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When a boiler is running flat out and not supplying the load, how low does boiler temperature get?

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VelvetFoot
VelvetFoot Member Posts: 48
edited October 2015 in THE MAIN WALL
Does it get down to the boiler protection level, say 135F or so?

That's my question, but I'll throw in some background as well.

I downrated my modulating pellet boiler for longer run times and hope to shave off the peaks with an oil boiler that is piped in parallel (and wood insert, but forget that for now).

Now, I want to figure out a way to kick in the oil boiler, with some comfort.
I figure some combo of thermostat, outdoor temp, and pellet boiler temp.
Setting the oil boiler as stage2, if pellet boiler can't keep room temp to within x of setpoint, maybe after a certain time, maybe if the pellet boiler temp doesn't go over 138 after an hour, maybe if it's less than 10 degrees out or whatever.

Throw in a setback recovery and it gets more complex.

Any insights would be helpful.

Comments

  • SWEI
    SWEI Member Posts: 7,356
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    A two stage thermostat might be enough.
  • hot_rod
    hot_rod Member Posts: 22,262
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    As far as the first part, the amount heat emitters connected to the boiler will determine the operating condition. All systems strive for, and arrive at thermal equilibrium, unless you interrupt the operation.

    When the boiler is oversized for the load, operating controls and limit switch on the boiler take control.

    When the boiler is undersized the temperature conditions in the system will drop until the balance point is found. Of course that could be a dangerous condition to run the boiler if it was not designed for low return temperatures.v

    Here is how it looks and a good explanation behind the physics.

    Also follow the link to idronics 10, there are some drawings in there very close to what you describe, along with piping and wiring schematics.

    We didn't show the pellet boiler schematics with backup, usually a modulating pellet boiler with a buffer tank matches most applications, and backup in not commonly added. But the wood fired schematic with fossil fueled back up would be the same concept.
    www.caleffi.com/sites/default/files/coll_attach_file/idronics_10_0.pdf

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • VelvetFoot
    VelvetFoot Member Posts: 48
    edited October 2015
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    Thanks for your nice reply Bob. It certainly points out the hazards of condensation, which I believe I'm protected against with the ThermoMix and the low temp shutoff on the boiler circ pump.

    As I read it, as outside temps drop to very cold, and I'm not running the wood insert, the boiler temperature on my pellet boiler will slowly drift down meanwhile as all zone circs are running constantly. Perhaps I could start the oil boiler when the pellet boiler reaches an equilibrium point of138, with the dropping outside temperature? I wonder what that could correspond to as far as building temperature? I guess if it started at, say, 68, it would stay there. No chance of setback recovery or domestic water unless a booster shot of oil boiler is given.

    I seem to have 57680 total baseboard emitter capacity spread out over two floors. My pellet boiler is derated now to 34,000 btu/hr.

    I am already seeing longer pellet boiler run times with the derate, in this mild weather, supplying a little heat to the zones and the buffer tank. I've been told that the longer the run time on the pellet boiler, the better.

    I was inspired by this article by John Siegenthaler: http://www.pmengineer.com/articles/91347-pellet-fueled-boilers-are-sized-differently-from-conventional-units

    Of course, he has a mod-con boiler as a peaking shaver. :)
  • hot_rod
    hot_rod Member Posts: 22,262
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    Guess I'm confused as to why you want to kick in the oil, when you have the pellet derated? Why not just allow the pellet to ramp up to meet the load on design days?

    How did you derate the pellet boiler?

    What is the load at design day? Is that the 57,680 you mentioned? If so can you derate the pellet to that output, then add a buffer to keep the long hot burns? I thought pellet boilers modulate to some degree by changing the auger speed?

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • VelvetFoot
    VelvetFoot Member Posts: 48
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    "How did you derate the pellet boiler?"
    There are dip switches that change the rating from 10 to 15 kW. The boiler modulates down to 30%.

    "What is the load at design day? Is that the 57,680 you mentioned?"
    That is the number I calculated at one time from the linear feet of conventional baseboard.

    The 10kW, or 34,000 btu/hr rating of the boiler is well below that. But maybe the 57,680 requirement is high, and maybe also including the insert, it'll be good for most days.

    " I thought pellet boilers modulate to some degree by changing the auger speed?"
    I believe they change the duration or frequency (I haven't tried to figure out which) of the periodic energization of the pellet auger motor, not the speed.

    It was the article above that gave me the idea to try this.
    I don't think there'll be any harm done with my little experiment, at any rate


    .
  • Steamhead
    Steamhead Member Posts: 16,901
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    You need to do a heat-loss calculation on your house. You may not need to run the baseboard at its full capacity if the heat loss is less than the baseboard's capacity.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • SWEI
    SWEI Member Posts: 7,356
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    Steamhead said:

    You may not need to run the baseboard at its full capacity if the heat loss is less than the baseboard's capacity.

    And that would be a good thing.

  • VelvetFoot
    VelvetFoot Member Posts: 48
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    Steamhead said:

    You need to do a heat-loss calculation on your house. You may not need to run the baseboard at its full capacity if the heat loss is less than the baseboard's capacity.

    Or, I could just wait 'til it gets real cold out. :smiley:
    But seriously, that's a good idea. Seems tedious though.
  • Quercus
    Quercus Member Posts: 61
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    But seriously, that's a good idea. Seems tedious though.

    It can be tedious but it would be a requirement for designing close to the edge and answering your questions. Blatant oversizing is the answer when you don't have the heat loss.

    If the whole house has the same type of insulation and construction and is a simple shape, you should be able to get the whole structure ballparked without a lot of detail. Room by room will let you see what is going on in your zones. I did my heat loss old school using the IBR manual but I see Slant-fin's calculator mentioned here. Put in the whole house size as if it's one room, add up all the windows and doors and see what you get. That should tell you if you have a big mismatch without too much time invested.