Thermostat issue (non-garage)

[lsrx101]

Although I wish my garage even HAD a thermostat, this question is about the 'stat in my GFs apartment.

It is a standard (non-AC) gas furnace controlled by the ubiquitous Honeywell Round thermostat. I recently replaced it with a new one but it seems both have the same issue.
I don't expect the temp graduations on the 'stat or the little thermometer pointer to be dead accurate but, at ~65 degrees ambient you have to dial it up to about 78 degrees for the furnace to light. The furnace will then cycle off at about 68 degrees, then cycle to maintain that temp. (The little thermometer actually seems to be about right). To get the place up to 70, the 'stat needs to be set at ~80.
The ambient temp, measured near the 'stat, never even approaches the set temp. There is about a 10 degree error.

I made sure the old 'stat was level and thought it was just old and out of range. However the new one is acting exactly the same. Is there any way to "calibrate" the unit so the set temp setting is more in line with the ambient temp?
I have the same thermostat (w/AC) in my house and it's just about dead on, even after 30+ years. Being a guy, I could easily get used to compensating for the error. However, the little lady is convinced that setting the 'stat to 78-80 will cause her gas bill to skyrocket.
Any thoughts or input would be very much appreciated.

 

[Hlidskjalf]

What makes you think it is only the thermostat? I would guess that there is a problem with the furnace, maybe locking out on high limit or another safety.

Those old mercury thermostats have a heat anticipator that should be properly set. I would suggest getting a digital one though, haha... far less work.

 

[nehog]

Two questions...

1. What model/make is the new thermostat?

2. It is not mounted on an outside wall by any chance is it?

 

[lsrx101]

1. Not sure of the model number but it's the dirt common, round Honeywell thermostat.

2. Nope. Mounted on an interior wall well away from any windows and such.

 

[nehog]

Sounds like it is out of calibration. I'd replace it with a newer programmable one and enjoy the savings in heating costs... Save the old one, and reinstall it when you leave.

 

[truckman5000]

The round mercury ones NEED to be level, or they do what you say its doing. The wight of the mercury moves a spring in the thermostat, wich makes the switch to kick the heat/ac on. If you through the new t-stat on the wall and didnt level it with a level, get a level and level it out. Or buy an electronic type of t-stat. The only other callibration these types have is a heat intisapator (used for heating systems so if you were to turn the t-stat on and off and on ect, quickly it dosent burn out the zone valve or controll on the heating system. You say it didnt work with a/c, im sure u just need to level out the t-stat.

 

[lsrx101]

The round mercury ones NEED to be level, or they do what you say its doing. The wight of the mercury moves a spring in the thermostat, wich makes the switch to kick the heat/ac on. If you through the new t-stat on the wall and didnt level it with a level, get a level and level it out. Or buy an electronic type of t-stat. The only other callibration these types have is a heat intisapator (used for heating systems so if you were to turn the t-stat on and off and on ect, quickly it dosent burn out the zone valve or controll on the heating system. You say it didnt work with a/c, im sure u just need to level out the t-stat.

Initially I thought the original 'stat wasn't level and causing the error. I checked it and it was right on. I also made sure to level the new one.
This has got me puzzled. It's not exactly rocket science.

 

[shougrider]

Does the t-stat have two stages of heat? If it does then the single stage furnace could be wired to the second stage switch in the stat. This could cause the large diff in temp where it comes on. Make sure that the wire is on the first or maybe only heat output.

Did you also try to jump out the unit itself to ensure that it fires as soon as it is told. This would rule out the unit or t-stat itself.