Usually the heat strips come on anytime you are more than about 3 or 4 deg
low. If the heat pump is turned up about 2 deg at a time they will not come
on , but if it is moved from 45 to 65 deg then they probably will.
Also at 20 deg the heatpump is not too efficiant and it might as well be
using the heat strips.
Funny that you ask this question.....I have to have done the
expereiment on three houses
Two in Orange COunty, Ca.
One is a 1 1/2 story 1930 ....no wall insulation but ~R30 in the
attics.....on cold (for SoCal) winter day of about 50F outside,
about 5 or 6F degs per hour
Second on is a one story ranch iwth blown in cellulose attic
insulation......about the same performance
the last home is an eastern sierra two story lots of wall & ceiling
insulation, propane heat...takes about 3 or 4 hours to go from 55F to
your temp rise of 2F deg per hours seems a bit on the low end in my
experience but not hugely unreasonable...a furnace needs to be able to
maintain the house at a reasonable temperature......how quickly it
gets there isn't really all that important.
what is the out side temp (I'm guessing in the 20's?)
an important thing to check would be the temp rise across the furnace
But I'm sure Bubba will jumping right in with a helpful addition
(along with his typical serving of insults) to this thread being the
furnace expert that he is
If I turned my furnace off & let the house get to 45- then turned it
on [while it was 20 with wind] I imagine it would take 1 -1 1/2 hours
of running to bring the house up to 65.
But my furnace was designed to keep the house at 70 when it was
30below outside- and that was with old, drafty windows & no
If 20 is about as cold as it gets where you live- and if it was 45 for
some time so all the mass had to be re-heated- and if you've got room
for improvement in your windows and insulation- then don't worry about
your furnace. Work on the insulation.
You say you "had the blower motor replaced". That implies that some
guy who knows a whole lot more about your setup was in your house.
What did he/she say?
Yes: when I find time, I'm going to be calling for a polyurethane
closed cell insulation install quote for the box band of the house,
which should be fairly easy/inexpensive, all factors considered.
The HVAC company that installed the furnace has been here four times
in just over 2 weeks. The first time was when the inducer blower died
on a Friday night; Saturday morning the tech checked a bunch of
things, decided the motor wasn't dead, and propped up the long
horizontal PVC exhaust pipes to correct a sag that apparently was
preventing gases from exhausting properly.
That got the motor going for another two weeks until this past
Saturday night when it died again. A second tech came over, checked
the system, determined the motor was dead, but was able to get it
going again. He told me to call again if it didn't keep working. It
died overnight, so I called Sunday morning (yesterday) and told him we
really needed to replace the motor, per his diagnosis. He was able to
get one and came over to install it, and all was more or less well,
except that the house took 7 hours to warm up when it had taken about
3 with the previous motor the previous episode.
This morning a third tech came over and replaced the condensate pump
(gratis as he broke off something on it) plus the vinyl condensate
discharge tubing (which I asked for, as ours was pretty gunked up
after almost 6 years in service). We agreed to do this after he found
several kinks that I'd asked the first tech about, which that tech
dismissed. Anyway, all seems well now; I will be watching tomorrow
morning to see how quickly the furnace brings the house to the
called-for temp (it took almost 2 hours to bring the house up 5
degrees F this morning and used to take one hour with the old motor).
Tonight will be low teens, tomorrow mid teens, so pray for us all!
Depends how many BTUs the furnace output is, as well as how much heat
loss the house has. Also depends how much thermal mass. If it needs to
heat up a lot of masonry it will take longer than heating up a lot of
stud-framed walls with good insulation.
You do know you have asked a trick question?
With the information you have given, it cant be answered.
Here let me give you and example or two so you might understand.
Where I live, we design furnaces to maintain an indoor temp of 70 at a
0 degree outdoor design temp.
What that means is: If my furnace is sized properly and my indoor
temperature is 70 and the outdoor temp is 0, then my furnace will run
24/7 until the outdoor temp begins to raise. If the outdoor temp
continues to drop to -10 or -20 below 0 then my house will begin to
get colder and colder and I will need to add some type of supplemental
On the other hand, with that same furnace, if it is 50 degrees outside
and 60 degrees in my home and I want to raise it to 70 I would
probably take much less than an hour.
What you are interested in is if the motor replacement you just got is
set properly. It should be set so that you get the proper "temperature
rise through your furnace as stated on the furnace equipment label.
Usually a temp in the range of 35 - 70 degrees.
More blower speed will lower this temp range. Less blower speed will
raise this temp range.
Why would more blower speed lower this temp range? It seems to me that the
faster the air, the cooler the air blowing across elements/heat exchanger
will be, and the more heat is getting transferred in.
Heat a pan on the stove. Quickly smack it with your hand. Now hold you
hand for 2 seconds. Now hold your hand on it for 10 seconds. Did the
fastest speed give you the most heat?
Same with the air blowing across the heat exchanger. There is only a given
amount of heat available and the longer the residence time, the hotter it
will get and it can them move the heat to another location. Cool air then
replaces the heated air. Check this out with your car heater as it warms up
on a cold day. Run the blower speed up and down and see how the temperature
You may have other issues with your furnace too. You mentioned a sagging
PVC pipe, kinks in the condensate drain, other parts were replaced. First,
it sounds like a hack did the original install. Some of these little errors
may be causing the heat exchanger to cycle off and on too frequently or it
is not reaching temperature.
While it is nice that the tech can resurrect motors from the dead, I think
perhaps, you need a new service company.
Delta T is highest when the air flowing over the hot surface is coldest. Max
delta T = max heat transferred. This happens when the airflow is fastest
because if the air flow is slower, it gets hotter because it is in contact
with the heated surface longer. Reduced delta T means less heat transferred
from hot surface to air and then to house. Which make me think that you want
higher air flow, not lower.
Hey Zoot. Think of it this way:
Lets take a 100,000 btu furnace and pull the blower and motor out of
it. Now install a bathroom fart fan in its place. Turn it all on. What
happens? You get an extremely high temperature with almost no air
Now, lets take that same furnace and install a 4 foot wide 4 blade fan
with a 50 hp motor turning at 30,000 rpm. Now tell me how much heat
you feel on the outlet side of that furnace.
It all needs to be done within a range. Thats why motors have 3 and 4
blower speeds. Its so you can set the heating blower speed and cooling
blower speed to fall within a temperature rise or drop across the heat
exchanger or cooling coil.
Clear as mud now or are you one of those guys with an EE degree?
Exactly correct. That info applies in automobiles also . You'll always
get colder air from your air conditioner if you drop the blower a notch
or two. We (as mechanics) would always measure the temp of an air
conditioner with the blower on medium, and the mode on recirc. (max)
No it will run cold and under perform due to not maintaining proper
operating temperature at least in the winter.
You can overheat one by upping the flow of water to the point that it
passes through too quickly to absorb the heat.
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