Level 3 Plumbing Revision: Design, Unvented and Faults

At Level 3 the questions stop being “what is this part?” and become “why did it fail, and how would you design it so it does not?” That is also the difference between a fitter and the engineer customers call when something has gone wrong at 2am.

Unvented hot water: the safety chain

An unvented cylinder stores hot water at mains pressure, so it must never be allowed to boil. Learn the three levels of protection in order:

1. The control thermostat, which stops heating at the set temperature.
2. The high-limit (energy cut-out) thermostat, which cuts the heat source if the control thermostat fails.
3. The temperature and pressure relief valve, which opens to discharge water if both thermostats fail.

Approved Document G sets out the discharge arrangement: a D1 pipe from the valves to a tundish that is visible and within 500 mm of the valve, then a D2 pipe with at least 300 mm of vertical drop below the tundish before any bend, running to a safe, visible discharge point. Expect at least one question on this. Our page on unvented cylinders shows what this looks like in a real airing cupboard.

Other parts to know: the pressure-reducing valve, the line strainer, the expansion relief valve and the expansion vessel or internal air bubble.

Designing systems

• Pipe sizing: use loading units and the flow and pressure available to choose pipe sizes, so that the worst-placed outlet still gets its design flow.
• Heat loss: heat loss in watts equals the U-value, multiplied by area, multiplied by the temperature difference. Add up each room’s walls, windows, floor and ceiling, then add ventilation losses to size radiators and the heat source.
• Bath temperature: new baths in new dwellings must be limited to 48°C at the outlet, normally with a thermostatic mixing valve.
• Efficiency: heating controls must include time and temperature control, zoning where appropriate, and boiler interlock so the heat source switches off when no heat is wanted.

Environmental technologies

Know the principles, benefits and limits of air and ground source heat pumps, solar thermal, rainwater harvesting and greywater reuse. Heat pumps run most efficiently at low flow temperatures, which is why they pair well with underfloor heating and larger radiators. Rainwater and greywater systems must be fully separated from the drinking water supply and their pipes clearly labelled.

A fault-finding method that passes exams and works on site

1. Listen: ask what happened, when, and what changed. Half the faults we find are explained in the first two minutes of conversation.
2. Observe: pressures, temperatures, sounds, error codes, water marks.
3. Isolate: narrow it down to one part of the system by testing, not by swapping parts.
4. Repair: fix the cause, not just the symptom.
5. Test and commission: prove the system works as designed and record your readings.
6. Explain: tell the customer what failed, why, and how to avoid it.

Write this list from memory before each exam. Examiners give marks for a logical sequence, and customers notice it too.

Commissioning and handover

Every system you install should be flushed, tested, set up to the manufacturer’s figures and handed over with instructions and paperwork. Practise filling in a commissioning record properly; it is the evidence that the job was done right, and the first thing an engineer like us looks for when called to a fault later.

Where to go next

Revisit our Level 2 notes for the foundations, and see our guide to boiler fault codes for how fault-finding looks from the customer’s side.