What are Atmospheric Modulating Boilers (Pre-Mixed)?
Modulating Boilers
Modulating atmospheric boilers are designed to control the output (size of flame) to match the boilers variable load requirements, during this process the burner is designed to stay at the correct fuel air ratios across the complete firing range ensuring maximum combustion and boiler efficiencies. Modulation refers to a boiler's ability to match firing rate - input - to meet the heating demand - output - of the system. Modulating boilers offer typical turndown ratios of 3:1 to 4:1.
Limited Turn Down Ratio:
- The "turn down ratio" is a function of the boiler's capacity to match the current base load of the system i.e. a 400kW boiler may be correctly sized for the application, for example the boiler at high fire will give a output of 400 kW (100%) and at low fire will give an output of 100 kW (25%) this would be referred to a "turn down ratio" of 4:1.
- If the above example is used and that the base load remains at above 100 kW the boiler will modulate without turning off and "dry cycling " will not occur. However, if the base load is below 100 kW the boiler will reach and exceed the set point and the burner will turn off and "dry cycle".
Other factors that will cause the boilers not to modulate:
With multi boiler applications the "turn down ratio" will be divided by the number of boilers i.e. 3 boilers rated at 300 kW each, with a turn down of 3:1 would give a total turn down of 9 :1 or from a minimum of 100kW to a maximum of 900 kW.
With modern building control (BMS) the boilers may be sequenced and controlled via weather compensation (VT), in this case the boilers can be subjected to lower flow rate and be unable to modulate under the minimum base load, this will cause the boilers to exceed the required set point and give cyclic operation ( "dry cycle" )
Summary:
Modulating boiler rarely operate as designed due to the above, further with the intervention of BMS control, the boiler can be turn off before the boiler has had time to fully modulate to low flame conditions or indeed at high fire. Historically 97% of modulating boilers do not have sufficient turn down or control to give constant operation and cause boilers to dry cycle, and therefore waste energy. By retrofitting M2G dry cycling can be removed - saving fuel costs and reducing CO2 emissions.
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