CENWAT Pipework Sizing and Installation — Questions and Answers
Question 1: What is the standard method for calculating the flow rate used for sizing domestic central heating pipework in the UK?
- Use the boiler manufacturer maximum flow rate only
- Calculate from the heat load using the formula: flow rate equals heat load divided by specific heat capacity multiplied by temperature difference (Correct answer)
- Use a fixed rate of 10 litres per minute for all systems
- Base it on the number of radiators only
Correct answer: Calculate from the heat load using the formula: flow rate equals heat load divided by specific heat capacity multiplied by temperature difference
Domestic central heating pipework is sized based on the calculated heat load. The flow rate is determined using Q = H / (SHC x DeltaT), where Q is the flow rate, H is the heat output required, SHC is the specific heat capacity of water (4.186 kJ/kg K), and DeltaT is the design temperature difference between flow and return.
Question 2: What is the recommended design flow and return temperature difference (DeltaT) for a condensing boiler system to maximise efficiency?
- 5 degrees C
- 11 degrees C
- 20 degrees C (Correct answer)
- 30 degrees C
Correct answer: 20 degrees C
For maximum condensing efficiency, a design DeltaT of 20 degrees C (for example 70 degrees C flow and 50 degrees C return) is recommended. This ensures the return water temperature is low enough, below the 57 degrees C dew point, to promote condensation in the secondary heat exchanger.
Question 3: When installing copper pipework for central heating, what is the minimum clip spacing for horizontal 15mm copper pipe?
- 0.6 metres
- 1.2 metres (Correct answer)
- 1.8 metres
- 2.4 metres
Correct answer: 1.2 metres
BS EN 1057 and industry guidance recommend that horizontal 15mm copper pipework should be supported at intervals not exceeding 1.2 metres. For 22mm pipe the interval is 1.8 metres, and for 28mm pipe 2.4 metres. Proper support prevents sagging and strain on joints.
Question 4: What is the purpose of inhibitor in a central heating system, as recommended by BS 7593?
- To increase the water temperature
- To prevent internal corrosion and limescale formation (Correct answer)
- To reduce the water pressure
- To improve boiler combustion
Correct answer: To prevent internal corrosion and limescale formation
BS 7593 recommends the addition of a suitable corrosion inhibitor to protect the system from internal corrosion (which produces black iron oxide sludge) and limescale formation. Inhibitor must be added at the correct concentration and checked annually during boiler servicing.
Question 5: What is the maximum velocity of water recommended in domestic central heating pipework to avoid noise?
- 0.5 m/s
- 1.0 m/s
- 1.5 m/s (Correct answer)
- 3.0 m/s
Correct answer: 1.5 m/s
The maximum recommended water velocity in domestic central heating pipework is 1.5 m/s. Exceeding this can cause noise from turbulent flow, water hammer, erosion of pipe walls, and vibration transmitted through the building structure.
Question 6: When installing a new central heating system, what cleaning procedure does BS 7593 require before adding inhibitor?
- No cleaning is required for new systems
- A hot flush with clean water only
- A pre-commission clean using an appropriate cleaning agent, followed by thorough flushing (Correct answer)
- Simply adding inhibitor to the system water
Correct answer: A pre-commission clean using an appropriate cleaning agent, followed by thorough flushing
BS 7593:2019 requires a pre-commission clean of new systems using an appropriate cleaning chemical to remove flux residues, oil, grease, and debris from installation. This is followed by thorough flushing until the water runs clear, then adding inhibitor at the recommended concentration.
CENWAT Pipework Sizing and Installation
What is the standard method for calculating the flow rate used for sizing domestic central heating pipework in the UK?