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does aquastat control pump and burner independently

timo888 Member Posts: 137
We have a Buderus G115/4 attached to radiators in an old uninsulated house. It is controlled by a Honeywell aquastat with the dial set to the 190 tick mark. Please see pictures.

Normally during the winter the burner runs for only a few minutes at a time and the return pipe never gets very hot. Today after the heat had been off for a week and the radiators were cold (inside house was 64F), the boiler ran for 45 minutes and the return got quite hot.

The DHW tank had been calling for heat about 1/2 hour earlier during morning showers, and the boiler gauge read 170 when I turned up the thermostat for the house. The zone valves opened, the circulator started, and the boiler gauge started dropping steadily 160..150..140..130..120..110..105 and the Riello started firing at 105. Where is that low limit set?

The boiler cut off at 30psi/200F. The temperature in the house hadn't yet reached the temperature set on the thermostat upstairs (67F). Is the zone valve supposed to remain open and the circulator supposed to keep running while the thermostat temperature is not yet reached, even after the boiler cuts off because it reaches/exceeds the high limit? Or when the boiler hits the high limit, does the aquastat shut everything off?


  • James Day_2
    James Day_2 Member Posts: 191

    When boiler reaches high limit, aquastat should shut off burner but keep pumps running as long as there is a call for heat.
  • timo888
    timo888 Member Posts: 137
    and what setting starts the burner firing

    Thanks for the answer to the question about the pump and zone valve remaining open while there's a call for heat.

    I was surprised that the Riello did not fire until the boiler gauge had dropped to 105. What condition(s) trigger the burner to start firing? I would expect it to be this:

    if CallForHeat = true AND boilerTemp < X°

    But I don't see any way to set {X°} on the L8148A. Is the temp at which the burner is told to fire a factory preset that cannot be changed? And is there a lag of several minutes between the boiler's dropping to that temperature and the aquastat's sending out the signal to the burner?
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    edited May 2011

    We call it differential, and it is not adjustable on that aquastat/relay package. I believe it is fixed somewhere around 5 to 10 degrees F below set point, so your boiler burner should have fired when the water was 10 degrees F below the setting.

    Could be a bad aquastat, or a bad sensor location, or an outdoor reset. But if it is and ODR, it is set way too low for your app.

    There may also be a time delay built in to avoid short cycling...

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • timo888
    timo888 Member Posts: 137
    edited May 2011
    lowered the aquastat to 180F

    Thanks for the additional clarification.

    No ODR.

    Aquastat on is sitting on top of the Buderus G115. Does the aquastat have a probe that must be fully immersed in water in order to work? Would a low-water situation cause it not to register correctly? Does it measure water temperature directly or use a diaphragm to measure boiler pressure? I guess it would be called a pressure-stat if it did the latter, but no harm asking.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Why, Why, Why

    Would a contractor install a 3-pass oil boiler such as this on cast iron without using the outdoor reset function available to get the most out of that boiler. Such a shame that your not using that boiler to its potential.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • timo888
    timo888 Member Posts: 137
    edited May 2011
    say what?

    First of all, I'm the homeowner trying to recover from a less-than-stellar job by the heating oil company who installed the unit. The "less-than-stellar" is not my judgment, but the judgment of multiple people here who know what they're doing.

    It has an aquastat. There's no controller that senses outdoor temperature (like the Logamatic). And I'm not about to spend $1000 for one, while some basic shortcomings about the plumbing need to be addressed.

    I don't know what "3-pass oil boiler" means.

    But maybe I misunderstood you???? Maybe we agree that the installers put in a a cadillac with chipmunk running on a wheel to give the passengers some breeze?
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