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  1. #1
    Join Date
    Jan 2005
    Posts
    9
    For a high effeciency heater, generally whats better for the unit:

    1) More cycles with shorter runtimes, and shorter space between cycles

    -or-

    2) Fewer cycles with longer runtimes, and longer space between cycles

    Is there any impact to the unit if its one vs the other, good or bad?

    Paul

  2. #2
    Join Date
    Mar 2004
    Location
    SE Michigan
    Posts
    17,948
    Short cycling= bad, long run time= bad.

    This is why it is important to have the system sized and installed by a professional. Thats why manual J and manual D were created.
    Behind the ostensible government sits enthroned an invisible government owing no allegiance and acknowledging no responsibility to the people.

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  3. #3
    Join Date
    Jan 2003
    Location
    Kansas City
    Posts
    1,482
    What is the concern relating to? Are you getting a new system or are you trying to modify an existing one?

  4. #4
    Join Date
    Mar 2005
    Posts
    39
    Its a question of comfort and cost.

    Short cylcing usually equates to higher cost (not always) but more comfort (its near the set temperture more ofter). Long cycling the other way around.

    As to which is "better" for the system, my belief is that experts would disgaree with reams of technical data. Not profesional is going to help you on that.

    In general if the thermostat lets you do it its probably OK. Then its largley a user decision.

    YOu may need to clarify what you mean by better: better reliablity or beter efficiany, etc

  5. #5
    Join Date
    Jan 2005
    Posts
    9
    Thanks. My problem is that I when bought the house, it came with 2 furnaces in it. They had been there for 10 years. It passed inspection, and I didnt think to do a deep dive. The inspector looked at the furnaces, said they looked ok. He thought the front one looked oversized but he wasnt alarmed by it. So I decided I would at least keep an eye on it.

    I observed the furncaces for a several months, and noticed the front furnance has cycles as low as 6 minutes, and the back furnace as long as 40. I am now montioring the ontime and cycle time of each. They are pretty consistent with the numbers I gave above, changing based on outside temp.

    I have a yearly inspection by an HAC pro that I trust, and he agrees this is not right. He beleives that how the ducting is laid out in the house and addition is right for 2 heaters, but that one furnace is too big, and one is too small. As of our last talk, he doesnt know how to help other than ripping out and installing new furnaces...

    So I am looking for options other the replacing them, since worst caset thats what I will have to do. Other than being told its wrong, I wouldnt know, they work. However if I can avoid replacing them, I want to explore that.

    Paul

  6. #6
    Join Date
    Jan 2005
    Posts
    9
    Originally posted by hoffa
    Its a question of comfort and cost.

    Short cylcing usually equates to higher cost (not always) but more comfort (its near the set temperture more ofter). Long cycling the other way around.

    As to which is "better" for the system, my belief is that experts would disgaree with reams of technical data. Not profesional is going to help you on that.

    In general if the thermostat lets you do it its probably OK. Then its largley a user decision.

    YOu may need to clarify what you mean by better: better reliablity or beter efficiany, etc
    I want to try to modify what I have, replacing 2 heaters is a going to be a lot$$ and I want the best mix of reliabilty and effiiency.

    I have thermosats that I can spread the set vs trigger point, and thats my current activity. The results thus far are addressing both. Comfort is off a "tad", but cycle times improved for both, and time beween cycles is tremonous. The actual ontime for the units per day has dropped.

    I wanted to be sure that by heading down the efficiency path, I wasnt making a bad reliability problem even worse. Sounds like I am heading in the right direction.

    I still welcome input on this and thanks to all who have commented so far

  7. #7
    Join Date
    Nov 2004
    Posts
    2,927
    Originally posted by pbibm
    I am now montioring the ontime and cycle time of each.
    Check your temperture rise.They should be about mid-range.
    (They should be printed somewhere on the furnace)

    Also,the location of your thermostats is critical.
    Sometimes there are compounding complexities of multiple variables that are not intuitively obvious

  8. #8
    Join Date
    Jan 2005
    Posts
    9
    Originally posted by jacob perkins
    Originally posted by pbibm
    I am now montioring the ontime and cycle time of each.
    Check your temperture rise.They should be about mid-range.
    (They should be printed somewhere on the furnace)

    Also,the location of your thermostats is critical.
    Jacob... thanks. I will look at the temperature rise. I'll get my HVAC pro if its not in the middle.

    I do know that the stats are placed right, at least I have two different professional opinions on that.

    Paul

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,082
    A longer run time is better for eff.

    How long do you plan to live in this house.

    If they are over sized, their using more fuel then they should.

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

  10. #10
    Join Date
    Apr 2004
    Posts
    7,680
    beenthere, dont make me pull out the ice cream again!

    Longer run times are for comfort more than efficiency, it costs more money to run the fan longer, technically. Short cycling is harder on the equipment and leads to uneven temps and large swings and if extreme can reduce efficiency since the time between firing and fan are fairly inefficient but its minimal.

    An oversized unit may waste some money in the efficiency losses up the chimney but the btuh output is the same over the course of an hour from either furnace. On just takes more of the hour to use it than the other.

  11. #11
    Join Date
    Jan 2005
    Posts
    9
    I will be here a long time... This is the house I want.

    Its really coming down to what I can controll, and what I can't. By spreading the setpoint vs trigger point on the stat, I can get the runtime longer on the big furnance. I actually have it go 1 degree above desired as the high point and two degrees below before triggering. Run times have gone from 6 minutes to 14 - 20, but....... Its a long time inbetween cycles now, up to 50 minutes.... thats a lot better.. And, because the space being heated overlaps heat from the front furnace moves into the back of the house. At least that looks like whats happening.

    The back furnance is harder. I can spread the setpoint vs trigger there too. Cycle times are still long, but it stays off for a long time in between.

    Overall the overall runtime per day of the units is down. The house is cooler in different spots in short spells as it cools off. So I need to work those setpoints to balance comfort and efficiency. And, for the big furnace, its running longer cycles.

    I beleive gas is more expensive then electricity now, so the fan run time should be made up by the furnace running less time. I'll continue to monitor that.

    Paul

  12. #12
    Join Date
    Apr 2003
    Location
    the Great Pacific Northwest
    Posts
    607
    Originally posted by coolwhip
    Short cycling= bad, long run time= bad.

    This is why it is important to have the system sized and installed by a professional. Thats why manual J and manual D were created.
    long run time= bad

    HUH???????? wouldn't the best system for comfort and eff. be the one that would run constantly and maintain set point, thats what I was taught.

  13. #13
    Join Date
    Apr 2003
    Location
    the Great Pacific Northwest
    Posts
    607
    Does the furnace that is suposedly "too small" keep up or does it run continously and loose control (space temp drops) if not it is not too small.

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