HVAC & thermostat question.

[snorko]

Woke up this a.m. and noticed when I went to turn up the thermostat for the day the temp was a coup[le degrees below what it was set. I put it at 63 at night and it was 61. This is a basic non-programable Honeywell model (TH5110D1022). The readout showed heat function was active but the furnace was neither burning nor blowing. I pulled the thermostat's batteries and snapped them back in. I also pulled the unit off the wall plate and snapped it back on. I then heard the thermo click and the furnace kick on. It ran long enough to raise it two degrees then shut off. Did the same thing again and it kicked back on, warming to the set 67 degrees. I heard it come on again a little later so I assumed function was back to normal.

During today I have had to mess with it twice more. Both times I noted temps were down 3-4 degrees, and after pulling and reinserting the batteries and the wall unit, function was restored.

Does this sound like the thermostat or could it be the signal unit (for lack of a better word) on the furnace?

This is a duplex with a small 1 bedroom unit attached to the main 3 br where I live. I actually live in the whole thing and have an interior connecting door. I am waiting for it to mess up again and I am going to switch the two thermostats to see if that makes a difference. I figure if I leave the connecting door open the small unit will get adequate heat from the large. The room in the large unit with the connecting door sits right under the attic placed furnace so it runs hotter anyway.

 

[Brandon]

Online diagnosis is get what you pay for.
Coule be thermostat is on its way out.

If you want to test it, take the red and white wire and hold the bare copper ends together when the tstat says its calling for heat but furnace isn't responding.
Its 24v so it won't hurt you.

Could be a board in the furnace.
Could be other things getting less likely as you go.

I have to ask, furnace filter clean?

Not using a filtrete 1500 are you? If so don't.


Also note, that if the red wire touches something metal you run the risk of blowing the fuse inside the furnace. Not a big deal, most likely a 3 amp fuse would have to be replaced.

 

[snorko]

Well I think it is the fan control on the furnace. Switching thermostats did nothing, and when I change the fan from auto to on nothing happens.

Can I assume the furnace is smart enough not to run if the fan isn't working? Stupid question but double checking.

 

[indyblue]

This same thing is happening to me.

It's 68 and I set temp to 73, it starts and runs for 20-30 minutes or so and then stops at 71. Cannot get above 71 for some reason, normally I get it to 75 in the afternoon when I'm lounging around.

Sometimes it says "wait" which should only be for a heat pump but I'm running only emergency gas heat (NG) since the heat pump is toast, it shouldn't need any "wait" states. Same old Honeywell touchscreen t-stat for years.

3 year old blower motor, 2 year old control board, new draft inducer a month ago. Don't know what else it can be.

Symptoms only started when it got under 5°

 

[snorko]

New experiment. I got it going as described above. While it is running I switched the fan from auto to on, hoping it will keep running after the heat cycle. Gonna stay up long enough to see if that is a temporary fix. Hate to do it since the supply ducts are in the cold attic and barely adequately insulated and the returns are bare metal in the basement.

 

[Brandon]

Well I think it is the fan control on the furnace. Switching thermostats did nothing, and when I change the fan from auto to on nothing happens.

Can I assume the furnace is smart enough not to run if the fan isn't working? Stupid question but double checking.

Worst it should do is run, overheat and shut off.

If thats the case, it may try to keep running, over heating again and again... may eventually throw a code and stop.

 

[snorko]

Well function was normal through the night. I ended the "fan on" trial since it seemed to blow only unheated air even when the heat function was on. But after that it has cycled as it should. I plan to call for a service check for later in the week. Appreciate everyone's input.

 

[Hoosier Carry]

Just an opinion here, but some thoughts:

A unit may have a built in safety to shut down after a long run time. When it is 20 degrees outside and your Tstat is set to 72, your unit can bring it to temp within the allowed run time. Now that we are setting at 0 degree temps, the unit runs and cannot achieve your 72 degrees and times out before the unit catches fire or overheats. Until temps rise, you may have to live with what your furnace can handle or find ways to better your heat loss through walls.


Look for any flashing codes on the unit or read your troubleshooting manual for suggested fixes. I think the outside temps are what you are fighting. Also thermostats need to be blown out with air from time to time as they get loaded with dust.


This is why there people are going to the extreme of making their homes air tight, to conserve energy.

 

[snorko]

Very good info but I do not think it applies here. Unit is set at 64 and has had no problem keeping up. When functioning it runs for 10-15 minutes getting to temp then will be off for a bit. It ran fine overnight but started being wonky again this morning. Service called and waiting for the overworked tech now.

 

[Hoosier Carry]

Very good info but I do not think it applies here. Unit is set at 64 and has had no problem keeping up. When functioning it runs for 10-15 minutes getting to temp then will be off for a bit. It ran fine overnight but started being wonky again this morning. Service called and waiting for the overworked tech now.

Love to know what you find out. I like learning.

 

[Sigblitz]

It sounds like it's shutting down because it's getting hot, tripping the high limit Klixon attached to the heat exchanger. Not enough airflow. I would check the furnace filter, blower motor and capacitor. Capacitor can be tested.

 

[snorko]

So far I've heard plugged condensate drain and maybe something else about the inducer assembly

 

[Michigan Slim]

My condensate drain plugged. Had water in the pressure switch. Same symptoms.

 

[snorko]

Condensate drain plugged at the pan - cleared.

Inducer motor replaced.

I am warm and thus happy

 

[snorko]

I need to add, though I have been a member of this forum for 15+ years, it is only now that I have started an HVAC thread, that I truly feel I belong.

 

[Hoosier Carry]

I need to add, though I have been a member of this forum for 15+ years, it is only now that I have started an HVAC thread, that I truly feel I belong.

I like the fix it threads. Knowledge is good, a warm house is more gooder!

 

[Sigblitz]

So far I've heard plugged condensate drain and maybe something else about the inducer assembly

Did not think about a high efficiency furnace because I have an old one.

 

[bocefus78]

I need to add, though I have been a member of this forum for 15+ years, it is only now that I have started an HVAC thread, that I truly feel I belong.

Churchmouse is smiling down upon you right now.

 

[DocIndy]

Had one early Sunday morning short cycling and giving a pressure switch fault. Same thing… trap on The old Trane high efficiency was plugged. The condensate sloshing around in the inducer was a dead give away. Cleaned the trap and all was well.

It’s the season. And change your filters! A dirty filter will stop a furnace from heating just as fast as freeze up a AC coil in the summer.

 

[snorko]

Had one early Sunday morning short cycling and giving a pressure switch fault. Same thing… trap on The old Trane high efficiency was plugged. The condensate sloshing around in the inducer was a dead give away. Cleaned the trap and all was well.

It’s the season. And change your filters! A dirty filter will stop a furnace from heating just as fast as freeze up a AC coil in the summer.

Interesting, mine is a 2005 Trane.