Boiler Over SizingBoiler Over Sizing

Anthony McGarry, Senior EngineerAnthony McGarry, Senior Engineer

RPS GroupRPS Group

Hotel AHotel AHotel AHotel A

• Maximum boiler capacity installed 3,100 Maximum boiler capacity installed 3,100 kWkW

• Capacity reduced to 1,800 kW by;Capacity reduced to 1,800 kW by;– Removing one Jet on 1,100 kW boilerRemoving one Jet on 1,100 kW boiler

– Reducing flow rate of existing jet by 11%Reducing flow rate of existing jet by 11%

• Actual capacity used is 450 + 480 = 930 kW for Actual capacity used is 450 + 480 = 930 kW for both heating, leisure centre and hot water.both heating, leisure centre and hot water.

– EPI for 2006 of 342 kWh/m2/pa which is at the EPI for 2006 of 342 kWh/m2/pa which is at the Good practice levels of 340kWh/m2/pa (260 Good practice levels of 340kWh/m2/pa (260 kWh/m2 – fossil fuels & 80 kWh/m2 – kWh/m2 – fossil fuels & 80 kWh/m2 – Electricity) based on CIBSE Guide F- Electricity) based on CIBSE Guide F-

• This was achieved by having a very large hot water buffer!This was achieved by having a very large hot water buffer!

Energy Consumption at Hotel AEnergy Consumption at Hotel AEnergy Consumption at Hotel AEnergy Consumption at Hotel A

– Difficult to say with certainty – could not contact designer, Difficult to say with certainty – could not contact designer, but the possibilities are;but the possibilities are;

• Heat loss not calculated i.e. U-values for wall, windows, floor Heat loss not calculated i.e. U-values for wall, windows, floor and flues etc.and flues etc.

• Heat loss factor too highHeat loss factor too high

• Maximum heat, hot water and leisure centre load added Maximum heat, hot water and leisure centre load added together with a large error margin and subsequently leisure together with a large error margin and subsequently leisure centre had its own dedicated boiler.centre had its own dedicated boiler.

• Boiler capacity duplicated with second back-up boilerBoiler capacity duplicated with second back-up boiler

• Design changesDesign changes

• Hot water demand per occupant too highHot water demand per occupant too high

• Nobody was sued for over sizing boilersNobody was sued for over sizing boilers

How was Boiler over sized in Hotel A?How was Boiler over sized in Hotel A?How was Boiler over sized in Hotel A?How was Boiler over sized in Hotel A?

– Elementary heat loss calculation to determine heat loadElementary heat loss calculation to determine heat load

• State all assumptions/specification made, any building State all assumptions/specification made, any building changes should be communicated and a revaluation of the changes should be communicated and a revaluation of the heat loss carried out.heat loss carried out.

– Hot Water load – best to have separate boiler for hot water.Hot Water load – best to have separate boiler for hot water.

• Again specification/assumptions stated, any future design Again specification/assumptions stated, any future design should trigger a revaluation of loadshould trigger a revaluation of load

– One Boiler for HW & Heating the ‘Maximum Simultaneous’ load One Boiler for HW & Heating the ‘Maximum Simultaneous’ load should be calculated not the maximum load.should be calculated not the maximum load.

– Boiler size (X2) at 50-60% of max load, modulating control down Boiler size (X2) at 50-60% of max load, modulating control down to 10% of design capacity to 10% of design capacity

– Hot water load typically 1 hour recovery, if less boiler size will Hot water load typically 1 hour recovery, if less boiler size will increaseincrease

How should Boilers be sizedHow should Boilers be sizedHow should Boilers be sizedHow should Boilers be sized

In RealityIn RealityIn RealityIn Reality• Heat loss guessed at Heat loss guessed at

• No time invested in engineering the boilerNo time invested in engineering the boiler

• Cheapest available boiler – Budget constraintsCheapest available boiler – Budget constraints

• Maximum load calculated plus excessive margin for heating Maximum load calculated plus excessive margin for heating efficiency, distribution losses, system poor insulation, future efficiency, distribution losses, system poor insulation, future expansions etc etc.expansions etc etc.

• Excessive boiler size margin for possible engineering changesExcessive boiler size margin for possible engineering changes

• No designer employed or changes made by M&E engineer, No designer employed or changes made by M&E engineer, Supplier, Customer etcSupplier, Customer etc

• Boiler sized duplicated for back-upBoiler sized duplicated for back-up

• The just in case syndrome! – Better looking at it that looking for The just in case syndrome! – Better looking at it that looking for it!it!

Thank YouThank You

Anthony.mcgarry@rpsgroup.ieAnthony.mcgarry@rpsgroup.ie

Boiler SizingBoiler SizingBoiler SizingBoiler Sizing