HVAC - Furnace and fan question

masraum · 11-27-2009, 08:43 AM

I have a gas furnace. I think my fan is failing to come on every time when the heat comes on. The fan will come on if I turn the fan to "on" at the thermostat.

To me, that says that there's a relay going out (intermittent fan).

I'd think that replacing the relay should be relatively cheap and easy if there was somewhere that I could get the relay.

There is a fan that blows hot air out of the vent (out through the roof). I've gone up when the thermostat said that the heat was "on" but the fan was not on and found that the vent and a couple of spots on the furnace were very hot. I think that's a bad thing.

UNtil I get this fixed, I'm just going to run the fan 100% of the time. Any ideas, should I try to find the relay and replace it myself or should I just call someone?

I think the furnace is about 10 years old give or take a year.

turbo6bar · 11-27-2009, 08:59 AM

It depends on how old the furnace is. If the furnace is really old (20-25+ years old), you may have a mechanical relay for the blower. If the furnace is newer, it likely has a circuit board controlling the furnace and blower.

Do the burners light on cue?

Can you provide a picture of the furnace (covers removed)?
jurgen

MotoSook · 11-27-2009, 11:09 AM

If the flue blower (the one blowing out the roof vent) does not come one and stay one, the main blower won't turn on. It's a safety detent.

You might want to clean contacts and check things over to be sure everything is in place and there are no obvious signs of loose connectors, etc.

Get down there and test with a multimeter. Every furnace has a wiring diagram on it. If it's beyond your ability, get a guy to come in and test things out. It might only cost you the cost of a house call, but then you'd know for sure what's broken.

If you are not freezing your butt off in texas, you might be able to track down the part and replace it yourself.

masraum · 11-27-2009, 12:11 PM

We've replaced the furnace since we've been in the house. So it's in the 8-12 year old range. I think it's 10.

Yes, the burners come on (as far as I know, like I said, the unit gets hot, and if I kick on the fan manually, the air that comes out is hot immediately as if the burners have been on.

The couple of times I that I went up to see what was going on, the flue blower was definitely blowing.

It seems to me (without getting up there with a multimeter) that the furnace is working except for the fan. It seems like only the fan isn't coming on.

I'll get up there and look around with the heat off and with a multimeter. I'll see if I can find the wiring diagram.

Thanks

dad911 · 11-27-2009, 12:17 PM

If it's only 10 years old, there is probably a wiring diagram inside the cover.

pete3799 · pete3799's Garage

Check on the furnace and see if you have a manual-auto switch on it.
You say you can turn the blower on (manualy) at the thermostat. I can only do that when the furnace is set on manual.

turbo6bar · 11-27-2009, 12:24 PM

Sounds like the low or medium speed on the blower motor is kaput. When you flip the fan switch to ON, you're triggering a higher speed that's used for cooling.

If you can read the schematic and read the wiring, try changing the speed. You're looking for the male spade on your circuit board marked HEAT. You'll want to throw another color (representing another motor speed) onto that spade. If the furnace then works properly, then you've isolated the problem to the motor. The blower motor can be changed with little trouble assuming the blower wheel isn't frozen to the motor shaft (doubt it, though). jurgen

rick-l · 11-27-2009, 03:40 PM

This is how it is supposed to work

The inducer fan comes on
the igniter heats up
the gas valve opens
all the burners light
the flame sensor detects a flame
the plenum heats up
at a set temperature the fan comes on.

does #4 (AND #5) happen?

does the controll have a blink code self test?

turbo6bar · 11-27-2009, 04:37 PM

5 assumes ignition vs standing pilot, correct?

masraum · 11-27-2009, 07:55 PM

Quote:

does #4 (AND #5) happen?

does the controll have a blink code self test?

Well, if the system says the heat is on, but the fan is not... if I kick the fan on, hot air comes out of the vents. So I can't say for certain, but I think so.

Blink code self test? Doubt it, but I'll check.

turbo6bar · 11-28-2009, 04:37 AM

Actually, he's referring to the sequence of events when your furnace is turning on. You're skipping steps to properly troubleshoot this. Could be your circuit board. Could be your blower motor. Could be the flame sensor, if you have an igniter. However, if your flame sensor is bad, the gas valve will close and the burners will go out.

You could pass over the troubleshooting and start replacing parts, hoping to get the right bit. You can dig in and start properly diagnosing the problem. OR, just call a repair guy.

turbo6bar · 11-28-2009, 04:40 AM

If you let the furnace go through a cycle, and it does not work properly, the circuit board will flash an LED corresponding to an error code. Count the number of blinks and check the manual or labeling on the inside of the furnace door.

masraum · 11-28-2009, 10:53 AM

Quote:

Originally Posted by turbo6bar

Actually, he's referring to the sequence of events when your furnace is turning on. You're skipping steps to properly troubleshoot this. Could be your circuit board. Could be your blower motor. Could be the flame sensor, if you have an igniter. However, if your flame sensor is bad, the gas valve will close and the burners will go out.

You could pass over the troubleshooting and start replacing parts, hoping to get the right bit. You can dig in and start properly diagnosing the problem. OR, just call a repair guy.

Right, I understood, and am grateful for the extra info. It's valuable.

masraum · 11-28-2009, 10:54 AM

Quote:

Originally Posted by turbo6bar

If you let the furnace go through a cycle, and it does not work properly, the circuit board will flash an LED corresponding to an error code. Count the number of blinks and check the manual or labeling on the inside of the furnace door.

Yeah, I'm hoping it has that feature. Actually, I think I may have the manual that came with the furnace when it was installed, so I'm going to try to find that today.

masraum · 11-30-2009, 02:09 PM

You guys were right, my furnace does have a control module/circuit board with a light that blinks. Right now it's blinking dim-bright-dim-bright which means that it's fine.

I also see the wires for the high speed (cool) and low speed (heat) blower circuits. And I found the instruction manual that details out the steps that Rick-I listed in his post. I'm going to monitor the system tonight so I can tell when it kicks on and off to see if it's working properly or not.

Thanks for all of the help.

masraum · 11-30-2009, 02:45 PM

Yep, just checked. The blower does not come on when the heat does.

So then I swapped the high and low blower circuits and kicked the heat on again and the blower still did not come on.

I swapped the blower circuits back and kicked the fan to ON and it's running fine.

So based on the blinking light (that indicates normal based on both the manual an the sticker on the outside of the furnace) and the fact that the blower never comes on by itself with the heat, there is something wrong.

Is it two different blowers or I assume a multispeed blower. Based on the fact that the blower will come on in the ON position, the blower should be good.

When I have the fan set to ON, the house stays warm. The house also stayed cool all summer.

I'm guessing that it's not a replaceable relay, but probably something built into the controller circuit board which means I'll have to get that replaced. Ugh.

Any new thoughts?

Brian 162 · 11-30-2009, 04:30 PM

The problem is the relay that brings on the motor which is on the circuit board is shot. You need to replace the circuit board.
If you have a multi-meter, cycle the furnace with no motor leads on the heating terminal on the circuit board. It should show 120 volts after 60-90 seconds. If no voltage is present after that time that tells me the board needs replacing.
You have proved by turning the fan on and changing speeds that the motor is fine.

pete3799 · pete3799's Garage

There should be a temp. probe mounted on the hot air plenum. This is what turns the fan on automaticly when it reaches the preset temp. Mine has an auto/manual setting on it. Check that it's set on auto. If you take the cover off you should find it has two adjustments one for
on temp. and one for off temp. I think this is where your problem is. When the furnace kicks on and runs for a while the air heats the probe and when it reaches the preset temp. it turns the fan on. The fan should continue to run after the furnace has shut off untill the probe cools down to the cut out temp.

NY65912 · 11-30-2009, 05:01 PM

it's called a fan limit switch.

masraum · 08:37 PM

OK, according to the directions

Quote:

1 Thermostat calls for heat. Control module verifies that limit switches and pressure switch are in normally closed position

2 Draft inducer relay is energized. Draft induce motor starts and pressure switch normally open contacts close, energizing the gas control

3 Gas control provides power to igniter and pilot valve to light pilot. After proving pilot-flame, the main valve opens and igniter turns off

4 Control module energizes main blower in 75 seconds

5 After the thermostat is satisfied, gas control is de-energized. After a 5 socend post-purge, draft inducer de-energizes and returns pressure switch to normally closed position.

6 Main blower remains energized for 180 seconds.

NOTE: This furnace will lock-out if the main blower fails in heating mode. During lock-out, the control module's diagnostic light signal will be 4 flashes.

It's definitely not doing any flashes, 1, 2, 3 or 4. So that's good, I guess. The thing thinks it's working fine.

Quote:

Originally Posted by Brian 162

The problem is the relay that brings on the motor which is on the circuit board is shot. You need to replace the circuit board.
If you have a multi-meter, cycle the furnace with no motor leads on the heating terminal on the circuit board. It should show 120 volts after 60-90 seconds. If no voltage is present after that time that tells me the board needs replacing.
You have proved by turning the fan on and changing speeds that the motor is fine.

I've got a couple of multimeters. I'll check to see if the terminal is getting power after 75 seconds.

Quote:

Originally Posted by pete3799

There should be a temp. probe mounted on the hot air plenum. This is what turns the fan on automaticly when it reaches the preset temp. Mine has an auto/manual setting on it. Check that it's set on auto. If you take the cover off you should find it has two adjustments one for
on temp. and one for off temp. I think this is where your problem is. When the furnace kicks on and runs for a while the air heats the probe and when it reaches the preset temp. it turns the fan on. The fan should continue to run after the furnace has shut off untill the probe cools down to the cut out temp.

Hmm, I do see a part in the exploded diagram called the primary limit switch which looks like a temp probe that sticks into the furnace.

Thanks all.