Replies

1. 
   kurt99 | Jan 03, 2010 11:17am | #1

   My bet is that it is something electronic in the furnace, either a control board or ignition controller that is going into lockout. Whether it is an actual fault with the electronics, with a sensor that it uses or an actual unsafe condition that it detects, I don't think there is any way to tell from a distance.

   Your best bet is if the furnace malfunctions between now and when the service person comes and you do not do anything to reset it, just shiver until he arrives. There is a much better chance of the tech finding the problem if it is doing it when he is there.

2. 
   hughmus | Jan 03, 2010 12:24pm | #2

   I had a high efficiency furnace that wouldn't fire. It would cycle the blower and then try to light, but wouldn't . I cleaned the sensor that sticks in where the pilot light is with an emory cloth. It did the trick for me. Good luck

   -h

3. 
   rjw | Jan 03, 2010 01:04pm | #3

   This could be a cracked heat exchanger which can be dangerous.

   Time to bite the bullet and get a good HVAC guy/gal in.

4. 
   DanH | Jan 03, 2010 03:07pm | #4

   You keep confusing us over the thermostat SETTING, vs the temperature the thermostat READS.

   If the thermostat SETTING still reads 68, but the actual temperature it shows is 63 (and the furnace isn't working hard to catch up), then there's probably something wrong on the furnace end. Normally with a high efficiency furnace this is a lockout condition, often due to too much flow resistance in the intake/exhaust system, but there should be a "code" showing on the LED display of the furnace to tell you.

   I forget -- is this a high-efficiency (condensing) furnace (vents out the sidewall in plastic pipe), or a standard unit with vertical metal flue?

1. 
   profengr | Jan 03, 2010 06:39pm | #5

   High efficiency condensing furnaces must drain the condenste (water) from the unit in the winter. If the drain line discharges to the exterior of the house, the condensate may be freezing in the drain pipe and preventing the water from draining adequately. As the water backs-up into the unit, a sensor in the furnace will stop the unit from operating and producing more condensate. The furnace will repeatedly try to start, but will not stay running and never reach the thermostat setpoint. Another symptom is the unit will sound as if it is surging. Resetting the thermostat will not cure the problem.

1. 
   Nuke | Jan 04, 2010 06:21am | #6

   The exhaust is a metal flue pipe that goes through the floors and out the attic. If I observed correctly about a week ago I was watching the roofers and noticed the flue for this zone with moist exhaust.

   This is a 9-10 year old unit. There is no LED panel on the furnace. And the behavior under observation yesterday is that it runs for a while and then shuts down before reaching the target.

   ***BUT*** somewhere around 5PM last night it reached its target (71ºF) and held it until we set it down to 62ºF for the overnight setting. It operated at that overnight setting just fine.

   Are there other indicators from observations that can be indicative of a specific condition? I noticed when it starts the blower the force of the blown air seems less than expected. That along with moisture in the exhaust ....

1. 
   DanH | Jan 04, 2010 07:20am | #7

   Moisture in the exhaust is normal in cold weather.

   I presume you have a unit with an igniter, and not a standing pilot. The igniter can fail during startup, but shouldn't be an issue once the burner has lit.

   The only thing I can think of that would cause a conventional furnace to shut down while burning -- before reaching the thermostat preset -- is overheat, usually due to a plugged air filter, but possibly due to blocked registers or a slipping fan belt.

   A failed igniter can prevent starting, and, once it's failed, may lock out until the thermostat's been reset. You might check your manual (online, if you can't find it in a drawer) and see if there's like a single LED that indicates igniter lockout.

   In a standing pilot setup moisture dripping down the flue can extinguish the pilot, but then you'd know that the pilot went out

1. 
   rjw | Jan 04, 2010 11:26am | #9

   >>The only thing I can think of that would cause a conventional furnace to shut down while burning -- before reaching the thermostat preset -- is overheat, usually due to a plugged air filter, but possibly due to blocked registers or a slipping fan belt.

   A cracked heat exchanger can cause short cycling, as well.

   I still maintain it's time for him to get a pro

   When I was young and foolish, I "maintained" my own furnance.

   Now I know enough to know that there are some real hazards in half-baked knowledge and incomplete information when it comes to furnances and I often bring in the pro's.

2. 
   rdesigns | Jan 04, 2010 11:44am | #10

   If you're still bent on
   If you're still bent on checking this out for yourself, as opposed to hiring a tech, you can give us, and yourself, a useful bit of info: use a thermometer to check the temperature rise.

   Let the furnace run for about 5 minutes, then stick the thermometer into the airflow near the return plenum and them compare this with a reading that you take on the supply plenum. You may need to drill a small hole for the probe. The DIFFERENCE should be in the range of about 35 to 65 degrees. A very high temperature difference will mean impaired airflow, which can be due to various things, but dirty filter is the first suspect. Next, check that all registers are open. If balancing dampers are installed (doubtful), they should still be at the original settings, unless they've been tinkered with.

   A high temp. rise can cause the furnace hi-limit switch to trip and can lead to premature failure of the heat exhanger. I don't know it that would explain everything that you're experiencing, but knowing temp. rise is always good, and anybody can easily do it.

2. 
   rjw | Jan 04, 2010 11:23am | #8

   In my area (NW Ohio) codes prohibit running the condensate line outdoors, I believe for that very reason.

1. 
   DanH | Jan 04, 2010 07:49pm | #12

   ????

5. 
   IdahoDon | Jan 04, 2010 02:08pm | #11

   I'm not an HVAC guy, but I'm
   I'm not an HVAC guy, but I'm the first to get a call from clients if their furnace isn't working and normally show up to see if it's something obvious. Trouble shooting the newer units isn't something I'm good at, but have learned from my HVAC guy that it's best to start looking for the obvious before assuming it's something more complicated.

   We know it's intermittant and dependant to some degree on outside temps or at least on the higher demand of the system. Blower not producing as much supply air as normal on restart.

   Condensate lines cause more problems to my clients than anything else so I check that. Blockages to the intake or exhaust are checked if easy to get to.

   What causes a slow blower? Sounds electrical to me. Obvious non-technical check is to see if unit is getting full voltage at the outlet and then at the blower motor itself. If the motor is getting full voltage but not blowing as hard as it should I'm thinking perhaps a 10 year old blower motor is on it's last legs and the sensor that provides feedback that the blower is blowing is tripped shutting down the system. With my luck the unit would have so many wires that I couldn't tell where to get a voltage reading so I'd wait until the guy who makes the big bucks shows up.

   Once upon a time an intermitant blower issue was a failing switch that cuts power to the blower when the inspection cover is taken off. I don't know squat about where the tangle of wires goes to or comes from, but I did find different voltage on one side of the switch than the other when the switch should have been closed. Supply air volume when operating was 100% so this is probably not your issue.

   Another intermediate blower issue was condensate that wasn't reliably being removed because of some remodeling debris that made it down the exhaust when the unit was set to the side. There was a drainage hole in the bottom of a P-trap looking bend in the exhaust the condensate is supposed to drain into but could only dribble through. Lots of condensate would block the exhaust. Removed the pebbles of plaster and solved the issue. Again supply air was 100% when operating so this is probably not your problem.

   edit: Another unit had an intermediate condensate pump so sometimes it would backup (someone handy fixed it so it wouldn't overflow if the pump failed) and the system would shut down.

   I'm betting on a bad blower motor or something electrical connected to that circuit. Having said that half the time my hvac guy finds the real problem to be something other than what I'm convinced it is! lol

   Also, just take a flashlight and look around at everything you can see to ensure there isn't something really obvious like a part that has let all it's smoke out.

1. 
   rich1 | Jan 05, 2010 12:07am | #13

   Just out of curiosity, is there a duct from outside directly connected to the return air?

   I really hate troubleshooting long distance, bwth, make, model and temp rise would really help. On the rating plate will be the temp rise range. That would help too. Is there an AC coil attached? Do the burners run the whole time and does the fan run the whole time.

6. 
   danski0224 | Jan 05, 2010 08:41am | #14

   >>>Note: It seems whenever the outside temp get's really cold the stat, or heating system, has this trouble.<<< I would check for loose or broken wires, especially at the stat connection. Look near the insulation. People that use utility knives to strip wire can cut the copper conductor, causing a problem. I would also make sure that the hole in the wall where the stat wires exit is not drafty. Cold air from an attic or outside can follow the wire path, affecting the stat sensor. I would not rule out problems caused by "high efficiency" pleated or washable air filters. If you are using these types of filters, pull them out and replace with the plain old woven fiberglass type. Pleated filters are too restrictive for 98% of the typical residential systems out there. Those washable ones are worse. Those are the easy things to check. If that doesn't help, then there is an intermittent problem with the furnace, and you should call a local pro.

7. 
   cussnu2 | Jan 05, 2010 09:15am | #15

   There is another possibility that happened in my new home. In our area the standard practice is to use stub bays as cold air returns. I woke up one really cold morning after moving in to find the house cold with the furnace running constantly and we also had a frozen pipe in the middle of our basement. Finally found out that a cold air return in a stud bay had found a path directly to the attic and outside air. The furnace couldn't heat the - air temp enough to warm up the house. I ended up cutting 2x4 blocks for all my cold air returns (that were not going up to the second floor) and pounded the 2x4 above the return grill height and then caulked it in. If you cut it tight enough, no need to toenail. Might be worth it to just pull your cold air returns grills off and see if you are getting air drawn from above the level of the grille in the room. Holes for wiring,plumbing etc can run horizontally all the way to the outside walls or to the attic and end up pulling in outside air.

   BTW, try just taking the filter out completely for a period of time to make sure you are restricting NOT air flow.