================================================== =========
(( 1 )) Problem with thermostat....
================================================== =========

I have a White-Rogers 1F82-54 Programmable Thermostat attached to a YORK Olympian 14 (14 SEER & 9 HSPF) & Chromolax HAF-324D (24kw/81900btuh) electric Furnace.
I have been having strange behaviour lately with the programming. It would appear that it is unable (lately) to bring the temp. back up to (23C/73F) from the setback temp (21C/70F) during the night. This only happens when the outside temp is at a certain amount.

When in normal HEAT mode and the temp hovers above or around 0C(32F) +/- 5C(9F), It is OK. Once the temp is around -10C(14F) +/- 5(9F) then it can take forever to come up. Below -15C(5F) (obviously) the furnace only runs. When it's furnace only then all is OK.

This past weekend is a prime example. I purposely backed up the startpoint 2 weeks ago after it started acting up, to 4:30 AM to ask it to go from 21 to 23. By 7AM it was still showing 21C(70F). It was -11C(12.2F) Sunday AM. I gave up and put it to emergency to bring it up. Once up I put it back to normal heat.

If I understand the thermostat owners manual, it moves the startpoint so that the temp. (some sort of smart technology) will be at the requested value at the designated time. If it misses it keeps adjusting until it hits it.

If I assume that most times it kicks in ~30 min.
prior (per the manual) to setpoint my system therefore ran 2.5+ hrs.? Does not the thermostat have a logic built in so that after a certain time, it gives up trying and the
furnace kicks in? I believe it does because I have noticed it do it. Could this "decision-making" circuit be faulty?

By-The-Way: I live in Montreal Canada. It gets cold here in the winter, as you can see by the temperatures I am describing above.

I have been to my local Home-Depot and saw HONYWELL (RTH8500D)replacmeent thermostats. If it's only that I would change it myself, instead of calling service, and be charged double. They (thermostats) appear simple to change.

Diagnostics already done:
=====================================

+ the filter is cleaned religiously every 30 days. I have a
electrostatic filter, so no airflow blockage.

+ I put my hand on the large copper tube (near the furnace) leading from the heat pump to the furnace. I cannot keep my hand on it from the heat, so the heatpump is definitely OK.

+ With the thermostat set to "emergency" it works like a charm. The furnace is therefore OK.

============

Just as added information the house was built in 1967 so the R-value of the house is a big question. I have been fighting with the idea of fluctuating the temp. or keeping it steady (because of the low insulation). By the way the heat pump was installed in 2000, the furnace in 1976.

The electric furnace is a 24Kw furnace. My house (30x40 semi-detached duplex) is 2 floors of 1200sq.ft. The lower (basement) has 400 sq.ft. (in-house) as garage. The garage is downhill into the basement level. That means that 1200 (top) + 800 (basement). The basement is finished by the way. The garage is heated independently but set low.

============

As an aside question !

(1) I have read different opinions about programmable thermostats and that they are not good for heatpump systems and should be left at a fixed temp.

Comments ?

=====================================

Any idea why this is happening ? Is it worth it to change the thermostat? This would be the 2nd WR brand I change in 4 years. I am tempted to get a HONEYWELL instead.