Hey there Home Energy Pros,

How do you calculate whole home heat loss from the ducts?  To put this in context, I know how to calculate heat loss from the envelope (walls, floor, ceiling taking into consideration insulation levels and air leakage) but I am stumped on how to calculate how much heat the home is losing through ducts located outside the thermal envelope.  A little help, please?

Tags: calculations, ducts, heat, loss

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Bob your point about surface area is right on. To elaborate, put in the smallest AC/furnace that will do the job, put the air handler in the center, run the ducts to high inside wall deliveries or center of ceiling, run the duct on the floor of the attic, not near the roof, Insulate the bejebes out of the ducts by burying them in insulation (applicable to dry climates and with modifications to wet climates) use R 38 or more!

Some pushback on the cycling comment. Flex ducts have very little mass and the amount of time it takes them to reach equilibrium with the supply air is small. The energy stored (or removed) in the duct mass is recovered when the appliance turns off and the fan runs for an appropriate amount of time. As you know, the basic physics (at steady state) is that the "losses" due to conduction are proportional to the temperature difference times the U value times the area times the time. If the U value is high and the area is large, you would want to get it over with in a big hurry. If the U value is low (large R) and the area is small, then the time (called residence time) is of way lesser importance. See http://www.proctoreng.com/utilities/sizing.html

There is always that hot/cold blast for a minute or so after the system starts. It takes awhile for the temp drop between the furnace/coil and register to stabilize. The less times it has to do that per hour the better.

Consider than a typical AC system takes 5-10 minutes to reach steady state conditions. Until it reaches steady state, it's operating at less than optimum efficiency. Less cycles per hour means longer cycles each time the system turns on.

Fan off delays do help, but aren't typically long enough to get ALL the heat/cool out of the ductwork. AC delays are typically 60 seconds, if they are used at all. Heat off delays are set to to shut off the blower at about 90 degrees register temp.

Bob You are hitting on a important point -- the length of the time delays are very important. The typical 90 second time delay on some ACs is set to take advantage of the mass of the evaporator coil and get a higher SEER rating. The applicable SEER test is using unreasonably dry indoor conditions. A 90 second delay in a moist climate under real conditions (moisture on the evaporator coil) is 90 seconds too long since it evaporates water off the coil driving up the humidity in the building (bad in moist climates). A 90 second delay in dry climates is too short since you want to evaporate the water off the coil and produce sensible cooling. In dry conditions, any dehumidification that the AC does is a waste of energy. In those conditions, the fan delay should be longer than 90 seconds and depends on how much water is on the coil (as well as the surface area, temperature differential, and R value of the ducts).

In heating, the length of the time delay is generally limited by the comfort sensation of the occupants.

See http://www.proctoreng.com/energy-efficiency/climate-specific-ac.html  

Then there is Oklahoma where it's wet one week and dry the next. Last weekend humidity was in the teens, this week it's about 50%. I've seen it get into the 80%+ range after a good rain. Ideally a system should be able to see the humidity and be smart enough to set the correct airflow/time delays as needed. Unfortunately few customers (myself included) are willing to spend the money on advanced systems.

My furnace OEM is set to turn blower on/off with compressor, I modified it using a sequencer to control the blower. 10 second on delay so the coil can start cooling before the blower clicks on, and 60 seconds to recover the remaining cooling from the refrigerant evaporating/coil mass at the end of the cycle. Thermostat is set to cycle twice per hour maximum. Allowing the coil to get cold before the blower kicks on offsets any moisture evaporated at the end of the cycle.

Bob Almost everywhere (except places like Riyadh) there are occasions of dry in wet climates as well as high humidity in dry climates. However predominantly in many parts of the US and elsewhere, the majority of the time when the air conditioner is operating is either dry or wet. (See the map at http://www.proctoreng.com/energy-efficiency/climate-specific-ac.html) Oklahoma has more wet days than dry days. So without a humidity sensor you are better off with the 0 second delay (and a lower airflow).

As you say the ideal would be a system that changes from one mode of operation to another as the indoor humidity requires. Such a system did exist (see http://www.proctoreng.com/innovative-products/concept-3.html) that was designed to switch back and forth based on the humidity signal from a thermidistat. Unfortunately they are no longer being produced.

The ON delay is important also. Allowing the coil to get below 50 degrees before turning on the blower allows the coil to start dehumidifying immediately instead of the typical 5 minute wait. Few PSC systems implement a blower on delay. Off delay should also be set for about a 50 degree coil for optimum dehumidification/efficiency balance. ON/OFF delays should be set at warmer temps for dry climates.

Manufactures could implement DIP switches on the furnace boards to accommodate dry or wet climates. If manufacturers really wanted to get fancy they could even run the blower on a lower speed for the first 5-10 minutes of the cooling cycle. But that would take away some of the advantages of the more expensive VS systems, now wouldn't it?

VS systems typically DO have a "Dehum" terminal on the board that changes the way the blower ramps up/delays. Thermidistats unfortunately are very uncommon, few customers are willing to pay for them.

I read the PDF you linked to, 1 cycle per hour is highly unrealistic but illustrates a point. According to page 3 It takes 5 minutes before the coil gets cold enough to start accumulating condensate. Getting the coil cold BEFORE turning the blower on drops wait time dramatically. Running the system longer each cycle will get the coil to spend more time on the "cold zone".


I ghink you are referring to two different issues.  One is the time per cycle to help keep efficiency high and that is what variable speed systems are all about  - keep the thing running so its efficiency stays near max.

The temperature inside the ducts is another issue.  If they are inside the house there is no problem, but if they are, say, in the attic, then the air in the ducts between cycles is being affected by the attic temps, winter and summer.  Another good reason to get the ducts inside the house.


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