Specific fan power, explained

What is specific fan power?

Specific fan power, or SFP, measures how much electrical power the fans use to move a given amount of air, expressed in watts per litre per second. A unit with an SFP of 1.0 uses one watt for every litre per second of air it moves. The lower the number, the less electricity the system uses to ventilate the home.

It is a like-for-like way to compare how efficiently different units move air, independent of how big they are. Two units moving the same air can have very different SFP, and over a year of continuous running that difference shows up on the electricity bill.

Why does specific fan power matter?

Because MVHR runs continuously, the fans are on all the time, so their efficiency drives the running cost. A low SFP unit in well designed ducting uses noticeably less electricity over a year than a high SFP one moving the same air. SFP is therefore the number to watch for running cost, separate from heat recovery efficiency, which is about heat rather than electricity.

People sometimes focus only on heat recovery efficiency and overlook SFP, but the two are different levers. A unit that recovers a lot of heat but has thirsty fans can still be expensive to run. For total running cost, SFP is the figure that matters most.

What is a good SFP for an MVHR unit?

Lower is better. Premium units achieve very low figures: the Zehnder ComfoAir Q, for example, quotes a specific fan power as low as around 0.53. Economy units, and more often poorly designed ducting, push the figure higher. Compare the tested SFP from the Passivhaus database or the PCDB rather than a best-case marketing number.

As with heat recovery efficiency, the tested figure from a database is more reliable than a headline claim, because manufacturers can quote SFP under favourable conditions. A figure well under 1.0 is good for a domestic unit; the best certified units sit around half that.

Sources: Passivhaus Institut; Zehnder Group UK

How does ducting affect specific fan power?

The unit rated SFP assumes reasonable ducting. In practice, long runs, tight bends and cheap flexible ducting all add resistance, forcing the fans to work harder and pushing the real SFP up. A good unit in bad ducting can perform worse than an average unit in good ducting, which is why duct design matters as much as the unit choice.

This is the single biggest reason a real-world system falls short of its datasheet. The rated figure is measured on a test rig with ideal ducting; your home is not a test rig. Keeping duct runs short, using rigid or semi-rigid ducting and avoiding tight bends keeps the real SFP close to the rated one.

Does SFP or heat recovery efficiency matter more?

Both, and they measure different things. Heat recovery efficiency is how much heat the unit keeps; SFP is how much electricity it burns. A unit can be strong on one and weak on the other. For total running cost, look at the two together, and remember that installation and ducting quality affect the real-world figures for both.

If you had to weigh them, heat recovery efficiency matters most in a very airtight, cold-climate home where heat loss dominates, while SFP matters most where the fans run hardest. In practice a good unit scores well on both, so use them together as a pair rather than choosing one.