Question on gas heat unit.

[horseman1]

Natural gas forced air, all outside unit, digital thermostat. Carrier
exh. fan will start, hot foot will glow, gas valve opens, burners start, main blowers runs, after a short period the main blower will stop, along with the burners, exh. fan still runs. Sometime the unit will re lite and run and sometimes the exh. fan will jsut run continious until I cut power.
Any suggestions??
I'm a member but my hard drive got zapped.

Thanks,

[mdshunk]

Very common problem with hot surface ignition units. I'm glad someone posted with this problem. The unit only tries for fire a certain number of times, before it "locks out" and just runs the main blower to alert the homeowner that there's a problem.

Since the main burner lights off, an assumption can be made that the conbustion blower fan or (or draft inducer fan) pressure switch is working. It seems that the flame sensor is likely what's causing your problem. I say this because you say that the main burner does light for a few seconds. Furnace controllers delay for a few seconds after the main burner lights before checking the flame sensor to see if there's really fire.

You should remove the flame sensor. It's a little "wire rod" in the combustion chamber connected with just one wire. It's generally held in with just one screw. Clean the flame sensor rod with steel wool or fine emery cloth. Replace the flame sensor and try the unit again. This flame rod is some special kind of steel, so you can't really screw it up or break it.

Your control module (under the bottom plate of a Carrier unit) has a light on it that flashed an error code that tells you exactly why the unit shut down. The "flash numers" are decoded on a sticker placed on the inside of the furnace door. Check the flashes and see why the furnace shut down. I'd almost bet "flame failure", and cleaning the flame rod will cure your ill.

If you have your hot surface ignition glow plug out for curosity or anything, be VERRRY gentle with it. They're ceramic, and fracture easily. They can also be a bit expensive if you break it (50 bucks).

Try this and see what you get. If no luck, repost what your error flashes are. Model and Serial numbers helpful for further help.

[Wgoodrich]

mdshunk, sounds like you have been there done that one a time or two.

Great reply

Wg

[horseman1]

I am very sorry, the unit is a YORK, the one in my shop is a carrier.... it's rainning now, but I will look at the flame sensorwhen it quits. The last time I looked at it the connector was rusty, but the probe was a silverish color (a few weeks ago). The hot foot seems to be working correct I replaced it about 2 yrs ago, an dput a new pc board and controller a little before that.

Thanks again and sorry for the mix up.

[horseman1]

I am very sorry, the unit is a YORK, the one in my shop is a carrier.... it's rainning now, but I will look at the flame sensorwhen it quits. The last time I looked at it the connector was rusty, but the probe was a silverish color (a few weeks ago). The hot foot seems to be working correct I replaced it about 2 yrs ago, an dput a new pc board and controller a little before that.
The unit seems to run for 5 mins or so befor the main blower cuts out. I did hear the burners refire and the blower came on a second time and then after a few mins. the blower went out again and I shut the system down.

Thanks again and sorry for the mix up.

[Unregistered]

With rectification flame sensing, it is very important to have clean, tight connections. Also, if your unit is a York, it may have a pilot burner bar for flame sensing that the ignitor lights, and then a main burner. Sometimes time little holes get crudded up on the long pilot bar. The main burner will light off, but the flame sensor may not be getting a good flame from the pilot bar. Physically observe to make sure that the flame rod is in some kind of flame. Also, since this is an outdoor unit, make sure that the covers are on tightly, so that ambient wind gusts aren't blowing the flames all goofy, causing intermittant flame sensing. A good ground from the module to the burner is also VERRY important for flame sensing. Since this is an outdoor unit, inspect the cleanliness and tightness of the grounds on the module and the burner. Crimp on new terminals as necessary. Sometimes, adding an extra ground between one of the module's ground terminals and good metal on the burner assembly will provide more reliable operation. York generally used a Johnson ignition module, which I think was kind of flaky in an outdoor application. I've had better luck replacing them with the Honeywell or White Rodgers Universal Hot Surface Ignition modules (only if the module is determined to be bad).