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Condensing Boiler and Outdoor Reset for Radiant Floor

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dcwittlo
dcwittlo Member Posts: 16
in Radiant Heating

My question has two parts.

  1. Is the return water temperature from a radiant floor system low enough to really take advantage of the latent heat (sensible heat vs latent heat) of the combustion products in a condensing boiler? It is my understanding that temperatures below 130 F are necessary to get a significant amount of condensation (caused by transfer of latent heat from combustion gasses to the circulating water). I live in the Upper Peninsula of Michigan and we get some pretty cold days in the middle of winter. Last year we had at least 2 days where early morning temperatures were -20 F and at least a week of low temps of -10 F. I already have a modulating/condensing boiler with baseboard heat emitters. The house stays warm but return temps are high enough that I don't think we are getting the efficiency of condensing operation.
  2. In radiant floor systems the advice is to keep floor temps between 80 F to 85 F or it will be uncomfortable on ones feet. Since this is a relatively narrow range of temperatures, does Out-Door Reset on the Supplied Water Temperature have much of an advantage over just a fixed temp at the higher end of the range? I think it is possible to set ODR with a very "flat" curve (slope?). My concern is the cost of a mixing valve with ODR vs a mixing valve with a fixed temp. I might need to keep some of the baseboard heat, so I might not be able to just lower the boiler output for the entire system.

Comments

  • GGross
    GGross Member Posts: 1,969
    edited 4:55PM

    with a mod/con if all your heat were infloor no need for a mixing valve. You would need to do a radiant design to determine your required supply temperatures, but most radiant floors will return condensing temps 100% of the time when set up right. Outdoor reset will generally help achieve more condensing operation, as you are running the lowest needed supply temperature for the given outdoor temp, therefore getting the lowest return water temp which equals more condensing operation. You can also just design your baseboard emitters based on condensing water temps. I'm in the tip of the mitt area and managed to be condensing through the entire winter season this year on baseboard (have a couple of panel rads as well) Not quite as brutal as some areas in the UP but we get pretty cold here too lol.

  • hot_rod
    hot_rod Member Posts: 27,829

    setting up the ODR takes some trial and error.
    if you know the heat load of every room, and the amount of fin tube in each room, use the manufacturer output tables to see what supply temperature is required. That gives you a starting point for the ODR setup.

    Heat load is not an exact science. Outdoor temperature, wind, sun exposure, internal gains, make that load number very dynamic, possibly changing minute by minute.

    Most of the mod cons have a boost function. If there is an ongoing heat call and the boiler temperature is not rising, it will ramp up the burner. So even you under size the ODR adjustment, the boiler will compensate. It is an adjustable function both time and temperature .

    There are a number of ways to pipe a two temperature system.

    If you have just one zone of low temperature radiant, run the boiler at the required high temperatures, based on ODR.

    A simple manual 3 way mix valve dusts the low temperature. It rides along with the reset curve. As the high temperature supply changes, so does the low temperature.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream

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