Estimating hot water demand and sizing a cylinder

From demand to storage volume

Sizing is about matching the store to how hot water is actually used. Work from the peak, the busiest short period, rather than only the daily total, because the cylinder must cover several outlets drawing together.

A structured estimate

1. Count the demand points. Bathrooms, en-suites, the kitchen and the number of occupants set the scale of the peak. A family bath uses far more hot water than a basin.
2. Identify the peak event. Often this is morning showers, or a bath being filled while the kitchen is in use.
3. Check storage and recovery together. Compare usable hot-water reserve and heat delivered during and between draws with the expected demand. Confirm performance from the manufacturer’s data at the actual conditions.

Heating a greater mass of water through the same temperature rise needs more energy. Standing losses also depend on surface area, insulation and the temperature difference from the surroundings. A larger cylinder can still run short if demand exceeds its usable reserve and recovery; a smaller one can be adequate when its combined delivery meets the use pattern.

Worked sense check

An 80 litre bath at 40 °C, mixed from water at 60 °C and 10 °C, draws a hot fraction of (40 − 10) ÷ (60 − 10) = 0.6. That is 48 litres of hot water, with 32 litres of cold, before allowing for heat losses. One draw does not set the nominal cylinder size: include other demand, useful storage performance and recovery.

Storage and recovery together

• Compare each expected draw with the remaining usable reserve and the heat being restored. Whether one or several baths can be served depends on their volumes, timing, mixed temperatures and the system’s actual output.
• Recovery depends on the heat the source can deliver and the exchanger can transfer at the actual temperatures and flows. A coil rating taken at different test conditions may not be the duty you will get.
• Apply a justified diversity method for the building and its use. Simultaneous operation of every outlet can occur where the usage pattern makes it realistic; do not discount demand arbitrarily.
• A store maintained hotter than its surroundings loses heat even with outlets closed. Compare the actual standing-loss data and avoid unnecessary storage that also turns over slowly.
• Heated water needs the specified expansion provision. An unvented cylinder may use an external vessel or internal air volume; routine heat-up should not rely on relief discharge.
• Stratification and tapping positions affect the temperature available at draw-off. Useful mixed-water output also depends on the temperature profile, heat input and demand; it is not simply the nominal volume of one layer.

Worked mixing example. The hot share of a mixed draw is (mixed temperature − cold temperature) ÷ (hot temperature − cold temperature). A 120 litre bath at 42 °C, mixed from stored water at 60 °C and cold at 12 °C, takes (42 − 12) ÷ (60 − 12) = 30 ÷ 48 = 0.625 of its volume from the cylinder: 75 litres.