Gas and Oil Home Heating Furnaces - Trane XL90 -- normal behavior?

Hi
We soldered a relay connection on our circuit board about a week ago and the furnace has worked well since then. However, I have noticed the following and wonder if it's normal for this to happen -- sometimes when the thermostat sends the "go" signal to the furnace, the exhaust fan starts immediately, and the furnace lights, etc. in about 80 sec. Other times however, the exhaust fan doesn't start up right away. I haven't timed it, but it seems like at least a minute or two, and eventually the exhaust fan kicks on and the furnace follows. Is this normal XL90 behavior, or another symptom of an ongoing problem? Thanks for your help.

THe inducer fan should come on right away when the T-stat calls for heat.

Some T-Stat will have a time delay to help prevent short cycling of your system.

How old of a system you have? The board may still be under warrenity.

Thanks Jay
The furnace was installed when the house was built, in 1987. We're the third owners. So I don't think anything is still under warranty, except for the heat exchanger, which was replaced the year before we moved in. I'm a little worried about the board because we noticed today that the furnace wasn't on and that it was set a degree higher than the temperature. Normally it kicks on in that situation, and nobody had recently done anything to the t-stat -- so that makes it look like the board might have a problem again. Would that be your take on this?

Since it was working fine after the solder job, I decided to wait and see how it went, rather than take it apart again and test whether the fan is drawing too much current and burning up the connection on the relay. I suppose I should test that sometime soon. Am I on the right track?

Thanks for your help.

Mike (and Emily)

In most cases the furnace should start the cycle as soon as the tstat sends the signal. The tstat itself may not send the signal as soon as you crank up the temp if it has protection against the furnace cycling too many times - usually specified in no of cylces per hour.

If the furnace does not come on as soon as the call for heat is made it would well be a problem. Since intermittent problems are hard to find and fix it may be best to monitor the situation and if it completely fails consider the new board as suggested originally.

Thanks rav12
We're now in monitoring mode. Something I've been wondering about -- We have a Robertshaw t-stat. If the temp in the house dropped to 68 and the t-stat is at 69, let's say the t-stat sends the go signal, but the furnace doesn't start. Would the t-stat do nothing else as the temp in the house sunk lower and lower? I've been wondering if the t-stat gets feedback from the furnace, or if it's completely controlled by the temp in the house, and whether it tries to start the furnace again if it doesn't fire up. Thanks.

Most tstats have no feedback from the furnace so if it thinks the furnace should come on it will send the signal to the furnace but nothing more. It will then just sit there sending the signal. It has no knowledge whether the furnace is running or not. I have a more advanced climate control in my house that uses a serial digital protocol to communicate but even this has no feedback.

If you have a multimeter you can check at the tstat to see if the signal is going out. If the furnace does not fire then this could indicate a possible fault unless the furnace control board itself has some protection against short cycling.

Thanks rav12