Results 1 to 13 of 13
  1. #1

    Furnace short cycling at set temperature

    Hello all.
    I badly need help diagnosing a two year old split system that is causing me to lose sleep. And money.

    I have a 850sf ranch house, insulated w r15 batt and foam insulation r36 in the attic rafters.

    I have a 2ton outdoor scroll compressor w TXV and an indoor heat pump and blower.

    I have recently noticed (since the compressor sits outside the window by my bed) that the compressor is making a loud noise when running at night (in the 30s and 40s here). Upon further study, I am realizing that it is bot short cycling (running less than ten minutes and restarting in less than a minute) and also running for twenty minutes or so, but also then stopping and restarting in less than a minute of compressor shutting off. Sometimes the blower has time to shut off first. Sometimes it starts again so fast it just stays on.

    The compressor isn't always this noisy. But is more frequently so at night when it is colder out.

    The entire time this phenomenon is occurring THE INDOOR TEMP MATCHES THE SET TEMP (tonight it is 68. Last night it was 70).
    I mean if it is cycling 3 or for times in an hour, running for the entire hour with 60sec (or less) stops ... The entire time the set temperature = the indoor temperature.

    The service tech has been out twice. Once in January. Once yesterday.
    In January the tech removed 3.5 pounds of refrigerant after hooking up a dual gauge to the compressor and taking a reading of 600 (should have been 450 to 500). This company was actually called out by me after I realized the company that did the original install was incompetent and lying to me. Trusting them, and not sure what was wrong now, I called them back again. He came today. I was worried that they had removed too much refrigerant. This time he took static pressure and got the cfms and several other measurements using the thermocouple and duct attachment. He said the previous tech definitely got the refrigerant within range and that the original installers must have severely overcharged the system. He said the cfms were low but at threshold of acceptable. Think he said 358. Threshold was 350?

    He persuaded me to leave it there since it would be less humid in summer if all else was okay. But looking in to other possible explanations for this phenomenon, it seems like the most plausible is cfms being too low.
    Possibly exacerbated as of late by my switching air filter types to pleated, and it took me a while to notice?


    Oh. And he replaced the thermostat today to rule that out as well.
    But the noise. Sort cycling. And odd restarting nearly immediately continue.
    All while the thermostat constantly reads that the indoor temperature is at the set temperature.
    This last part I CANNOT understand.... If the thermostat is showing the indoor temp,to match the set temp WHY IS THE FURNACE RUNNING FOR NEARLY A SOLID HOUR?
    PLEASE HELP???

    This is what I noticed when I got in bed tonight around 12:56am
    12:56 running loud
    1:04 cycled off
    1:04 (+30sec) Cycled on
    1:14 cycled off set 68 at 68
    1:14 and 40 sec cycled on
    1:45 cycled off set 68 at 68
    1:45 and 30 sec cycled on
    AT 68 for entire 45 minutes of observation
    Loud the whole time as well.

    ANY THOUGHTS PLEASE???

  2. #2
    I want to add that I just followed up on a notion the tech and I had earlier regarding the possibility that the noise was itself a symptom of the short cycling and not a cause or indicative of other problem.

    I just dropped the temperature down three degrees to keep the compressor off for at least five minutes, and when the compressor kicked back on NOISE WAS GONE.
    So the noise is being CAUSED by the pressure created by the short cycle.
    But I am still at a loss for the cycling and constant running when the stat says indoor equals set. ? And my tech says the stat should lock the unit out for five minutes after shutting off to stop compressor pressure related problems. Why is it turning back on in a matter of seconds? There IS a "pigtail" junction by the furnace where the thermostat line is spliced. COULD THAT BE THE CAUSE?
    Does this sound thermostat related or airflow related or system design/insulation related?

  3. #3
    CFM WAS 691 (original post was incorrect. I checked info on invoice)


    Here is another time chart
    Adjusted stat to get hvac to stay off for over five minutes.
    Off since 2:05
    2:12 on NOISE GONE
    2:38 off
    2:39 on
    2:40 off
    2:41 on
    2:42 off
    Fan remained on whole time since 2:12. No noise.
    Fan off at 2:43
    Indoor temp equalled set temp of 67 entire time from 2:12 to 2:43
    2:49 on
    3:07 off and fan off

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,875
    They should probably recover the charge, and then weigh the proper amount back in.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,040
    What's the thermostat?

  6. #6
    Join Date
    Jul 2000
    Location
    Northern Wisconsin
    Posts
    2,003
    You say you have a heat pump, but you never mention anything about you noticing or realizing that one or more of the cycles it's going through could be (or you identified it as) a defrost cycle. That's the first thing you need to realize about a heat pump. The outside condenser unit will (depending on control types and setup) go into defrost from time to time. The defrost cycles should or will be controlled by the outdoor temperature and the amount of humidity in the outside air. If you don't understand the reasoning and mechanics behind the defrost cycle I suggest that you search on here or on the web for more information. I'll list the basics for you:

    Defrost of a heat pump system is needed due to different reasons. One of the reasons is that if the temperature and humidity outdoors is "just right" the outdoor coil can become covered in frost. This frost accumulation reduces the amount of air flow through the outdoor coil and because of that it reduces the amount of heat that can be "pulled/removed" from the outdoor air. If the outdoor unit (and the compressor) were to continue to operate with the coil partially covered/plugged the efficiency and amount of heat it can deliver to the home goes down, thus wasting electricity.

    The unit via settings of controls will shift into defrost mode when either a certain amount of running time has passed or sensors monitoring the temperature of the outdoor coil indicate that a condition exists that "could" mean the coil has become frosted. Note that this same set of parameters can also be met by low outdoor temperatures which can fool the unit into thinking the coil is actually frosted up. It's not a perfect way of doing things, but it's the way most defrost cycles are controlled. Some are controlled simply by X amount of time having passed.

    When your unit goes into defrost the outdoor fan will stop running, the indoor fan will continue to run, the compressor in the outdoor unit may or may not stop running for a very short amount of time, a valve in the refrigerant lines in the outdoor unit shifts to reroute the refrigerant in the system to now take heat from inside your home and use it to warm up the outdoor coil to remove any frost build up (if it's there) or in some cases just warm up the coil sensor to trigger the unit to go back to normal operation, at the same time all this is happening outside the fan inside continues to run and the auxiliary heat (in your case I'm guessing electric heating elements) is turned on so the air doesn't come out of the registers cold in your home. Once the defrost control is happy the system reverts back to it's basic operation all which "can" happen without the indoor unit's fan ever stopping.

    That's the very basic description of the defrost function. And like I said each system does it differently and at different timing etc. depending on how it's set up and what kind of controls it has.

    You have a very small home which sounds by your description to be well insulated. In a perfect world every house would have the perfectly sized heating system installed in it that was capable of putting the EXACT amount of heat into it 24/7 that was needed. Trouble with that perfect scenario is that unless you have installed a system capable of delivering that exact heat the equipment will cycle on and off. The cycle times and in your case defrost times are totally dependent on outside conditions that change hour to hour and from day time to night time.

    I didn't sit down to write a book, but to throw out a suggestion of "do this or that" I felt would do nothing but add to your concerns and worries.

    Short cycling can sometimes be caused (in any heating or cooling system) by a thermostat being located where it too easily senses the heat or cold that is being delivered to the house. The thermostat should not be located where it is the air path of a supply register. Sometimes in smaller homes it's hard to find the perfect place to locate the thermostat, but it's placement takes more thought than "This is the easiest place to run the wires. or This is where the old one was." . New equipment that replaces an old system can often create totally different air flows in the home. This difference could result in needing to move where the thermostat is placed..... and who knows, the original place could have been the worst place to have it, but with the older system it didn't matter as much. Also with the new digital thermostats some can be overly sensitive and react to much to this condition.

    From what you've posted I can't deduce enough information to say for a fact what your problem is or if there even is a problem that needs to be fixed. What I can deduce is that what is happening has you concerned. At this point I can only suggest that you concentrate less on the actual cycle times and take a little time to fully understand all of the things that are going on and what may be causing them. I'm not trying to say that the cycle times of fans etc are not important, it's just that without understanding the full story..... you've picked the one thing that is as generic as they come when trying to diagnose what is going on with a heat pump system.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  7. #7
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,723
    if the thermostat has a 5 minute delay the short cycling could be caused by a safety in the unit itself.
    try leaving it off for a while then turn temperature up 4 or 5 deg and see if unit runs constantly to reach temp or if it short cycles on the way up to temp.

  8. #8
    Join Date
    Mar 2001
    Posts
    1,486
    Quote Originally Posted by t527ed View Post
    if the thermostat has a 5 minute delay the short cycling could be caused by a safety in the unit itself.
    try leaving it off for a while then turn temperature up 4 or 5 deg and see if unit runs constantly to reach temp or if it short cycles on the way up to temp.
    X2
    One way to outthink people is to make them think you think. They'll think you're not really thinking what you're trying to get them to think you think...........

  9. #9
    @firecontrol
    i understand what you are saying, and have read about (and had explained to me by the techs) the defrost cycle. but please, tell me, what the defrost cycle has to do with the following run pattern:

    2:12 on NOISE GONE
    2:38 off
    2:39 on
    2:40 off
    2:41 on
    2:42 off
    Fan remained on whole time since 2:12. No noise.
    Fan off at 2:43
    Indoor temp equalled set temp of 67 entire time from 2:12 to 2:43
    2:49 on

    NOTE: the condenser runs for 25 minutes, then turns off at 2:38am. It then comes on again at 2:39, then turns off again seconds later, then turns on again seconds later again, then turns off a final time and stays off for almost 10 minutes.

    How could that possibly be defrost related?
    Would the defrost cycle not run when the machine was intending to RUN for a period? not turn off? this thing was running for a while in the cold -- admittedly a condition which could call for a defrost cycle -- but then does the following over less than 2 minutes: 2:38 off 2:39 on 2:40 off 2:41 on 2:42 off
    what was that function? It just decided to defrost for 40 seconds so it could run for 40 seconds?
    and AGAIN: THE ENTIRE TIME THE MACHINE IS RUNNING, THE THERMOSTAT REGISTERS SET TEMPERATURE = INDOOR TEMPERATURE.
    *** WHY IS IT RUNNING AT ALL ??? ***

    ps -- my thermostat is in the interior hallway of the house, no windows, no supply registers ... THERE IS A FLOOR RETURN located about SEVEN FEET from the thermostat. But i have been unable to find information indicating that a RETURN vent is a problem related to a thermostats position. ??? The thermostat WAS moved in to the hall way after a recent remodel (old location no longer suitable because of movement of an interior door) ???



    @T527ed
    this comment sounds so simple, but i am having trouble wrapping my head around what you mean by that. can you (or someone) walk through a scenario where this causes the problem i described?



    @badloonie
    it is a non-programmable base model Honeywell.
    honeywell th3210d1004

    ???

  10. #10
    uhh.... so i just took the thermostat off the wall and in to the living room to look at the model number.

    Upon reinstalling, with the house having been set to 67 all night, and the indoor temperature reading 67 all morning, when i plugged it back in to the wall the thermostat read 72???
    I'm going to keep watching it, but over two minutes later it is still showing 72.
    ??? again. set temp = 67heat, indoor temp was 67 heat.
    removed unit. reinstalled. says 72.

    this is the temperature my lap \ hands
    has warmed it's sensor to
    and it just is slow to adjust back down or WHAT?
    (just dropped to 71)

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,875
    Common for them to show a higher temp after you handle them. Takes a few minutes sometimes for them to show the room temp again.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  12. #12
    Join Date
    Jul 2000
    Location
    Northern Wisconsin
    Posts
    2,003
    Quote Originally Posted by Ifiwasakid View Post
    @firecontrol
    i understand what you are saying, and have read about (and had explained to me by the techs) the defrost cycle. but please, tell me, what the defrost cycle has to do with the following run pattern:

    2:12 on NOISE GONE
    2:38 off
    2:39 on
    2:40 off
    2:41 on
    2:42 off
    Fan remained on whole time since 2:12. No noise.
    Fan off at 2:43
    Indoor temp equalled set temp of 67 entire time from 2:12 to 2:43
    2:49 on

    NOTE: the condenser runs for 25 minutes, then turns off at 2:38am. It then comes on again at 2:39, then turns off again seconds later, then turns on again seconds later again, then turns off a final time and stays off for almost 10 minutes.
    ???
    I'll give it a shot. Understand that this is full of assumptions and guesses from not being there to see and monitor what is actually happening with each of the half dozen components (at least) that are all working together or independently to keep your home warm.

    2:12 On: Everything is running normally and this continues until 2:38 with normal noises
    2:38 Off: I asked if just the fan was turning off during these short off times or if both the compressor and fan were shutting off. I don't read where that question was answered. I'm going to assume here that you are saying the fan went off and not the compressor. Why? Because you never have mentioned the reversing valve changing which would be indicated by a whoooshing noise of refrigerant in the outside unit.
    2:39 On: The unit has finished a defrost cycle and the outdoor fan has again been energized to continue supplying heat to the home.
    2:40 Off: The unit's controls are for some reason putting the unit back into defrost cycle.
    2:41 On: Again the unit has finished a defrost cycle and has returned to heating.
    2:42 Off: The thermostat is satisfied in the home and the delay on the indoor fan turning off is simply a time off delay built into the unit to get the last little bit of heat left in the indoor unit
    2:49 On: The home has cooled down enough to need more heat.

    I'm guessing here, but I'm thinking there is a problem with the defrost controls on the outdoor unit. The first off cycle seems (from here) to be ok, but for whatever reason the controls are "bouncing" the unit in and out of defrost once the initial need is seen.

    When the unit goes into defrost the indoor emergency heat is brought on to keep cold air from blowing out of your registers and this added/extra heat is satisfying the heat need in the house during the multiple short defrost times to eventually satisfy the heating need of the home. Once this is done the thermostat turns everything off waiting for the temperature in the house to drop again.

    You need a skilled technician that understands heat pumps, defrost controls and your specific type of equipment to come and look things over and actually sit there and watch and check things when it's doing what it's doing. Either that or an experienced tech might be able to see something that isn't correctly installed or listen to your description of how it's working and "guess" at what is causing it and act accordingly.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  13. #13
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,040
    Many stats will run the equipment with stat at temp. It may read at temp but really be shy since stats rarely show tenth of a degree.

    With that stat you can adjust cycle rate to reduce the number of cycles. Factory setting is 3. Factory setting for time delay is 5 minutes. Since the stat isn't following that, sounds like it is the culprit though 527 has a point that something in the heat pump can be also.

    We had a caller whose HP short cycled. 3 years old, competition wanted 1/2 way to 4 digits to change a faulty control under warranty. Our tech changed the filter. The heat pump was cycling on high pressure switch due to the plugged filter.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event