Dual fuel - Geo and oil boiler question

Discussion in 'General Discussions' started by kmckinley, Jul 25, 2008.

  1. kmckinley

    kmckinley New Member

    I'd like to have my oil fired boiler in the system to either back up my heating or augment the geo it if necessary. I'm planning on a heat only water - water system (5 ton) with an 80 gallon buffer tank that will supply a small amount of radiant heat and two hydro air handlers. My plan is to either find a twin coil buffer tank or use two buffer tanks, one for the geo and one for the oil burner. What would the best method be to install the dual tank set up. My thought is to have return water from the heating system enter the boiler buffer tank first then the geo tank then out to the the supply manifold. an aquastat on the geo buffer would call for additional heat if need be and the boiler would fire to heat it's own buffer tank. basically a pre heater for the geo tank

    MY design parameters call for the geo system to supply 130 degree water to it's load buffer tank.

    Ultimately, I'd like to have an outdoor thermostat regulating the system

    SO...is this feasible? smart? I'd like aquastat to vary the call for boiler pre heat (it may NEVER be required) based on OAT. that said, on a 40 degree day 110 supply in the geo buffer might be OK with no preheat but on a minus 20 degree day I might need the preheat to get the 130 degrees and have the aquastat call for the boiler at say 120-125 degrees.

    Probably thoroughly confused every one by now!

    So the questions are:

    1. dual tanks, twin coil tank, something totally different
    2. what do you guys recommend for a temp control system that works with OAT?



    ps I know if the geo system is properly engineered (and it is) the boiler augment won't be needed but I definitely want it as a backup.
  2. TechGromit

    TechGromit Member

    A. Geothermal systems generally only can heat water up to 120 degrees.
    B. Entering water temperature for the geothermal systems in heating mode shouldn't be any hotter and 80 degrees F, They can't extract more heat from water that hot and the higher pressures make the system shut down to protect itself from damage.

    I'm not sure I completely understand what you want to do, but it sounds like you have two completely seperate systems (with seperate water loops). The lines from the two systems are coiled together so they transfer heat to one another (or the hotter to the colder) so your preheating the water to the geo system.... why? Geo is the most efficent system, why would you want to use an oil based system to help it along? Any savings involved would be eaten up with the fuel the boiler requires. Although warmer water would help the geothermal system be more effiecent, anything hotter 80 degrees would force the system to shutdown or damage it.
  3. kmckinley

    kmckinley New Member

    EVERY Unit I've looked at can produce 130 degree water with a load side EWT of around 110 degrees. Injecting hot water from an oil burner into the hydronic loop would have no consequence in the load loop of a water-water system as it just goes to the buffer tank and back to the geo unit. The buffer tank IS the load. My hydro air and radiant would be heated by the buffer tank. This is a standard retro fit W-W installation method.

    In the unlikely event that my buffer tank can't get my hydronic loop running at 125 degrees (on a cold night) I'm planning on augmenting the hrdronic loop with my boiler.

    There are several ways to do this. Injecting hot water from the boiler into the hydronic loop or creating a another closed loop similar to the geo to buffer tank loop except it would be a oil burner to buffer tank loop, this would be added to the hydronic loop

    You can look at it this way. There are 3 loops

    1. the open source loop from my well to the geo unit
    2. a closed load loop from the geo to the buffer.
    3. A closed hydronic loop from the buffer tank to my air coils and radiant heat

  4. kmckinley

    kmckinley New Member

    I don' understand this.

    B. Entering water temperature for the geothermal systems in heating mode shouldn't be any hotter and 80 degrees F, They can't extract more heat from water that hot and the higher pressures make the system shut down to protect itself from damage.

    the entry source entry water temp is 50 degrees from the well. Heat is not extracted from the Load entry water it's added
  5. TechGromit

    TechGromit Member

    Depends on weather you talking about heating or cooling. In heating mode, the water entering the Unit should be no hotter then 80 degrees F.
    In cooling Mode, the entering water should be no hotter then 100 degrees F. When your heating, heat is extracted from the entering water, and when you cooling cold is extracted from the water. As for "Every system I've seen provides 130 degrees" statement, I would be interested in knowning where you got your information, the Highest I've seen is 125 degrees, usually its 120.

    I guess I don't understand what your trying to do, a Diagram would be helpful.

    The units shall be designed to operate with entering fluid
    temperatures between 50oF (10oC) and 110oF (43.3oC) in cooling and
    temperatures between 25oF (-3.9oC) and 80oF (27oC) in heating.

    Water Limits--------------------- Cooling------ Heating
    Min. Entering Water ______ 30 _______ 20
    Normal Entering Water ____50-110 ____30-70

    Max. Entering Water ______120 _______90


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