Domestic Hot Water System - a Design Procedure

A design procedure of domestic hot water systems

The Demand of Hot Water - quantity and temperature

Hot water is normally supplied to the fittings and consumers at 50 - 60^oC. For canteens and professional kitchen a temperature of 65^oC are required to satisfy most hygienic standards. Hot water should not be stored at temperatures below 60^oC (140^oF) to avoid the risk of legionella.

Where a lower temperatures are necessary for safety reasons - as in kindergartens, centers for disabled etc. - the hot water temperature should not exceed 40 - 50^oC.

Note! The hot water can be stored at higher temperatures and reduced to supply temperature by mixing with cold water in blender valves. Storing hot water at a higher temperature increases the systems overall capacity and reduces the need of storage volume.

The quantity of hot water is determined by number of occupants and their consumption habits.

The type, capacity and heating surface of calorifier - or heat exchanger

A hot water accumulator will reduce the required maximum heat supply. The heat supply capacity to a system with an accumulator can be calculated as:

H = 4.19 V (q₁ - q₂) / t (1)

where

H = heat supply capacity (kW)

V = accumulator volume stored (liter)

q₁ = temperature of the cold feed water (^oC)

q₂ = temperature of the hot water (^oC)

t = available time for the accumulated volume to be heated (sec)

(1) can be modified to express heated accumulated volume if heat supply capacity and available time for heating is known:

V = H_a t_a / 4.19 (q₁ - q₂) (1b)

where

H_a = heat supply available (kW)

t_a = heating time available (sec)

With an instantaneous heater with no accumulating calorifier the heat supply can be calculated as:

H = 4.19 v (q₁ - q₂) (2)

where

v = required volume flow (liter/s)

The heating surface of a heat exchanger can be calculated as:

A = 1000 H / k q_m (3)

where

A = heating surface (m²)

H = rate heating (kW)

k = overall heat transmission coefficient (W/m²K)

q_m = logarithmic mean temperature difference (K)

Heat transmission coefficient depends material and construction of the calorifier.

The boiler

Boiler with the correct rating must be selected from manufacturer catalogues where

Boiler rating = Heating capacity of calorifier + safety margin (normally 10 - 20%)

Design pipe scheme and size pipes

The maximum volume flow through connection pipes to fittings and other equipment is determined by the maximum demand of each consumer or fitting.

The maximum volume flow through main pipes is determined by the maximum demand of the fittings and statistic demand based on the number and types of fittings and equipment supplied.