Day & Night brand of Furnace - blower comes on before heat builds up

My 35 year old Day & Night furnace, model 100U42-19A series 321 has just developed this issue: I turn up the tstat, hear the burners fire and the blower immediately comes on before the heat has a change to build up, blowing barely warm air. I can never get the temp. up above 57 degrees whereas it used to heat the whole house in a few minutes with very warm air. in the past I know that there was a delay before the blower would come on, but now there is not. someone suggested it might be the "limit switch".

 

Even if the blower came on right away....you should still get warm/hot air after a bit of operation as long as the burner stays on.

What is the airflow like? Same as before? Is the filter clean?

 

Those older furnaces had a mechanical fan and limit switch, sometimes combined. They were nice because you could set the air temp you wanted unlike today's that are on timers.
It probably is the fan/limit switch.
What kind do you have. Make & model or pics would be good.

Did you try to adjust yours to see if it makes a difference.

Honeywell makes a fan/limit switch with different insertion lengths into the furnace or plenum.

Good Luck,

 

Have you replaced the thermostat? If yes, check the back and look for the setting ...gas/electric, it must be on the gas setting

 

New filter, but the problem is the air never heats up as it should, just blows cool, barely warm air. I have something on the furnace called a camstat with a temp range arm. Should this be at the upper end or lower end of the temp settings ? Would this be my problem ?

 

What are the numbers & is it adjustable. If possible set it at 150° and if that's your fan switch the blower should come on @ 150°. What is it set at now. Is it one dial or two. There should also be a control that shuts the furnace off if it reaches a high temp.

Can you take pics.

Good Luck,

 

not at home so can't take pics but currently set at 105. Will try 150 tonight. I know the high end was 220 on the dial/gauge.

 

I was wrong. just called the house and had someone look. the low end if the dial on the camstat is 50 and the high end is 120. Currently set at 105 and the blower kicks on immediately when the burners ignite. Is this part just shot and needs to be replaced ?

 

Joanne,
The camstat is your fan/limit switch with a delay.
I believe yours has 6 wires. 2 for the fan, 2 for the limit and 2 for the delay.
There are different models with numbers on the part itself. That 50/120 setting is the point where the fan turns off. So 105 is fine or you can raise it to shut off earlier.
The problem you're describing seems to be because the delay isn't working, but what I don't understand is the air never getting warm. It would seem if the furnace continues to run the air should get warm.
From the information I'm finding camstats are a thing of the past but are still available as replacement parts with the model number.

At any rate it appears the control needs replacing.

Good Luck,