Are HVAC ducts a factor in heat loss?
user-1087955
| Posted in Energy Efficiency and Durability on
I am in the whole house fan business (http://www.invisco.com) and just released an R60 duct adapter. I notice that there is nothing like it for HVAC ducts. Are those ducts too small to be of concern? My design can be used to add insulation to ceiling connections to meet R38, R49 and even R60 for hvac ceiling interfaces if that would be of value.
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Replies
Kurt,
Q. "Are HVAC ducts a factor in heat loss?"
A. Yes -- especially when the ducts are located outside of a home's thermal envelope.
I imagine that ducts insulated to R-60 would be cumbersome to install. It makes more sense to simply locate the ducts inside a home's thermal envelope.
The R60 ducts I have released are not cumbersome, they are simple, inexpensive, and very easy to put in. And they are intended for older energy upgrades. So I think you answered my question - they are a factor, so now I must ask, can you help me quantify the factor? I know that standard ducts are usually rated about R6. So heat can travel up the duct and be lost through the duct walls in the attic. Has anyone calculated the amount of heat loss?
If I may, there are at least 4 ways that a duct system can experience heat/ cooling loss:
air leakage via equipment;
air leakage via boots & returns;
air leakage through duct seams;
conduction through duct insulation.
although I have nothing but experience to footnote, energy loss through conduction would be the least of my worries ... a little bubble wrap and R-value upgrade and done ... wouldn't pay an extra $.10/ft for r-whatever ducting ... jmho! Once heat or cool activates, duct temp is almost immediate
Kurt, if you're designing product, why do you not have some engineering capability available to you?
Paul, your statements are puzzling. Sounds like you think that conductive loss from ducts is minimal. There is often a very large delta T between air inside and outside a duct. You can use a very simple equation to find how many BTUH are lost given delta T, surface area, and insulation value. And, I'm interested in this bubble wrap stuff, tell us more please.
David, I have one of the best engineers I know assisting me in product design and he can do your calculation in a flash. He has assisted me in design of the product I have released. It is not duct, and it has no bubble wrap. It is a carefully engineered damper assembly that combines two butterfly dampers that are thermally disconnected, aerogel, a pocket of trapped air and off the shelf cellulose to offer up to R60 and better.
I posed this question because I have seen nothing yet that discusses thermal loss up through standard room hvac ducts. My damper design is useable for all duct connections, both hvac and whole house fans. I sits at the ceiling connection and interfaces to standard R6 hvac duct.
I am guessing that the surface area presented by an hvac outlet in a room is such a small heat loss source that it is deemed negligible by energy conservationists. If it becomes an issue in the future my product will solve it.
Thank you for your interest.
Kurt,
You wrote, "I posed this question because I have seen nothing yet that discusses thermal loss up through standard room hvac ducts."
Check out these articles:
Creating a Conditioned Attic: "During the summer, attic temperatures often exceed outdoor temperatures. Attic ducts almost always have thinner insulation than ceilings, in spite of the fact that the delta-T (that is, the temperature difference) between the air in the ducts and the air in the attic is even greater than the delta-T between the inside of the home and the exterior. ... The bottom line: running ducts through an attic saves money for the builder, but costs the homeowners dearly in increased energy costs."
Forced Air HVAC Systems: "It makes little sense to install ductwork with R-6 insulation in an attic. During the summer, when the air conditioner is struggling to cool the house, the attic temperature will usually be significantly higher than the outdoor temperature, making the system work that much harder."
Is Bubble Wrap Duct Insulation a Good Idea?: "Any claim that bubble wrap is the equivalent of R-6 duct insulation is a scam and a fraud."
Martin, your comments are interesting, but they don't address the opposite thermal extreme, cold attics. My investigation is looking for data on the loss of heat in the winter up through the duct interface. As you correctly point out, the temperature difference is huge. And since heat rises, loss in the winter is likely to be a significant factor.
About bubble wrap, as you know, air is at the heart of every insulating system except vacuum systems. R6 can be achieved by any system that contains air and has little else to conduct heat. Bubble wrap should be an excellent method IF it is installed thick enough to offer the R rating needed.