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Multi-speed equipment and attic ducts
Hello everyone. This is my first post. Thank you in advance for reading and responding.
Many of our customers have systems with duct work in the attic, where temperatures can drop to 20F or less in the winter, and 130F or higher in the summer. Energy codes require insulation over the ceiling to be r-38 (here), but only r-8 on the duct work. Obviously, the attic is not a desirable place for your HVAC system or ducts. But… add to that a multi-speed system! Now, when a system is operating at a low speed, you have air moving more slowly through over-sized ducts. I am sure that in some circumstances the lowest operating speed produces no net heating or cooling due to the high duct heat losses or gains.
A premium model digital thermostat's algorithm should eventually figure out that it is useless to operate in the lower speeds when it is hot/cold out. So now we have a customer that paid a premium for a multi-speed system that at best only runs on high speed, or at worst is of a much lower efficiency than they thought.
The conclusion I come to is that variable speed equipment, and possibly even 2-speed equipment, is not appropriate for systems with ducts that are in extreme environments, though I can find no literature that backs this up. Anyone care to weigh in on this?