I have a Rudd furnace which is 15-16 years old. Lately, it's giving problem when the temperature goes around zero degree outside. When outside temp goes to appx 55-60 deg. It will start to work again. I called maintenance/repair person to look at it and he couldn't find any reason. My son looked at it and he thinks it's gas valve problem. This morning, it did it again and eventually, I took a small fan heater and put near the furnace, in 10-15 minutes, heater started and continued working until set temperature.
Do you have any clue for what can be wrong?

When outside temp hits zero it stops working and when it gets to 55-60 outdoors it starts working again? How could that be? Quits in jan. and starts in April?

What I mean is, during evening and night time the outside temperature goes to 0F or below 0F, the heater stops heating but when the temperature reach around 55F degree during day time, it would start working to reach 70F set on thermostat. If it is a cold day and the high for the would not go above freezing temp, then heater/furnace wouldn't start at all.

Fuel type?? LP/propane/or natural gas?

Natural Gas.

I took a small fan heater and put near the furnace, in 10-15 minutes, heater started and continued working until set temperature

Where is the heater located. Is it in an area that is affected by the outdoor temp? (does unit see below freezing temps). This is a long shot, but here goes. If the unit is a 90plus and has condesated drain trap, if the area the furnace is in gets colder than 32 degrees, the drain will freeze, causing the furnace to shut down because it can't drain the condensation away. Only reason I mention this is because the small fan heater made the furnace work. Like I said, it's a long shot. Condensing furnaced should never be installed in areas that will see below freezing temps. Very curious problem you are having, please let us know what you find.

Where is the unit located? If outside then I would also suspect the gas valve since they are not usually designed to operate at a cold temp outside. Now all outside design units like roof tops and packaged units will have a low temp compatible gas valve from the factory BUT they do go bad.

The unit is located in Attic. The person who came to do maintenance talked about clogged drain pipe. What is the solution to keep the condensation away so it would not freeze in cold temperature?

Hate to say it, but somebody made a boo boo by installing a condensing furnace in an attic wear temps fall below freezing. No way to keep condensation out of a high effiecency furnace, its just the nature of the beast. All you can do is find a way to keep the area the furnace is in heated above freezing. Maybe build an area around the unit , insolate it, and put a register from the enclosed area to the living area to allow heat from the living area to keep the drain system from freezing. Or just keep that little fan heater up there during extremely cold weather.

In this area they use special heat strips to prevent the drain from freezing and they also add insulation around the heat strips. Check with a local know good contractor for further assistance since all of the electric heaters for that purpose are not created equal.
I agree an attic is no place for a condensing furnace and the enclosure idea will work if done properly.

In the code it states that any unit installed in the attic space or any other space for that matter must be listed and labeled for that usage listing must be posted in the install instructions.

I was thinking heat tape and insolation too, but never actually run across this problem before.. well, I did once(heres were I tell another tail of learning from the school of hard knocks). About ten or so years ago, we installed a 90 plus unit in a church in a small town about 30 miles from my home town. Everything was just dandy, till we got the first call back on a nice cold day the next winter. Well, we had installed the unit inside the church, heated area, but what we neglected to ask, was, do you keep the church heated 24/7. Well, if I would have had my brain working when I did the install, I would have realized that this was a very small, old church, no plumbing, hence, no running water or bathrooms, so no need to keep the building heated except for sundays. So hear comes Sunday, and guess what, no heat. I thought about the heat tape idea, but then my boss stepped in and said, nope, just tear the unit out, install an 80% and ill bend over once again. Definitely a learning experience for me, and I'm just glad my boss is a great enough person(instead of reeming my ars) just says, these things happen.

Actually it was the salesman's fault for not anticipating that situation. Usually installers just do exactly what they are told since you have no idea of a no heat situation. That question was on our salesman's list of things to check during a sales presentation and it was also listed on the invoice. (Heat must be maintained at all times in this structure unless other circumstances have been listed on this invoice or the equipment warranty is void at our discretion).

The reason for this is simple. When you have a 100,000.00 boiler system and they let it go down or turn it off I will not take the blame since it is there responsibility to arrange temp heat to protect there building and contents.

I was a firm believer in CYA