Question 1

What factors determine the choice of hot water system?

Accepted Answer

Property size, number of occupants. System selection depends on: property size, distance to outlets, number of occupants and their demand, number of outlets, fuel type, installation/running costs, and mains pressure.

Question 2

What is the most common hot water system in UK domestic properties?

Accepted Answer

Open-vented indirect system with double-feed cylinder. The double-feed indirect system is the most common. It uses a cylinder with a heat exchanger coil, two cisterns (CWSC + F&E), and separates primary and potable water.

Question 3

Why is it now standard practice to install indirect cylinders even if central heating is not connected?

Accepted Answer

The indirect flow and return can be capped off ready for future connection to a heating system. Indirect cylinders are future-proofed. Even without CH, the flow and return connections are capped off for later use. This avoids replacing the cylinder when CH is eventually added.

Question 4

What is a direct hot water system?

Accepted Answer

A cylinder with NO internal heat exchanger. Direct = no heat exchanger. The water in the cylinder IS the water that comes out of the taps. Heated directly by a boiler touching the water, or by immersion heaters inside the cylinder.

Question 5

Why must a boiler in a direct system be made of copper, stainless steel, or bronze?

Accepted Answer

the boiler water IS the drinking water. In a direct system, boiler water goes straight to the taps. A steel boiler would corrode and produce rusty water. Only corrosion-resistant materials (copper, stainless steel, bronze) are acceptable.

Question 6

How does water circulate from the boiler to the cylinder in a direct system?

Accepted Answer

By gravity circulation. Gravity circulation: hot water is lighter and rises through the flow pipe into the cylinder. Cooler water is heavier and sinks through the return pipe back to the boiler. No pump needed.

Question 7

A direct system with immersion heaters typically uses how many elements?

Accepted Answer

Two 3kW elements. Bottom element heats all contents (usually overnight on cheap tariff). Middle element provides daytime top-up of just the upper half. Both are 3kW. Wired to a time controller.

Question 8

What temperature is recommended for immersion heaters?

Accepted Answer

55 deg C - balances running costs with scale prevention. 55 deg C is the recommended immersion heater setting. Higher temperatures increase scale build-up and energy costs. Lower temperatures risk inadequate hot water and Legionella.

Question 9

Why can a direct system NOT be used with central heating?

Accepted Answer

There is no heat exchanger to separate primary water from potable water. Central heating water contains inhibitor chemicals and sludge (fluid category 3). Without a heat exchanger, this contaminated water would mix with the drinking water. Indirect systems solve this.

Question 10

What is a gas circulator?

Accepted Answer

A small gas-fired hot water boiler designed to heat water directly in a direct system. A gas circulator is a small dedicated gas boiler used in direct systems. It heats the water directly (no heat exchanger) so must be copper, stainless steel, or bronze.

Question 11

What is thermal stratification in a cylinder?

Accepted Answer

Cold water enters at the bottom. Stratification = temperature layers. Hot water at the top (lightest), cold at the bottom (heaviest). The hot outlet is always at the top to draw the hottest water first.

Question 12

What makes an indirect system 'indirect'?

Accepted Answer

The stored water is heated INDIRECTLY via a heat exchanger. Indirect = heat transfers through a coil wall. Primary water (from boiler) flows INSIDE the coil. Potable water (for taps) is OUTSIDE the coil in the cylinder. They never mix.

Free unvented hot water (G3) exam questions

1. What factors determine the choice of hot water system?

2. What is the most common hot water system in UK domestic properties?

3. Why is it now standard practice to install indirect cylinders even if central heating is not connected?

4. What is a direct hot water system?

5. Why must a boiler in a direct system be made of copper, stainless steel, or bronze?

6. How does water circulate from the boiler to the cylinder in a direct system?

7. A direct system with immersion heaters typically uses how many elements?

8. What temperature is recommended for immersion heaters?

9. Why can a direct system NOT be used with central heating?

10. What is a gas circulator?

11. What is thermal stratification in a cylinder?

12. What makes an indirect system 'indirect'?

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