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Falacy of Outdoor Reset

Gilmorrie
Gilmorrie Member Posts: 186
edited December 2015 in THE MAIN WALL
The idea of outdoor reset is to reduce the hot-water set-point during mild temps, and increase it during colder temp.

Let's assume that your radiation is oversized for the coldest day (which many, including mine, are). If on the coldest day, let's assume that you can keep you house warm with 160 deg water - which, for the sake of argument, let's assume is just warm enough to avoid any condensation in the flue. Why would you then want to jack up the boiler temp higher than 160 deg, which reduces boiler efficiency?

I'm talking about a conventional hot-water boiler, not a condensing unit.

Comments

  • Steamhead
    Steamhead Member Posts: 17,387
    Less standby loss in the piping is one reason. Also, a continuous supply of water that is just warm enough is more comfortable than the intermittent heating-up and cooling-down you get without reset.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
    CanuckerSWEIJean-David Beyer
  • SWEI
    SWEI Member Posts: 7,356
    This would be what we refer to as a "partial reset" strategy. A Taco SR-501-OR-4 will do this for under $200 and is worth every penny for systems that can use those temps IMO. Full reset on a conventional CI boiler will require mixing, which requires at least 2x that in materials cost.
  • Gilmorrie
    Gilmorrie Member Posts: 186
    "The only caveat is your hot water production. Trying to heat an indirect with 150°F supply water temperature is going to take forever."

    I don't use my boiler for heating domestic hot water. I have a stand-alone gas-fired water heater.
  • Zman
    Zman Member Posts: 7,611
    If the domestic is piped as a priority you can run a hotter temp for domestic.
    There are quite a few indirects out there that have significant surface area and will function just fine at 150.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 23,405
    Zman said:

    If the domestic is piped as a priority you can run a hotter temp for domestic.
    There are quite a few indirects out there that have significant surface area and will function just fine at 150.

    And don't some boiler controls allow you to set the high temperature limit for DHW priority function?

    I know with enough plate style HX surface area you can get a 2-3° approach.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Gordy
    Gordy Member Posts: 9,546
    edited December 2015
    The OP's question is why you would want to turn the aquastat higher than 160, if 160 is a high enough SWT to accommodate the heat loss. On a CI boiler with out an indirect.

    You would not want to turn it up higher. Unless your experiencing a high system delta of 30-35* that brings your return temp down to sub condensing temps for a CI boiler.

    Not sure how this relates to the title of the post?

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