Hello, I have a Day&Night (Carrier) model 518A048 furnace and A/C unit. Need some help to get it fixed...my wife just got laid off, and I'd really, really like to avoid calling someone in if possible...
The other day we noticed it got cool in the house and that, although the thermostat was calling for heat, the furnace was not blowing warm air. just blowing as if on a "low" fan setting.....Hmmmm what could this be? So I went out to the garage and the furnace circulation fan was running, but the furnace was not burning/running.
Also noticed that the fan kept running, even if the thermostat was set to "off".
After unplugging the 110V cord from the wall, I removed the cover from the control board assembly (the board to which the thermostat wires attach) and looked for fuses - there are none.
So I bypassed the safety interlock cover switch, now fan is running again, Tapped on the two relays w/screwdriver handle, to see if there was anything intermittent going on with them...nope. So I got out my DMM and checked to see if the transformer secondaries were putting out any voltage, and there was no 24V as expected.
I'm thinking, Aha! That's my problem, ye olde transformer has simply given up the ghost, after 23 years of faithful service....
I congratulate myself on my troubleshooting prowess, and order up a new transformer (luckily it has been warm and we could do without heat for a few days) and just installed it. Furnace behaved same way.
Checked the secondaries and I have a solid 25V.
Also set thermostat to call for AC, no response from the AC compressor. I let it run (fan running) for a few minutes, thinking that maybe the circuit needed to time out, reset, or something, and after running for a few minutes, I heard a faint "Pop" ...and then decide to check the transformer secondaries...dead.
So whatever it is, the transformer is not the cause of the problem (good thing I picked up a spare - I'm putting an inline fuse on the next one!)...
So, I am wondering what else it could be....I'm thinking that there is a good chance that the board is toast...and there is a dead short on it that is killing the transformer....Are these boards even available any more?
Any HVAC gurus out there who have experience with this equipment?