I apologize. I should have said what it was, OIL. Not so common
these days. AIUI, it doesn't burn unless the spark transformer is
running and providiing a continuous spark, and defintitely, no oil
comes out of the nozzle unless the electric pump is running. I have
on a couple occasions turned the switch off, and the furnace stops
immediately, including the noise of the fire.
I was also unclear in that I would still be using the same thermostat
I have now. It's connected to the furnace 24V transformer, of course.
The X-10 switch would be in place of hte 110 votlt AC toggle switch
which tunrs off all the power to the furnace (and all the power to the
AC except for the 220 volts that goes to the compressor, but is
controlled by the part of the furnace control board.) Nothing woudl
be changed about the setup except this switch.
Okay, iwrt your concern about it turning off without my turning it
off, I could put in a toggle swtich in parallel with the X-10 and if
I went out of town in the winter, turn that switch on, so the furnace
always had power. In fact I coulld leave the swtich on all winter.
It's only in the other three seasons that I open t he windows.
Does this relieve your concenrs or change your warnings at all?
Thanks for raising this. I meant to say something aobut this to the
OP. Posted separately.
I read the read the rest and didnt' delete anything that follows.
THAT'S THE CRUX OF THE PROBLEM...AIUI!!!
We understand almost zero, cause the OP disclosed little.
The things you understand aren't the things that will bite you.
It's all those little things you don't understand or interpret
wrongly. An HVAC system is a SYSTEM. The parts work together.
When you start messing with it, unintended consequences occur.
it doesn't burn unless the spark transformer is
My concerns don't need to be relieved, cause I'd never do something like
I'll go off on a short rant about interweb advice.
If a person knew what to do, he'd not be here asking the question.
He gets conflicting authoritative advice from people who've never seen
and have no idea how it's configured.
If the wrong answer results in your viewing the wrong movie, it's not
such a big deal.
If the wrong answer sets your house on fire, it IS a BIG DEAL.
I don't mean wrong according to accepted practice for a skilled licensed
contractor using components approved for the application and inspected
local building inspector.
I mean wrong relative to an unknown system of unknown configuration being
modified by a person of questionable skill and knowledge according to
random interweb advice...
They wouldn't be here if they knew what to do. They also have no way of
determining whether a particular chunk of advice will be helpful or
harmful in THEIR situation.
While it is statistically unlikely that the place will catch fire,
that's small comfort to the person who IS on fire.
Most everything you
buy today has multiple safety systems and is thoroughly tested for safety by
Yet we still find the need for multiple smoke detectors and the fire
department on speed dial.
One has to look at the cost benefit ratio. Do you REALLY want it badly
risk life and limb. Is it really that hard to walk down the hall
and flip the switch on the thermostat to turn off the system?
Just 'cuz it would be cool doesn't mean it's
the right thing to do.
Often, the best advice you could give is, "don't mess with stuff that
breathes fire and has the (however slim) possibility to burn down
the biggest investment you ever made and put Granny in the morgue."
Want a second opinion? Call up your insurance carrier. Tell them you're
gonna mess with the control system on your furnace by adding some
unreliable stuff you read about on the web. You won't have the result
inspected. Ask how that affects your fire insurance. Maybe you can
purchase an "idiot" rider. Better to learn
the answer now. It's too late when you're standing in a pile of
smoldering rubble and the insurance adjuster is shaking his head".
Call me conservative...
I have no idea what I'm talking about, so some expert will likely
It's not about hysteresis in the thermostat.
It's about balancing the heat supplied by the furnace with the heat
lost thru the walls, etc.
The longer the thermal time constant of your house, the more stable
the temperature will be. So, Let the system stabilize at some
For fixed external temperature, plot the internal temperature vs time
with the furnace off. That'll give you the thermal time constant
of the house (at the location you monitor). You want the cycle time
of the furnace to be much shorter than the time constant of the house.
Infiltration can also make a BIG difference. You don't want any, for
this calculation...but you do want some for health. It's a tradeoff.
I have a Honeywell TH8000 on a high-efficiency gas furnace.
It has a zillion setup functions that I never bothered to understand.
I don't know how it is supposed to work, but I can tell you the symptoms.
The temperature readout never changes (after it stabilizes).
I thought it was broke, so I put another thermometer next to it.
It don't change either (one degree F resolution).
A thermometer on the external wall next to the window does
change with the house's thermal time constant, local heat loss
and differences in air flow.
The thermostat modulates the furnace run time and the cycle time.
I have a PDA that tracks the on-time, cycle-time, and the duty factor
for each cycle. It graphs the duty factor.
Duty factor is directly proportional to BTU/hour.
The thermostat runs the furnace for about 5 minutes and modulates the
cycle time. As the in-out temperature difference increases, the
cycle time gets shorter. When the cycle time gets down to about an
on-time starts increasing as the cycle time decreases.
The graph of duty cycle tracks the internal-external temperature
difference (for fixed other heat sources).
When I turn off my computer and go to bed, the duty factor goes up
to account for the 200W of heat that is no longer generated by the
computer system. It's also interesting to watch the duty factor
track down during the day due to insolation, but with a phase
shift due to the thermal time constant of the house.
If I raise the setpoint, the duty factor overshoots and rings as
it settles to a new duty factor value consistent with the heat loss
at the new internal temperature. Classic feedback control system
behavior. It's interesting that the thermostat temperature readout
changes rather quickly to the new setpoint value, but it keeps running
the furnace on that
first cycle. Takes about 10 minutes of extra run time to bring the house
up one degree F. The temperature readout does not overshoot,
but the duty factor does.
It seems to be more about tracking average heat loss than the hysteresis
of the thermostat. The thermostat predicts future behavior based on
past behavior and applies minor tweaks based on sensed temperature
to keep it at the setpoint. This is not your grandfather's mercury
Your fuel injected car works the same way. For a given throttle
position, yes there are many other inputs too, the computer looks up an
injector pulse width in a table and runs with it. The oxygen sensor
determines whether the mixture is rich or lean and tweaks the injector
on-time up and down slightly around the optimum point to keep the
average mixture about right. It doesn't even try to make it exactly
right. There isn't any hysteresis required, but the mixture "hunts"
around the correct value.
I think, if you make theis thermostat cycle too quickly, you mightt
damage the compressor in the summer time, by what Mike called short
cycling. . Maybe if you can get it to work for heat, you would at
least need one thermostat for the furnace and the current stat for the
Any decent thermostat has a lockout period of 5 mins or more
during which it will not allow the AC to restart. That gives the
pressure time to equalize.
So, while cycling the compressor
with a differenct thermostat more frequently isn't
a good idea and will lead to more wearing over time, it isn't
going to just suddenly burn it out. The disadvantages to shorter
cycles for the AC are about the same as for the furnace.
For the OP, it would be interesting to know how much the
temperature is varying with the existing thermostat. You
say you want it to cycle more, but don't give data as to
what the temp swings are. Have you measured the actual
temps with a thermometer? And where are the objectionable
swings in temp occuring? At the thermostat or at some
room remote from the thermostat?
I've had a variety of
thermostats in my home over the years and I've yet to
find one that caused temp swings that were objectionable.
I have seen lots of situations where there were temp
problems caused by other factors, like unbalanced or
oversized systems. Which brings up another issue.
The typical thermostat is accurate enough compared to
the rest of the system. Meaning that the system is
balanced as best as possible so that when the temp
at the thermostat is X, it's close to X at all other points
in the house as well. That variation is probably more
than the hysteresis of the thermostat. So, even if you
put a super accurate thermostat on such a system
it's questionable if it will do anything to locations rooms
farther away from the thermostat. In fact, with shorter
run times it might make it worse.
As HomeGuy pointed out, the temp problems are classic
with an oversized system, but I guess we can rule that out
since you say the system is undersized and takes a
long time to raise the house temp up.
As far as thermostats go, the Honeywell VisionPro
models let you directly set the number of cycles per hour that
the system will run. I know you can set it to at least 5.
How exactly that then translates into the turn on/off points
isn't spec'd. But it is a totally different approach than other
thermostats I've seen.
Cycles per hour is not something I would like to have. The number of cycles
has nothing in common with temperature controlled lab experiments, and
might only have to do with efficiency, where too many cycles is a bit less
You have to read up on thermostats. Most I have used go to a minimum one
degree sensitivity. That's only a start. Some have learning modes so the
unit actually functions as a true PID controller that initially learns the
needed curve to avoid overshoots, and really good ones adds fuzzy logic to
add needed programing as it ages. The specs on most furnace stats tell
little. I do think some honeywell thermostats do indeed act smart. Drafts
and placement on walls trick the thermostat so that needs to be addressed.
Multiple zone sensors as well as personal portable remote sensors make
temperature more personal and more accurate. The size of the source of heat
or cold unit will change the time constants, and of course best to size
properly. Even though. The thermostat, say is going to go plus and minus
degree adjustment, a good one will be measuring tenths of a degree
resolution to make computations.
Actually and simply no matter how sensitive the 'stat, electro-
mechanical device such s furnace, ac unit won't respond with
equal sensitivity. Your system and your house itself need energy audit
may be. Up here we can no longer install a furnace with efficiency
rating less than 90% by law.
I believe you want smaller temperature swings, right? I have electric
resistive hot air heat and I bought a Honeywell CT31A1003 thermostat.
It is not programmable. It's a simple design and uses two magnets to
help it "click" from on to off to on...
There are two screws set from the factory for on and off positions. All
you need to do is adjust the screws so the two contact points are closer
together. Simple. Now my heat turns on and off with smaller
temperature swings. The only tricky part is setting the heat
anticipator properly and that took a little tweaking to get it to my
here ya go, a high dollar one that looks purty and it's smart....
Probably won't work any better than what you have but the economy
needs the money, buy one.
Yes, slightly more stable temps. I am not looking for anything drastic.
Like I said, most electronic thermostats have adjustments for sesitivity, I
just want one setting more sensitive than the most sensitive setting I have
had in the past. I'm really not sure what the settings H1, H2, or H3 mean
exactly when it comes to temp. swings. If H1 allows a .5F swing and H2
allows for a 1F swing and H3 allows for a 1.5F swing then I guess what I'm
looking for is something less than .5F If H1 means 1F swing, H2 means
1.25F swing, and H3 means a 1.5F swing, I guess I'm looking for a setting
which allows for a swing of less than 1F.
I have to believe that there has to be some variation between the
manufacturers of thermostats and that there is one out there more suited to
me or maybe one with much greater control, but not to the point of causing
damage to the system.
You talk a lot about "settings", but what settings? Various (and most)
thermosats have a hysterisis control and an on/off temperature setting. The
cycle setting is right n the thermostat and the other temp on/off settings
are in the furnace. I can't help but wonder if you aren't shot-gunning t
here when the answer is rght in front of you if you look at the right access
There's a gradient across the room due to losses on the outside walls.
There's a gradient top to bottom due to stratification.
The air coming out of the heater vent is a LOT hotter than the setpoint.
You probably have a single point temperature measurement for the whole
Given all this, I'd be AMAZED if you could tell the difference between
.5 and 1.0 degrees hysteresis in the thermostat.
Methinks you're addressing the wrong problem.
I think the Honeywell VisionPro 8800 will do what you ask. I just don't
think it will solve your problem.
Exactly what I've been questioning all along. We don't even know
what the objectionable temp swing is. Any decent thermostat that is
properly installed will control the temp at the thermostat to 1.5 deg
so. And if that is objectionable at the thermostat, then what about
the rest of the house, where the temp swings are almost always
greater? In fact, with short cycleing, which is what he appears to
want, the temp fluctuations in the rest of the house might be worse.
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