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Old 11-19-13, 11:14 AM   #7
AC_Hacker
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Originally Posted by MN Renovator View Post
I'm not sure that modulating furnaces solve the problem. At lower burn rates, the heat exchanger is colder and will exchange slightly less heat, a condensing furnace will still be over 90% efficient so the loss isn't that great though. Another point is that any losses to the ductwork have now become constant ductwork losses instead of intermittent ones. If going with a modulating furnace, I'd make it a point to use a variable speed or X13 motor and see if you can set it to have a very low temperature rise, if possible, to minimize ductwork losses. The trouble is that you'd need to be sure that you downsize your furnace as much as possible to do this and make sure you have enough ductwork to make sure that you'll get the airflow without raising power use or increasing static pressure too much. I personally wouldn't do a two-stage or modulating furnace unless I could be sure that my temp rise is something like 40-50 degrees or so. That takes a pile of air though 578 CFM for a 25000 BTUhr output (low stage of a two stage Bryant 40k furnace) for a 40 degree temp rise. The lowest air flow setting on that particular furnace is in the 550ish CFM so it seems they've got it right. I'd imagine the X13 type electrically efficient ECM motor like that uses about 200 watts with that kind of airflow but that's just a guess based on my PSC blower that gives me about 620cfm from 288 watts, which isn't so good for me because my 57000BTUhr output puts that temp rise at 85 degrees, so I up the furnace speed so its consuming 324 watts and a slightly more respectable temp rise.

Does anyone have a good idea on how much power an X13 or variable speed ECM blower uses? Does anyone have one of these furnaces where they have measured this or has a link to how much power they use at a specific CFM? I have yet to come across this data yet. Reducing cycling losses by raising CFM but staying within the manufacturer temp rise spec will help reduce losses through ductwork and you'd get more use out of your fuel at the expense of extra electricity. I don't think it would make sense to do this with a PSC motor and definitely not with a shaded pole furnace motor like the one I used to have. I'm actually thinking of upping its speed to the max in January (408 watts) which is about 800 CFM to bring the temp rise to about 65 degrees which puts towards the lower end of the 55-85 temp rise range for my older furnace. I think I'd get pretty close to 55 if I swapped my pleated filter out for a cheap fiberglass one for the airflow but I'd rather do what I can to not clog my A-coil.
Thank you for this useful post and detailed post.

It had made me more certain than ever that I am on the right track, and that I am truly doing the work of god, in disassembling and removing my gas furnace, which I haven't used in four years.

I'll no longer bang my head against the sheet metal elbow duct that stuck out from the wall at the bottom of the basement steps.

I'll no longer have floor ducts that have eaten bits of corn chips, gallons of dust, and escaping screws.

I'll no longer need to engage in territorial war against the squirrels over who actually owns the chimney that has gone cold... they can have it now.

I'll finally be able to rip out all of those horrible ducts and be able to properly insulate under the living room and kitchen floors.

The removal of the furnace will enlarge the space I need to stage work on installing radiant floors in my house.

Thank you.

-AC
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