There is no description of why the draft inducer was installed or
whether the boiler can be safely run without it.
I have a condensing boiler that has a draft inducer. Some of the features:
- pre-burn purge run to make sure there are not combustible gasses in
- a pressure switch confirms that the draft inducer is running; also
confirms that the pressure switch has not failed
- post-burn purge run to clear the boiler and flues
RBM commented that you might need a "post purge" run if you do not run
the draft inducer continuously.
OK -- long, long ago *I* was a "ham" (but got bored because
all that the people talked about was their equipment. But
of course if I had *designed* and built my own transmitter,
I wouldn't be bored, but fascinated!)
So I know what a DPDT switch is OK.
But what's a "center-off" one?
Or, at bare minimum, a "break before make"? (Well, that one
seems self-evident. Question: aren't ALL dbl-throw switches
break before make? I guess not. Please elaborate.)
Which leads to two further questions:
(1) What's an *example* (and thinking behind) some switching
being make before break?
(2) (Begging the question), what then is a "reversing duty" switch?
(Lotsa vocab here!)
On Sat, 11 Feb 2012 19:35:03 +0000 (UTC), firstname.lastname@example.org (David
The heater control swith on many cars in make before break so the
heater does not shut off between speeds, causing arcing. Hi-Low would
be double throw. Most are "multi-throw". Other similar applications -
but like I said, generally "specialty" switched.
A switch that is used to reverse a motor - like a drill reverser, or
the up/down switch on an electric garden tractor 3 point hitch, or the
suck-blow switch on a ceiling fan, They are generally a DPDT switch
with a jumper on them. MOST are "center off" but some use a separate
switch for on-off and simply handle direction
Seems interesting, but not really clear to me.
What two places does the jumper connect, and what effect does it have.
FYI I drew this crude picture of a dpdt switch:
Crude drawing of DPDT switch:
Where would the jumpers be? With what effect? (What to google for
wikipedia or other
I say, there are LOTS of ideas comong out of this suprisingly long
and detailed thread, that I should be thinking about.
Knowing what to google for, or having direct urls, to places
that tutorially explain some of this would nice to see, both for
me and some others on the thread.
Using your diagram:
A B C
D E F
A connects to F, D connects to C.
A-D wires to one side, B-E wires to the other side.
The switch just reverses the connections (the same as a 4-way switch,
which can also be used).
On an AC motor you connect A-D to the start winding and B-E to the motor
connections for the start winding. It only reverses when the motor starts.
For a DC motor you connect A-D to the field winding and BE to what the
field winding connects to. Reverses while the motor is running.
Center-off would not be a good idea in these applications.
"Open" is not used. It is a failure mode.
A DC motor has an armature (that rotates) and a field (on some motors
the field a permanent magnet). The rotation is produced by interaction
of the magnetic fields of the armature and field.
A series motor has the field connected in series with the armature.
Shunt motors have the field wired in parallel with the armature (both
are connected across the supply).
You can think of the armature as a resistance in series with a
generator, which is the generator action of the armature rotating in the
magnetic field of the field.
Just looking at a shunt motor, when you start the motor there is just
the resistance (of the wire) and the armature current is high. As the
armature rotates faster the 'generator' action opposes the supply
voltage and the current decreases. At some speed the armature current
would drop to zero. The speed stabilizes at a current large enough to
supply the torque the mechanical load requires. If you lower the field
current, the field magnetic strength is lowered, and the armature
'generates' less voltage. The armature has to spin faster to 'generate'
more voltage. Field weakening is used for speed control.
If you open the field connection the 'generator' action produces very
little voltage and the armature has to spin really fast, and will have a
high current. Low torque may limit the speed, but the high current
remains. The control circuit for a shunt wound motor may disconnect the
motor if there is no field current.
A center-off switch in the off position will disconnect the field coil.
A single phase AC motor with a center-off switch on the start winding
will run OK, but in the off position it will not start. It will draw
high current when turned on.
On Thu, 02 Feb 2012 17:16:11 -0600, email@example.com wrote:
By "thermostat", do you mean thermostatically controlled valve?
That, and the thermostat, are in the same circuit and would be powered
by the transformer which is powered by the AC.
And the ignitor also. It's not enough to turn on the gas, you have to
light it too.
My first month in my 4-year old house, the AC failed, because the
furnace transformer failed. I went to a heating supply house and
since the transformer was part of the furnace control panel, his
reflex was to sell me a new control panel, for 400 dollars (in 1983)
I whined and he sold me a 24 volt transformer for maybe 30 dollars**,.
AFAIK all that is necessary is that it be 24 volts and big enough, to
power the furnace controls and the AC controls. Since the AC
compressor is on a separate cirucit, and the furnace air circulation
fan are on separate cirucits from the 24 volt control circuit, the
transformer doesn't have to be that big. Are you sure it has to be
made for the furnace? In my case it was a generic 24 volt
transformer, and I probably could have gotten it for less than the 20
or 30 dollars I paid at Radio Shack if they had one big enough, that
is, not a wall wart, not universal, and enough amps.
**I'm still using the same transformer 28 years later. It was too big
to fit where the original transformer was, so I mounted it on a
shelf/panel inside the furnace, though it could be mounted outside the
This is not a furnace, or furnace/AC, it's a steam boiler, and a crude
one at that. The 24 volt gas valve is controlled by the wall thermostat.
There is a pressure switch or switches that limit the steam. That's it,
no ignitor, it's standing pilot, but he does have a draft inducer fan
that needs line voltage.
Looks like I need some education here:
What's a "furnace" vs a "steam boiler"?
The thing we have is this big boiler, I guess, with a pipe
going (out of the top, I think) *up*, to the floors above.
Provides zero heat (via hot water or steam or whatever) to
the same floor (basement) it is sited in.
Now, what's a "furnace", and how does it differ?
Finally, what's a "furnace/AC". I suppose that's one
of those horrible! "heat only via hot air" things that
uses the same air-conduits for hot or cold air.
("horrible", because there's no heat via *radiation* --
just via conduction from nauseatingly-hot air surrounding
your (clothed, ie insulated from the hot air) body,
WHILE at the same time your body has net OUTWARD radiation
to the windows and, in old houses, walls. OK for Florida,
*maybe*, but a real loser for anywhere it gets cold.)
A boiler heats water, a furnace heats air. You have a basic steam boiler
with a 24 volt gas valve. The draft inducer you have, was not installed
properly, probably because it was done by a plumber. The typical draft
inducer setup is that the fan gets a constant feed of 120 volts, and in
your situation it gets a 24 volt control circuit that intercepts the
power from going to the gas valve until the inducer is running, then it
allows the 24 volts to continue to the gas valve. The unit may or may
not have a post purge, which would allow the fan to continue to run for
a few seconds after the burner stops, to evacuate the flue pipe.
All of this can be wired to a generator transfer device. I doubt any
local electricians are going to just install an outlet and plug for you
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