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Short cycling causes?

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Mom's ancient Hydrotherm HC100, propane.

Been short cycling, using lots of fuel. Maintains high temp of 180°F, but flame goes on & off every few minutes.

I don't know what aquastat is on there, if memory serves it is a single high limit. Probably the original control from late 1960s.

What should I look for that could cause short cycling?

There are 2 zones, altho 2nd zone has been disconnected.

Not sure if thermostat anticipator setting is culprit? It's an old round manual stat that has the mercury switch. I don't know which pump control is there, but it's within 18 to 20 yrs old.

Any tips?

What is the usual temp differential for (single?) High limit aquastat before the flame should come on again?

Comments

  • Harvey Ramer
    Harvey Ramer Member Posts: 2,261
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    Is this a new problem?
    Make sure the pump is working correctly you aren't only working on gravity flow.

    Does the high limit have a adjustable differential?
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
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    I thought I noticed it some month ago but thought it was bc it was cold Winter. I'm not there visiting very often. My brother noticed it when he was visiting last week.

    I don't know if there is an adjustable diff or its fixed. That's part of what I am asking, is it typical for it to be a fixed diff? What should the diff be?
  • JUGHNE
    JUGHNE Member Posts: 11,111
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    HW round T-stats have an adjustable heat anticipator. Sometimes they short out and can cause short cycling.

    Easy test is to jumper T-stat wires and see what the boiler does.
    It should shut off on high limit with T-stat jumped. It should try to overheat the house if your pump is working....water flowing.

    Could the pump be a B&G 100.....maybe coupling broken ......or pump stuck??
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
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    Taco 007. I thought the aquastat works independant from circulator controller on this boiler?
  • Gordy
    Gordy Member Posts: 9,546
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    If this is a new development like Harvey said circ may not be working, or impeller partially damaged inhibiting proper flow rate.

  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
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    House is plenty warm. It just short cycles even during stsndby.
  • Gordy
    Gordy Member Posts: 9,546
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    Has it been mild?

    Is it zoned?
    If so are zones small?
    Is boiler over sized for the heat loss?
  • Gordy
    Gordy Member Posts: 9,546
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    That's the conceptual start to a flow chart isn't it.....
    SWEI
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
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    Can anyone tell me, what *SHOULD* the temp differential be for a high-limit aquastat on this boiler?

    If aquastat set at 180* and boiler temp rises properly to 180* and gas flame shuts down ..... at what lower temp should the aquastat fire up again?
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
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    Thanks for the answer.

    I'll have to go to mom's and watch the boiler temps for awhile and see how low the temp drops on standby before it fires again (without call for heat).

    I don't recall ever seeing an adjustment for the differential on this boiler, juse a high limit. Not sure which model came with Hydrotherm HC100 propane version.

    Lousy cellphone pic from few yrs ago (boiler was shut down for outside gas line repair). Maybe someone recognizes the aquastat ?
  • Zman
    Zman Member Posts: 7,607
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    I would first want to know if the short cycle is being initiated by the t-stat or if it related to the boiler control.
    If you turn the t-stat way up does it still do it?
    If it on the boiler side, check the circulator , then look at the controller.
    A controller with adjustable differential would be a nice upgrade in either case.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    Leon82
  • Leon82
    Leon82 Member Posts: 684
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    My Honeywell thermostats used to nuisance call for heat. Literally 20 seconds one time.
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
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    Could a gas valve be faulty and open on its own? Yeah, that sounds unlikely after I see it written!

    Is it more likely that the aquastat is some how faulty, turning the gas valve on too soon before the temp drops more than the aquastat differential?

    For some reason, I think that on this boiler the aquastat is not connected at all with the circulator controller. I'll have to get over there to verify but I wanted to be armed with more info beforehand.
  • Paul48
    Paul48 Member Posts: 4,469
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    "I'll have to go to mom's and watch the boiler temps for awhile and see how low the temp drops on standby before it fires again (without call for heat)".

    @Patchogue Phil
    Is this boiler cold-start or maintained?
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
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    Temp is maintained, not cold start.
  • Paul48
    Paul48 Member Posts: 4,469
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    Then it is a triple-aquastat, with hi, lo and differential. The differential works with the lo-limit, and hi-limit has a fixed 10* differential. Definitely look at the anticipator setting. It can cause rapid on-off cycles when there is a call for heat. As for abnormal cycling when in standby......is it possible you're getting gravity circulation?
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
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    I don't think it is a triple aquastat, only one temp setting with a steel gear wheel setting. No hw coil.
  • Paul48
    Paul48 Member Posts: 4,469
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    Hmmm...........I wonder why and how it is maintained then.
  • rick in Alaska
    rick in Alaska Member Posts: 1,458
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    TT on the aquastat is probably jumpered and unit is running hot all the time. Aquastat maybe just lost its differential? Instead of ten (?), maybe five?
    Rick
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
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    Regardless of what the differential was when it was new, I can almost guarantee it's not that any more. Mechanical controls are famous for losing set point and differential accuracy over time.

    As for hot deck operation, that in and of itself can be the cause of short cycling. Having a block of metal at 180 with any kind of decent stack height is going to compound standby losses. If you are worried about condensing the boiler, then I'd say 140 degrees F as a standby temperature should keep you relatively free from condensation production. If you can keep the pump off line until the block hits 160, you should be good to go, and reduce energy consumption to boot.

    Take a new aquastat with you.

    ME

    There was an error rendering this rich post.

    Gordy
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
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    Yes, I know this is an old thread. Same problem, but now I can fix it. My mom wouldn't let me or anyone else fix it. She was rather difficult to reason with in her old age. She passed away recently, so now I *CAN* fix it.

    Attached is pic of aquastat, and controls. Looking for recommendation for good aquastat, maybe new control - whatever will solve the short cycling problems.

    First, I'm thinking it needs a good flushing out. Hasn't been done since my Dad passed in 2002. Like I've said, Mom wouldn't let anyone fix anything.

    It's hot water, only for baseboard fin-tube heat.
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
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    Inside of control
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
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    Sorry to hear about your Mom.

    It probably has a fixed 10° differential. More important is to figure out why it's bouncing off the high limit.

    Adding on to all the other advice, you (and/or your heating professional) just have to go thru and check each component. ----Check that the system is properly filled.
    -Check that it's purged of air.
    -Check that the circulator is working properly and moving water.
    -Check supply/return water temps, see if they are matching up to the aquastat settings.
    After all of that, add up the btu's of the emitters vs. the btu output of the boiler.
    It just may be that your severely oversized.

    There was an error rendering this rich post.

  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
    edited October 2018
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    Thank you.

    Could be oversized (like most very old systems) , but it's been the same setup for over 45 yrs. Just a new circulator maybe 20 yrs ago, and new fill/pressure reducer about same time. It didn't short cycle except for last 3 years.

    I'm hoping someone can recommend an aquastat to use here.
  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited October 2018
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    Hi Phil,
    Sorry to hear about your Mom as well.

    Although not related to your original post, you seem to have some exhaust leakage in the area indicated by the arrow. You may want to have that looked into before putting any additional $$ into that boiler.




    I had the same boiler in my 1963 home in mid Suffolk (they probably installed thousands of them here on Long Island back in the 1960's). It developed an exhaust leak between the sections and was not worth fixing in 2016. It was leaking low levels of CO into my basement. Luckily the leak was very near the draft hood, so once the draft was established it pulled most of the CO up the chimney with the hot flue gasses. After running for 45min in the morning I could measure 12PPM CO in my basement. Sometimes it was higher if the boiler didn't run for long periods.
    It was 53 years old when I replaced it.

    Please be safe......
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,845
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    @patchogue phil

    You said 1 zone was disconnected that is probably some of the short cycling cause
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
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    > @EBEBRATT-Ed said:
    > @patchogue phil
    >
    > You said 1 zone was disconnected that is probably some of the short cycling cause

    That disconnected zone was added on approx 1977. Was disconnected in the mid 1990's. No short cycling until more recent time. I do not think the disconnected zone is the cause.
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
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    > @NY_Rob said:
    > Hi Phil,
    > Sorry to hear about your Mom as well.
    >
    > Although not related to your original post, you seem to have some exhaust leakage in the area indicated by the arrow. You may want to have that looked into before putting any additional $$ into that boiler.
    >
    >
    > Please be safe......

    Thank you.

    I recently read your thread about having same Boiler.

    I don't know what caused that scorch/soot mark. It's always been there since the mid-late 1970s when we moved in.

    For more than a decade I tried to convince mom to replace the boiler. She didn't see a need, didn't want to spend money altho she had enough. Woulda paid for itself in quick time even if a simple atmospheric but new system.

    Time to talk with my siblings about a new system. Again. I had brought it up recently. But it will be cold soon, gonna need heat. A new system will make the house more attractive to sell.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,578
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    If you are intent on selling the house, I would compare very carefully the cost of fixing vs replacing the boiler.
    It’s hard to know how far to go in improving a house for sale, as there are many things which also may need “updating”. For instance, new shingles may be the wrong color for the prospective purchaser, a new kitchen may have the wrong layout, and etc. The new owner may view the house as a “scraper”, and not care about the heating or anything else!
    If you will live in the house, then that is another matter.—NBC
  • Patchogue Phil_2
    Patchogue Phil_2 Member Posts: 304
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    > @nicholas bonham-carter said:
    >
    > If you will live in the house, then that is another matter.—NBC


    I get what you are saying. Not going crazy remodeling. Just clean out the house. Repair anything minor that could dissuade a buyer (non-flipper, non-lowball offers). Having reliable heat that's not wasting fuel is the idea too, we don't want huge expenses while carrying the house til sale.