I have a ducane furnace which on very cold days and nights will shut off early and is unable to maintain set temp. It will start up again but by the time it does, after about an hour, it never can get back to temp before shutting off again. If I either turn the thermostate to the off and then on (heat) or cut all power to the furnace and then turn it back on the furnace will restart and run normal but still cannot keep temp. In other words if it is really cold out the furnace cannot bring the temp over 65 degrees no matter what I try. Does anyone have a solution to this problem?

When was the last time your system was professionally serviced?

It was serviced about a year ago last winter because of the same thing and it fixed nothing so we just toughed it through last winter but we don't want to do that again

First the system could be to small for the area it is trying to heat.
Second the gas pressure at the valve might need to be adjusted to allow the unit to fire at its maximum capacity according to the rete plats attached to the unit.
The cut off and back on situation would have to be tracked down by testing On Site since it is electrical in nature and all info from here would be guess work.

If it was professionally serviced last year and they did not fix the problem then the company that did the work should have come back until the problem was taken care of. Looks like if further action on your part is not take then you will suffer from another cold winter.

The gas pressure adjustment requires a special tool and unless you have access to one you cannot do that but you can check to see what size the unit is,where you live, and the size of the home. That could tell if the furnnace is large enough to do the job.

Well hopefully the size is right, I believe that it is, we just got it about 4 yrs ago. Today it is a bit warmer outside so it is keeping the temp with no problem. We only have this problem when it is below 20 degrees or so outside. The furnace is equipped with a smart valve which flashes a code when there is a problem. The code flashes that there is a soft lockout due to recycle count exceeding 5 on the call for heat, whatever that means. But as I said this only happens when temps are below 20 outside.

The term "Smart Valve" is a crock. Try this: observe unit from start-up, if LED flashes nudge the square connector on the "Smart Valve" (I feel dumb even saying it). If blinking stops and unit operates normally, the "Smart"... oh God, I can't... valve is bad.