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There are various types of water heaters for hot water consumption available in the market today. These hot water heaters can be classified by a series of technical characteristics that give the different kinds distinct features.

    1. Direct versus Indirect Fire:
      • –Direct Fire Heaters
        As the name indicates, direct fire heaters heat the water directly, with either a flame or the heated combustion gases of a burner coming into direct contact with the tube through which the water circulates or the container which stores the water.
        Characteristics: Domestic hot water normally contains dissolved salts and minerals. The walls of the tube in which the water circulates or the container in which the water is stored, upon being heated, reach high temperatures. At temperatures higher than 158°F, the dissolved salts and minerals become embedded, sticking to the heat transfer material. These salt and mineral deposits form an insulating layer called scale, which impedes the proper heat transfer required to cool down the tube or container material. The heat transfer mechanism is thus gradually reduced and with it, the efficiency of the boiler, until eventually, the heat transfer material becomes perforated or burned.
        Efficiency: As mentioned above, the efficiency of direct fire heaters steadily declines, drastically reducing the useful life of the heater.
      • Indirect Fire Heaters
        With indirect fire heaters, domestic hot water is heated by means of water or steam in a closed circuit, thus preventing major scaling and its negative consequences.
        Characteristics: The flame or combustion gases first heat the water or steam in the closed circuit which then transfers the heat produced to the domestic hot water with the help of a heat exchanger. The domestic hot water is heated indirectly, thus eliminating or drastically reducing the problem of scaling and the negative consequences that are brought about by direct contact with high flame or combustion gas temperatures.

        In order to achieve a long useful life and a high level of heat transfer efficiency that will remain constant over the years, an indirect fire heater is required. For optimal performance results, a larger, more expensive heater will be necessary.


Calderas Powermaster recommends using only indirect fire heaters to ensure a long useful life, high efficiency levels throughout its life, lower fuel consumption, reduced maintenance costs, and the greatest reliability.

    1. Pass versus Deposit:
      • –Pass Heaters
        The exchange or heating of domestic hot water is carried out directly in the water pass through the heat exchanger, which is normally heated by direct flame.
        Advantages: Pass heaters do not require a water storage tank and as such provide space savings.
        Disadvantages: When domestic hot water is being used for multiple things simultaneously (for example, showers), there will be large temperature fluctuations which cause obvious annoyances for the end users who must continuously adjust the water temperature. Pass heaters also have shorter useful lives.
      • Deposit Heaters
        *Heaters with a large container, or if necessary, a hot water storage tank
        With the help of a hot water recirculation pump the water, upon being heated, is recirculated between the heater heat exchanger and the water in the container or storage tank.
        The heat exchanger can be installed within an indirect fire water heater or within the hot water storage tank.
        Advantages: Having a large volume of hot water stored eliminates temperature fluctuations in the consumption outlets (e.g. showers). The quality of the bathing experience is greatly improved as nuisances caused by temperature fluctuations are eliminated. Deposit heaters also have longer useful lives.
        Disadvantages: Deposit heaters require larger equipment and therefore necessitate a larger machine room.


Calderas Powermaster recommends using a pass heater only if there is one end user (e.g one shower) and a deposit heater in the event of more than one end user, this to guarantee a more constant water exit temperature and sufficient supply to cover any water demand surges during peak hours, thus achieving a longer useful life for the equipment and a more comfortable and high quality bathing experience.

  1. Condensing versus Non-Condensing:
    • Condensing Heaters
      Condensing heaters are those in which combustion gas temperatures reach temperatures below the bulb temperature, causing the evaporated water contained within to convert to its liquid phase (i.e. to condense) and with that to deliver the latent energy of the phase shift to the domestic hot water to be heated.
      A condensing heater must be designed to support the corrosion caused by the condensed gases and to drain the corresponding accumulation of water. Normally, a condensing heater requires low water temperatures at entry (around 59°F) in order to achieve condensation. This is usually not the case. When the entry water temperatures are higher and combustion gas condensation does not occur, a condensing heater functions the same as a non-condensing heater.
    • Non-Condensing Heaters
      Non-condensing heaters are those in which the equipment design does not contemplate combustion gases arriving at temperatures lower than the bulb temperature of the gases and therefore does not require special materials to support the corresponding corrosion and drainage. Normally, all deposit heaters are also non-condensing in their design.


Calderas Powermaster recommends using a condensing heater only in the absence of a water reserve deposit because when a deposit exists, the temperature of the water at the boiler inlet will be greater than the maximum required to achieve condensation and as such the unit will perform like a non-condensing heater.