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  1. #1

    Question how short is short?

    Just had the heat running for the first time this season.
    The system is a cast iron oil boiler with copper fin and tube baseboard radiators. I haven't really paid much attention to the cycling time of the boiler before, but I'd like to check and see if it is in an acceptable range.

    It wasn't all that cold out last night, about 40degF, but enough to run the heat. The boiler was running ON ~2 1/2 minutes and off for 10 minutes. It looked like the thermostat stopped requesting heat as the boiler temp only went up to ~165F before it shut off. (When the water heater runs, it goes all the way up to 195F or so)

    I'm guessing that it will start running longer once the outside temp drops.
    Does this sound reasonable or should I expect to be calling the boiler svc co?

    Thank you.

  2. #2
    Join Date
    Mar 2007
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    The colder it gets outside, the more it will run. The warmer you keep it inside, the more it will run. To give you any kind of answer, We need to know your current outdoor temperature, your indoor temperature, and the design outdoor temperature for your city. Where your house is may help as well.

    Your question was vague. Our answers will be also. GIGO.
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  3. #3
    I will try to provide more info.

    House is located in southeast MA. The nearest cities would be New Bedford, Fall River and Boston.

    Yesterday the outdoor temp at night was 40F and inside was set at 70F.

    The house is ~1000 sq ft, one floor with full basement. It's kind of a long rectangle with the long sides facing east/west. There are 3 heat zones. One for each end of the house upstairs and one for the "man cave" area in the basement. The basement heat doesn't get used on much. House is fairly old and some windows will be replaced next season.

    If there is anything else that may help, I'll do my best to post it.

    I guess I'm looking to see if that kind of on / off time is OK for the boiler. Don't want to stress it any more than is necessary. Also, maybe there is something we could do about the temp settings? The thermostats are the old style honeywell mercury switch type.

    Thank you very much for the input.

  4. #4
    Join Date
    Jan 2004
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    Lancaster PA
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    Some what common on zoned systems. Where the boiler maintains water temp for domestic hot water.

    Might want to have your HVAC company install digital thermostats. They can have their CPH set. to help prevent those short cycles.
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  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
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    6,274

    Thumbs up INCREASE YOUR COMFORT LEVELS

    Quote Originally Posted by brainslice View Post
    House is located in southeast MA. The nearest cities would be New Bedford, Fall River and Boston.

    Yesterday the outdoor temp at night was 40F and inside was set at 70F.

    The house is ~1000 sq ft, one floor with full basement. It's kind of a long rectangle with the long sides facing east/west. There are 3 heat zones.

    I guess I'm looking to see if that kind of on / off time is OK for the boiler. Don't want to stress it any more than is necessary.

    Also, maybe there is something we could do about the temp settings? The thermostats are the old style honeywell mercury switch type.
    1. Turn the Boiler temperature down SIGNIFICANTLY for about 7 weeks.
    2. Change out the t-stats

    RESULT: Less equipment stress, less Owner stress and a lot more Comfort.
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  6. #6
    Thank you for the input. I'll be making some calls here to see what I can have done. Will post back with some results, probably in a couple of weeks or so.

  7. #7
    Join Date
    Sep 2002
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    Boilers are one type of heating equipment where there can be several right answers to one question, it all depends on how your system is set up and what type of control system is installed, translated "there are just about as many ways to install and control a boiler as there are plumbers (who probably should not install them to begin with) to install them". Old plumber told me that one day when I was trying to figure out some aborted multi zone system with two boilers siamesed with one common header, ten circulators, two 24 hour timers, half a dozen outside thermostatic set backs and more non color coded low voltage relays and wires than I could count.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  8. #8
    Join Date
    Nov 2008
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    1,673
    Quote Originally Posted by brainslice View Post
    It wasn't all that cold out last night, about 40degF, but enough to run the heat. The boiler was running ON ~2 1/2 minutes and off for 10 minutes.
    I'm guessing that it will start running longer once the outside temp drops.
    100(2.5/[10+2.5] = 20% duty cycle for an inside/outside ΔT = 30F, so for a 100% duty cycle your outside temp. would have to be
    -80F?
    Sounds like it's oversized.

    Definitions can be hard to come by, but from another forum, on times of 6 to 10 min for cast iron boilers probably mean short cycling.
    But with the same 20% duty cycle, 6 min. on time may overshoot your setpoint temp.

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Quote Originally Posted by WhoIsThat? View Post
    100(2.5/[10+2.5] = 20% duty cycle for an inside/outside ΔT = 30F, so for a 100% duty cycle your outside temp. would have to be
    -80F?
    Sounds like it's oversized.

    Definitions can be hard to come by, but from another forum, on times of 6 to 10 min for cast iron boilers probably mean short cycling.
    But with the same 20% duty cycle, 6 min. on time may overshoot your setpoint temp.
    Its a zoned system. If only 1 zone is calling. Then only 1/3 of the load the boiler is sized for was calling/losing heat. So it may very well be properly sized.
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  10. #10
    Join Date
    Sep 2002
    Location
    Northeast Ohio
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    Quote Originally Posted by beenthere View Post
    Its a zoned system. If only 1 zone is calling. Then only 1/3 of the load the boiler is sized for was calling/losing heat. So it may very well be properly sized.
    Here in lies one of the problems when it comes to boilers, oil boilers in particular. This can be especially bad with a zoned system that does not maintain temp during no call for heat (no domestic coil) and the boiler cold starts during milder weather.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  11. #11

    Wink

    Indeed, I only turned on the heat in one zone the other night. Should probably try out the other ones to make sure everything is functioning before we actually need it.

    About the plumbers: On the right side of the boiler is a nice, clearly marked metal plate that says, among other things, to not obstruct access to that side of the boiler. Of course the plumber located the circulators and the water heater right next to it. The service man was cursing that one a few times. To make it better, there is another 10 feet to the right and left of the boiler, go figure ??

    About the sizing of the boiler... I don't have any calculations for the house to see where it should be. The system was installed long before I moved in here. One thing about the boiler I do know is that prior to the most recent service visit (details posted in another thread) it was over-fired by about 15-20% for who knows how long.

    I have wondered about the cold start and if it's putting extra wear on the boiler. During the summer it can go 16 hours or more off and the temp drops down close to room temp.

    So many options, so much info to process. All is much appreciated.

    After all the reading around here, you guys make me wish I could just toss the whole thing in the scrap yard and get a nice proper new system installed Maybe after the truck is paid off But for now I'll start by seeing what I can have done about the old thermostats and go from there.

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