The winter is approaching, I noticed that the furnace in the attic might have problem, here's the symptom: The fan was running while the thermostat was set "heat on" with room temperature lower than the temperature set, the vents kept blowing cool air. Hence the fan didn't stop by itself, so I had to turn the thermostat from heat to off manually to avoid having the fan to run for long time, while no fire was in the furnace I assume. Later I turned the heat back on and things looked normal and the rooms were warmed up. The issue happened several times per week, but I could hardly reproduce the issue when I tried to troubleshoot. I went up to the attic and I saw no error light on the system board(which normally gives an error code), and when I was at the attic to test and monitor, it always worked. I replaced the frame sensor rod with a new one last week, but the issue has happened another 3 times since then. Any idea or suggestion?
Also, when I was in the attic and had the top cover removed, I noticed a brownish liquid running(really quickly, just about one second and no more, and I only saw that once, although I believe it could happen often hence we can see the brownish leaking line), please see the attached pictures below, does it indicate another problem?
Thank you so much!
Last edited by PJmax; 11-23-21 at 05:13 PM.
Reason: added arrow to 2nd pic
I added a red arrow to your second picture. That rubber hose must be clean. Take it off the end away from the pressure switch. Clean the little tiny hole at the bottom of the ferrule. It must be clean all the way thru.
Thanks for your suggestion about cleaning, I will study it.
How about the main issue, about the heating turns off and keeps blowing cool air until I manually stop it from the thermostat, any idea about the cause? Thanks again!
You should be getting the fault from the LED.
The pressure switch failing to prove is a very common problem especially with that condensation leaking into the inducer.
finally caught the LED error: 4 flash = open high temperature limit switch
I am going to buy a new switch to replace it.
I couldn't find any air filter used for this furnace(in the attic), not at the bottom as well, is that normally? I did replace two air filters on the celling of the 2nd floor, but they are connected to the vent, not directly to the furnace. I worry about problems which could cause overheating, but I will go ahead to replace the switch first.
Any suggestions? Thanks!
New update(problem persists after high temperature limit switch replaced):
1. In the afternoon, I bought a new high temperature limit switch from Grainger, installed it, started the furnace, no more error light, house warmed up as expected;
2. early evening, I found the the thermostat was set 70F, while the room temperature stayed at 68F, fan heard blowing, but cool air, NOT good, problem persists just like it happened in the past several weeks. I left the fan running, and went up to the attic, saw 4 flash again(open high temperature limit switch). The fan was still blowing, however, I noticed the small fan in the inducer stopped. At this time, I used digital multimeter to check the newly replaced limit switch, it was in low ohm(0.2~1.0 ohm, so it looked "closed" not open to me, weird). I powered off the furnace(turned off the main switch), otherwise the blower might continue running for hours and days…(I am leaving home for a few days from tomorrow, I may have to shut off the furnace to avoid the blower running for days when the problem happens again…)
3. Within a minute, I powered the furnace back on, inducer fan started, a few seconds, it stopped, saw 4 flash error light again. (I can't remember if the blower started or not)
4. Waited for another 6~7 minutes, I powered cycle the furnace, ignition worked as expected, room warmed up to the temperature set, then fan stopped as normal.
Limit switches rarely go bad. Yours wasn't which means you have an overheat problem.
There may be more than one limit switch. There could be several in series.
Where was that switch in the furnace ?
What's the model number of the furnace.
Yeah, when I was at the store this afternoon, the staff checked the old limit switch and said it was good. But it was just $35, so I bought it anyway just in case.
Please see the model number(P3HUA12N04801C, York) in the picture below and the location of the limit switch. I also worry about overheat problem, because it's about safety. There is no air filter with the furnace, but at the two intake vents at the celling(I think), I checked those two air filters weeks ago they looked OK but I replaced them new anyway. And I thought if there is an overheat, then the limit switch should be open, but I checked with digital multimeter it read low ohm(but I could be wrong, maybe it was a wrong timing to check the number).
Thanks again!!
When you were getting the 4 flash code..... did you check the circled limit switch ?
That type of limit is self resetting so if the blower keeps running and the burner stops.... it will reset quickly.
If you didn't check it.... run the furnace until it fails and then check it. We need to make sure there are no other limit switches. The manual shows two flame rollout switches on either side of the burner. Those should set a different code and have a reset button on the back.
If the unit fails on high heat.... try running it with the blower door off. You'll need to tape the safety switch in. Only run the furnace like this when you are there. It must be attended.
If I was there my next step would be to use a thermometer to measure the return air and measure the discharge temperature. The difference is the heat rise and that is listed on the ID tag as 25°-55°f. That tells you if the furnace has an airflow issue.
If you get a chance... post the wiring diagram on the back of the blower door.
When you said "circled limit switch", did you mean the high temperature limit switch I newly replaced, or the Flame rollout limit switch? I will post the wiring diagram as soon as I get it(will be out for Thanksgiving for a few days).
When the unit failed, I didn't feel it hot at all. Even when it was working well, the air blowed out was just a bit warm. I will figure out how I can "measure the return air and measure the discharge temperature", and get back to you.
One thing I just found it new, the furnace may need an air filter within the furnace next to the blower! I didn't know that until I saw this https://www.repairclinic.com/ProductDetail/1385456(all parts for my furnace Model P3HUA12N04801C), at the bottom of the page, there is a part "York Furnace Air Filter" and "York Furnace Filter Support" on page 2. Then I compared it with my furnace(I took a picture a while ago and attached below), there is a filter support, but no filter there. Could it be a problem without air filter? I would guess that in fact helps airflow without the filter in place(but I could be wrong).
One filter is all that is required. Two will restrict airflow.
It doesn't look like the filter would stay in position well there being so close to the blower opening.
The wiring diagram is in the shared album, a bit dirty but I had it in high resolution, you can zoom in and see it clear, if not, please let me know.
Here's the finding I got since last night:
1. My wife told me the furnace failed to work again(blowing cool air, room temperature didn't go up), so I switched the thermostat "off" and the blower stopped in a few minutes. Then I went up to the attic, took the upper cover off, and asked my wife to switch the thermostat back on heat. I took a video in the shared album, 0:19 second one: I could hear the blower and the inducer fan started, but within a few(or just couple) seconds, the inducer fan stopped(as you can see from the video), I waited but no ignition, so I checked the error light, and found 4-flash(same as I told you last week). I should have checked the small window for the error code before I asked my wife to turn on the heat, maybe it had already got the 4-flash error, as my guess. The inducer fan stopped so quickly I really doubt there was an airflow issue and caused overheat.
2. Then I powered off the furnace by switching off the whole unit in the attic, waited couple minutes, switched it back on. After about 15~20 seconds, the blower and inducer started(the unit detected "heat on" from the thermostat my wife set minutes ago before #1 test above), I took the video 1:36 in the shared album, things looked good, I put back the upper cover, and went down. It took about 20 minutes to get 72F from 66F without stopping, good sign. So it looked to me the problem might not because of a real overheat, because previously(#1) it failed quickly when the furnace was still cold, however when it was working, it could blow heat for a quite a while.
3. This morning, I noticed the room temperature was same as the programmed one, which indicated it might have worked in the morning. In the afternoon, I noticed it kept blowing cool air, so it failed again, then I switched heat off from the thermostat. Hours later, I switched the heat back on, it worked and made 72F from 67F. However, I noticed the blower didn't stop, minutes later, it started blowing cool air(I believe the fire already became off!), I left it running, the temperature dropped to 70F from 72F(because the attic was cold), I had to turn the thermostat to "OFF" to avoid the temperature going down further. Then I turned the thermostat back to "HEAT", it worked again and brought back 72F, I waited a few minutes, noticed cool air blowing out again, I manually turned OFF the thermostat. So this was a new finding about the issue today, very weird. And as I mentioned last week, when it started blowing cool air, it would never stop until I set heat off from the thermostat, this problem persists.
There is no air filter with the furnace unit, but there are two in the return on the celling. You can refer to the shared album for the 5 pictures about the returns and the vents, with their temperatures measured while the furnace was working. The size of the filter is 16x16x1.
Your furnace has three flame rollout sensors and three limit switches.
You replaced one of three limit switches. I've highlighted them on the diagram.
They form a loop from pin one to pin seven. Any one switch opens cause a high heat code.
You need to trace the wiring to locate the sensors.
Thanks Pete! I traced the cables and found the other 2 limit switches in the blower(lower) section of the furnace. Please see the 5 attached photos below for the switches(LS1 is the one I newly replaced). "1" in the diagram is labeled "HLO" on the circuit board, "7" is labeled "HLI"(also see the photo below).
Besides the Limit Switches pictures I posted on Monday, I tried to use digital multimeter to measure LS2 and LS3 today, while the furnace was OFF(because the lower cover was removed, so the unit was off), both read low ohm(0.1~0.3, so it looked closed). However, I am not sure if it's useful because the furnace was off and it's a loop.
Please advise what I should check next. Thanks!
BTW, I had a few more tests since Monday: mostly the blower couldn't stop automatically when it reached to the set temperature, until I manually switch to OFF from HEAT on the thermostat; but in a few times the blower stopped soon after the set temperature was reached.
Those are called flame rollout switches because they are manually resettable like rollout switches are. Most high heat limit switches are self resetting..... no red pushbutton. They can still be bad without tripping. That's why you check the loop from pin 1 to 7. There should always be 0 volts or very close to 0v measured.
In the picture are what look like the actual rollout switches. Typically close to the burner.
Hi Pete,
What should I check next? The problem now looks to be the heat can turn on, but after it reaches to the set temperature, the blower doesn't always stop(when the blower fails to stop, it gets cool air from the attic and lower the room temperature). Thanks!
Problem persists(the fire keeps shutting off before the set temperature is reached, and the fan blows cool air after that), I am going to replace the limit switches LS2 and LS3 (pictures in my last post), when I tried to figure out the right part number for it, I referred to this page for all parts for my furnace(P3HUA12N04801C): https://www.repairclinic.com/ProductDetail/1385456
But how can I tell which one/spec is the correct one? 185 degrees F? 320 degrees F? or 350 degrees F? Please advise. Thanks!!
Previously I discussed checking the safety loop. When I have an intermittent problem I put my meter right at the control box on the 9-12 pin plug and watch for the loop to open (go to 24v) when the burner fails. If it doesn't open... it's not fault. If it does open then it's one of the limits.
Thanks Pete for the response.
I was trying to do so and provide you more info, but 10+ times when I was up in the attic with my meter, the furnace worked fine. The only chance I got the error light when I was in the attic, the bottom cover(which covers the section with the control board) was not yet taken off, so when I slide the cover off, the furnace shut down as expected. I could hold the button to keep power on to monitor the whole process, but so far it has always been working. The failure happened about 20% or less in November, about 30% in December.
So the option leaves to me I would guess is to replace all the limits(I did one, two more to go), or please advise if I missed something here.
My last post was about the spec of limits to order, can you tell me which one I should order? 185F, 320F or 350F. I tried to get the model number from the existing limit part, but I could hardly see the numbers… I wonder if there is another way to identify the spec of the part to be replaced.
Thanks again!
I eventually replaced the limit switches LS2 and LS3 with brand new switches, so I replaced all the three limit switches(LS1 ~ LS3 in the diagram). Unfortunately, the problem persists. When it worked, it could warm up the room for 2~3 days, but then it failed again: before the set temperature was reached, fire turned off and started blowing cold/cool air for half an hour until I noticed that and manually turn the heat off(from the thermostat), just like before.
Hello,
Thanks in advance for any assistance. This year my furnace started acting up. When the heat is called from the thermostat, the burner ignites and the blower then starts all as it usually does.
My issue is that intermittently (more often than not) the burner will turn off before the temperature is reached (thermostat is still calling for heat) but the blower motor will stay on continuously forever afterwards. The fan switch is set to "auto". When in this state, even if I physically remove the thermostat from the wall leaving the wires exposed and not connected the fan still continues to run. Even if the heat is called again from the thermostat, the burner will not ignite again and the fan just continues to run.
The only way to reset this is to switch off the power to the furnace then turn it back on again. If the heat is being called at that moment, the burner will then re-ignite with blower to follow, only to repeat itself again with staying on until the power is cut.
As stated, this is intermittent though Sometimes the heat cycle works as expected with reaching temperature and blower turning off as it should be only give the issue again on the next cycle.
I have not noticed any diagnostics codes on the main panel's LED. To be honest the light doesn't even turn on at all indicating any normal operation. It will flash once quickly once the power is turned on, but then remain off during all other cycles.
Any suggestion on what the issue can be?
Will this thermostat work with aprilaire wired indoor sensors using the S1 / S1 terminal?
I have two sensors that was hooked to an older Perfecttemp thermostat. One down and one upstairs. I suppose I can just buy new wireless C7189 units. Curious
11-23-21, 02:40 PM
Heating turns off
