I am interested in purchasing a Rheem high efficiency furnace. Two
options have been presented to me, the two-stage furnace and the
modulating furnace. The modulating furnace is $00 more, and has an
efficiency of 0.5% less. What is the advantage of the modulating
furnace if the two-stage one is more efficient and cheaper?
On Sat, 17 Oct 2009 20:36:17 -0700 (PDT), Miguel Watler
The efficiencies quoted are derived from laboratory tests. In
practice the modulating furnace should be more efficient. Providing it
is not oversized. It also depends on the turndown ratio (ie ratio
between full output and it's low output, the greater the ratio, the
You will need a good man to accurately determine the right size of
furnace for your house. If he comes along and just sort of looks &
then pulls a figure out of the air, kick him out. It's a couple of
hours work to come up with a furnace size for any premises. Unless
there is an identical house he has worked on previously of course.
If your furnace is correctly sized, in the coldest weather, it will
run at high output almost continuously. If it is cutting out more
than once every few hours, it has been oversized because your
installer is ignorant. The ignorant oversize the furnace to play
safe. (Too late for you then of course, you've paid the extra money
out and losing energy to boot)
Every time your furnace cuts out it wastes energy. A modulating
furnace stays running for longer (it turns down as the set temperature
is approached) and thus saves energy.