Furnace kicking out before temp reached - http://www.hvacmechanic.com/ Forums
Original message
| Sean69
 |  "Furnace kicking out before temp reached"
, posted Sun 9 Nov 17:26  
Hi, I have a Lennox Value Series gas furnace M/N 80UHG2-75A-2. It works fine until it's required to heat more that a few degrees (celcius) gap in temperature. We have the heat set at 17 overnight and 21 during the day (Honeywell 7 day prog. thermostat), and in the morning it rarely makes it to 21 deg before cutting out. All you need to do is go downstairs and turn the power to it on and off and you're back in business. I've had a service guy here, but of course it wouldn't do it for him.......... Any ideas?
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Replies:
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| Xenos
Webmaster 
| "Re(1):Furnace kicking out before temp reached" , posted Sun 9 Nov 20:23
There are things you could check.
1- check the temperature difference between the return and supply. If the A/C coil or filters are dirty the unit may be tripping on it’s high limit. Verify the reading against the TD rating on the furnaces name plate.
2- Clean the flame sensor. Look of a rod in the burner area with a single wire on it.
Xenos.
The best way to escape a problem is to solve it.
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| | Sean69
| "Re(2):Furnace kicking out before temp reached" , posted Sun 9 Nov 22:03
I haven't checked the temp difference, but it should be fine because the cold air return (8"x14") ends about 12 feet from the furnace in the basement, and it's quite a bit cooler down there (4 to 5 deg. C). We aren't running A/C (It hits -30 here alot more than it hits +30!!!). I have an el cheapo high flow filter in there because I thought it may not be getting the flow it needed. I cleaned the flame sensor very recently because I thought the same thing about that too.
I have a feeling it's the high limit sensor. Is this something I can do myself? (I'm pretty mechanically inclined..)
Thank-you!!!!
P.S. Something I should add - the furnace is probably about 6 or 7 years old but has only been in use for two. It sat for about 5 years without use in the house that wasn't completely weather tight while it changed hands under construction, so corrosion may be a factor.
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| | Xenos
Webmaster
| "Re(3):Furnace kicking out before temp reached" , posted Mon 10 Nov 17:12
You may be right. I however do see this quite often, let me explain. Today most furnace manufactures use 4 speed fan motors. When the furnace is first installed it’s the installer’s responsibility to check the TD across the furnace and then set the speed of the motor to match the recommended TD, Some do others however have no idea about TD. Too much or too little is not good. This next part doesn’t pertain to this case, however you got me started. Now it’s July and a new installer installs a central air conditioner on the furnace but neglects to check the heating TD once the A/C’s coil is installed. As time passes the A/C’s indoor coil picks up dirt due to the home owner using inexpensive filters and since the heating speed has been reduced due to this added coil the furnace now start to over heat. Normally it is quite common for a furnace with a cracked heat exchanger to have been running with too high a TD and a rusted one to have too low a TD. Even to verify the limit you will need to know the temperature the furnace is running at. It’s a small test that can save you a fortune in future repairs.
The problem with changing a limit your self is where to buy it.
Xenos.
The best way to escape a problem is to solve it.
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| | Sean69
| "Re(4):Furnace kicking out before temp reached" , posted Mon 10 Nov 19:01
O.K., I'm going to have a go at it. How do I test the TD? Can i assume It's too high or too low from my problem? If I can't do the test myself, which way should I go first (I should mention that I think the furnace should be more efficient than it is, if that helps). What's the best way to determine what temp it's running at? I really apprecite the help!! One last thing, is there a company that you back or would recommend in Winnipeg, Manitoba (Canada), that I can call to have work done if necessary? I haven't had very good luck so far...........
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| | Xenos
Webmaster
| "Re(5):Furnace kicking out before temp reached" , posted Mon 10 Nov 19:49
Check the temperature of the air leaving the furnace and subtract it from return temperature. If it is the problem it will be too high but just the information alone is a good.
Local private companies normally provide the best service.
Xenos.
The best way to escape a problem is to solve it.
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| | Sean69
|  "Re(6):Furnace kicking out before temp reached"
, posted Tue 11 Nov 13:14
TD is as follows: I get 130F (thermometer bottomed out) about 4 feet above the burner in the hot air duct (hot air return to humidifier) and 65F with the thermometer placed just before the filter in the cold air return (filter in place). So I measured a 65 deg TD, but it's probably more, because 1) the thermometer bottomed out on the high side, 2) I don't open the hot air to the humidifer all of the time, therefore the return temp will be cooler, and 3) it's still fairly warm here (20 to 30F), so the basement will get cooler, and draw cooler air thru the basement return when it hits -20 to -30F here. What now?
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| | Xenos
Webmaster
| "Re(7):Furnace kicking out before temp reached" , posted Tue 11 Nov 18:45
The Td still concerns me seeing as though your thermometer bottomed out. The maximum TD allowed on that furnace is 75 and you say your running at 65+ the question is it over 75? Now the theory about the colder weather will not effect the TD. Ie if the return air is 65 now with a TD of 65 means your supply air temp will be 130. If the return air lowers to 60 the td will remain at 65 and you will only supply 125. Please note this may be a problem this however doesn’t mean its the problem.
I just noticed something reading the manual. This furnace has trouble codes. The next time it trips before resisting look at the led’s. If it’s a limit the first led will flash slow and the second one will remain on.
Xenos.
The best way to escape a problem is to solve it.
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| | sean69
| "Re(8):Furnace kicking out before temp reached" , posted Sat 22 Nov 15:28
O.K., It fianlly started doing it again. It's very sparatic, and seems to happen more when it's colder outside. I had a look at the 2 green LED's on the circut board thru the "peep hole" before taking the bottom panel off, and the top one was on, and the bottom one was blinking. I took the panel off, and #1 is on the bottom, so #1 was a slow blink, and #2 is on. So it makes it a limit switch. This is when I call a service guy or is it something i can do myself?
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| | Xenos
Webmaster
| "Re(9):Furnace kicking out before temp reached" , posted Sat 22 Nov 18:52
I could be a defective limit but you had mentioned earlier that your thermometer bottomed out therefore you never did get a good TD reading. If your furnace TD is too high it will cause this. Now if the supply air temperature is 130F then yes you most likely need a limit.
Xenos.
The best way to escape a problem is to solve it.
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| | sean69
| "Re(10):Furnace kicking out before temp reache" , posted Sun 23 Nov 12:16
One last thing before I call the repair guy. Is it safe to kick the fan up one speed (I assume this is how to bring down the TD) to see if the problem goes away? If so, is there any thing I should be aware of when doing it? It seems fairly straight forward to hook up a different wire........
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| | Xenos
Webmaster
| "Re(2):Re(10):Furnace kicking out before temp " , posted Sun 23 Nov 15:35
Sure, but I would recommend checking the TD. If you kick up the speed and if the TD is too low the combustion products will condense too early and rot out your unit. This of course is over a long period of time. If you have air conditioning Just turn on the thermostat to the fan On position. This will be High speed.
Xenos.
The best way to escape a problem is to solve it.
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