N ATIONAL D IPLOMA M ECHANICAL AND E LECTRICAL S ERVICES C ONSTRUCTION Hot Water Pipe Sizing and Boiler Recovery Times

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION Aims To understand the process of calculating the correct diameter of pipe for a domestic hot water system. Objectives To understand the terms primary flow and return flow. To investigate the hot water system in a domestic situation. To complete a calculation based on a domestic hot water system.

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION In any hot water system, the hot water travels through the system by means of convection. This rate of movement is relatively slow, about 0.2 m/s. This is dependent on the length of pipe and the location of the boiler and hot water cylinder. However in the more modern systems, a we usually fit a circulation pump, to increase the velocity of the hot water from 0.2 m/s to between 0.5 and 3 m/s. If the speed of the hot water increases above this, it could lead to excessive noise, and will also lead to damage to the water pipes in the form of corrosion.

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION In any system hot water is pumped from the Boiler, known as primary flow, circulates around to the cylinder, and back to the boiler, known as primary return. The temperature difference between primary flow and primary return is normally around 10°C for a system that incorporates a circulation pump. 20°C for a system that relies purely on convection.

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION To calculate the size of the hot water pipe required we use the formula Mass Flow Rate (kg/s) = Boiler net heat input SHC x Temp Diff (pf – pr) Boiler net heat input = size of your boiler in kW. SHC = Specific Heat Capacity of water = 4.2 kJ/kgK K = degrees Kelvin temperature interval Temp Diff (Primary Flow – Primary Return) For these calculations assume a water temperature of 80° C

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION Example – A large domestic property with a 30kW boiler will require what size of pipe? (in this example we will assume the system has a circulation pump, with a velocity of 1m/sec).

Mass Flow Rate (kg/s) = Boiler net heat input SHC x Temp Diff (pf – pr) Mass Flow Rate (kg/s) = x 10 Mass Flow Rate (kg/s) = Mass Flow Rate = (kg/s) Referring this figure to the Copper Development Design Chart, this would indicate a 35mm pipe.

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION If we take the same example and calculate the size of pipe required for a system relying on convection only, of a velocity of 0.2m/s Mass Flow Rate (kg/s) = Boiler net heat input SHC x Temp Diff (pf – pr) Mass Flow Rate (kg/s) = x 20 Mass Flow Rate (kg/s) = Mass Flow Rate = (kg/s) This would indicate a pipe size of 54mm.

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION With any hot water system, the size of the boiler is based on the number of people, and the building arrangement as to its size and performance. In addition the capacity of any hot water storage vessel must also be able to meet the requirements of the building. Two important considerations for any heating system is the rate of energy consumption, (measured in Kilo watt / person) And the time it takes for a system to reheat the water. A good example of this is to calculate the time it takes a hot water system to reheat the water, after you have just filled the bath.

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION Building PurposeStorage Capacity (Litres / person) Energy Consumption ( kW / person) Dwellings: Single Bath Multi Bath Factory / Office50.10 Hotels Hostels Hospitals Schools / Colleges Day50.10 Boarding Sports Pavilions351.00

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION In our first example we talked about a large domestic property and calculated a pipe size of 35mm for the hot water system. If this large domestic property had 5 bedrooms and two bathrooms, a hot water system that was supplied by a 30kW Boiler, how long would it take to reheat the system if the bath had been filled with hot water? The formula we would use is Boiler Power = Kgs of Water x SHCx Temp Rise Time in Seconds However we need to transpose this formula for Time

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION Time in Seconds = Kgs of Water x SHC x Temp Rise Boiler Power Kgs of Water = from the table we identify the storage capacity for the number of people living in the house. For this calculation we will assume there are 6 people in the house. SHC = Specific Heat Capacity of water 4.2 kJ/kgK Temp Rise. = What do we want the temperature to be (60°C) – The temperature of the cold water refilling the cylinder. (in summer this maybe 15°C or in Winter it could drop to 5°C. The boiler power for this example we have already identified as 30kW.

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION Time in Seconds = Kgs of Water x SHC x Temp Rise Boiler Power Time in Seconds = (6 x 45 ) x 4.2 x (60 – 15) 30 Time in Seconds = 270 x 4.2 x Time in Seconds = Time in Seconds = 1701 (However we need to convert this from seconds to minutes by dividing by 60) Time = 1701 ÷ 60 = minutes (29 mins)

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION Time in Seconds = Kgs of Water x SHC x Temp Rise Boiler Power Time in Seconds = (6 x 45 ) x 4.2 x (60 – 5) 30 Time in Seconds = 270 x 4.2 x Time in Seconds = Time in Seconds = 2079 Time = minutes (35 mins)

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION So how could we speed up this process and make the hot water system heat up quicker than 29 (summer) and 35 (winter) minutes. Install a bigger boiler. So have a go at using a 40kW boiler in the same system and calculate how much quicker the system would heat up.

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION Time in Seconds = Kgs of Water x SHC x Temp Rise Boiler Power Time in Seconds = (6 x 45 ) x 4.2 x (60 – 15) 40 Time in Seconds = 270 x 4.2 x Time in Seconds = Time in Seconds = (÷ 60) Time = mins. (summer)

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION Time in Seconds = Kgs of Water x SHC x Temp Rise Boiler Power Time in Seconds = (6 x 45 ) x 4.2 x (60 – 5) 40 Time in Seconds = 270 x 4.2 x Time in Seconds = Time in Seconds = (÷ 60) Time = 26 mins. (winter)