No A/C


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Old 05-28-11, 06:44 AM
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No A/C

Hello everyone, I'm new to this forum & had no idea it was this LARGE! Just about anything you can think of is covered here. Maybe I can put my two cents in sometime being a retired finish/trim carpenter. Anywho, my problems lie in the A/C unit (central) at my campsite. This year when I turned the thermostat down to start the a/c the only thing I got was a hum at the furnace. I also tried to run just the fan mtr on the thermostat & the same thing happened. After opening the furnace unit, I could hear the hum (electrical sounding). Neither mtr on the blower or the compressor will start. Also, the thermostat used to light up when rotated. It no longer lights. I pulled the capacitor on the blower mtr (which was really hard to get to) & tested it with my meter for capacitance. It read around 10.1uf, & it's a 10uf cap, so I guess it's OK. I also pulled the cap on the compressor which has 3 connectors. I had various readings between the contacts. Not sure what they should be as all the writings have either faded or wore off. It's nice & rusty & It has your average cake of mud built up on the top. I hate to start buying caps not knowing for sure whether they are bad or not. Also, the unit failed around 5 or 6 yrs ago, & a friend replaced a transformer on the comp which worked great. That friend is disabled & lives in another town now. What should I be looking for or test for to get my A/C running again. I am somewhat electrical minded with an autoranging meter & an old digital one. This unit is your regular run of the mill home compressor (Intertherm), nothing 3 phase or commercial. At the camp I have a mobile home, so the furnace unit can be challenging to work on. Thanks a million in advance.
 
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Old 05-28-11, 06:54 AM
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if the fan does not run in the on position at the t-stat, and you hear a hum when you switch it from auto to on, then that is most likely your problem the blower motor is bad.
 
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Old 05-28-11, 06:59 AM
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hvactech....
Wouldn't it be odd that both comp fan and air handler both failed? I'm wondering if its not a low voltage transformer issue...esp since the light on the stat isn't coming on?

Uncle Jam....Welcome to the Forums.

Is the hum you hear coming from the actual motor or just something in the vicinity?
 
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Old 05-28-11, 07:08 AM
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I believe it's something in the vicinity. I actually do not hear it from the mtr. I had a transformer replaced on the comp once before. And neither mtr will start but turn freely by hand. It worked fine to the end of the season. I don't use the furnace during the winter. This is just my campsite that I visit during the summer months. Like I said, when I went to first start it this year, it just wouldn't come on. No issues before.
 
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Old 05-28-11, 07:26 AM
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How can I test that transformer? (it's on the comp). Do you know what the voltages should be on the primary & secondary sides? I do remember when He replaced it years back, he couldn't get one like the original, so he brought a different trans & spliced some wires & said it would be OK>.
 
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Old 05-28-11, 08:11 AM
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Primary should be 120VAC...secondary is 24V (I THINK). I was hoping one of the Pro's would weigh in. I know the basics and my own system...but no HVAC expert.

There should be a rating on it for how big (power wise) it needs to be...bigger won't hurt...smaller will burn it out or not supply enough current for reliable operation.
 
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Old 05-28-11, 09:12 AM
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Thanks for replying, I'm going to mosey over there this aftn & check a few things. One more question about the caps, the blower mtr tested within range, but I'm not sure about the large cap that came off the compressor. It has 3 connectors w/4 wires. the only markings on it says Herm at one of the terminals. The rest are faded beyond recognition. When I measure across one set of connectors, I'm reading 29.82uf. When I measure across the other of connectors I read 4.21uf. Do these values seem to sound like they're in range? The compressor is a Intertherm split system if that's of any need.
 
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Old 05-28-11, 09:32 AM
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I'd guess thats a 5/30 cap and IIRC that is a pretty common size. Also (IIRC..again) caps can be off by 10% or so and still be fine. And since neither fan is working, that wouldn't be my first thought unless there are obvious problems such as bulging or leaking, though newer caps use different chemicals (thanks EPA) and have been reported as not lasting as long. If you've gone that far into it...you can check the transformer voltage at the contactor in the outside unit (Careful there is 240VAC in there) on the small control wires.

If you have a helper...have them turn the stat to cool and lower the temp...you should see and hear the contactor pull in.

One thing...have you checked all breakers and possible fuses in disconnects?

You should be able to isolate the humming pretty easily just by ear and touch.
 
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Old 05-28-11, 10:01 AM
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My wife is going to go with me, so she can help. But that Tstat has a digital dial that lights up. There's no light & no numbers that should be visible. If that contactor doesn't pull in, what's the next step? Is there a way to bypass the transformer to get that contactor to engage? Or is the transformer the only reason why the Tstat isn't operating?
 
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Old 05-28-11, 10:24 AM
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The transformer supplies power to the stat (though it may also have batteries for LCD display and memory) and the voltage/current from the xfmr is used to control the contactor and control board in the air handler unit. Check to see if it has batteries...not that it would affect system operation..but it might bring back your display and light.

No way to bypass unless you supply power some other way, which....would involve a transformer..lol.
 
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Old 05-28-11, 11:21 AM
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sorry I was gone for a while. I've skimmed the posts so far so, Lets check the low voltage first. should be 24V AC at the air handler between R & C
 
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Old 05-28-11, 11:42 AM
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Thx for checking back in Kevin....I tried to reply and catch you before you took off this morning. Like I said..I know basics...but not with all the experience you Pro's have.
 
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Old 05-28-11, 12:52 PM
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Just got back from camp, here's what I found. I isolated the "hum" noise. It's coming from the 24v transformer in the funace/ac unit. It does have 120v going in, & 24-26 coming out. The Tstat is lighting & working when it is switched to Heat or off. When switching to Cool, it will go out. Doesn't make any difference where the fan switch is. There is no voltage at the primary side of the transformer at the compressor. There is a fuse inline, but it checked out OK doing a continuity test. All breakers on with disconnect connected. It almost sounds like a mouse or something ate thru a wire, but I'm sure they couldn't get thru 14 or 12ga. At least I don't think so. Not sure why the trans would be humming in the furnace/ac unit. Both mtrs spin free by hand. I'll be back out there Monday. What should I look for next? Thanks again.
 
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Old 05-28-11, 01:03 PM
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need to check the low voltage wiring to the outdoor unit... check for breaks in the wiring.... a quick check would to be to remove the wires from the stat a twist R, Y, and G together.....this will call for outdoor unit and indoor blower operation. I'm leaning toward stat since you have voltage.
 
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Old 05-28-11, 01:20 PM
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Thanks man. I was going to crawl under that trailer & chase down the wiring, but I didn't have my knee pads with me & it's sitting on gravel (ouch). I never thought about the Tstat going bad out of the blue, but I guess anything is possible. Hopefully, it'll be something simple & less costly. I'm putting this place up for sale & didn't want to get into too much money. I'll get back with you in a few days.
 
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Old 05-30-11, 11:31 AM
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OK, I got the units running, FINALLY! After getting voltage everywhere it belonged, I manually started that compressor contacts with a screwdriver. It ran & so did the blower fan. At first, it wouldn't start by itself. I started taking some more voltage measurements & all of a sudden it kicked on & scared the $$#&@# out of me. It was also buzzing pretty loudly. The fan setting was on fan. When I switched it to auto, it shut off again. I manually started it a few more times pushing the contactor in, & it eventually stayed on in the auto mode. I noticed when I pulled lightly on one of the contact wires, the buzzing would stop. I let it cool the place down, & it kicked back off 1 degree sooner then the stat was set. I turned the stat down, & after a very long delay, it started to run. And it's still buzzing at the contacts. What's causing this long delay in starting? Is that contactor going bad? Also I ran a continuity test across the trans on the compressor. One side was 0 ohms, but I got a reading of around 85.6 ohms on the secondary (I think that was the one). Isn't there suppose to be a dead short across those windings?
 
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Old 05-30-11, 04:08 PM
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If the t-stat is digital, then it may have a 5 min. compressor delay built in. Sounds like you need a new contactor to resolve the buzzing.
 
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Old 05-30-11, 05:40 PM
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Yes the stat is digital. I don't remember it taking that long to engage before, but then again, my memory isn't what it used to be either. I believe it's a Honeywell, the round type. I kind of thought that contactor would be needing to be replaced. I guess it wouldn't hurt anything to just leave it alone till it quits, would it? That buzzing is outside and it doesn't bother me all that much as long as it's working. Or could that also be the culprit for the long delay on starting?
 
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Old 05-30-11, 07:05 PM
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I would replace it, it could be the cause if its not allowing proper voltage to flow thru
 
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Old 05-31-11, 04:17 AM
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Thanks for all your help man, you've been a true God-send. Now to find a place that sells these parts, I live in a pretty rural area & these small towns don't handle parts like this. That Intertherm unit's label is old & completely faded, so I have no idea what model this thing is. Are these contactors pretty standard, or different from model to model? Or should I just remove it & try to match it up with another one? Thanks again!
 
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Old 05-31-11, 03:40 PM
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search for HVAC parts online and order online if you want. just a standard 2 pole 30A contactor should work.
 
 

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