When is a High Efficiency Furnace Really an Energy Bust?

When it’s attached to ducts that look like this!

In existing buildings, the energy upgrades which are easiest to apply often aren’t the right ones to do.  Or at a minimum, some upgrades should be done in tandem with other measures.  Otherwise the true impact of an upgrade can be considerably less than you’d think.

Take this attic duct system as an example (see picture).  It is not well insulated – just buried in some spots under blown-in insulation.  It’s also not air-sealed with mastic, which seals up openings at the joints which are part of all duct systems.  So the duct loses heat (or gains it in the summer) through conduction, and directly blows heated (or cooled) air into the attic through leakage.  We can assume that a duct system like this loses roughly 40% of the energy it’s supposed to transfer to the living space.  Not a pretty picture.

If we have an old furnace supplying this duct system and replace it with a 90 AFUE unit when the old unit is ready for replacement – that’s a good thing right?  After all, the new furnace is about 90% efficient which places it among the good furnaces in the market.  But we also need to remember that the existing ducts – which deliver the heated air – are only 60% efficient (they lose 40% of the energy they carry).  So the overall system efficiency for the furnace + the ducts will be 0.90 * 0.60, or just 54%!

This doesn’t mean the high efficiency furnace is a bad selection – in fact it’s a very good choice as long as the ducts are also addressed. Attic ducts like this can be accessed for air-sealing with mastic, and also insulated with duct wrap or by having spray-foam applied to them.  This would make the ducts closer to 80% efficient.  Ducts which are inaccessible can also be air sealed with an aerosol sealant system that seals the leaks from the inside-out.

So the best results in improving home energy performance are gained by looking at an entire system.  By using this approach contractors and homeowners alike can make strategic upgrades which deliver the most impact.

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