On Saturday, March 1, 2014 6:03:05 PM UTC-5, Stormin Mormon wrote:
You have it backwards. With a lot of the air handler air flow blocked
off, you have less air moving, the air in the plenum get *hotter* and
the temperature delta across the heat exchanger is lower, ie the plenum
temp is closer to the temp of the combustion gases. Hence, you recover
less heat and more of it goes out the exhaust.
Also as pointed out previously, if the furnace then kicks off and
recycles on and off due to the high limit being hit, during the
period it's cooling down, most of that heat is being lost up the vent,
or could be lost to an unfinished basement, attic, etc. I think that's
largely a moot point, because I believe any modern furnace is going
to trip and stay off with a fault code if the high limit is exceeded.
But if it does continue to cycle on and off, no question you're taking
a hit on energy loss.
There is no "sweet spot" for the heat exchanger efficiency. Setting the blower on HIGH blowing more air over the heat exchanger will always be more efficient then lass air.
It's only a sweet spot in terms of comfort and not feeling a draft in your home, not in terms of heat exchanger efficiency.
On Fri, 28 Feb 2014 22:35:34 -0500, Stormin Mormon
Was it before or after you bought them that they converted to Mormonism?
Almost in the same way. I think?, that driving short distances and never
letting a car's engine warm up to proper operating temperature. In
the case of the furnace, it may get that warm, but because it turns off
soon, a higher percentage of the On time is at the lower temperature.
On Sat, 01 Mar 2014 06:48:01 -0500, Stormin Mormon
FTR, I thought we were talking about closing the warm air supply to some
rooms, but actually I don't think there is much difference.
But they don't run all day. It might be 10 or 20 minutes. Yeah I see
your point that, assuming as was said that they warm faster, they get to
operating temp sooner than normal, but they might then zoom though that
temp range on to overheated soon thereafter.
I think they can but maybe there is no general rule. Maybe each similar
furnace setup is the same but ....
Well I don't know what the but is, but I've been bothered since
yesterday, per my previous post, wondering if that was why my furnace
was turning off. In the short period between replacing the igniton
transformer and having the next problem, I was sitting at my computer in
the room next to the furnace and for about an hour I logged when it
started, when the blower started, when the fire stopped, and when the
blower stopped. The notes are somewhere in my Agent outbox so I have
to find them and read them.
The rest of this post is about my 1979 furnace and may not interest most
people, but though many things have changed over the years, I doubt the
final actions described here of the control panel, etc. have changed.
I've also been trying some more to understand the furance wiring. There
are a couple omissions to the wiring diagram (2 resistors), and one or
two more things** I haven't figured out yet, but most I do understand
and when the heat limit switch opens, that interrupts the 120 volt black
wire to the furnace. which will turn off the burner motor, the ignition
transformer, and the control board power transformer (which powers the
furnace relay so that too turns off the furnace) . The fan wiring I
find complicated but I don't think the fan turns off because it's needed
to cool off the plenum etc. which we just said had overheated. Anyhow,
after the house reached 168, the furnace never ran for more than 20
minutes iirc and that started to worry me.
Since I've had trouble with the control panel, what I've been doing is
turning the furnace on by hand (with a stick*** holding down the relay
armature) for 2 or 3 hours when I wake up in the morning, and for 2 or 3
hours just before I got to bed. (Less time during the couple days it
was warmer out) I get the second floor of the house up to 178 or even
180 before I got to bed (where I often listen to the radio for a couple
hours and then sleep 8 hours) and it's 164 to 168 in the morning.
Under the covers it's always warm enough, but a couple days, I've had to
dress quickly and go turn the furnace on.
If my furnace could overheat in 20 minutes, surely it would overheat in
3 hours and the high limit swtich would turn off the whole furnace.
But it doesn't. I have to turn it off by taking the stick out. So it
must not be overheating.
***The stick was originally used to post an illegal advertising poster
on public land. Baltimore County law now allows anyone to remove such
signs. Many are held up by wire things, but when it's wood, I save the
wood. About 2 feet long by 1 by 1/2" with a point at one end.
**Things I don't yet understand about the schematic. The switch that
turns the furnace off when there is no flame (or maybe too much black
smoke??) has four wires going into it, but the schematic shows nothing
about how they are connected inside -- it's a black box -- and that
makes it hard to understnd the main relay's operation, since two of
those four wires are connected straight to the main relay coil. Also
the fan relay, which is in the fan & limit control enclosure, I think, I
have just started to figure out. The schematic does not use the same
exact symbols that radio and tv schematics do. I think I've found a
double throw switch but I'm not sure. (Maybe I'll post the diagrams)
It also doesn't match my own fan, which has 3 speeds but only one is
connected, the same fan speed for everything. This one from the web for
my model may have high speed for AC and low or medium for heat.
It also appears that 24 volts are provided to close the main relay, when
the thermostat calls for heat, but maybe that's lowered to 12 volts to
keep the relay closed once it has closed. Can't tell because of the
I have also found that sometimes when the house is cold enough that the
thermostat should call for heat, when I start to close the relay by
hand, when the armature is half-way there, it's pulled shut
magnetically****. But it didn't shut without my help. That goes
with something I think you said, that there's a problem with the main
relay coil. So I've decided to replace the control board (including
the main relay coil) first and worry about the thermostat later.
****Other times when the house is cold, there is no magnetism in the
relay. How can that be? Maybe that's a problem with the thermostat or
maybe it's gremlins.
(ONE DAY WHEN I GUESS I HAD THE STICK just right, the furnace went on
and off by itself for 24 hours or so, and the house was always 68. I
guess the stick wasn't holding the armature closed but it was holding it
close enough to the relay coil that the coil could pull it the rest of
the way. Nothing else makes sense.)
I also remembered that my thermostat is electronic or semi-electronic,
not like the round honeywell thermostat that was used initially in 1979
and is in the schematic, and disconnecting the stat and expecting to
find zero ohms across the red and white wires when the house is cold
might be wrong. Maybe it needs its 24 volts to work right.
On Fri, 28 Feb 2014 21:16:31 -0500, firstname.lastname@example.org wrote:
Hmmm. That's a serious concern. Thank you. How short do you think
too short an ON portion of the cycle would be?
Also there should be some way to know if the furnace is turning off
because the thermostat reached the desired temp, OR becaues the high
limit switch turned it off. I realize now that I don't know why my
furnace turns off.
If the furnace shuts down while the thermostat is still calling for
heat, you have a problem, and it is short cycling.
What kind of thermostat? If it is an electronic digital stat it
usually has an indicator telling when it is calling for heat.
Mine ( a 2 stage furnace - high stage really too big for the house)
will occaisionally short cycle if it goes on high.
On Sat, 01 Mar 2014 13:16:42 -0500, email@example.com wrote:
Well, right now I'm not using the thermostat, but now that this issue
has come up, I'm going to up the priority of the control board
replacement and I should know soon.
Although with a day to think about it, it seems to me from theory and
looking at the wiring diagram for my furnace installation that if the
high limit switch opens, the furnace would shut down, except for the
blower needed to cool it off, and it never shuts down until I do it
manually, even after 3 hours. That's certainly not a short cycle.
I have maybe the first residential setback thermostat sold, from 1984
maybe. It's electronic or semielectronic, The clock is digital but the
time and temp settings are mechanical switches. Anyhow, no indicator.
But when I replace the control board on the furnace, I'll put in a
couple wires that extend beyond the furnace, so I'll be able to use a
voltmeter to check without having to get my hands in among hot wires.
On Fri, 28 Feb 2014 07:36:32 -0500, Stormin Mormon
I said at the top that I assume I'm saving money.
For just this reason.
I too can't really tell if the cost has gone down because I didnt' keep
records before I closed the vents, and I'd have to keep track of degree
days from one year to the next and gallons of oil delivered and it's too
When I say at the end I don't think it will change anything, I mean I
don't think it will change the character of an oil (or gas) fire to
either make the air in the house dirtier or make more carbon monoxide,
or something else I wouldn't like. DD3 says the same thing, iiuc.
But someone here in the last 15 years once said it would, and iirc no
one posted a disagreement.
Hah. You Yanks really are in the dark ages,like a hundred years behind the
rest of civilisation.
My house needs no heating most of the Winter I have two feet of insulation
in the walls.
The TV and freezers keep the place warm
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.