This summer, I upgraded my walls to R20, attic to R40, replaced metal single
windows with double low-e argon vinyl, replaced all wood doors with
insulated steel, air sealed the house and replaced a 30 year old 120k gas
furnace with a 75k condensing unit. The old furnace used to run every 5 out
of 15 mins at 0C, now it runs 10-15 mins every 3-4 hours. Something doesn't
seem right here as my neighbors runs 2-3 times per hour and it's a 100k
furnace in approx the same sized house with similar insulation (maybe 20%
less) I have my tstat set to +/- .5C and CPH disabled.
Don't follow the question (if there is one?). What doesn't seem
I see a former case of running 1/3-rd the time, now only about
1/16.8-th of the time (using mid-points of on time and interval given).
That seems pretty good to me. It seems to be taking 2-3X the "on"
time to raise the temperature from "on" to "off" with something under
2/3-rds the capacity of previous (assuming same efficiency which was
probably also worse for older unit) so that also seems quite
Trying to then do something w/ the neighbor's "data" w/o any more
factual basis than this seems futile.
So, I'm at a loss as to what you're asking/expecting...
Old furnace was 65% AFUE steady state, much less on average, new is 92%.
The heatloss rate seems far too low (slow), most tstats have a CPH setting
whereas my cycles are PER DAY!
Maybe the problem is that the furnace btu output is too high for the rate of
heat loss. I did calculate a loss of 40k at -5C so I'm about 50% oversized
which should be ok.
My old furnace ran nearly continuously before we did the upgrade to 96%
and did the walls doors and windows, now it hardly ever comes on, but
the temperature is where I want it. I'm delighted, less run time = less
I question the decision to go with steel doors, I took a nice Stanley
steel door out of the living room and went with a wooden six panel. The
steel door was just transferring too much heat both in in the summer
and out in the winter
That should not be the case since a "steel" door is just surfaced in
steel on each side. In the middle is insulation and a wood perimeter
frame which provides a thermal break between the inner and outer steel
Some early and/or inexpensive didn't have complete thermal breaks.
I noticed even with one that did that while the actual heat loss
through the door was probably less than the wood it replaced that the
metal seemed to "feel" colder when near it. I suspect it's the same
effect as the hardwood or tile floor--the heat transfer from the
surface is just so much better it makes a sensation difference.
The Stanley had the thermal break & was a wood core with steel surface.
On a hot Texas day with the sun beating on it, the inside knob (solid
brass) got so hot you could burn yourself & the surface of the inside
was (guestimate) around 90. I moved it to my office and it is out of
the sun there out of the weather really, under a car port. The office
can stand it better anyhow as it is a converted garage and has concrete
1/4 of the way up the outside walls & requires very little heating and
cooling, kind of like a basement.
"When you upgraded the insulation so much ( I assume - you didn't say
what it was), you should have downsized the furnace as much. Also,
older furnaces were frequently oversized anyway"
Exactly what I was thinking.
That only seems to be a_good_thing (tm) to me. Other than perhaps some
condensation or other air quality issues from a "too tight" house, the
objective is to minimize heat loss/gain. I don't see any reason for
concern just because you're doing far better than average.
The furnace output rate will only have an effect on how long it takes
to raise the temperature from the "on" setpoint to shut off when the
demand has been satisfied. The external gain/loss is independent of
the heat source and only a function of the temperature differential and
the effective overall heat transfer coefficient and direct loss terms
(air leakage, doors/windows open, etc.).
If the temperature swing from "off" to "on" isn't excessive to cause
discomfort or the air doesn't feel "stuffy" from stagnation, I can't
see any reason to not feel good you're saving a significant amount of
fuel costs paying back your (undoubtedly not inconsiderable)
If there is a problem, might try simply letting the fan run to
circulate the air.
What kind of exterior temperatures are you experiencing now?
It's currently -10C, setpoint at 17.5 and drops .5C in 1 hour, therefore
cycles every 2 hours. Air quality is not an issue. I had leakage measured
pre retrofit at 11 ACH 50pa and 5 ACH post retrofit. A tight house requiring
an air exchanger is <2 ACH. Note that the greater the temperature
differential the greater the heat loss rate. ie: heat loss is not linear.
Undoubtedly I would have been more comfortable with a smaller furnace
(40-50k input) but neither me nor my contractor believed such a small
furnace could heat 3000sqft.
When you upgraded the insulation so much ( I assume - you didn't say
what it was), you should have downsized the furnace as much. Also,
older furnaces were frequently oversized anyway.
We should all have such problems.
Condensing furnaces from 50k to 100k are all about the same price ($1000 +/-
$100) However, I don't think this one is grossly oversized as it does not
short cycle. Run time at -10C is 20 mins every 2 hours at a fixed span of
1600sqft on top floor plus a 1400 sqft finished bsmt 2ft below grade in
front and 6 ft in the back. I believe insulating and airsealing the
uninsulated foundation walls made the biggest difference as the bsmt is now
7C more warm than last year. This also significantly reduced the "stack
effect", cold air infiltrating thru the bsmt towards the warmer upper floor
of the house. The heat loss model I used also supports this observation.
To get some insight into where you stand with your new insulation values and
your furnace sizing you could address your concerns by running or having
someone run a Manual J on your home. Manual J calculates the heat gain heat
loss of your home for proper sizing of the heating and cooling equipment.
Until you have those values to work with you can't make any real
Online short form
If someone could build the perfect furnace it would vary the fan speed,
heating and cooling input to maintain almost constant running of the
equipment. High end equipment built today do just that by using variable
speed fans, compressors and modulating gas valves in a effort to do just
Your run and cycle times suggest you have a very well insulated, tight home.
While in a perfect situation you might want to see the furnace run longer
because maximum efficiency is achieved when you are running a furnace that
has already "warmed", still your bills should be reduced by your efforts.
The only immediate concern you might have is that you may not have enough
air turn over to maintain good air quality. I've seen really tight homes
have problems with high humidity indoor during the heating season.
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