A/C Crankcase Heaters (on a Trane)

Discussion in 'HVAC Heating & Cooling' started by jadnashua, Aug 16, 2012.

  1. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,832
    Location:
    New England
    My neighbor had an issue where his a/c unit tripped the breaker. The guy that came out found that the crankcase heater had overheated and the insulation had burned off, making a direct short. He removed the crankcase heater and told him on a typical residential setting, it isn't needed. It's my understanding that while this may be true most of the time, it is designed to protect the compressor from trying to start on a cooler day when there might be liquids on the inlet...compressors don't like to try to compress liquids!

    Mine recently started to trip the breaker, but resetting it would start it right back up again. It would run for one or two cycles, then trip the breaker again. I haven't been there when it tripped, just notice that the house is warming up. Since the thing always started right back up again when I reset the breaker, and the only thing running when the compressor isn't is the crankcase heater, I just tried removing those leads and am experimenting to see if it stays on.

    If it didn't cool well when it does run, I'd suspect maybe low refrigerant.

    I did call a serviceman, but all I got was his answering service, and it's been a day with no callback. May call someone else soon if this doesn't help.

    If disconnecting the crankcase heater lets the thing stay on, how critical is it to replace the thing? Also, based on the fact that it always does start up when I reset the breaker and cool (as far as I can tell) in a normal fashion and performance, what else might cause this sort of thing?

    The old CB didn't feel quite as strong, and didn't 'click' when resetting like a new one, so I did replace it with the same type just to see if it was failing...no change.

    The wiring to the outside breaker box is fed by an older 30A circuit. The a/c unit calls for a 15A breaker, and the wiring from that breaker to the unit is 12g with a 15A breaker. With the associated wiring, I could change the breaker to a 20A, and that might have enough margin to let it work, but I'd rather keep it where they called for in the installation manual!?

    Thoughts?
  2. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
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    The crankcase heater is only needed/used in COLD weather for the heat pump operation. If it is turning on in the summer, you are wasting a lot of energy, and should disconnect the leads/
    Last edited: Aug 16, 2012
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,832
    Location:
    New England
    The specs don't say when the thermostat on the crankcase heater should turn on, so it might be both are bad. I didn't feel like tearing the top and covers off to dig in there unless I found that was really the problem - disconnecting was much easier...just unplug a couple of leads.

    It's been running awhile now since I disconnected the heater, so that may have been the culprit. I'll have to wait awhile longer to be sure. If it makes it through a full day or so, I think I've nailed it.

    How critical would it be to replace in the long run? Or, can I just leave it disconnected forever? At what temps are you liable to get liquids there and lock up the compressor?

    Well, update, the breaker still tripped...now what?
    Last edited: Aug 16, 2012
  4. DonL

    DonL Jack of all trades

    Messages:
    3,820
    Location:
    Houston, TX
    The heater thermostats that I have seen are 65 deg F.

    As jimbo said they are more important for heat pumps.

    If you do not run your unit when it is below 65F outside, Then the heater would be off anyway.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,832
    Location:
    New England
    Well, there are times when it may be needed at that outside temp, so maybe once this gets resolved, I'll leave it hooked up.

    But, anyone give me some ideas why the a/c unit would pull excessive current and trip (after running for awhile) the CB? It does seem to cool properly, but I haven't measured the outlet temp.
  6. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    My two stage (two compressor) Rheem unit has crankcase heaters. These pull 40W each (measured indirectly at meter.) I don't know if they have a temp limit or not to shut them off. That's what I wish they had, but I don't think they do. Manual says they kick on after 2 hours idle by the respective compressor, and the compressors rotate cycles.

    I asked the installer and he told me it was fine to flip the breaker to the AC during heating only months (this is not a heat pump.) He indicated that the crankcase heaters are really only intended for winter heat pump operation as I suspected. I figure saving 80W of wasted juice for more than 6 months a year is a good thing. He noted that the heaters would probably last longer if I did this. I've read complaints of early heater failures on many brands.

    My 4 ton system has a 40 amp circuit inside, not sure what is outside. I've had nothing but trouble with 15A breakers in 110 service, but this is a different matter.

    Oh! Just occurred to me why you might be getting a trip after a cycle or two. If there is a timer it is probably resetting when you flip the breaker! When it tries to kick the heater back on again later, it trips again.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,832
    Location:
    New England
    Well, I still eventually tripped with the heater circuit disconnected, so that's not it (unless there are two problems!). So, while I probably should have done this already, I'm going to go check the voltage drop across the contactor when it is running to see if that might be presenting a low-voltage situation to the compressor.

    The guy I called has now had nearly 48-hours and he hasn't called back, so I guess I need to call someone else! Maybe if I'm lucky, the contactor is bad, and I can just replace it. I'll keep you posted.

    Any other thoughts for simple things to check would be appreciated. I may just also pick up an induction current probe - not something I've really needed before, but handy in this case. Just don't want to spend a lot on tools I may not use and just let someone who has them, fix it right. Don't mind paying a pro, but like to check the 'easy' stuff first. Luckily, with the added insulation I put in this spring, the house doesn't get anywhere near as hot as it used to, so this isn't a major issue, but still annoying - I don't like it when things don't work right!
  8. DonL

    DonL Jack of all trades

    Messages:
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    Location:
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    Not sure what your model number is ?

    A 15 amp breaker seems small for many home units, but if it worked before then it should work now.

    Something a dirty Condenser or the Fan not running at full speed can increase the current. (Or too much refrigerant, but low refrigerant normally makes it draw less current.)

    Normally the compressor start-up current will pop the breaker, If you have compressor problems.

    A Clamp on amp-meter is handy to see what is happening.

    If the time delay relay has some problem and the compressor drops out and tries to restart before the Head pressure drops, will cause excessive current. Most smart Thermostats have built in protection for this also.

    A bad Fan Run capacitor can cause the condenser fan to run slow.

    You may want to look at the in coming line voltage and see if it is low. The voltage can vary a lot when everyone is running their air at the same time.

    If your model is the same as your neighbors then your could very well have the same problem. Did disconnecting the heater cure your problem ?

    Their are a lot of variables, and a volt and amp meter is needed to really see what is happening.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,832
    Location:
    New England
    Line voltage was a nice 240vac and when the compressor was running, I had zero volts across the contactor, so it's not the issue (i.e., high resistance contacts). Disconnecting the (40W) crankcase heater didn't make a difference. Assuming the thermostat is calling for it, it always starts and runs for awhile (not sure how long, actually) when I flip the breaker back on. It's long enough to get the house to the setpoint, anyway. It seemed to stay on all last night, but then it was a cool night and may not have had to run.

    15A is what the installation manual calls for, and it has been working for years on that, so don't think that is the issue although the size of the feed wires would allow me to bump it to a 20A CB. It's a 2T XLi 16 series, maybe five years old or so (well, maybe a little older, I'm not sure anymore).

    I'm going to pull the cover and see if there's anything obvious - maybe some mice decided to chew on some wires that I can't see from the utility panel. Then, I'm going to have to call someone else, as the guy I did call hasn't returned my call in two days...so much for a good reference!?
  10. DonL

    DonL Jack of all trades

    Messages:
    3,820
    Location:
    Houston, TX
    That is a nice unit.

    It may still be under warranty.

    I would look at the capacitors. Some Swell and some leak their electrolytic, you can test them.

    The two stage units get more complicated.

    Sounds like you have a good reason to buy a new piece of test equipment. Can never have to much.


    Good Luck.
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,832
    Location:
    New England
    Finally got a tech out today...not sure we found the problem, but maybe. The start cap was running hot, so he changed it (no extra charge over the service fee). Measured the start and run current, and run was around 7A, but once it peaked at 19 momentarily when starting, which popped the breaker. We had been turning it on and off with not that much time between, so locked rotor may have been the issue.

    The manual called for a 15A breaker (HACR), and the wiring from that breaker to the unit is 12G. He said they normally run these things off of a 30A circuit and the external is often just a disconnect (often, not fused). The internal wiring in the a/c is 12g, so he suggested replacing the breaker with a 20A one, or switching the wiring to 10g and use a disconnect. I picked up a 20A breaker, but may wait to install it to see if the cap may have been the issue. He said they normally start to leak when failing, and it wasn't leaking.

    Found a Fluke clamp-on ammeter that I liked, model 324 - true RMS a/c current with the ability to check capacitors. It's a new model, and not that many places have it yet, so there's not much competition on the pricing. Just didn't want to spend money on a tool that wouldn't get used very often. Amazon has the older version (without the cap check) for a fair discount, but it's also not RMS. Guess that really doesn' tmatter that much if you're just doing comparisions, but prefer RMS capability. Don' twant to spend the premium to get one that can do DC current, too. The price almost doubles!

    Here's hoping this fixes things...the thing has been running fine for 7-years with a 15A circuit, so I was leary about changing it. Their electrician said not a problem...if it continues, swap it out.
  12. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    As it ages it will likely pull more amps. So 15A might be okay early in its life, but in its "golden years" it could be marginal or less than that, especially when it tries to start. They put larger capacitors on the AC when the caps start failing to start the unit, but with an undersized breaker it might go first.

    How many tons is the unit? I would expect it to be pretty small for a 15A.
  13. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,832
    Location:
    New England
    It's only a two ton unit. It ran about 6 hours (off and on), but it was tripped just now. I'll put the bigger breaker in tomorrow and see what happens. Nothing else checked out marginal. Maybe something intermittent, and, maybe all it needs is the bigger breaker. Time will tell.

    Considering the internal wiring is 12g, while I know they don't follow the same rules as house wiring, I'm not sure bigger is necessarily better feeding it - it was 240vac while running and there's no dimming or anything in the house when it starts, so bigger doesn't seem like it is needed. Maybe the bigger breaker will cause the weak thing, if there is one, to finally fail, making it easier to fix.
  14. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    I was guessing it would be around 2 tons. 15A doesn't seem out of line considering the 30A for the our 4 ton system.

    As long as you don't exceed the rating of the wiring it shouldn't hurt.

    I'm trying to recall what all happened on our old 4 ton before we replaced it. We weren't the original owners and it was on its last legs. This is my recollection of the sequence: It was having a lot of trouble starting (repeated clicks/attempts). I think replacing a cap did the trick. The HVAC guy said normally he would have put in a bigger capacitor, but it was already maxed and that it looked like someone had already done that previously. A season or two later a post burned off the terminal block, but he had an old unit to cannibalize to replace the block. I ran it another season (really pushed my luck) before replacing the whole unit in the late Fall. When they tried to drain it, the compresser seized quickly.
  15. DonL

    DonL Jack of all trades

    Messages:
    3,820
    Location:
    Houston, TX
    The compressor has its own over current protector for Locked rotor.

    Normally it will open before the main breaker.

    They can use smaller internal wiring, because they know the length and the voltage drop that is has.

    Being on a 20 should not cause a problem for a 2 ton unit. Actually 20 would be the norm.

    You can always install a hard-start kit as last resort. But It may be ok.
  16. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,832
    Location:
    New England
    Well, it still tripped, so I replaced the cb this morning with a 20A one, which was the limiting factor for the supply wiring. So far, other than the drain plugging up and leaking all over the carpet, it's been working...hopefully for a long time.

    I did search out a condensate trap to replace the pieced together one that's there that can't be cleaned easily. I bought one of these http://airtec.rectorseal.com/condensate/eztrap/cleartrap from an e bay vendor pretty cheap. Looked a little around town, but couldn't find anything and didn't want to waste gas looking further.

    BTW, looking inside at the compressor, the heater thermostat and body of the compressor and wiring was pretty pristine - not bad for 7-years. I usually do shut the breaker off over the winter - the 40W heater, probably continuous over the winter, still adds up.

    Thanks to all that responded - I don't deal with these things too often other than as a user - adjust the thermostat!

    Now, the (25-year old) power protector I had in the system died a month ago, and I bypassed it. I can buy the same model, but the replacment is this one: http://www.patriot-supply.com/files/icm491_ss.pdf. I have an ICM490 in there now. Question is, is it worth installing a new one? Biggest advantage to me, seems like during a power outage, and reapplication of power, it would prevent the system from operating until things stabilized - cheap protection. I had it interrupting the 24vac circuit so neither the air handler nor the compressor would turn on until the power stabilized. Not sure if it ever kept things off from a severe brown-out - I didn't have the low-level cutout adjusted very low. The 491's price is lower than the old 490, so seems like that would be the better choice.
  17. DonL

    DonL Jack of all trades

    Messages:
    3,820
    Location:
    Houston, TX
    I hope you have things working Jim.

    If you need a good Meter that works and do not need to do a drop test, then take a look at this one.

    The Fluke has been overrated for years.

    Compact True RMS Clamp Meter
    Perfect for general purpose applications, this compact clamp meter measures AC/DC voltage, AC/DC current, resistance, frequency and duty cycle.

    http://www.mcmelectronics.com/product/72-7226

    It is not a Fluke but it gets the job done for us poor folks. I take care of my test equipment and have had no problems with this brand.

    MSRP is $119 and it can be had for $49.99.

    PM me if you need a code to get that price.
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