Furnace Q

[Teken]

This is going to sound pretty retarded so please bare with me. But, does a furnace at times produce different levels of heated air when it operates?

There have been a few times this week that when I place my hand over the vents the air coming out is cooler than I would expect?

Now, I know there is a pre stage, and also a cycle down where the furnace keeps blowing out air and no heat is being dispensed. This is NOT the case of the two.

This is when the furnace is running at 100% capacity and operating.

Can someone who is a HVAC pro please chime in and let me know the 411 as to what the hell is happening . . .

 

[rickairmedic]

The limit switch in the heat exchanger is shutting off the burners when the heat exchanger hits a certain temperature . Should the thermostat temperature not be met yet the furnace will continue to throttle the burners on and off as it runs to keep from overheating the heat exchanger .


So in short yes you may get slightly cooler air at times as it is running .


Rick

 

[brewchief]

This is going to sound pretty retarded so please bare with me. But, does a furnace at times produce different levels of heated air when it operates?

There have been a few times this week that when I place my hand over the vents the air coming out is cooler than I would expect?

Now, I know there is a pre stage, and also a cycle down where the furnace keeps blowing out air and no heat is being dispensed. This is NOT the case of the two.

This is when the furnace is running at 100% capacity and operating.

Can someone who is a HVAC pro please chime in and let me know the 411 as to what the hell is happening . . .

What type of furnace? Gas? Heat pump? Electric?

Multi stage gas furnaces are becoming pretty popular around here along with heat pumps using gas or oil backup, those along with staged electric heat will cause the symptoms you have when operating normally.

Brand and model# of your equipment will help out a bunch here.

 

[930dreamer]

The heat pump we have does this in Emergency Heat. When the outside temp is below a set temp the full furnace kicks in. I don't like tepid air temps in the winter.

 

[Teken]

One would think it would be easy to simply state the model number. But I looked high and low on the door, inside the unit, in both top and bottom access doors. The closet thing I found is this -> G43UF-36B-070-08 it is also a Lennox gas fired furnace.

I don't believe this unit is a dual stage unit but someone who is a pro could advise either way.

I did read the replies from Rick / Brewchief. What I am not clear on is are you guys saying that this furnace will output varied heat output like a oven?

 

[rickairmedic]

Ok to simplify it . There is a temperature switch inside the heat exchanger of the furnace . Lets say this switch is set to open at 200 degrees and close at 175 degrees . What this does is when the heat exchanger temperature hits 200 * the switch will open and turn the burners off while the fan is still running . When the heat exchanger temperature gets down to 175* it will turn the burners back on . This cycle will continue until the temperature you have the thermostat set at is reached . Then the burners shut off and the fan continues to run until a second switch says the heat exchanger is cool enough to shut the fan off .

Rick

 

[Teken]

Ok to simplify it . There is a temperature switch inside the heat exchanger of the furnace . Lets say this switch is set to open at 200 degrees and close at 175 degrees . What this does is when the heat exchanger temperature hits 200 * the switch will open and turn the burners off while the fan is still running . When the heat exchanger temperature gets down to 175* it will turn the burners back on . This cycle will continue until the temperature you have the thermostat set at is reached . Then the burners shut off and the fan continues to run until a second switch says the heat exchanger is cool enough to shut the fan off .

Rick

Rick,

As always I thank you for your insight and consideration . . .

 

[mitymouse]

The lennox
g43 doesnt use a high limit like this, its a make or break through the board..if you are tripping a high limit it is not a good thing and should result in a trouble code on the two diagnostic leds see in the sight glass of the blower door

 

[mitymouse]

start by checking filters and for a plugged a coil, both can limit heat transfer, if to much heat restriction occurs it will trip the "high limit" due to the fact your not getting enough heat transfer from the exchangers.
If it is a hit and miss the watchdog part of the board should indicate a problem,if it happens more often it will go into a hard lock out which equals no heat

 

[brewchief]

One would think it would be easy to simply state the model number. But I looked high and low on the door, inside the unit, in both top and bottom access doors. The closet thing I found is this -> G43UF-36B-070-08 it is also a Lennox gas fired furnace.

I don't believe this unit is a dual stage unit but someone who is a pro could advise either way.

I did read the replies from Rick / Brewchief. What I am not clear on is are you guys saying that this furnace will output varied heat output like a oven?

G43UF-36B-070-08 is the model number, it's a basic 90+% single stage furnace, 3 ton blower 70,000 btu input(well actually 66k).

It should not hit the limit during operation, if it is there is a problem.

What kind of filters do you use and are they clean? Avoid the high dollar 3M ones, they are very restrictive even when new.

 

[Teken]

G43UF-36B-070-08 is the model number, it's a basic 90+% single stage furnace, 3 ton blower 70,000 btu input(well actually 66k).

It should not hit the limit during operation, if it is there is a problem.

What kind of filters do you use and are they clean? Avoid the high dollar 3M ones, they are very restrictive even when new.

I just changed the filter a few days ago and its still nice and clean. I am using those cheap filters that come in a pack of three for $4.**.

I was initially going to use one of those really hi tech hepa filters but for the price of just one. I could buy and change 50 of these filters over and over and come out ahead.

I really appreciate you looking up the model and the tech info. The EnerGuide says this furnace is a 92% efficient model. I was on the fence in spending more for a 96% model but many people said the ROI wasn't even worth it.

 

[jvitez]

When the furnace first fires and the fan kicks in, all the air in the ducts are at room temp. The air coming out of any floor grill will feel cool at first, then gradually warm up until it hits a steady state temp, then cool off once the burner shuts off. Did you perhaps feel the air temp during the initial firing stage? The heat exchanger and plenum are at room temp upon initial firing, then they absorb heat until they reach a steady state too. There may be a small temp rise in the discharge air during this time, but I'm not sure anyone's hand could feel the difference.

If during this steady state period you're feeling temp variations, something doesn't sound right in a single stage furnace. Check the circuit board for flashing LED's, check the fan compartment for dirt/construction debris, and if everything seems OK it might be time to call a heating contractor to check it out. Not unreasonable this time of year in any case.

 

[mrobins297aaa]

its not normal for the burner to cycle during a call for heat.

easy way to check is to turn the thermostat way up and sit and watch the burner, it should run continously.........if the burner shuts down before the thermostat is satisified you have a problem.

 

[Teken]

When the furnace first fires and the fan kicks in, all the air in the ducts are at room temp. The air coming out of any floor grill will feel cool at first, then gradually warm up until it hits a steady state temp, then cool off once the burner shuts off. Did you perhaps feel the air temp during the initial firing stage? The heat exchanger and plenum are at room temp upon initial firing, then they absorb heat until they reach a steady state too. There may be a small temp rise in the discharge air during this time, but I'm not sure anyone's hand could feel the difference.

If during this steady state period you're feeling temp variations, something doesn't sound right in a single stage furnace. Check the circuit board for flashing LED's, check the fan compartment for dirt/construction debris, and if everything seems OK it might be time to call a heating contractor to check it out. Not unreasonable this time of year in any case.

its not normal for the burner to cycle during a call for heat.

easy way to check is to turn the thermostat way up and sit and watch the burner, it should run continously.........if the burner shuts down before the thermostat is satisified you have a problem.

I will check tomorrow night when I have more time. Much thanks for all the insight and follow up.

Teken . . .