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Modulating oil fired boilers

wpmike Member Posts: 13
I have come across some oil fired hot water boilers for heat that "modulate", for instance the ones I found are the Buderus G115WS and the Burnham MPO-IQ. Not sure if there are others.

As far as I know, burners have a constant firing rate, none of literature I read explains how it changes output based on the outside temperature. These units do use an Outdoor Reset. Does anyone know how they achieve this modulating?

More importantly what are your opinions on these type of units? Does it actually work? I suppose a limitation is it can't lower the return water temp. below 130 F because of condensation issues. Are there any other manufacturers / units that you like or recommend. Any opinions or recommendations are appreciated.



  • STEVEusaPA
    STEVEusaPA Member Posts: 6,132
    edited January 2019
    Modulate is probably an incorrect term relating to firing rate.
    Only 2 stage burners modulate, and it's not much, until you get to the big commercial burners.

    The 2 boilers you mention use controls with outdoor reset to lower the target supply temperature. Buderus allows lower return water temperatures than Burnham.
    It technically 'works' but in practice not very well if without the use of a buffer tank, unless the boiler is properly sized and you don't have many zones-and even then, not very well in the shoulder seasons.

    For instance my MPO w/ODR has severely short cycled without a buffer as it targets 140° most of the shoulder season and goes up to 180° only at design temp. Most of the winter it's only targeting 150°-160°, which it can get to in minutes.

    Combine that with it targeting 90°to 120° for the injection mixing loop for the radiant, and the boiler would only fire for a minute or two to bring the supply temp from 130° to 145° to hit the radiant loop target and get above minimum return temp. Then the injection pump does it thing, the boiler supply temp hits the 130° minimum rather quickly, the injection pump stops, the boiler fires for a short time, and over and over until the call for heat is satisfied.

    With the Tekmar boiler control (like many others), it holds off injection mixing when you hit the minimum, so tremendous short cycling bouncing between those 2 numbers.

    So buffer tank with large differential, and boiler supply control sensor in the buffer tank, and having the boiler's only job to charge the buffer tank. All zones work off the buffer, except an indirect. This seems to me to be the best way to go.

    No true modulating/condensing oil equipment has really been introduced that people will get behind in USA, although Europe is taking a poke at it.

    Unless you go with an Energy Kinetics, whole different (better) approach/concept.
  • Jamie Hall
    Jamie Hall Member Posts: 21,116
    The problem with true modulating (firing rate) oil burners is that while it can be done, one not only has to somehow modulate the oil flow while keeping a decent nozzle spray pattern -- no mean trick -- but separately modulate the air to match. Not that it can't be done -- consider an aircraft gas turbine engine, for example -- but that it isn't worth the money required for the necessary sensors and controls.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,132
    edited January 2019
    There are 2 stage residential burners, that modulate via fuel pressure and air, just another solenoid for the oil pump, and the control changes the air gate.
    The problem is their range is quite limited, unless you go to the big commercial burners.
    I think Dragon is using/testing/experimenting with one in his home (or he used to).
  • wpmike
    wpmike Member Posts: 13
    edited January 2019
    Thanks guys, I kind of guess that, the only way to control the water temp., the burner would have to be turned on and off....

    So it sounds like there boilers are a type of Kludge to get them to "modulate" the water temp. No insult with the Kludge term.