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Old 11-25-13, 02:49 PM   #34
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Originally Posted by Servicetech View Post
Back to the original topic, it does appear that duct losses are significant in the cycling loss calculations. Not sure hot much the 45 second burn time to heat up the exchanger costs in efficiency, and how much is recovered during the 180 second blower off delay after the burner shuts off. 90%+, 80% and old school pilot light furnaces have different cycling losses.
The real problem as it relates to this thread is that when using old-school, forced air "blast" furnaces, the duct losses are calculated separately in the design. The AFUE or thermal efficiency measurements are done for the heating unit only, under a tightly controlled set of conditions. They basically consider what heat is available to distribute, and tell nothing about the ductwork outside the box. Once it leaves the unit, there's no telling where the heat may go, only how much and how hot it is, at what pressure.

In the real world, the professionals in the industry are left to solve the problem of delivery. Any mechanical contractor or engineer will tell you that it's all about trade-offs. Finding the balance of airflow vs temperature vs raw btu's delivered here or there is an art in itself. It would be great to have a wide open, straight path to follow, but bottlenecks are unavoidable due to the nature of the installation. Working around or through the real world situations is what these pros are supposed to be devoted to. As always, your mileage may vary.

There are lots of ways to try to approximate the "transient" losses of ductwork, as well as "static" losses. Due to the uncertain nature of (especially residential) on-site distribution equipment actually installed, it is commonly much easier to just take measurements. The main idea is this: the more restrictive, leaky, and uninsulated the ductwork is, the less heat that will make it to its destination. As with anything, everything you do to improve the system adds up.

As with many other aspects of making a building more efficient, most of the remedy is a one-time expense. After it's done, it just keeps working for free. But it's the work that turns people off. Nobody wants to go crawling around in a dark, dusty, cramped space. Not even once. Don't even mention dragging materials in and out of there!

Last edited by jeff5may; 11-25-13 at 02:58 PM.. Reason: grammar
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