At the time the furnace was purchased, Attic was only 2 inches loose
full basement non-insulated. No heatload calc was done. they had this
use. I figured should be similar to
Previous 'dinosaur furnace was 100KBTU'.
The NEW furnace installed is Rheem 80% 2 stage: model RGPK-O7EAMER
As per sticker on the furnace:
rated 1st stage: 52500BTU
2nd stage : 75000 BTU
House is typical box with roof (barn type?)
1140 sqft area.
After a year, I had added insulation to the attic
(added R12 bats 3 layers). Also insulated the basement walls to R14
sealed the sills tried to eliminate air drafts. I havent touched the
yet. Just used clear plastic to minimize draft.
This winter found out that running the furnace in 1st stage only, the
furnace is able
to maintain temperature. It appears like I don't need the
2nd stage. The temperature does not dip even at coldest outside.
The 1st stage do run much longer before it stops when temperature
I guess my question is, do I have an oversized furnace?
Furnace oversizing is not uncommon, Canada, USA, Europe
Heat load calcs are rarely done. The format shown is 'good' enough for
most installers without an HVAC ARI certified engineer on staff.
I live in a 1982 vintage 1200 sq ft home. Insulation has been added,
athough we are not at R40 ceilings nor R20 walls. Windows are recent
vintage double pane glass and we have done a pressure door test to
reduce air leaks.
This table says that I need a 45K BTU input furnace to heat my home, yet
a 70K unit is installed. When the heater is active, the fan is in low
speed mode way over 50% of the time. Today, we are at the low temp
point of the year, and it will rarely get much colder than it is here
today, and the furnace is running maybe 10% of the time.
You neglected to mention your location. Are you actually in Saskatchewan?
I am, and apparently the design temperature around here is -35. In
Saskatoon at least we haven't actually reached that this winter, so I'm
not surprised that you're not kicking into 2nd stage very much.
I just got a new 95% two stage furnace (and 2 stage tstat), and it has
only kicked into the second stage first thing in the morning to recover
from setback, or if I manually bump up the tstat a lot.) So either
you're not unusual, or my new furnace is oversized as well. (Which is
This next bit is my on opinion, so take it for what it's worth.
Anyways, I think that with a 2 stage furnace the question of it being
oversized is less critical than for a single-stage. And with a variable
speed blower motor, the issue of oversized blowers is dealt with as well.
Also, according to the Canada Mortgage and Housing Association, while
there are data showing efficiency improvements when properly sizing
conventional (~60% efficiency) furnaces, there are no good data for
Finally, a report prepared for the National Association of State Energy
Offices comments, "Contrary to conventional and mid-efficiency furnaces,
where efficiency decreases with furnace oversizing, some literature
suggests that condensing furnaces are actually more efficient when they
are oversized and run for shorter periods." While this doesn't apply to
your case, it may be of interest to others.
With a dual-stage furnace I doubt you'd be looking at much of a
performance penalty, if any -- 52,500 BTU/H is half the size of your
previous furnace and perhaps not unreasonable given temperatures in
your area can fall as low as -30 and even -40C (especially if your
home is exposed to high winds).
Even if it were somewhat oversized there may be some benefit to this;
e.g., peace of mind (frankly, I'd worry a whole lot more if I had too
little capacity as opposed to too much); faster recovery after
nighttime setback; and, additional capacity to meet future needs
should you later add on to your home. And in the event of an extended
power outage, powering a slightly larger furnace may help minimize
Thanks for the replies. now I can feel at ease.
i'm now going to have a 'smart response thermostat installed'.
The present thermostat is a 'Rite Temp' 8085C' 2 stage thermostat.
2nd stage is by timer comes up after 15 minutes of 1st stage and
then stops furnace when desired temperature is reached.
I'm presently operating it in Heat pump mode. This way it always
runs in 1st stage. Except when temperature difference is 2 degrees
and more (then it will go to 2nd stage).
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