Can bad heat pump capacitors cause power consumption to double?

Discussion in 'HVAC Heating & Cooling' started by jmaland, Jul 28, 2010.

  1. jmaland

    jmaland New Member

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    Hi,

    my latest power bill was double from last summer (due to the watts consumption doubling from what was used the previous summer) where I have a 2600 sq ft house in Tucson, AZ and the bill was ~$800!!! The power company is already going to change out the meter (on my request, agreeing that something seems wrong, and it's 30 yrs old), but they claim that it's most likely not the meter, but an appliance drawing the extra current.

    Nothing major has been added or change onto the grid at the house over this last year and nothing funny has been occurring (no breakers tripping, nothing seems to be running constantly, and everything seems to be working as usual). The only thing to change was two weeks ago I had my heat pump unit looked over and the tech said both caps (the inside and outside ones) were not working correctly and replaced them (for ~$400), but note that the unit seemed to be working properly and cooling the house even before he replaced them (and wasn't running constantly or anything like that).

    So, could these bad caps cause the power consumption to double over a month? The previous month's bill was also slightly higher (but not double), so these caps might have been bad a while and I didn't notice the slight increase.

    Also, one other quick question. The outside part of the unit has a buzz sound when it is not running, which I think is normal, like a transformer, right?

    Thanks for any input,

    Jim
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    As I understand it (and I'm not absolutely sure) the caps in the thing are to aid the motor in starting. If they were shot, the compressor might have more trouble starting up. this could draw more current - once it actually does get running, they don't do much of anything. If they were shorted, you'd pop the breaker. If they were open, the motor might have trouble starting.

    A heat pump has a valve that gets switched to change the things function from heating to cooling (i.e., it reverses the operation). This is probably controlled by a solinoid valve. That might be what you are hearing. Depending on the type, it may need to be energized for cooling, and when 'released', switch to heating, or it could run the opposite way (these have a spring to return it to the 'rest' state when deenergized). Some get moved to one position, then the power is removed. To move it the other way, it reverses the polarity, and the thing goes back the other way. These do not have power applied all the time. If this was sticky, I don't think it would double the power, unless it ended up in the middle...then the unit might 'run', but not be doing much for that time.

    Unless the unit is running nearly twice as long, on most, if it has the proper CB, it would normally pop the breaker, as the CB is not normally twice as big as the normal load.

    Most a/c units also have a heater in them to ensure it doesn't have moisture that could end up frozen. This might humm, but normally doesn't.

    Someone who has more intimate knowledge can probably give you some other thoughts.
    Last edited: Jul 28, 2010
  3. jimbo

    jimbo Plumber

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    Try disconnecting the crankcase heater, and see if the buzzing stops.
  4. Dana

    Dana In the trades

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    The caps won't be doubling the power consumption, but it might be burning an extra percent or so in the power wiring between it and the meter if it shifts the phase relationship between the voltage & current draw by a lot.

    But a stuck heater element would use about 0.720kwh x rated power per month. (For every kilowatt or 1000W of heater power it would use ~720 kwh/month, if stuck on 100% of the time.)

    Comparing a particular month's bill to the same month last year is truly apples & pears unless you've correlated it to the total cooling degree-days (CDD) accumulated between the meter readings for both bills. It's probably less of an issue in a place as dry as Tucson, but in humid locations you'd also have to factor in the average dew point during those billing periods to calculate and factor in the latent loads as well. If (as is true with many utilities) the bill gives the average outdoor temperature for the billing period as well as a daily use average, you can calculate the power used per CDD using 65F as the base, eg: If your average use was 55kwh/day, and the average temp for the period was 89F, your average CDD was 89-65=24CDD, and you used 55/24=2.3kwh per CDD. At lighter degree-day loading the other power use will cause that ratio to shift, but typically the cooling will begin to dominate when the average daily CDD is 15+. If last year's CDD average was half that of this year (which it might be on any given 30 day period), the bill won't be cut in half, since you're probably averaging about 1kw (720kwh/month) for things other than cooling. (This will vary by quite a bit- power scrimpers can get it down to 400kwh/month- those who like to live in the dark, even less, but people who have 2 Tivos, 3 computers and a game machine on 24/7 can see well over 1000kwh/month of baseline power use.)

    If your bill doesn't show average temps, you can get the weather history from a number of sources online. Uncorrelated to random dates, but the caledendar monthly averages can be found on wunderground.com, which reports that in Tucson the average temp for June '09 was 84F (19CDD per day), whereas June '10 averaged 87F (22CDD per day), which is a 16% difference between this June and last. If you normally keep your thermostats at ~80F rather than ~75F you should probably be using base 70F, not 65F for your CDD calc, in which case the difference is even more pronounced- 14CDD , to 17CDD, for a 21% increase this year over last. This will explain part of it- your cooling load was ~15% bigger, maybe more, but clearly not a 2x factor on the whole bill.

    A $400 increase is quite bit of extra power enough extra power to really cook something (almost enough to heat my New England home with resistance space heaters!) At Tucson's ~9cents/kwh rate that "extra" $400 represents a constant extra background load of about 6000 watts, which should be fairly easy to trace by any number of methods if it's real. The utility company should be able to test the accuracy of the old power meter and may be willing make some adjustments to the billing if it's more than a few percent off. (The odds that it's off by a factor of two are very small, but not impossible.)
  5. 3m

    3m New Member

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    What kind of furnance do you have gas or electric
  6. jmaland

    jmaland New Member

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    Thanks for all the feedback and research! Everything in my house is electric.

    They haven't replaced my meter yet, so over the last few days I've been trying to get a reference on the number of rotations of the disk (I have a 30 yr old mechanical meter) for different appliances in my house. So far, I'm not convinced this extra consumption is real, but I keep reading online that even 30 yr old meters usually slow down, not speed up 2-3 times the actual power consumption!

    The trends I see with my appliances seem normal, where the big hogs are the heat pumps ac units, followed by the water heat, where other things seem relatively low by them self in revolutions (pool pump, fridge, dryer, etc). And the revolutions I see on the meter are every 2-3 secs for my heat pumps (which is mostly driven by the larger 3 ton, energy star unit that's only 8 yrs old), does that seem normal (I don't have any idea what it was last summer, but it seems really fast)?

    So if my heat pumps units are really sucking in double the power consumption they did last summer (since weather temps were relatively comparable), wouldn't that fry my unit, trip breakers, or at least be running constantly (which it isn't)? Power has to be consumed by something running, right?

    Jim
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Try turning off the auxilliary heat CB in the air handler. If, for some reason, the thing was trying to turn on the heat while it was running the a/c, you could easily use lots more power. Is the outlet of the air registers as cold as it used to be? There are some plug-in test devices you can buy that will tell you how much power an individual device is using. Makes you think twice about leaving some stuff on that you normally wouldn't.

    A refrigerator that had the defrost cycle stuck, or the door seal heat on all the time could add a lot of energy use. Did you buy a small frig to sit by the chair in the office or den? Some of the smaller ones aren't that efficient. Depending on how old the frig is, the newer ones are LOTS more efficient. My electric bill went down probably 300Kw per month when I replaced mine. Is someone using hot curlers, and leaving them plugged in all the time? Somebody (maybe many) using a blow-dryer a long time with long hair? Not huge, but in aggragation, it can add up. Did you buy a new, huge plasma display? They're better than they used to be, but they can draw a fair amount. The LED ones are probably the best, but projection can be costly as well - the lamps are pretty big. Have you got an electric barbeque grill? Using it a lot this summer? Heater for the pool?
  8. jimbo

    jimbo Plumber

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    Rather than count turns, you can start by just reading the name plate data on the items ....look for the amps or Kilowatt numbers. Remember that water heaters, electric heat, etc are 220 and watts is AMPS times VOLTS. You PAY for WATTS.

    No secret that air conditioning costs a lot to run. Did you find out yet if either the electric heat or the crankcase heater is stuck on? An electrician with a clamp on meter can find out where a lot of amps are going.
  9. jmaland

    jmaland New Member

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    The thing is that we just had a service agreement guy come out to check out the main 3 ton system (where he said the caps were bad and replaced them) and he seemed to test everything else out (even the temp blowing out the vent), so since that seems to be the biggest power consumer from the meter (where the smaller older heat pump is less than it, so the a/c size to meter revolutions trend seems consistent) I'm at a loss to what else is pulling so much power. A friend thought that I might have some major current leakage somewhere, but I don't think I could have that large of a leak (2-3 times my normal summer consumption) and not notice something weird. I've done a number of minor home repair/remodeling things (even rewiring a smallish kitchen), so I don't get why I can't easily find this mysterious power source.

    And other new appliances or more usage out of anything, I just don't see it. Nothing major was added or different or seems to run always. In the early morning only minor power is consumed compared to the heat pumps turning on (or the water heater for brief moments).

    I called a electrician that I'm going to have over to help sort this out if the meter isn't the problem. Hopefully they will switch the meter on Monday and test it soon after that.

    Jim
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    That amount of energy use would create some heat. Induction a/c current probes aren't all that expensive (assuming you already have a multimeter that can accept one), and might be a good tool. Something like this http://www.degreedays.net/kill-a-watt-meter might also help you track it down (but only on things that you can plug in, not wired items, where the current probe would work).
  11. jmaland

    jmaland New Member

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    FYI, a quick post to end the chain on what the issue was ... it turns out that when the a/c company serviced my heat pump in the Winter (where the outside unit wasn't turning on and they added a "jumper wire" in the thermostat to get it working right) the jumper wire was still activating the strip heat now in a/c mode, so that was the mysterious double power was going (it was around 20 amps!). And now I'm in the process of seeing what this crappy a/c company will do to "make it right" (maybe even going to small claims court) since not only did they misswire the therm, but when they came out to service the unit a month ago, they didn't catch the heat strip activating (and instead sold me new caps for ~$400 to help with my issue)! With my extra power consumption and the service contract (and $500 I paid a month ago for those new caps), the total is ~$2k!!!

    Thanks for all your help on here. I actually did go buy a "kill-a-watt" tester at home depot and tested everything I could when I saw it wasn't a power company meter issue after the power company replaced my meter and my power consumption was still around double. Everything I tested was all good, which led me to get the a/c company out again.

    Jim
  12. nukeman

    nukeman Nuclear Engineer

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    Glad to got it figured out. I bet the AC works a lot better now too. :)

    Hopefully they will make things right.

    BTW: the capacitor(s) is/are about a $20 part (at least on the outdoor unit). About 2 years ago, the run capacitor died on me. The compressor/outdoor fan would try to turn on (you could hear click/buzz), but wouldn't kick on. I verified that the bearings in the fan were not seized and did a little research on the issue. Found the the capacitor was shared between the fan and the compressor, so I knew that was the likely problem. I wrote down the capacitance/voltage of the capacitor, picked up a new one, and installed (replacing the cap was about a 5 minute job). Good to go after that.
  13. jimbo

    jimbo Plumber

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    I would not describe the capacitor as "shared". It is very common to find a DUAL capacitor. Just easier to mount 1 device than 2 in the assembly. There are 2 separate capacitors in the can. One is for the compressor and one for the fan. Now, since they are inside one can, I would suspect that if one overheated or otherwise "acted up" the other is affected, but in any event you replace it as a unit of course.


    capacitors are the "unsung hero" of the HVAC world. The compressors are extremely robust, as are the fan motors. Both of those items are replaced thousands of times each year ( maybe each day) when they are OK...the cap. is bad! Manufacturers are well aware of what the "shade tree technician" HVAC world costs them, and it is of course built into the total price they charge for the unit.

    By the way.....don't know if it is the higher seers, change in manufacturing, or what....but this year more than ever....you will save yourself a callback if you just install a hardstart kit on every new condensing unit or new compressor. They just seem to need it.
  14. nukeman

    nukeman Nuclear Engineer

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    Bad choice of words, I guess. What I mean is it is a single part and if it dies, it can cause both the fan and the compressor not to work. May have been a power surge, etc. I don't know. Things worked fine during the previous cooling season and this started at the beginning of the next cooling season. The capacitor looked okay (not leaking, etc.), but I knew it had to be the problem (I didn't have a cap tester on hand to test it).

    When checking things, I could give the fan a quick flip with a screwdriver and it would then run for a couple minutes, but then you would still hear a click/buzz from the compressor trying to start (and no cooling was being done), so I knew the issue was something related to both the fan and the compressor. I could have simply used the home warranty that came with the house (the previous owners were not the DIY types, so they bought a warranty each year to cover repairs). However, it would have been $75 service call, plus waiting with no AC. Fixed it myself for $20 and a lot less time.

    jimbo: I totally agree with you. Many times the cap dies and the HVAC tech says "You need a new compressor". They tried that on my mom. She said that the compressor was one 2 years old, etc. They went back and "checked again" and found it was the cap. Makes you wonder how many good compressors are replaced when they just need a new cap.
  15. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    At the suggestion of the installer, I installed one of these monitor devices on my hvac system a long time ago. I think the company that made mine (I bought it at Grainger) may have been sold, as the name is different, but the form-factor of this one is almost the same (mine is like 20-years old, and still working fine), but adds a few new features. http://www.docs.hvacpartners.com/idc/groups/public/documents/techlit/570-992.pdf I thought mine was made by IDC, which may still be true, this could be a rebranded item. I have mine setup to inhibit the 24vac control circuits from operating until the incoming a/c power is stable. You could use it to control a contactor that controlled actual power application, but for me, it was easier to just break the 24vac circuit through it, which prevents the thermostat from enabling either the heat, a/c, or fan control.
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