Got out of the TV business back when the prices of new TV's dropped to a point where there was no money to be made repairing them. Glad switched to HVAC, TV prices have just continued to plummet ever since I got out of the business. Saw an LG 60" flat screen for $800 recently, 42" TV's under $400.
I just read over your original post for specifics about your area. The usual temperature range for the Tacoma, MD (District of Columbia area) is to provide 70 degrees indoor temperature when the outdoor ambient temps are at 17 degrees. Once again, your furnace may be fine for your area, but if replaced, should be of a lot less Btu input value.
In your area, if your home is comfortable with a 3.5 ton (42,000 Btu) cooling system, it will kept warm with less than double that amount of heat (84,000 Btu), which it is possible what your existing furnace is producing. If you currently have a 3.5 ton cooling system, I would recommend a 90k condensing furnace as a nice replacement. If your cooling system is a 3 ton, and that amount of cooling is keeping your house comfy, go with a 70k condensing furnace (it indicates your house is tightly built and decenlty insulated.)
RoBoTeq, you live closer to the OP than I do, interesting theory on sizing furnaces based how much AC it takes to make a house comfortable. Here in Oklahoma we're closer to a 1:1 ratio. Normally the blower size dictates the minimum furnace required, which usually exceeds the 1:1 ratio. Some contractors take it one step further and use the high BTU for a given size blower because that's what the supply house has in stock. The result is a furnace that rides the high limit because the contractor doesn't upsize the ductwork when they install the oversized furnace. Unless the customer has a system powered thermostat that goes blank when the system hits high limit the customer have no idea the furnace is cycling on high limit unless they happen to be watching the burners. As long as the house is warm and warm air is coming out of the vents you will rarely get a call. Since the furnace is typically 2X the size needed the house will stay warm even when the burners are shut off 1/2 the time on high limit.
John01 understand that steady state efficiency and AFUE are 2 different things. Your furnace could very well be 80% while running, but cycling losses and pilot light (if your uses one) reduce AFUE to 65-70%. Newer furnaces have much lower cycling losses and no pilot lights therefore the 80% Input/Output ratio is closer to AFUE.
The 123k input rating was when the furnace was new, it may or may not be that now. Ideally it should be the same now as when it was installed, but gas burner orifices can partially clog, and the gas valve can loose it's calibration. You can "clock your gas meter" and see how much gas it's actually burning.
Not Tacoma, Maryland, but Tacoma, Washington...
Yes, it does have a pilot light. And, I don't have any cooling attached. This is just a furnace.
Now, I just did a seat of the pants check of furnace cycling, because I don't have a data logger to capture the data. I did not clock the meter, as suggested by 54regcab. Didn't think to do it.
The current outside temperature is 39 f.
The thermostat set temp is 71 f.
When it drops to 70, the furnace turns on. It runs for fourteen minutes, and raises the temperature to 72 f.
It takes an hour and 25 minutes for the inside house temp to drop from 72 to 70, and then the furnace turns on again.
Is that helpful information?
I don't know what clocking the meter would tell you other than how much gas you are using per hour, which is strictly input data that has no relevance to heating the house.
The longer cycles will even out the air temperatures throughout your house and you overall feel more comfortable while saving money on energy.