Due to duct leakage, only 57% of heat generated in a home furnace is delivered to conditioned spaces. OMG! Run for your lives. Big deal.
I have overheard conversations at conferences and other networking gigs that include statements like, “We are so focused on envelope tightness, insulation, energy-saving windows, and efficient appliances, but meanwhile we are wasting almost half the heat produced in an energy efficient furnace.” Not so fast.
In mechanical engineering, we have the first and second laws of thermodynamics. The first law is like a change machine – conservation of money. Put a piece of cotton-infused paper in and get four discs of metal – they represent the same thing: one dollar, one hundred pennies, four quarters, and so on. The first law is actually conservation of energy. Energy in any form – electricity, heat, fossil fuel, fried cheese curds enter an imaginary or real box/container, and it is either stored, converted to something else, or leaked, piped, blasted, or oozed out of the box. We nerds call the box a control volume.
The second law, in case you were wondering, is: your refrigerator doesn’t work unless you plug it in. Heat doesn’t move up hill from cold (ice cream) to warm (your kitchen) without adding energy. Water doesn’t flow uphill without a pump. Wealth is not grown for real by taking money from somewhere and showering it from helicopters.
Engineers who cannot explain the first and second laws of thermodynamics a dozen different ways each – do not waste your time or ours looking for employment.
Consider the conditioned space of a typical home to be the control volume or imaginary box described above. Across the boundaries of the box cross water, electricity, natural gas, heat transmission, and air leakage.
Consider only heat fueled by natural gas. Within the home, the furnace system includes natural gas, combustion air and return air in; combustion gases (exhaust) and warm air out – as depicted in the cartoon. That’s it. Natural gas, combustion air, and air from the space go in, combustion gases (exhaust) and warm air leave the furnace.
As is typical with most homes in regions of the country with considerable heating needs, all the ductwork is within the conditioned space, basement, or in the case of my house, also running through the wall cavity to the second floor with “blown in blanket” (BIB) insulation packed around the duct, between it, the studs, and exterior sheathing.
With any decent heating system design, heat is delivered where the losses are – on the perimeter in this case. Return air flows to the middle, depicted by the blue lines and back to the furnace.
So if only 57% of the energy leaving the furnace makes it to the conditioned spaces, where is the energy going?
- Out the exhaust as unburned fuel? No. The EPA would have a fit.
- Out the exhaust as hot combustion gas? No. That amount of wasted heat would melt the exhaust.
- Out of the ducts upstream of the room diffusers? Some of it, yes. But so what? Spaces need heat anyway. Perhaps you have seen duct socks? The entire “duct” is a cloth diffuser. It “leaks” everywhere. They distribute warm air through conditioned spaces, as with a house.
- Some heat is also stored in the ductwork itself, just like water pipes and the water within them cools over time; it takes a non-negligible amount of heat to heat up the ducts. But again, the heat dissipated from the ducts is not lost to the environment. It stays in the home.
It doesn’t matter what the climate is, “quality installation” needs to include ductwork running in internal spaces so leakage, heat gain, and heat loss contribute to home heating and cooling needs. Running ductwork outside, in crawl spaces, or in the attic is a bad idea.
As pathetic as it sounds, an outstanding installation would include a total heat (and cooling) load calculation to size heating and cooling equipment. Room by room load considerations would be as common as an IRS agent confessing they know anything about the goings on in their fiefdom.
Analyzing the room by room heat delivery design in my house – every room has heat supply whether it is needed or not. The bathrooms, for example, have virtually no heat loss by design, but too bad; they have a warm air supply. Every window and door gets a diffuser of roughly the same capacity, even in our great room which has a very high ceiling. Result: spaces with practically no heat loss like bathrooms and other smaller spaces get gobs of excess heat and the great room is woefully under supplied. Fortunately, homes are quite small and leaving doors open results in reasonable temperature distribution to mask the fact that thoughtless robots install this stuff.
Lastly, this rant pertains to single family, duplex, quads, and similar, only.