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-   -   Intermittent blower operation (http://www.diychatroom.com/f17/intermittent-blower-operation-122376/)

Illroy 11-04-2011 04:39 PM

Intermittent blower operation
 
Not sure if this is the appropriate forum for this question. If not, let me know.

I have a very old package unit, Carrier, I believe, over 20 years. Just trying to milk the last few miles out of it before replacing, and having an issue with the heating.

Sometimes works normally - pilot starts, fan starts, house heats to specified temp, gas shuts off, fan shuts off.

Sometimes now the blower cycles on and off regardless of the thermostat setting and switch settings (Auto/On, Heat/Off). The cycle time is all over the place - somtimes on for 10 minutes, off for 5, sometimes on and off every 30 seconds, or 10 seconds, then back to normal. Removing the thermostat unit completely makes no difference.

The schematic inside the unit is hard to read, but I think I've identified 2 or 3 thermal switches wired in series. One is mounted over the burner. When I take the door off to check these switches the fan keeps running although I don't see any mechanical switch associated with taking the door off and if there was, it seems it would turn off, not on. If I disconnect the thermal switch over the burner, (burner off) it doesn't interrupt the fan. The switches test closed, so obviously not hot enough to open - assuming they are normally open at temp.

Would probably help if I could locate a better schematic, but does this sound like a thermal switch issue or a logic board issue? The logic board is clean - no discolored/overheated components.

Thanks for any advice/assistance. If you know anyone reasonable and reliable in the Van Nuys, CA area who will get me through the next few months on a tight budget, let me know.

biggles 11-04-2011 05:39 PM

the last thing to go offf after the stat satisfies is the supply fan...controlled by a metal disc or that silver covered control right above the burner.the setting has the fan on at the end the fan reaches the setting within. the fan goes off then the left over heat turns the FAN LIMIT control to start the fan again 2 or 3 times is no big deal as long as you don't have it running till the next cycle with drafting...the reason if you pull the stat it stays running :wink:the fan cycle is with in the furnace control not the stat in heating mode..even in HEAT-FAN AUTO.during the summer the stat controls the fan run thru the cooling call....your normal no big deal:whistling2:

Illroy 11-04-2011 06:14 PM

Quote:

Originally Posted by biggles (Post 763910)
the last thing to go offf after the stat satisfies is the supply fan...controlled by a metal disc or that silver covered control right above the burner.the setting has the fan on at the end the fan reaches the setting within. the fan goes off then the left over heat turns the FAN LIMIT control to start the fan again 2 or 3 times is no big deal as long as you don't have it running till the next cycle with drafting...the reason if you pull the stat it stays running :wink:the fan cycle is with in the furnace control not the stat in heating mode..even in HEAT-FAN AUTO.during the summer the stat controls the fan run thru the cooling call....your normal no big deal:whistling2:

All makes sense. Except I guess I need to clarify that when the intermittent fan cycling issue starts, I can't get the normal heating cycle. It just blows cold air and sometimes the fan won't switch off for long periods. When that happens, I cut the power at the breaker panel. Next time I need heat - say after a day or so, I switch the breaker back on and I can sometimes get a normal heat cycle or two before the intermittent fan issue comes back, after which I cannot get a heat cycle.

biggles 11-04-2011 06:30 PM

do you have this type of limit controller http://www.bestbuyheatingandaircondi...ry_Code=fl-5fl do you always leave the breaker off then repower it to run the furnace...if you don't have a control module in the furnace the fan is totally heat reacting if you shut the stat off and leave the breaker on what happens

Illroy 11-04-2011 07:14 PM

Quote:

Originally Posted by biggles (Post 763950)
do you have this type of limit controller http://www.bestbuyheatingandaircondi...ry_Code=fl-5fl

I don't know for sure, but I think not. I'll look at it tomorrow morning. Raining now :)

Quote:

Originally Posted by biggles (Post 763950)
do you always leave the breaker off then repower it to run the furnace...

Only when it starts acting up, which lately has been more often.

Quote:

Originally Posted by biggles (Post 763950)
if you shut the stat off and leave the breaker on what happens

The fan turns itself on and off at random intervals.

biggles 11-05-2011 04:28 AM

what about during the cooling season with the fan in AUTO/cool and the system is off satisfied at the stat?if the answer is nothing happens then its either the ctual heat or somethong on the heating side of the system..let me know about that silver gizmo limit controller:whistling2:

Illroy 11-05-2011 04:51 PM

4 Attachment(s)
Quote:

Originally Posted by biggles (Post 764137)
what about during the cooling season with the fan in AUTO/cool and the system is off satisfied at the stat?if the answer is nothing happens then its either the ctual heat or somethong on the heating side of the system..let me know about that silver gizmo limit controller:whistling2:

Can't tell because I don't use AC. The comp is NG.

New info though. I went up to check and take pics. The top of the package unit housing over the burner compartment seemed unusually warm. When I took the cover off of the burner compartment I could hear what seems to be the pilot burning. This is with the stat and breaker switched off. Here are pics of the burner compartment. The sensors in the second and third pics appear to be wired in series.

Doc Holliday 11-05-2011 04:58 PM

round disc with red wires in burner compartment is a flame rollout, the other wires going into the backing plate of a high temperature limit switch. Both do the same thing, cut the furnace off at a certain temperature. A certain too hot temperature that is.

Doc Holliday 11-05-2011 05:17 PM

I'm suspecting there are more temp switches than what you are seeing. Once you find them all test continuity through them. They should be normally closed when not tripped meaning, as you've already stated, showing continuity through them. You'll need to pull the wires off of switches to test continuity.

Illroy 11-05-2011 06:21 PM

OK, I have a new theory.

I was suspicious about the pilot being on while the burner and the burner exhaust fan were not. So I shut down the whole system (breaker off) and turned off the gas valve inside the burner chamber - waited a few minutes, and turned the gas valve back on. I didn't hear any hissing, so no unburned gas emanating at the pilot. I replaced all the covers, reset the breaker and turned the stat to HEAT and 75 deg. Pilot fired, then burner fired and finally the blower kicked in after a minute or two. Heater is now working normally.

There is a red high voltage wire - looks like a spark plug wire going from what looks like a solid state ignition box to the pilot - and a small gauge white wire also going to the pilot. I'm guessing the white wire is connected to a sensor that tells the ignition module whether the pilot is on or off. If the sensor is bad, it doesn't tell the ignition module that the pilot is still on, so the system won't reset. If so, the blower keeps cycling on and off to evacuate the hot air created by the pilot.

Sound plausible?

biggles 11-05-2011 08:06 PM

the pilot should only light in the beginnning of the heat call the thick orange wire is the spark ingitor(like a spark plug wire) it sparks the pilot lights the spark stops and the white wire signals the main gas to open....pilot can't produce enough heat to cycle a fan limit

Illroy 11-05-2011 09:05 PM

Since my previous post the heater has been working normally.

Makes sense that the pilot wouldn't normally seem hot enough to trigger the fan. But the flame coming out of it earlier today was unusually large - like almost 2 inches and the top of the unit over the burner compartment was very warm to the touch. After resetting it, it seems OK.

I wonder if the pilot jet is bad and why when the pilot was excessively hot there was no gas supply to the burner.

Anyway, thanks all for the feedback :)

Illroy 11-06-2011 09:30 AM

Update: The pilot is definitely not turning off.

Heater operates normally after a pilot reset, but once the stat setting is satisfied and the burner shuts off, the pilot stays on and burner will not start on next heat cycle until pilot is reset again.

Seems like I have to figure out if it's the ignition module or the pilot assembly :confused1:

P.S. It's a Day & Night unit.

biggles 11-06-2011 09:57 AM

the pilot should shut off after the main burner comes on and stays of till the stat calls again.....that ignition module with the orange cable boot is the spark ignitor and senses the pilot then knocks it off,and kicks the main gas in...you need to get a service guy in either the module isn't producing the swing on whats OFF or ON or the pilot gas feed within the gas valve.just to think if the pilot gas feed is leaking it will always be lit from the main burner and stay lit after burner goes off?and then the fan thing get it checked...and you reset and it runs normal...repeating myself the pilot should not be on in the off cycle

biggles 11-06-2011 10:05 AM

see the pix of that 2 wire limit stamped 180F remove it and clean what i think is a heat resistor looks like a fan limit,but not a hi limit where it is located..anybody want to kick in on that:huh: i see the roll out on the upper left disc where is the other limit....up in the heat exchanger?is there anything else above the burner with a pair of wires on it and a temp valve showing.


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