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Optimum Water Temp

kenb1 Member Posts: 1
I have a Burnham PV-85WC oil burning tankless furnace that provides hot water to baseboards via insulated 1/2 in copper tubing to approx 13 zones (every room) in my home. I would like to know the lowest water supply temp I can use to most efficiently burn oil and avoid issues that I have no idea of because I'm not knowledgeable. The reason I ask is because I am supplementing the temp of the boiler's return water with heat from a heat exchanger on my solar hydronic system that only provides 130-145 degrees F max on sunny days in the winter. Problem is, the boiler's min supply temp is 170 I think (may be 165), but anyway, at that lowest setting the return water temp is about 135-140. Still above the avg. temp of the heat exchanger on my solar system's heat exchanger, so I get 0 benefit from it. I am purchasing a manual controller for the boiler that will allow me to set the oil burner's supply and return temps manually versus the current factory-limited digital controller. I will attempt to lower the supply water temp so that I can get a benefit from the solar system's heat exchanger on the return water temp.

The bottom line is this: is it better (more efficient) to maintain a room at say, 68 F by making the boiler produce lower temp water (140) for a long time, or produce scalding hot (200+) water for shorter time? Because if it's the latter, I may as well give up my solar hydronic heat exchanger. Does anyone know?


  • Paul48
    Paul48 Member Posts: 4,469
    Feeding water below 140* back to the boiler will make the boiler condense and rot the boiler out.
  • wogpa67
    wogpa67 Member Posts: 238
    edited March 2015
    What is the primary function of the solar?
    Are you using a storage tank?
    Pics of all near boiler piping and solar HX and piping would be good.
    The factory control for the boiler is fine. Don't buy another control.

    What's controlling the solar?
  • icesailor
    icesailor Member Posts: 7,265
    This is confusing. If the boiler is as you describe, the factory supplied digital control will mimic any analog control made. It replaces about every type of control made. I hope you can send the new control back. You have two systems. You have to figure out how to make them converse with each other in a language they understand.

    You need someone that understands controls to help you.
  • wogpa67
    wogpa67 Member Posts: 238
    edited March 2015
    Start by reading this.

    You need as much storage as you can afford to put in.1.5 to 2.5 gallons per sq ft of total collector surface area Remember you can only move that heat inside when you have daylight.
  • SWEI
    SWEI Member Posts: 7,356
    Upgrade your emitters so they can use the heat from your solar tank.
  • hot_rod
    hot_rod Member Posts: 22,261
    SWEI said:

    Upgrade your emitters so they can use the heat from your solar tank.

    I agree, the solar will only have value if the heat emitters can deliver enough heat at those solar temperatures you are recording.

    In the mean time the solar still has DHW value :)

    Drive the solar tank as low as possible, use the differential function on the solar controller to call on the boiler.

    The boiler can handle low return temperatures if it has low temperature protection like a thermostatic valve that re-circulates thru the boiler until the return temperature is adequate.

    All boilers start from a cold return temperature condition. With low mass emitters like fin tube, the boiler gets up to a safe operating temperature quickly. With high mass radiant for example, not so quickly.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream