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HVAC advice needed

D

daveymark

Lifer
An HVAC repair tech is coming to look at the problem, I figured I'd ask here in case I might be able to cancel the appointment.

I noticed a few days ago that cold air is blowing when the furnace is on.
I went downstairs in the basement to check out the furnace. I took the panel off, turned the furnace off, then turned it back on. the little ignition coli heated up to a bright red, as soon as it sparked or clicked, the gas lit up and started blowing heat. heat was coming out of all the vents in the house then. about 20 minutes later, the hot air started going cold, and sure enough, I went downstairs and there was no fire in the furnace, but the blower was blowin air.

anyone have any ideas on what the issue might be?
 
Does the blower ever shut off?

Some furnaces have a two stage blower that stays on to blow all the warm air out of the ductwork. You could also have the thermostat (blower) set on the "on" mode instead of the "auto" mode.
 
Originally posted by: bctbct
Does the blower ever shut off?

Some furnaces have a two stage blower that stays on to blow all the warm air out of the ductwork. You could also have the thermostat (blower) set on the "on" mode instead of the "auto" mode.
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my guess too...

How old is the furnace, what brand and type is it?

You have a service guy coming out on a Sunday?
 
..therostat/filter/airflow sensor. Is this an old heater? does it have a fault logger port? The more modern HVAC's have OBD (on board diag's) and fault codes can be downloaded for troubleshooting.
 
Originally posted by: IGBT
..therostat/filter/airflow sensor. Is this an old heater? does it have a fault logger port? The more modern HVAC's have OBD (on board diag's) and fault codes can be downloaded for troubleshooting.
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Thats they way mne is. It uses an LED and the troubeshooting codes are listed inside the inspection panel.
 
You realize that the furnace will still blow once the burners go off. It is trying to get all the efficiency it can out of the heat exchanger.
 
Post heat cycles increase efficiency but if a relay or TDR is acting up the blower can just run. The thermostat can be acting up too! Check between red and green and make sure you read high ohms when the blower is supposed to be OFF. If not, your blower is being called on by the thermostat.
 
There is a control inside that tells the fan when to kick on and when to turn off. When the flames initially start, the fan doesn't blow until the temperature hits a certain level. The fan continues to blow until the plenum cools to a certain level. It's a 30 second fix. Bummer that you called for a repairman first.

Oh, and if you're having trouble with the part, get a can of electrical cleaner. Those parts can cost a fortune (relatively), but there's not much to them. Three times I've been able to fix furnaces simply by cleaning that part.

(it's on the tip of my tongue, but I can't remember the name of the part...) I'm 95% certain that this is the problem though.
 
Originally posted by: DrPizza
There is a control inside that tells the fan when to kick on and when to turn off. When the flames initially start, the fan doesn't blow until the temperature hits a certain level. The fan continues to blow until the plenum cools to a certain level. It's a 30 second fix. Bummer that you called for a repairman first.

Oh, and if you're having trouble with the part, get a can of electrical cleaner. Those parts can cost a fortune (relatively), but there's not much to them. Three times I've been able to fix furnaces simply by cleaning that part.

(it's on the tip of my tongue, but I can't remember the name of the part...) I'm 95% certain that this is the problem though.
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Snap disk switch like this one
 
From how it works
The limit switch is a safety control switch and is located on the furnace just below the plenum. If the plenum gets too hot, the limit switch shuts off the burner. It also shuts off the blower when the temperature drops to a certain level after the burner has shut off. If the blower runs continuously, either the blower control on the thermostat has been set to the ON position, or the limit control switch needs adjustment.

To determine the problem, check the thermostat. If the blower control has been set to ON, change it to AUTO; if blower control is already on AUTO, the limit switch needs adjusting. To do this, remove the control's cover and find the toothed dial underneath. One side is marked LIMIT; don't touch this side. The other side is marked FAN. There are two pointers on the fan side; the blower turns on at the upper pointer setting and shuts off at the lower pointer setting. Pointers should be set about 25 degrees apart. Set the upper pointer at about 115 degrees Fahrenheit and the lower one at about 90 degrees Fahrenheit.
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Originally posted by: DrPizza
There is a control inside that tells the fan when to kick on and when to turn off. When the flames initially start, the fan doesn't blow until the temperature hits a certain level. The fan continues to blow until the plenum cools to a certain level. It's a 30 second fix. Bummer that you called for a repairman first.

Oh, and if you're having trouble with the part, get a can of electrical cleaner. Those parts can cost a fortune (relatively), but there's not much to them. Three times I've been able to fix furnaces simply by cleaning that part.

(it's on the tip of my tongue, but I can't remember the name of the part...) I'm 95% certain that this is the problem though.
Click to expand...

flame sensor? The HVAC tech is coming on monday
 
Originally posted by: Minerva
Originally posted by: DrPizza
There is a control inside that tells the fan when to kick on and when to turn off. When the flames initially start, the fan doesn't blow until the temperature hits a certain level. The fan continues to blow until the plenum cools to a certain level. It's a 30 second fix. Bummer that you called for a repairman first.

Oh, and if you're having trouble with the part, get a can of electrical cleaner. Those parts can cost a fortune (relatively), but there's not much to them. Three times I've been able to fix furnaces simply by cleaning that part.

(it's on the tip of my tongue, but I can't remember the name of the part...) I'm 95% certain that this is the problem though.
Click to expand...


Snap disk switch like this one
Click to expand...

Are those pre-set without the ability to adjust?
Oh, I see from the title that it's the high limit... That'll turn off the gas if the plenum gets too hot.

Daveymark, I'd take pics and post them for you, but I don't have a forced air furnace in this house. If you look inside where most of the controls are, you can trace back to it from the blower motor. The ones I've seen were enclosed and required a cover be taken off to get to the control part. I'll look for a pic of a typical one as well as the name..
 
"fan limit control"
Minerva, are there fan limit controls like that as well? I can understand the high limit being preset that way; it's a safety device to keep the house from burning down.
 
Yes a high limit is there in case the fan motor is locked. If the burner is firing with no airflow the heat exchanger will bust wide open! The high limit will cut power and shut down the control side. Fuel valves ALWAYS fail closed so this squelches the fire.

If a high limit opens the system should always be checked because even a slight crack in the heat exchanger is bad particularly on gas burners. With oil it's pretty obvious if the exchanger is cracked. (WTF is that smell everyone will cry when the box rumbles! hehe)
 
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