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Whole Building Heat Loss Calculator

HeatLoss.pngThis heat loss calculator is designed to provide the total heat loss for a building and incorporates some deviations from generally accepted practice. These deviations are used because this software is aimed to help those installing biomass boilers in particular. The basis of the design is given briefly here but a more detailed explanation will be posted on this site as soon as it is available.

This program does not include a 15% increase in boiler size as an intermittent heating allowance.
The biomass boiler does not turn-down to less than about 30% MCR (maximum continuous rating), so to reduce intermittent firing we install an accumulator or buffer vessel with the boiler. As a rough guide this vessel would be around 100 litre/kW for log boilers and 25  litre/kW for chip or pellet boilers. The large volume for log boilers is because they have to burn out their charge whereas the chip and pellet boilers have automatic ignition. Also, some wood chip burning biomass boilers will kindle, which means that they never go out but simply keep the fuel bed hot by small regular injections of fuel and air, As an aside this system works less well with variable quality fuel and is slightly less efficient than boilers with automatic ignition. Currently automatic ignition is limited to 500kW with most manufacturers. Back to the solution. A 2,500 litre accumulator with a 100kW chip boiler would store around 58kW.hr. which means that it can provide an early morning boost of  around 129kw for a period of two hours. So when sizing the biomass boiler do not add an intermittent heating allowance.

The heat loss from the building is not the sum of the heat loses from each room or sector.
The program calculates the heat loss through the various parts of the building fabric. The glazing is simplified to a selection of either more than or less than 20% of the external walls. The heat loss through the floor is taken as a function of the perimeter and not the area.

The ventilation allowance for calculating the heat loss is two air changes.
The ventilation allowance in the program is a function of the exposed envelope of the building i.e. the total area of the external walls and the roof but not the floor. It is debatable whether the floor should be included in the envelope. As most properties have solid or at least sealed ground floors we have decided not to include the ground floor in the calculation. The result is that, as the size of the building increases so do the number of rooms, so the ventilation rate decreases. The heat load due to the ventilation  is a substantial part of the total heat load in modern buildings which have low fabric losses. This is certainly an area for further discussion.
The formulae used are ; VentHeatLoss = AirChange x Volume x TempDiff x 0.33 and AirChange = Envelope / Volume x f / 0.33 where the factor f is thenumber shown in the selected "Ventilation rate".

This program is offered free of charge in the hope that it will help heating engineers, consultants, architects, and designers to design and accommodate biomass boilers. We do not offer any warranty with the use of this software but trust that some will find it useful. If you have any comments, criticisms, or suggestions then please use the e-mail contact below.

Thank you for your interest,
George Fletcher

George Fletcher