. But it does it by passing both airstreams through a heat exchanger, adding equipment and ductwork that need keeping clean to keep working. Here is energy recovery ventilation explained for facilities managers.
The short answer
Energy recovery ventilation (ERV), also called heat recovery ventilation, saves energy by transferring heat between a building's outgoing stale air and its incoming fresh air. Normally, a building expels air it has heated (or cooled) and brings in outside air it must then condition from scratch, wasting the energy in the exhausted air. An ERV passes both airstreams through a heat exchanger, so the outgoing air pre-warms the incoming air in winter (or pre-cools it in summer) without the two mixing - reclaiming much of the energy that would be lost. This is a genuine efficiency gain. But it adds a heat exchanger and additional ductwork to the system, and those - like all ventilation ductwork - accumulate dust and dirt over time, which reduces the heat exchanger's efficiency and the airflow. So for an FM, an ERV is an energy saver that depends on being kept clean: a fouled heat exchanger or dirty ducts lose the efficiency the system was installed to gain.
The energy a building throws away
To understand energy recovery ventilation, start with the waste it addresses. A building needs a constant supply of fresh air for the people in it, which means continually expelling stale indoor air and drawing in fresh outdoor air. But the indoor air being expelled has had energy spent on it - it has been heated in winter or cooled in summer to keep the building comfortable - and throwing it out throws that energy away. Meanwhile, the fresh outdoor air being drawn in is at outdoor temperature, so it has to be heated or cooled from scratch to condition it. So ordinary ventilation is continually discarding conditioned air and conditioning raw air, wasting energy at both ends.
This waste is significant, because ventilation air is a large part of a building's heating and cooling load - the energy used to condition the incoming fresh air is a real and continuous cost. The more fresh air a building moves, the more energy it wastes this way. So there is a lot of energy going out with the exhaust air, and a lot being spent conditioning the incoming air - and the two are, in a sense, opposite: the outgoing air is warm (in winter) while the incoming air is cold and needs warming. Energy recovery ventilation exists to bridge that gap - to use the energy in the outgoing air to condition the incoming air, instead of wasting one and paying for the other. That is the opportunity it captures.
How the heat exchanger works
Energy recovery ventilation captures that opportunity with a heat exchanger - the core of the system. Both airstreams, the outgoing stale air and the incoming fresh air, are passed through the heat exchanger, which transfers heat from one to the other without the two airstreams mixing. In winter, the warm outgoing air gives up much of its heat to the cold incoming air, pre-warming the fresh air before it enters the building, so less energy is needed to bring it to temperature. In summer, the process runs the other way, the cooler outgoing air pre-cooling the warm incoming air. Either way, energy is transferred from the air being thrown away to the air being brought in, reclaiming much of what would be lost.
The crucial feature is that the heat is transferred while the airstreams stay separate - the heat exchanger passes heat between them without letting the stale outgoing air mix into the fresh incoming air, so the fresh air stays fresh. Different heat exchanger types achieve this in different ways, but the principle is the same: heat crosses between the airstreams, air does not. The effectiveness is measured by how much of the energy is recovered - a good system reclaims a large proportion of the heat that would otherwise be lost. So the heat exchanger is what makes ERV work: it is the component that transfers the energy from the outgoing to the incoming air, turning wasted energy into a reduced heating and cooling load. Everything else - the fans, the ducts - serves to move the two airstreams through it.
What it adds to the system
The efficiency gain comes at the cost of added complexity: an ERV adds a heat exchanger and additional ductwork to the ventilation system - both airstreams have to be ducted to and through the heat exchanger, so there is more equipment and more ducting than a simple extract-and-supply arrangement. And all of it, like any ventilation ductwork and equipment, accumulates dust and dirt over time. The incoming fresh air brings in outdoor dust and pollen; the outgoing air carries indoor dust; and both pass through the heat exchanger, where dirt collects on its surfaces. So an ERV has more surfaces gathering dirt than a simpler system, including the critical heat exchanger itself.
This matters because the fouling directly undermines what the ERV is for. A heat exchanger with dust and dirt built up on its surfaces transfers heat less effectively - the dirt insulates the surfaces and obstructs the airflow, so less energy is recovered, eroding the efficiency gain the system was installed for. Dirty ducts and a fouled heat exchanger also restrict the airflow, so the fans work harder and move less air, adding energy cost and reducing the ventilation. So the dirt that accumulates in an ERV is not just a hygiene matter - it degrades the energy performance that is the whole point of the system. An ERV left to foul up gradually loses its efficiency advantage, until it may be recovering far less energy than it should, while the FM assumes it is still performing. Keeping it clean is keeping its efficiency.
Why it matters to the FM
For a facilities manager, the practical implication is that an ERV is an energy-saving system that depends on being kept clean to deliver its saving. The heat exchanger and ductwork need cleaning as part of the building's ventilation maintenance, to keep the heat exchanger transferring heat effectively and the airflow unrestricted. Neglected, the system fouls up and quietly loses efficiency - still running, still ventilating, but recovering less energy and costing more to run than it should, with the loss invisible unless the performance is checked. So the FM who wants the ERV to keep delivering its energy saving has to include it in the maintenance and cleaning regime, not treat it as fit-and-forget.
This sits alongside the general reasons for keeping ventilation ductwork clean - air quality, hygiene, airflow, fire safety where relevant - but for an ERV there is the specific, measurable reason that cleanliness preserves the energy performance. A clean ERV recovers the energy it was designed to recover; a dirty one does not. So keeping the heat exchanger and ducts clean is, for an ERV, directly protecting the return on the investment the system represents. For the FM, that makes ERV cleaning a straightforward efficiency measure: the system was installed to save energy, and keeping it clean is what keeps it saving. Understanding the ERV as equipment that needs maintaining to perform - rather than a passive box that saves energy forever - is the key to getting the value out of it.
The takeaway
Energy recovery ventilation saves energy by transferring heat between a building's outgoing stale air and its incoming fresh air, through a heat exchanger that passes heat between the two airstreams without mixing them. In winter the outgoing air pre-warms the incoming air, in summer it pre-cools it, reclaiming much of the energy that ordinary ventilation throws away as it expels conditioned air and conditions raw air. It is a genuine efficiency gain, and a large one given how much of a building's heating and cooling load is ventilation air.
But it comes with a heat exchanger and additional ductwork that accumulate dust and dirt over time - and a fouled heat exchanger transfers heat less effectively while dirty ducts restrict the airflow, so a neglected ERV quietly loses the efficiency it was installed to gain. So for an FM, an ERV is an energy saver that depends on being kept clean: cleaning the heat exchanger and ducts preserves the energy performance, protecting the saving the system represents. The system was installed to save energy, and keeping it clean is what keeps it saving - which makes ERV cleaning a straightforward efficiency measure, not just a hygiene one. Include it in the maintenance regime, and the ERV keeps delivering; neglect it, and the saving quietly erodes.
Questions
A ventilation system that saves energy by transferring heat between a building's outgoing stale air and its incoming fresh air. Both airstreams pass through a heat exchanger, so the outgoing air pre-warms the incoming air in winter (or pre-cools it in summer) without the two mixing - reclaiming much of the energy ordinary ventilation wastes by expelling conditioned air and conditioning raw air.
It transfers heat from one airstream to the other without letting them mix. In winter the warm outgoing air gives up heat to the cold incoming air, pre-warming the fresh air so less energy is needed to condition it; in summer it runs the other way. The heat crosses between the airstreams while the air itself stays separate, so the fresh air stays fresh.
Because it adds a heat exchanger and ductwork that accumulate dust and dirt over time - the incoming air brings outdoor dust, the outgoing air carries indoor dust, and both pass through the heat exchanger where dirt collects. That fouling reduces the heat transfer and restricts the airflow, eroding the efficiency the system was installed to gain. Cleaning preserves the energy performance.
It quietly loses efficiency. A fouled heat exchanger transfers heat less effectively, so less energy is recovered, and dirty ducts restrict the airflow, so the fans work harder and move less air. The system keeps running and ventilating, but recovers less energy and costs more than it should - a loss that is invisible unless the performance is checked. Neglect erodes the saving.
No - for an ERV there is a specific, measurable reason beyond hygiene: cleanliness preserves the energy performance. A clean ERV recovers the energy it was designed to; a dirty one does not. So keeping the heat exchanger and ducts clean directly protects the energy saving the system represents, making ERV cleaning an efficiency measure as well as an air-quality one.
Include the ERV's heat exchanger and ductwork in the building's ventilation maintenance and cleaning regime, rather than treating the system as fit-and-forget. The system was installed to save energy, and keeping it clean is what keeps it saving - so regular cleaning is a straightforward way to protect the return on the investment the ERV represents.
An ERV only saves energy while its heat exchanger and ducts are clean - our duct cleaning keeps them clear, so the system keeps recovering the energy it was installed to save.
