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QUESTION:We live in a 3,000 sq foot home in the Denver suburbs and are looking
to replace our 18 year old Rheem gas furnace with a Lennox G60V. It is
the Lennox signature collection at 80% efficiency with a variable
speed blower. The dealer told us we could go from the 150,000 BTU our
Rheem currently has to 135,000 BTU since it is so much more efficient.
The best bid we have is $3,172 installed. We have an Amana air
conditioner also. Hopefully they are intelligent enough to know how to
hook all this up correctly. My questions are:
1. Is Lennox worth the extra cost?
2. Is a two stage furnace (gas) with a variable speed blower really as
important as all the dealers seem to think?
3. Is there any other advice you can share with us on this purchase?
ANSWER: Lennox is not my favorite brand but be that as it may, yes it would
be worth it.
Your current furnace is quite likely an "Atmospheric" or Catagory I furnace,
this is just a fancy way of saying it uses air from inside the house for
combustion, while it's in operation and assuming it's relativly clean it
will usually burn fuel(gas) at 80% efficiency, even the superhigh efficiency
units burn the gas at 80% efficiency.
The problem is that a lot of homeowners fail to grasp the difference between
the 2 efficiency rateings. Your 80% efficiency rateing is "Static" and only
looks at one aspect of the heating process, specifically the combustion
itself. The normal air/fuel ratio is 10:1, so for every cu.ft. of gas your
furnace burns 10cu.ft of "primary air" is intermixing with the gas, there is
also the phenominon called "secondary air" where air not going thru the
burners themselves is present to aid in the combustion process, secondary
air starts out low volume but increases as the heatexchanger and stack
heatup, so about 8-10 mins into the "on cycle" you might have 3 or 4
cu.ft/min secondary air passing thru the combustion area along with the
10cu.ft. of primary air. And unless you have an automatic fluedamper that
closes off the stack in the off cycle your system will continue to suck 10
or more cu.ft./min of air during the entire off cycle. The actual amount is
dependent on the length and number ells in the smokepipe as well as the
temperature difference between the inside air and outside air.
The true or "Dynamic" efficency of your furnace is actually somewhere
between 45-60% because it uses a draft to expell the heated gases from the
house. Once the T-stat is nolonger calling for heat the burners go off but
air you just paid to heat is still being sucked out of your house.
The furnace your looking at is a Catagory III "true" 80% efficiency furnace,
it still uses inside air for the combustion process, but the heat exchanger
is so tight that it has to have a draft induction blower just to pull the
heated gases thru. When the combustion blower is off very little air
migrates thru the furnace thus drastically increasing the dynamic efficiency
of the furnace. The tubes are made of materials designed to maximize the
amount of heat being transfered to the "living" air as it passes over the
tubes and into the duct system.
That furnace is also a 2stage heat, which means it starts out in lowfire and
changes over to highfire either on a timer or by means of a 2stage Tstat.
Think of it sort of like your car, you don't always have your foot tromped
down on the gas, sometimes you only want to go a little fast. Any single
stage piece of equipent knows only 2 states On or Off, the 2stage knows 3
states Lo/Hi/Off, the fan speed is also staged, lowfire/highfire fan speed.
Lastly the variable fan increases the overall efficiency by starting out
with minimal airflow in cooling mode and gradually increases the speed to
normal amount, this causes water to condense out of the air much faster, so
the air is drier by the time you reach full speed on the blower.
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