Coolant min. T
The minimum operating temperature of the heat medium in the boiler system when operating in heating mode.
Coolant max. T
The maximum operating temperature of the heat medium in the boiler system when operating in heating mode.
DHW max. T
The maximum temperature of domestic hot water supplied by a dual-circuit boiler. For comparison, we note that water begins to be perceived as warm, starting from 40 °C, and in centralized hot water supply systems, the temperature of hot water is usually about 60 °C (and should not exceed 75 °C). Accordingly, even in the most modest models, this figure is about 45 °C, in the vast majority of modern boilers, it is not lower than 50 °C, and in some models, it can even exceed 90 °C.
Also when heated to a given temperature, the temperature difference ("ΔT") may be different — depending on the initial temperature of the cold water. And the performance of the boiler in the DHW mode directly depends on ΔT; see below for performance details.
Efficiency
The efficiency of the boiler.
For electric models (see "Energy source"), this parameter is calculated as the ratio of net power to consumed; in such models, indicators of 98 – 99% are not uncommon. For other boilers, the efficiency is the ratio of the amount of heat directly transferred to the water to the total heat amount released during combustion. In such devices, the efficiency is lower than in electric ones; for them, a parameter of more than 90% is considered good. An exception is gas condensing boilers (see the relevant paragraph), where the efficiency can even be higher than 100%. There is no violation of the laws of physics here. It is a kind of advertising trick: when calculating the efficiency, an inaccurate method is used that does not take into account the energy spent on the formation of water vapour. Nevertheless, formally everything is correct: the boiler gives out more thermal energy to the water than is released during the combustion of fuel since condensation energy is added to the combustion energy.
Expansion vessel capacity
The capacity of the expansion tank supplied with the boiler.
The expansion tank is designed to drain excess water from the heating system when the total volume of liquid increases as a result of heating. It consists of two parts connected by a flexible membrane: in one, hermetically closed, there is air under pressure; in the other, excess water enters, compressing the membrane. In this way, a catastrophic increase in pressure in the heating circuit is avoided. The optimal volume of the expansion tank depends on several system parameters, primarily the volume and composition of the coolant; detailed recommendations for calculations can be found in special sources.
Expansion vessel pressure
It is a pressure in the hermetically sealed part of the expansion vessel (for details on the design, see Expansion vessel capacity). The required pressure in the expansion vessel must be approximately 0.3 bar higher than the initial pressure in the system. The initial pressure, in turn, directly depends on the total height of the heating system or, rather on the difference between the height of the highest and lowest points of the heating system. It can be derived using the approximate formula P=H/10, where P is the initial pressure in the bar, and H is the height difference between the highest and lowest point of the system in metres. Thus, if the height difference is 2 m, the initial pressure in the system is 0.2 bar, and the pressure in the expansion tank must be at least 0.5 bar.
