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Old 06-14-2009, 06:36 PM   #1
Dom
 
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Lennox A/C unit not starting


Let's start from the beginning. Last year, my Lennox unit (Model # HS20-261-2P, dated Dec 1990, charged with R-22) started making a badbearing noise and after about 2 weeks quit working (I was getting married and didn't have time to look at it). So we let it be and turned off the AC. Warm weather was over in about 2 weeks, and life was good again.

This spring, I had time to look at it and diagnosed that it was the fan motor making the noise. Replace that with a new one ($225). Now, everything work if I depress the contact switch on the unit itself (Both the compressor and the fan kick in). It does not however start when the thermostat says so.

Lennox A/C unit not starting-controlpanel.jpg

In the view of the side panel, The capacitor from the BLUE arrow (top) was added when I swapped the fan out (universal fan). When I push the contacts near the YELLOW arrow (middle), everything sounds fine and the fan spins, and the compressor hums and I can start feeling a change in temp of the lines going into the house. I'm getting 28 volts at the wires with the RED arrow (bottom) when the thermostat is set to cool, and 0v when the thermostat is off. No surprise there except that it's not working.

Lennox A/C unit not starting-inside-without-fan-motor.jpg

I traced the blue wires from the RED arrow on the side panel to the compressor. They each attached via wire nuts to a pair of wires running to the top of the compressor (the larger blue wires and the 2 smaller wires pointed to by the RED arrows again), running under a sealed (or at least siliconed) cap. I replaced the wire nuts (under the plastic cover on the side of the compressor), and nothing changed. It was a long shot, but I was slightly hopeful

The best guess I have is that the wires on the top lead to some sort of pressure switch. Does that mean I need a recharge. I doubt it was ever done before (I've owned the house 8.5 years, and the unit is from 1990). How do I know for sure?

If it does need a charge, how much is that typically, and is the age of the unit a concern. I can't really swing a replacement. Also, is there anything else it could be? Is there anything else I should check?

Thanks in advance

Dominique

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Old 06-14-2009, 06:55 PM   #2
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Lennox A/C unit not starting


Those wires on top of the compressor are for an thermal overload protector. Have replaced a few those. Do you have voltage across the 24v connections on the contactor. Another way to check is to bypass the overload, If it runs the overload is bad. However I do not recommend running the unit with the overload bypassed except for testing purposes.

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Old 06-14-2009, 06:58 PM   #3
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Lennox A/C unit not starting


sounds like you could have a bad contactor. From the pics i see no pressure switches. Test the contactor coil for ohms, see if the coil is open.
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Old 06-14-2009, 06:59 PM   #4
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Lennox A/C unit not starting


contactor should still pull in and fan should run even if ol is open
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Old 06-14-2009, 07:08 PM   #5
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Lennox A/C unit not starting


Won't pull in on this model if the overload is bad. It's wired in series with the contactor low voltage wiring. Check out the first pic, you can see the low voltage connected to the blue wire.

Last edited by SKIP4661; 06-14-2009 at 07:11 PM.
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Old 06-14-2009, 10:08 PM   #6
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Lennox A/C unit not starting


Well, that does make perfect sense. Since the fan went out last season, it may have caused it to overheat. And yes, it is wired in series. I'll stop by the store tomorrow if I can get out of work early (otherwise tuesday evening) and pick up a replacement. I'm assuming it's field replaceable with some silicone or glue or something. I'll post afterwards and let you know how it went.

Thanks

Dominique
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Old 08-16-2010, 03:15 PM   #7
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Lennox A/C unit not starting


Had a related issue with my Lennox HS29-024, thought it was the 35/5 dual cap, so I got a new one and replaced it (2nd time in ~10 years, life of unit), and still no go.

prodded the contactor switch which is spring loaded, and appears NO (normally open, with springs pressing outward), and fan and A/C went on fine.

Then checked the tstat lines and wasn't getting 24V (or anywhere near).

Went back inside and traced from tstat down (what a rat's nest we have, tstat lines flow through several walls, and split off at furnace for fans and humidifiers), and finally got the thing running.

What I haven't figured out is if there is supposed to be a reset switch for one or either of these sides of the contactor switch. Can't see any buttons to press. Seems like with the condensor being outside and the contactor not sealed so well, that relying on the low tstat vac to close the spring NO switches could be a reliability problem waiting to rear it's ugly head again.

I blew it off with compressed air, along with some spiderwebs in there, but am wondering if I should keep a spare connector handy, along with the normal 35/5 - 440 cap?

Thanks for any thoughts on this.
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Old 08-16-2010, 09:05 PM   #8
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Lennox A/C unit not starting


Normal for 24-V from t-stat to be all that is required to close the contactor. You won't find a reset switch as there isn't one. May find one on the transformer in the fan coil though.
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Old 08-16-2010, 11:30 PM   #9
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Lennox A/C unit not starting


All is well now, but I wonder what the glitch was that stopped it. Anyways, have traced 24 VAC from tstat through walls to furnace/blower and it's all good now. May replace the tstat since we never ever use the fancy features of this programmable honeywell, and I'd love to have a simpler one.

Thanks.

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Originally Posted by Haldog View Post
Normal for 24-V from t-stat to be all that is required to close the contactor. You won't find a reset switch as there isn't one. May find one on the transformer in the fan coil though.

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