Heat stays on; Blower shuts of after 30sec.

[rayblazak]

I have Ruud furnace ~17 years old. The problem we've been having is that when the furnace kicks on in an attempt to reach the desired temperature (from a digital programmable thermostat) the blower shuts off after about 30 seconds. When I checked the furnance and the house hasn't reached the target temperature, the blower has stopped, but the furnace is still on - full flame. It then kicks back on after another minute or so and keeps doing so - blower off, blower on - until it reaches the temperature setting. Any suggestions?

[albright]

I Would Say Your Fan High/ Low Limit Is Bad. It Has A Coiled Piece Bimetal In It That Gets Distorted Over Time.

[suprheat]

The fan control could be bad or you may be just able to reset the temperature setting a tad bit higher. However, you could have a problem with the gas. The pressure may be low or the valve may not be working properly.

[Phelps]

This problem is occuring because you are setting the stat way back at night or for other reasons, probably, eh? In such cases what can happen is the cold return air is extra cold, and then as it passes the heat exchanger, it drops the temperature at the fan switch located in by the heat exchanger. "Souprheat"'s first sentence solution may be what is in store for you, to cure such a problem. Or, if you have some worn out fan switch like "albright" suggested, due to weakness over time, the same end result woud apply.

The problem is magnified if the stat is set lower, and then runs. Especially early on. With some people's similar complaints, this problem may occur for the first and or second cyclings, but then will go away after that (as warmer air starts entering the cold air return). But if yours does it all the way to "set point", then what ever is off in the fan switch is REALLY off, with yours.

[suprheat]

It usually goes away not because the return has been heated up - how much colder do you set it at night? 20 degress? Then maybe so but most people only set it back about 5 to 8 degrees. This wouldn't trigger the "too cold air intake syndrome". But what does occur is that your furnace hasn't run for a while at night and then you kick it up and after a cycle or two it stops because the furnace heat exchanger is staying warm between cycles. Oh I forgot to ask the key question - Is this a NEW problem or has this always done this?

[Hube]

Rayblazak; the suggestions that this problem could be due to the amount of "set-back" is completley off base.
Your problem is most likely an improperly set fan on/off control, or a fan on/off/limit control that is in need of replacing., one or the other.
If its the control's adjustment setting, reset it to 125- 130 F on, 95 F off.
Otherwise simply replace the whole control, and you should be ok.