Lennox G20 heat issue

Problems related to residential installations.

Lennox G20 heat issue

Postby threejeeps » Tue Feb 12, 2013 3:11 pm

I have a 15 yo lennox G20, 100,000BTU furnace in which the blower won't come on after the furnace fires when thermostat is in AUTO.

Here is the - thermostat in Auto, calls for heat. Damper door is open, gas ignites and fires full for maybe 10 seconds, then shuts down.

The other - Fan is manually ON (from thermostat switch) and running, thermostat calls for heat, furnace fires and continue to fire until temp setpoint is reached. Burner extinguishes and damper door closes and fan continues to run...at some point the damper door opens.....next call for heat the cycle repeats.

I have checked the following switches and all appear to work - damper door position, pressure switch, exchanger overtemp switch, roll out switch.

My thought is that the flame sensor is not operating, but I don't understand why the furnace fires when the fan is in the manually ON position?

Any suggestions as to what may be the problem or next troubleshooting steps?

Appreciate your time and suggestions

-Bill
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- Lennox G20 heat issue

Postby Freon » Tue Feb 12, 2013 3:54 pm

You have a very curious problem. Let's try a simple test. Go to the furnace and where the thermostat wires connect to the furnace control board, label each wire CAREFULLY as to which control board terminal it connects. You need this information to reconnect them.

Now turn off the power to the furnace. Carefully disconnect all thermostat wires connected to the furnace control board (the wires you labeled). Now place a jumper wire between W and R. If you do not see W and R please post back. This jumper will call for heat just as the thermostat would. Now turn on the power to the furnace realizing it will IMMEDIATELY start the heating cycle.

Let us know if the flames extinguish after 10 seconds as before or if the furnace runs normally... flames heat the heat exchanger for a minute or so then the main blower starts.

After the test, TURN OFF the furnace power and reconnect the thermostat wires.

Let us know what happens.
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- Lennox G20 heat issue

Postby threejeeps » Wed Feb 13, 2013 10:55 pm

Freon -
> You have a very curious problem. Let's try a simple test. Go to the furnace
> and where the thermostat wires connect to the furnace control board, label
> each wire CAREFULLY as to which control board terminal it connects. You
> need this information to reconnect them.
>
> Now turn off the power to the furnace. Carefully disconnect all thermostat
> wires connected to the furnace control board (the wires you labeled). Now
> place a jumper wire between W and R. If you do not see W and R please post
> back. This jumper will call for heat just as the thermostat would. Now turn
> on the power to the furnace realizing it will IMMEDIATELY start the heating
> cycle.
>
> Let us know if the flames extinguish after 10 seconds as before or if the
> furnace runs normally... flames heat the heat exchanger for a minute or so
> then the main blower starts.
>
> After the test, TURN OFF the furnace power and reconnect the thermostat
> wires.
>
> Let us know what happens.
I won't get back to this for a couple of days, will respond with results asap.
I see that you are removing the thermostat connections and jumpering the contact that calls for heat.
Am I correct in assuming that blower Auto control is done locally at the furnace, and blower ON setting at the thermostat over-rides the Auto function at the furnace? e.g. Thermostat blower ON closes a contact in the thermostat which basically applies 24vdc to the blower signal on the furncce control board, and when the thermostat is in AUTO, the blower relay contact at the thermostat is open, permitting the blower local furnace controller board to operate?

Thanks
J
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- Lennox G20 heat issue

Postby Freon » Thu Feb 14, 2013 12:26 pm

Blower ON at the thermostat directly powers the G terminal at the furnace and that's the blower. When the Fan is set to Auto and you call for heat the thermostat directly powers the W terminal and then the control board at the furnace does the blower control.

By removing the thermostat from the system via my test, we can see if the problem is at the furnace or at the thermostat and its wiring to the furnace.
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- Lennox G20 heat issue

Postby threejeeps » Mon Feb 18, 2013 11:24 pm

Freon -
> You have a very curious problem. Let's try a simple test. Go to the furnace
> and where the thermostat wires connect to the furnace control board, label
> each wire CAREFULLY as to which control board terminal it connects. You
> need this information to reconnect them.
>
> Now turn off the power to the furnace. Carefully disconnect all thermostat
> wires connected to the furnace control board (the wires you labeled). Now
> place a jumper wire between W and R. If you do not see W and R please post
> back. This jumper will call for heat just as the thermostat would. Now turn
> on the power to the furnace realizing it will IMMEDIATELY start the heating
> cycle.
>
> Let us know if the flames extinguish after 10 seconds as before or if the
> furnace runs normally... flames heat the heat exchanger for a minute or so
> then the main blower starts.
>
> After the test, TURN OFF the furnace power and reconnect the thermostat
> wires.
>
> Let us know what happens.

So here is what -
So I did the following and the results are -
1) Turned furnace off using on/off switch mounted on furnace
2) Disconnected thermostat by prying it off of the mounting frame
3) jumpered R and G ( Blower test) at the BCC terminal strip
4) Turned furnace power ON
- observed blower running (note - I *believe* have a single speed blower)
5) Turned power to furnace OFF
6) Removed jumper R-G
7) Jumpered R - W at the BCC terminal strip.
8) Turned power ON to furnace, observed following -
a) Damper motor rotated damper door to full open ( full open at about t+5s)
b) Pilot ignitor began sparking - about 4-5 times
c) Pilot valve opens and pilot ignites (about t+7s)
d) Pilot fires for approximately 3-5 seconds (ends at t+10s)
e) Main gas valve opens and burners fire fully (about t+13s)
f) Furnace continues to fire....at about the 50 second point (from when I powered the furnace ON) I hear a relay click (at t+50s)
g) Furnace continues to fire.....at t+120s I turn the power off since the blower has not started (as I assumed it should)

Since the blower did not start, I tried the experiment again, and reproduced 8a-8g and noted the -
There was never any 120VAC across N-H (Neutral to Blower Heat connection)
There was never any DC control voltage (e.g. 24vdc) between T (connected to T on BCC also connected to TR on Robertshaw controller) and V (V on BCC which is also connected to TR on Robertshaw controller). I measured 0.5 VDC between T & V the entire time.

Prior to doing all of this, I reseated the connections on the Front of the BCC, e.g. Limit, W, 24v, T, and COM.

I am not quite sure what to make of this behavior. I suspect the BCC but according to the troubleshooting chart I should 'check wiring'.
(as an aside, how can I determine the bcc number for my furnace without removing the existing board?)

I found the schematic for my unit tucked into a pocket on the inside frame. I need to study it a bit. I cant figure out how to attach a .jpg file here...
-John
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- Lennox G20 heat issue

Postby threejeeps » Mon Feb 18, 2013 11:28 pm

What is it with this web site that when I type most punctuation marks, such as a dash, question mark, ampersand, percent sign, and a few more, that they are changed to asterisks (I'd type a few question marks here but they would be converted to asterisks...lol)
-J
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- Lennox G20 heat issue

Postby Freon » Tue Feb 19, 2013 9:31 am

"f) Furnace continues to fire....at about the 50 second point (from when I powered the furnace ON) I hear a relay click (at t+50s)"

I have a feeling this click is the fan relay. See if you can find it on the control board. It is possible the 120 volt leads to the relay on the printed circuit board may be loose... bad solder joint. But your test does indicate the blower relay is the cause. Since you hear the click it seems the coil is being energized. So either the 120 volts on the line side of the relay (power in) is not there or the relay contacts are bad and there's not 120 volts on the load side (power out to blower).

Do not use dashes, slashes or question marks. Due to spammers these punctuation marks will cause an * do be displayed.
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- Lennox G20 heat issue

Postby threejeeps » Tue Feb 19, 2013 12:19 pm

Freon -
> "f) Furnace continues to fire....at about the 50 second point (from
> when I powered the furnace ON) I hear a relay click (at t+50s)"
>
> I have a feeling this click is the fan relay. See if you can find it on the
> control board. It is possible the 120 volt leads to the relay on the
> printed circuit board may be loose... bad solder joint. But your test does
> indicate the blower relay is the cause. Since you hear the click it seems
> the coil is being energized. So either the 120 volts on the line side of
> the relay (power in) is not there or the relay contacts are bad and there's
> not 120 volts on the load side (power out to blower).
>
> Do not use dashes, slashes or question marks. Due to spammers these
> punctuation marks will cause an * do be displayed.

thank you for your insight. I am somewhat in agreement, but, what prevents me from being 100% sure it is related to the BCC, is the fact that I can run the blower by jumping the R and G terminals. I am assuming that there is only one blower relay. The 24vdc control voltage could come from two differnt logic sources (e.g. manual blower on/off, and local bcc blower control on/off). The 'line side' (120vac through the relay contacts) feed to the relay contacts to the blower motor would only come from one source (and only one circuit path on the bcc).

The Figure from the lennon manual shows that the time delay relay, K36, feeds the blower red (heat) wire via contact K36_1, and the relay K3 feeds the blower black (cool) wire via contact K3_1. Soooo there _are_ two separate relays and K3 is infact controlled by the Blower Manual On (connected to G) at the thermostat. So now I am more conviced that the K36 relay contacts are not working properly. I've worked with relays for over 30 yrs and on only one occasion have ever seen relay contact fail....switching and inductive DC load that was not properly snubbed, so I find it weird that in switching an AC load, the contacts would get carboned up....I doubt dirt b/c I assume they are sealed - or should be....and the contacts should be properly sized so there is not detoriation due to excessive current, although the designer may have not considered 6xto10x FLA in rush at startup....

So, how can I upload a picture here (question mark)
Wish I had the schematic of the bcc. Know where I can get one (question mark)

A quick Google search seems to indicate that, there is one replacement - bcc3 which is backwards compatible with bcc2 and bcc1 and variations. Can you confirm (question mark)
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- Lennox G20 heat issue

Postby Freon » Tue Feb 19, 2013 11:33 pm

There will be 2 relays as you have discovered. One will be in the heating circuit with the time delay and the other will be energized by the G terminal when powered. The relay you want to look closely at is the one you hear click at the t+50 seconds. Look for a bad connection on the PC board or elsewhere.

Check the line side of the relay for 120 or 240 volts AC (depending on your motor). If you have voltage on the line side of the relay then you know the relay is the problem. If there's no voltage at the line side, keep back tracing until you find out why there's no voltage. Any visible fuses.

You have the control board make and model and can find a replacement if you determine the relay is beyond repair. Good diagnosis work. If you're concerned, call a local HVAC supply house like Johnstone and check the replacement to be as you suspect.
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- Lennox G20 heat issue

Postby threejeeps » Thu Feb 28, 2013 9:12 pm

Freon -
> There will be 2 relays as you have discovered. One will be in the heating
> circuit with the time delay and the other will be energized by the G
> terminal when powered. The relay you want to look closely at is the one you
> hear click at the t+50 seconds. Look for a bad connection on the PC board
> or elsewhere.
>
> Check the line side of the relay for 120 or 240 volts AC (depending on your
> motor). If you have voltage on the line side of the relay then you know the
> relay is the problem. If there's no voltage at the line side, keep back
> tracing until you find out why there's no voltage. Any visible fuses.

Well this didn't turn out as planned...got a replacement board and the furnace behaves the exact same way. I hear the relay click at around the t+50 sec mark but no blower. I was short on time so I didnt check the voltage on the red 'H' lead coming off of the BCC, which should be energized with 120vac when the relay clicks...
So I am scratching my head at this point.....Can rule out the BCC but now what? bad motor winding? Is the start capacitor a suspect?- I can't figure out if it is in the start circuit for the Heat line or the Cool line...*? Suggestions? Other things to check???

-J
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