Quote:
Originally Posted by jeff5may
As long as the system spends most of its cycle time burning gas and blowing hot air, it isn't short cycling. Many apartment and condo units have furnaces that have very little "dead" time waiting for heating calls in between run cycles. I lived in one a number of years ago that I swear cycled fifteen or more times an hour. Vent 1 minute, light burner, blower kicks on, burner shuts off 2 minutes later, blower shuts off 1 minute later. 3 Minutes later, vent kicks on and here we go again. Not a well designed heating system.
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I'm not sure this gathers the whole efficiency equation though. Let me explain. What I've done is stuck an analog dial-type stick thermometer through the rubber sheath above the AC evaporator coil and also put a digital thermometer probe in the duct with the shortest run to the furnace.
What I found was the first 5 minutes of blower run time the temperature is still ramping up. Once I hit 10 minutes it is mostly there and starts to settle off but it doesn't reach the full temperature rise(supply temp subtract return air temp) until 15 minutes.
This also doesn't include the 75 seconds that the furnace runs the burner without blowing air. My furnace runs the blower for 2 minutes after the burner shuts off but it is still putting out a 40 degree temp rise by the time it shuts off and this is with the highest (800CFM nominal) blower setting for a 60k nominal output furnace. I found that manually leaving the blower on until I hit 3 minutes would allow the temp rise to drop to about 20 degrees. Keep in mind that the heat exchanger and metal ductwork is absorbing some of this heat before the full temperature rise ends up coming through the ductwork and out the register and likewise after it is shutdown.
I figure efficiency would be boosted if I could get a cycle time of at least 20 minutes and modify things so the fan kicks on after 30 seconds of burner time and doesn't turn off until 3 minutes after the burner shuts off.
My thermostat has a maximum span of -1f/+2f, so if I have it set to 70 degrees, it will turn on at 69 and shut off at 72. With that 3 degree span I get a 12 minute call for heat which equates to a 10 minute burner time.
If anyone ever finds a thermostat that has a customizable minimum run time setting that would allow me to choose 20 or 30 minutes or a span that I could push out to get such a run time, I really want to know about it. When I manually run a 30 minute cycle, the temperature in the house is comfortable before and after the furnace runs so I'd like to configure the furnace like that. It would be especially nice to have a larger span during setback when I'm away from home too so I'd get a more efficient run.
So basically my opinion is that based on the experience from my furnace, I'd like to see at least a 20 minute cycle time to be reasonably efficient. If I could go for a 30 minute cycle time, I'd drop the thermostat down a degree or two to get roughly the same shut off temperature. I think the Honeywell default of 6 cycles per hour(at 50% load) is an atrocity to the lifespan of a furnace and to efficiency. I'm not sure how anyone could want their furnace to fire up every 10 minutes and run for 5 minutes each time at half load.
Oddly enough at half load, which I could get to before really starting air sealing work my furnace will fire up for a 30 minute call for heat and shut off for 30 minutes when it was -20f outside and set to 70f inside. So under all conditions my furnace will never fire up more often than once per hour. Now that I've done air sealing work and some extra insulation I've got my heat load down to 17k BTU/hr at design load(-11f). Definitely less run time on the furnace but I'd still like to enforce a minimum run time anyway to stretch out the lifespan of the furnace and get a little extra efficiency.