Furnace (not blower) short-cycling
Bryan Beeman
which has a 1-year-old central heat/AC retrofit.
Since we (and everyone else in N. America) have been having
a cold winter, we have been giving our furnace a workout.
The problem is that the burner/gas valve is short-cycling.
Initially, the thermostat is turned on; a few seconds later
the furnace burner will come on for some warm-up period
(about 15-20 sec.). Then the fan will come on.
Normal operation, right?
However, after some time the furnace (not the blower fan)
will abruptly stop the gas, pause for about 5-10 seconds,
and then woof! turn the gas back on. This behavior will
then continue indefinitely: woof! gas on for 10-20 seconds,
gas off for 5-10 seconds (repeat).
All the time the blower fan continues to run non-stop.
My question:
Does the limit switch also control the gas valve?
I understand that the high setting is the one which keeps the
house from burning down, the middle setting causes the blower
to come on, and the low setting causes the blower to go off.
Is it possible that the high setting is set too low, causing
the burner to cycle--effectively operating between the middle
and high limit switch settings ?
NOTES:
- the natural-gas furnace (a Heil/Intercity unit)
is in the attic
- there is one 16x20" air return for the
whole house in a hallway ceiling
- house is about 1700 sq. ft.
Any ideas?
Thanks everybody!
Bryan
SolowM
it looks like a flame sensor is shutting you down,maybe the limit switch,
only if the furnace has the one you talk about but i don't think that.
Blower fan will come on 30sec or 60-90sec delay,some furnaces like carrier
will operate blower in safety mode.
Gary & Rita Lloyd
>Any ideas?
>Thanks everybody!
>Bryan
The information you give is sketchy, but let's give it a shot. If the
fan/limit is the one you describe, then I would assume that you have
an atmospheric burner rather than induced draft. What are the settings
on the fan/limit switch?
Since the unit is new, I also assume intermittant pilot. Does the
pilot also short cycle? If so, does the spark ignitor immediately
spark during shutdown, or does it wait for the 5-10 seconds?
Have you checked the temperature of the air leaving the furnace?
Given the aforementioned assumptions, the unit is cycling either on
the limit or pilot flame sensor. The limit would kill the spark
ignitor until it (the limit) resets itself. It would seem unusual for
the flame sensor to produce such a steady on/off pattern. Usually when
the limit cycles, it is because of an air flow problem, but the
setting may be too low as you suggest. Since the furnace is new, and
you just acquired the house, we don't know if it ever worked right. If
it did, then the setting is probably OK.
Let us know what you find.
Gary
================================================
Everything should be made as simple as possible, but not simpler.
-Albert Einstein-
Gary R. Lloyd CMS - TECH Method Training
HVACR Trouble Shooting Books/Software
Tel 313-671-0188 Fax 313-676-9262
http://www.gatecom.com/~tmethod
tme...@gatecom.com
tme...@aol.com
Ray Matthews
>The problem is that the burner/gas valve is short-cycling.
>Initially, the thermostat is turned on; a few seconds later
>the furnace burner will come on for some warm-up period
>(about 15-20 sec.). Then the fan will come on.
>Normal operation, right?
>
>However, after some time the furnace (not the blower fan)
>will abruptly stop the gas, pause for about 5-10 seconds,
>and then woof! turn the gas back on. This behavior will
>then continue indefinitely: woof! gas on for 10-20 seconds,
>gas off for 5-10 seconds (repeat).
>All the time the blower fan continues to run non-stop.
>
>My question:
>Does the limit switch also control the gas valve?
>I understand that the high setting is the one which keeps the
>house from burning down, the middle setting causes the blower
>to come on, and the low setting causes the blower to go off.
>Is it possible that the high setting is set too low, causing
>the burner to cycle--effectively operating between the middle
>and high limit switch settings ?
If the temperature in the plenum exceeds the limit switch setting, it
will shut down the burner until the plenum cools enough to turn it back
on. So that it one possible problem.
However, the symptom you are describing is the exact same thing that
happened to my furnace earlier this winter, and I had a totally different
problem. In my case, my furnace has a regular 3" galvanized stack pipe
for the exhaust gasses, that goes to the chimney. The furnace has a small
blower that pushes the exhaust gasses out through this pipe. There is a
small tube right at the outlet for this blower, that goes to a pressure
switch inside the furnace. This switch looks like a metal can, about 1
inch thick and about 3 inches in diameter.
Anyway, the purpose of this switch is to verify that the exhaust
gasses are being properly vented, by sensing a positive air pressure at
this tube. But if the switch is bad or if the small tube is plugged or
leaking, the switch will shut off the burner thinking that the exhaust
isn't venting, which would be a carbon monoxide hazard. In my case, the
switch had become too sensitive, and was cycling the burner during each
heating cycle. Replaced the switch and the rubber tube, and problem went
away!
Doug Geddes
>which has a 1-year-old central heat/AC retrofit.
>
>Since we (and everyone else in N. America) have been having
>a cold winter, we have been giving our furnace a workout.
>
>Initially, the thermostat is turned on; a few seconds later
>the furnace burner will come on for some warm-up period
>(about 15-20 sec.). Then the fan will come on.
>Normal operation, right?
>
>However, after some time the furnace (not the blower fan)
>will abruptly stop the gas, pause for about 5-10 seconds,
>and then woof! turn the gas back on. This behavior will
>then continue indefinitely: woof! gas on for 10-20 seconds,
>gas off for 5-10 seconds (repeat).
>All the time the blower fan continues to run non-stop.
>
>My question:
>Does the limit switch also control the gas valve?
>I understand that the high setting is the one which keeps the
>house from burning down, the middle setting causes the blower
>to come on, and the low setting causes the blower to go off.
>Is it possible that the high setting is set too low, causing
>the burner to cycle--effectively operating between the middle
>and high limit switch settings ?
>
> - the natural-gas furnace (a Heil/Intercity unit)
> is in the attic
> - there is one 16x20" air return for the
> whole house in a hallway ceiling
> - house is about 1700 sq. ft.
>
>Any ideas?
>Thanks everybody!
>
>Bryan
Hi Bryan, You will probably get some specific Bryant experience from others
here, but here are a few possiblities.
1. The anticipator (if an anticipator is present) setting of your thermostat
may be set too low.
2. Electronic thermostats don't use an anticipator but electronics to control
on-cycle lengths, possibly the thermostat is defective.
3. The furnace could be cycling on high limit because of an airflow blockage
of some sort - do not adjust the high limit setting.
4. The induced draft fan may not be working properly.
5. The pressure switch that senses a blockage in the vent may be defective.
6. There may be a blockage or restriction in the vent - check outside outlet
as well - sometimes the vent pipe can be too long or have a sag where water
builds-up and creates just enough of a restriction to cause the pressure
switch to activate..
7. If a direct vent system (uses outside air for combustion) the inlet may be
obstructed.
That should get you started.
Doug Geddes
Mark Roberts
the initial start up period of the cycle. Check the flame sensor at the
burner. It is possible that the furnace has falsely sensed that the
flame has gone out. It waits 30 seconds or so to disipate any remaining
gas and then tries to restart? Just another thing to check.
--
Regards, Phone: 612-736-0536
Mark Roberts Reply: mrob...@mmm.com
Opinions expressed herein are my own and may not represent those of 3M.
Jean Seguro
temperature inside my place is only 60 F and is noway to make it go up higher, I
have even set the thermostat at 90 F but it doesn't help....
I have been checking the cycle and it seems pretty short, it is like this (the
times are approximate):
60 sec. fire on
100 sec. fire and fan on
80 sec. fan on
45 sec. fire and fan OFF
45 sec. fan on
80 sec. fire and fan OFF
15 sec. fan on
And now the cycle is repeated.....
It seem to me that every thing is working fine, but I don't have idea about the
furnace operation, so I think that the fan and burner should be on longer. I
don't know if it is the control logic that is programated but does anybody has a
clue of what is going on?????
By the way the furnace has a capacity of 30000 BTU/hr my place is a basement of
about 820ft^2 and the furnace is about 30 years old....
Appreciate coments,
Jean
Tom Geletka
:Recently I am having a similar problem, even when I set my thermostat at 75 F the
:temperature inside my place is only 60 F and is noway to make it go up higher, I
:have even set the thermostat at 90 F but it doesn't help....
If it used to work, I think it is time for a new thermostat. The
electronic ones are nice.
Gary & Rita Lloyd
>I don't think that the thermostat is bad, because the contact still close when
>the furnace go off...., I have it on 90 F and it never release the contact...but
>the temperature in the home is around 60 F....
>My guess is that the logic of the control was made in too long periods of rest and
>too short periods of heating....so it is not delivering enough energy to the
>room per unit of time... BUt I don't know if it is possible to change...
>If some body know please tell me...
>Regards
>Jean
The problem with trying to trouble shoot this, and similar systems, is
lack of information. Consequently, those who take pride in their
knowlege and skills are reduced to the level of guessing. Having said
this, here is my guess:
Judging from the size (BTUs), age of the system, and duration of the
off cycle, I would guess that this is a wall mounted unit with a
propellor type fan, an atmospheric burner, a standing pilot, and a
klixon type limit switch. let us know if any of this is incorrect.
Given the above assumptions, the unit is probably cycling on the limit
switch. If this is the case, the first thing to do is clean the heat
exchanger on both the air and the combustion sides. Also clean the
flue pipe/chimney. This should be done periodically, anyway. If the
problem then persists, replace the limit switch.
All of this should be done by a qualified service technician, as a
furnace can be a dangerous thing.
Gary
===================================================
Mark W. Gardner
>You might also check that the exhaust flue is not blocked or that the
>flame is not rolling out of the burner ( there probably is a sensor
>for this also). I recently had this problem (6 yr old Heil).
>roger
>>
>> The information you give is sketchy, but let's give it a shot. If the
>> fan/limit is the one you describe, then I would assume that you have
>> an atmospheric burner rather than induced draft. What are the settings
>> on the fan/limit switch?
>>
>> Since the unit is new, I also assume intermittant pilot. Does the
>> pilot also short cycle? If so, does the spark ignitor immediately
>> spark during shutdown, or does it wait for the 5-10 seconds?
>>
>> Have you checked the temperature of the air leaving the furnace?
>>
>> Given the aforementioned assumptions, the unit is cycling either on
>> the limit or pilot flame sensor. The limit would kill the spark
>> ignitor until it (the limit) resets itself. It would seem unusual for
>> the flame sensor to produce such a steady on/off pattern. Usually when
>> the limit cycles, it is because of an air flow problem, but the
>> setting may be too low as you suggest. Since the furnace is new, and
>> you just acquired the house, we don't know if it ever worked right. If
>> it did, then the setting is probably OK.
>>
>> Let us know what you find.
>>
>> Gary
>>
>> ================================================
>> -Albert Einstein-
>>
>> Gary R. Lloyd CMS - TECH Method Training
>> HVACR Trouble Shooting Books/Software
>> Tel 313-671-0188 Fax 313-676-9262
>> http://www.gatecom.com/~tmethod
>> tme...@gatecom.com
>> tme...@aol.com
>
>
>BTU output of furnace should not exceed 1.5 to 2 times BTU rating
>of your air conditioner. An example is if A/C is 3 ton (36,000 BTU)
>and does a good job the gas furnace should be no larger than 52,000
>BTU output. Oversizing is a common problem in retrofits.
Roger Knutson
Jean Seguro
SCreel5818
may be a problem with the heat anticipator on your stat. If this is the
case check the anticipator setting, it is located in the stat and should
have some numbers like .2 to .8 on it. If it is set too low (.3) it will
cause a short cycling problem. Without checking the circuit amperage it is
hard to tell you what it shoul be set on, but an setting of .6 would do.
At a setting of 90 deg. the unit will still shot cycle because the
anticipator will get warmer than 90 deg.
If this is not the case there may be a restriction in the duct work or
some other problem causing the unit to go off on high limit (if your
furnace is the newer type).
DaveC
James Storm
during start up ), some of the older models used the same ignitor as a flame sensor after
flame on.. It doesn't always work as a good sensor and will shut down flame and recycle the
furnace. Short fix is to replace ignitor. Have a service company replace it and to verify that no
other flame sensor is present. There is also a kit available to add the required sensor. Of
course this work needs to be done by a qualified tech.
P.S. sorry Gary if you get the mail on this post but I replyed to your reply
--
Stormy (jst...@ptd.net)
Robert Thompson
be correct.
Tim Keating
>Recently I am having a similar problem, even when I set my thermostat at 75 F the
>temperature inside my place is only 60 F and is noway to make it go up higher, I
>have even set the thermostat at 90 F but it doesn't help....
>I have been checking the cycle and it seems pretty short, it is like this (the
>times are approximate):
>60 sec. fire on
>100 sec. fire and fan on
>80 sec. fan on
>45 sec. fire and fan OFF
>45 sec. fan on
>80 sec. fire and fan OFF
>15 sec. fan on
>And now the cycle is repeated.....
>It seem to me that every thing is working fine, but I don't have idea about the
>furnace operation, so I think that the fan and burner should be on longer. I
>don't know if it is the control logic that is programated but does anybody has a
>clue of what is going on?????
I diagnosed and fixed a similar problem back in September...
Spider webs in the fire box thermostat(controls both fan and burner
on/off) was changing the set points.
Thermostat should be mounted on furnace somewhere near the fire box.
(has a little dial, usally under a retangular cover).
Suggest you first record the current set points.. Shut off furnance..
Then move all the settings to one extreme, then the other, finally
restore them to the orginal settings..
Turn furnace back on..
good luck..
Tim K.
Rob-L
: However, after some time the furnace (not the blower fan)
: will abruptly stop the gas, pause for about 5-10 seconds,
: and then woof! turn the gas back on. This behavior will
: then continue indefinitely: woof! gas on for 10-20 seconds,
: gas off for 5-10 seconds (repeat).
: All the time the blower fan continues to run non-stop.
Is this a condensing furnace with a combustion blower? Well, anyway...
Sounds like the limit switch. Try running the unit without an air filter
for a few minutes. Could be that the filter's dirty or too restrictive.
Could also be undersized ducts (open the dampers!), or the main blower needs
to be connected to the high-speed tap. Could also be that the limit
switch itself is too eager. Depending on the type, you may have to pull
it out to adjust it. First, make sure the failure is happening while the
thermostat remains switched on, so you can rule it out.
To confirm that the limit switch is cutting out, you can short it during
failure and it will turn the burner back on (same with other safety
cutoffs). Make sure you know the electrical schematic so you're not
guessing. The switch is not a high-power device, it just controls the
relay.
Invest in a dial thermometer that goes up to 200F. Then you can poke it
through a bit of cardboard in place of the limit switch, and see what
temperatures you're operating at. 170F is where my furnace is set to
trip, and I had cycling problems which turned out to be duct/a-coil
related. None of the other measures could cool it enough. Had to
temporarily bump up the limit and let it run about 10 degrees hotter.
The burner jets will have to be changed to the alternate (lower) input
rating to run cooler, all else being maxed out.
Condensing furnaces have an extra blower and some more safety cutoffs
that can cause trouble, and those can be tested the same as the limit switch.
Dave Marulli
exact same symptoms that you decribe: Gas/flame cycling ever 15-20
seconds.
In the case of my dryer, it turned out that the exhaust vent outside
the house was clogged with lint. My assumption is that with the vent
clogged, the controls did not sense the correct air circulation and
shut the gas off, then the thermostat turned it back, ad nauseam.
As soon as I cleared the vent, problem gone.
Maybe you have cold air return or exhaust problem.
(Isn't there a concern that wind blowing across the exhaust vent can
cause problems with new high efficient furnaces?)
ken
thing lock out? I was under the impression that ALL newer units had to
lock-out after a specific number of re-starts in a given time.
I had a similar problem with a 2 yr old Rheem unit, and after
discussing the problem with the mfg, they decided it was a flame sensor
problem. The sensor had NO deposits on it, it was dirctly in the
flame, and all the electrical connections were tight. I checked the
polarity of the system, and it was OK. This was a shot in the dark,
after all the system WAS working up to this point, and no one was
screwing with the elctricity. They were mistaken, it was a defective
circuit board. As it turns out, the new board was not identical, and had
to be rewired, to alleviate a problem the board was prone too. Rheem was
apparently well aware of the problem with the board, but since it was in
warranty, opted for a new flame sensor. I got a little pissed at this
point, and had them ship me a new board, next day air.
The behavior of the Bryant unit suggests flame failure, but
regardless I think the unit should lock out the gas valve. It's VERY
unlikely that its a high temp safety, due to the fast recycle time. It
COULD be the combustion blower pressure switch, but I think THAT should
lock out the unit as well. Beats me, but at this point, I'm opting for
a circuit board.
Either way, I hope Bryan lets us know what the final verdict
is !! It MUST be fixed by now anyway, it's REALLY COLD OUT THERE !!!
ken
Robert Gracie
Your dryer was actually over heating. What happens is, because the venting
system is clogged and cannot get rid of the heat fast enough the Safety
thermostat was unable to stay cool,(Lower then specs) then pop; the safety
opens up and you loose voltage to the Gas coil or heating element until it
cools, normaly taking 1-5 Min. Providing your dryer is a catagory I Appliance.
The new Underwriters Laboratory listing now requirs all new Catagory I dryers
made; to be equipt with a thermal link. This is a none resetable fuse that when
overheated (meaning when the safty therm. fails) it shuts the whole dryer down.
The problem is that the manufactures don't use safty thermosats with tighter
specs. These dryer safety thermostats have a peak around 250f. with a
differential of +or- 40-60 degress. So when the safty allows the temp to reach
to high in a overheating cycle the thermal link (which seem to be made with
much tighter specs.) does it's job and opens, then thats when I come in the
picture.
So in stead of just having to clean out the venting system, the customer now
has to buy a new safety therm. and fusable link. This situation can also be
caused by a venting system that is too long :-) Here in Maryland we have a big
problem with the architects doing this :-(
Sorry one of my pet peeves!!!!
Robert Gracie
In article <4f527l$7...@kodak.rdcs.Kodak.COM>, mar...@rdcs.kodak.com says...
Bill Harris
is at least one other potential cause. We are wrapping up a remodel, and
we have a new gas furnace. After about a month of enjoying its heat
when compared to electric forced air, I noticed that the unit was
short-cycling. It would cycle 4 times and then shut off totally; I would
need to cycle power to get it going again (our furnace will shut off like
that if it believes it can't ignite the gas successfully, according to
the manual). Believing it was a bad logic board or sensor, we had the
furnace people back out to fix it. They took one look, pulled the
filters and cleaned them, and reminded us of the importance of routine
maintenance! (embarrassment) I hadn't counted on that much build-up in
a month, but the dust of a remodel did tax the system. It seemed to work
fine after that.
Bill
Liberty or Death
SNIP...
>NOTES:
> - the natural-gas furnace (a Heil/Intercity unit)
>Bryan
Bryan, In my experiance Heil/Intercity units have had several troubles with
thier boards. But, as with any system using a board, a GOOD ground is an
absolute requirement. L.D.
Liberty or Death
js44...@longs.LANCE.ColoState.Edu says...
SNIP...
>I don't think that the thermostat is bad, ...
>Jean
Jean, If it's the mercury type T/S the heat anticapator can be failing or
dead.
L.D.
Rob-L
: The behavior of the Bryant unit suggests flame failure, but
: unlikely that its a high temp safety, due to the fast recycle time. It
: COULD be the combustion blower pressure switch, but I think THAT should
: lock out the unit as well.
High-temp cutoff can cycle fast - mine did with a 10-15 second cool-down.
There is no lockout on the natural gas models of my furnace, only the LP
ones (yours may vary). Rule out the pressure switch by sticking a
screwdriver or jumper clip across its terminals when the flame cuts
out. Then do the high-limit switch. Simple test.
Jean Seguro
Yesterday a technitian went to see my furnace, basically he did two things, one
increases the speed of the fan, and two, change the set of the limit/fan control.
Before it was 160-limit, 140-fan, now is 180-limit, 120-fan. The furnace seems to
be working much better now, but I wont know until another ccold front comes down.
He told me that the heat exchanger does not need to be cleaned (without even
looking at it) because with the high temperatures every thing get too hot, burned
and blowed out....I don't know if it is true....
Anyway I hope that my problem is solved now....thanks to all that answered my
questions...
Regards,
Jean Seguro
alan
>Recently I am having a similar problem, even when I set my thermostat at 75 F the
>temperature inside my place is only 60 F and is noway to make it go up higher, I
>have even set the thermostat at 90 F but it doesn't help....
>I have been checking the cycle and it seems pretty short, it is like this (the
>times are approximate):
There could well be a problem which is causing a limit switch to shut
the furnace down -- something over-temp, for example. This means it
doesn't run anywhere as long as the thermostat wants it to.
If it's 30 years old, I'd have a good furnace technician who is familiar
with that brand and model give it a thorough going-over. They may be
able to find the problem.
Also -- does it have any filters, or cleanable parts? Are they clean??
Alan
Most people stumble over the truth, now and then, but they
usually manage to pick themselves up and go on, anyway.
---- Winston Churchill
Gary Lloyd
>Hi every one,
>Regards,
>Jean Seguro
Let's hope that the technician at least checked for flame rollout at
startup, which would indicate a dirty combustion chamber/heat
exchanger. If so, and if the system now runs until the thermostat is
satisfied, then its probably OK. A cold front doesn't change the way
it operates. It just runs longer.
Gary
===================================================
Barry Fowler
and repaired it. It was due to an oxidized flame sensor. The analog
input of the computer controller chip was not receiving a signal (actually,
a resistance) that the flame was present. New flame sensors are not
thermocouples as they used to be but simply a metal rod that is in the
flame path. This metal rod is used to sense the resistance *of the flame*
to ground. When oxidized, the computer "thinks" that there is no flame
and shuts the gas solenoid to avoid the possibility of an unlit gas source
which could cause big problems.
The problem was remedied by cleaning the flame sensor rod with emory cloth.
This was a situation where a cycle of purge/ignitor power/shutdown
occured repetitively.
Barry Fowler
: This was a situation where a cycle of purge/ignitor power/shutdown
: occured repetitively.
Correction of my post, it was a cycle of purge, ignitor power, ignition/
shutdown. ^^^^^^^^
:
That is, I was obtaining ignition but it wasn't being sensed.
@alaska.net Tom Bierle
pluged filter resticting air flow through furnace causing Heat Exchanger
to heat up to quickly and shutting down on High Limit.
Or Check Fan Belt if its broken or sliping. It could also be the Heat
Anticipator on Thermostat. Set it to match the gas valve amperage. One of
these is probably the case.
Check the owners manual for a troublshooting guide.
There are a few others but you need to be familiar with Electricity.
SolowM
he should have investigated in more detail why there was a problem in the
first place
Kevin F Schama
: Recently I am having a similar problem, even when I set my thermostat at 75 F the
: temperature inside my place is only 60 F and is noway to make it go up higher, I
: have even set the thermostat at 90 F but it doesn't help....
: I have been checking the cycle and it seems pretty short, it is like this (the
: times are approximate):
: 60 sec. fire on
: 100 sec. fire and fan on
: 80 sec. fan on
: 45 sec. fire and fan OFF
: 45 sec. fan on
: 80 sec. fire and fan OFF
: 15 sec. fan on
: And now the cycle is repeated.....
: It seem to me that every thing is working fine, but I don't have idea about the
: furnace operation, so I think that the fan and burner should be on longer. I
: don't know if it is the control logic that is programated but does anybody has a
: clue of what is going on?????
: By the way the furnace has a capacity of 30000 BTU/hr my place is a basement of
Pat Wedel
>SNIP...
I would also check the hi-temp cut off and if your thermostat has a
heat anticipator adjustment set it to ZERO.
Joseph & Anna Orgeron
>Hi every one,
>Yesterday a technitian went to see my furnace, basically he did two things, one
>increases the speed of the fan, and two, change the set of the limit/fan control.
>Before it was 160-limit, 140-fan, now is 180-limit, 120-fan. The furnace seems to
>be working much better now, but I wont know until another ccold front comes down.
>He told me that the heat exchanger does not need to be cleaned (without even
>looking at it) because with the high temperatures every thing get too hot, burned
>and blowed out....I don't know if it is true....
>Anyway I hope that my problem is solved now....thanks to all that answered my
>questions...
>Regards,
>Jean Seguro
I donot know who the tech is but he needs to check both the flame and
air side of the heat exchanger. I have seen heat exchangers that have
had an appricial buildup of dust on the air side of the heat
exchanger.
You also didnot say what he did to trouble shoot the system. I hope he
checked all the usual components, heat anticipator, etc.
Hope you have good luck.
Joe
Anna Orgeron, USAF Retired & Wildlife Rehabber & Hunter
Joseph Orgeron, USAF Retired & Hunter & NRA Life Member
e7...@tesser.com
"Half Bleed Cajun, If I was full Bleed, I coud't stood Myself." (Justan Wilson)