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  1. #1
    Join Date
    Jun 2004
    Location
    New Haven county, Connecticut
    Posts
    132
    I currently have two Honeywell Chronotherm IV digital programmable tstats connected to my zoned forced air heating/cooling system.

    After reading some info on this forum, it seems that Honeywells .5 degree of differential may be too restrictive. Ive noticed this during the heating season, when the system blower stays on after the burner shuts off (normal) but then by the time the blower is done cooling off the heat exchanger, the Honeywell senses a .5 degree difference, and turns the burner back on. Depending on how cold it is outside, this could happen occasionally or a lot. I assume this is sort of short-cycling.

    My question is, would I be better off switching to a newer/diffrent brand thermostat to help alleviate this issue? I notice that the White Rodgers stats can be set to a 1.6 degree differential, possibly stopping my system from short off-times. Or could I even just try switching the Honeywell to less cycles per hour? (6 is suggested for my forced air heat, but the t-stat supports as low as 3 or 4 I think).

    Thanks!
    -Chris

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,897
    I'd adjust the Honeywells if they are cycling too much. Honeywell suggests 3 cph for 90s, faster for 80s and older.

  3. #3
    Join Date
    Jun 2004
    Location
    New Haven county, Connecticut
    Posts
    132
    Ill give that a try.

    We have an 83% furnace, so obviously the setting I have it on (I think its 5 cyles/hour) is correct. But I dont know if its the way the zones and/or their programming interact or what, but it always seems that the burner should just stay running longer, instead of turning off and then back on before the fan timeout is even complete.

    I guess the cheapest thing to do is try to reduce the cycles and see what happens. At $2.50/gallon of oil, Im willing to try a lot to see if it helps conserve.

    -Chris

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,897
    With a zone panel involved, hard to say. New panels have plenum temp sensors to shut off the burner if the leaving air gets too hot. Could be what's happening to you. Or could be that fan is still purging the last zone that called when another one calls bringing the burner back on.

  5. #5
    Join Date
    Jun 2005
    Posts
    44
    I'd go for a thermostat with a larger differential, (see my post on "short cycling problem solved). Even with my Honeywell stat set on 3 cph, it still seemed like it was always on,off,on,off. My Robertshaw stat is adjustable from .5 to 3.0 in 1/2 degree increments. It's nice now, set at just 1 degree differential, it stays on long enough that you forget its on, and stays off much longer than before. It's at the point where it should be, you forget it's even there.

  6. #6
    Join Date
    Jun 2004
    Location
    New Haven county, Connecticut
    Posts
    132
    lyork - Your posting is actually what got me thinking. I had an idea that my system might be short-cycling occasionally, but it wasnt constant and in the middle of last winter I wasnt about to go mucking around with it I was looking at some White Rodgers t-stats, but the differential isnt as adjustable as teh Robert Shaws (only 2 settings: .6 and 1.6). The Honeywells are .5, I knwo that much, so Id probably want to run with 1.6 on the WR.

    BaldLoonie - yeah, I figure my zoning panel changes a lot of things. Unfortunately (or fortunately, I guess) its a very old setup (circa 1984) with almost no features. Mastertrol Mark V I think. I believe that all the sensors involved are attached to the furnace/burner/blower which is much newer (replaced in 1999). Ive been able to figure out how it works, mostly. I get a call for heat which fires up the burner. Once the burner gets up to temp the blower comes on and sends heat. Once the call for heat is satisfied the t-stat shuts down the burner but the blower remains running for a preset amount of time (Im told this is normal, to cool down the heat exchanger). I havent tracked the exact amount of time, nor do I knwo how to change it (I probably cant, I think its all determined by the heat exchanger temp). After the temp goes down enough, the blower shuts off. Now, at an indeterminent amoutn of time afterwards, the blower comes on briefly and then turns off. Someone mentioned this indicated that perhaps the exchanger wasnt completely cooled down and it turned the blower on again. Ive also been told this is fairly normal but could resolved if it bothers me (it doesnt). What you said about purging one zone when another calls might be correct. However, I kinda figured (hoped?) that having a larger differential might cure this because I have very little time during the day when both zones are programmed for heat, so the few times both zones should be calling might be resolved by them not being so "sensitive" to changes in temp. Ive tried a bunch of things with the programmed on/off temps, and no matter how I set them (equal, far apart, etc) it seems that this happens only because the t-stat is sensitive (if you stare at the display, the stat comes on even at the set temp, which proves that it hasnt even lost a degree before the stat calls for heat).

    Im trying to get my HVAC guy out, but he keeps blowing me off (no pun intended). Thanks for all the help though, you guys are probably a better source of information anyway!

    -Chris

  7. #7
    Join Date
    Apr 2005
    Location
    Kingston Ontario Canada
    Posts
    1,210
    Originally posted by chrisexv6
    lyork - Your posting is actually what got me thinking. I had an idea that my system might be short-cycling occasionally, but it wasnt constant and in the middle of last winter I wasnt about to go mucking around with it I was looking at some White Rodgers t-stats, but the differential isnt as adjustable as teh Robert Shaws (only 2 settings: .6 and 1.6). The Honeywells are .5, I knwo that much, so Id probably want to run with 1.6 on the WR.

    BaldLoonie - yeah, I figure my zoning panel changes a lot of things. Unfortunately (or fortunately, I guess) its a very old setup (circa 1984) with almost no features. Mastertrol Mark V I think. I believe that all the sensors involved are attached to the furnace/burner/blower which is much newer (replaced in 1999). Ive been able to figure out how it works, mostly. I get a call for heat which fires up the burner. Once the burner gets up to temp the blower comes on and sends heat. Once the call for heat is satisfied the t-stat shuts down the burner but the blower remains running for a preset amount of time (Im told this is normal, to cool down the heat exchanger). I havent tracked the exact amount of time, nor do I knwo how to change it (I probably cant, I think its all determined by the heat exchanger temp). After the temp goes down enough, the blower shuts off. Now, at an indeterminent amoutn of time afterwards, the blower comes on briefly and then turns off. Someone mentioned this indicated that perhaps the exchanger wasnt completely cooled down and it turned the blower on again. Ive also been told this is fairly normal but could resolved if it bothers me (it doesnt). What you said about purging one zone when another calls might be correct. However, I kinda figured (hoped?) that having a larger differential might cure this because I have very little time during the day when both zones are programmed for heat, so the few times both zones should be calling might be resolved by them not being so "sensitive" to changes in temp. Ive tried a bunch of things with the programmed on/off temps, and no matter how I set them (equal, far apart, etc) it seems that this happens only because the t-stat is sensitive (if you stare at the display, the stat comes on even at the set temp, which proves that it hasnt even lost a degree before the stat calls for heat).

    Im trying to get my HVAC guy out, but he keeps blowing me off (no pun intended). Thanks for all the help though, you guys are probably a better source of information anyway!

    The Carrier Thermidistat that I have acts the same way as you descrbe. If the sent point is say 70 it comes on when it still says 70 and shuts off when it still says 70 or blinks once at 71, but settles back to 70 almost immediately. It does have a ajustable anticipator, which I have tried, but make hardly any appreciable difference.

    Thorton

    -Chris

  8. #8
    Join Date
    Jun 2005
    Posts
    44
    My Robertshaw is adjustable in .5 increments from .5 to 3.0. The Honeywell that I replaced it with apparently had a .5 differential, and I never saw it move from the setting of 74. I now have my new stat set at 1.0 differential, and the Robertshaw reads to .5 degrees. So when my unit shuts off it is 74.5, and when it kicks back on it reads 73.5. Really has made all the difference that 1/2 of a degree.

  9. #9
    Join Date
    Feb 2005
    Location
    Richmond
    Posts
    480
    My 2 Lux Thermostats let you adjust the cycles per hour, and automaticly adjusts the number of degrees +/- the setpoint before the burner comes on.

  10. #10
    Join Date
    Jun 2004
    Location
    New Haven county, Connecticut
    Posts
    132
    Thanks again for the info. Im looking into trying out a Robertshaw t-stat but I noticed that most of they say "works with all standard 24VAC heating systems". However, I dont have 24VAC coming to my t-stats (I assume mine is a "millivolt" system). Can I still use the t-stat in that case? I apologize that this is sort of a DIY question, I know how to hook up the wires and such, I just wasnt sure if the t-stat itself is compatible with millivolt systems.

    Thanks!
    -Chris

  11. #11
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,897
    With zoning, you would have standard 24v. Millivolt is an ancient conversion burner for example.


  12. #12
    Join Date
    Jun 2004
    Location
    New Haven county, Connecticut
    Posts
    132
    Oh, OK. I was under the impression "24VAC" meant I had 24VAC available at the t-stat (which Im 100% sure I do not). But I do have a transformer for the system itself on the furnace (feeds 24VAC to the zone panel which then passes it thru to the dampers).

    The manuals for the Robertshaw t-stats made it look like it needs to be powered off the system and not the batteries.

    -Chris

  13. #13
    Join Date
    Jun 2005
    Posts
    44
    Check your current thermostat for 24 volts across the "R" and "C" terminals. That will determine if you have 24 volts going to your thermostat. Some of the Robertshaw therms are battery operated, if your current therm has batteries, chance are you will need one that does use batteries. (I'm not an HVAC person so somebody that is, feel free to chime in) "R" and "C" are L1 and L2 on my stat on the schematic.

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