Unit 333 is the biggest thing on the qualification and the one that separates a fitter from a designer. It is also the only unit where getting the arithmetic right is not enough — you have to get it right in the correct order.

The short answer

Unit 333 — Central heating system planning and design — is 95 lessons and eighteen sections, and it is the longest unit on the Diploma by a wide margin. It runs the whole way from "how much heat does this building lose" to "why is that one radiator cold".

The spine of it is a calculation chain, and every part of the unit hangs off a link in it:

heat loss → emitter size → flow rate → pipe size → pump duty → boiler output

Change one input and everything downstream moves. That is why the exam asks you to work through it rather than to recall the end of it.

How the unit is shaped

The five parts of Level 3 Unit 333 central heating system design
The biggest unit, and the one with the heaviest calculation load.

System types and controls. One pipe, two pipe, S-plan, Y-plan, sealed and open vented, zoning, and what the Building Regulations require by way of control. The control questions carry more marks than most people expect.

The design calculations. Fabric heat loss, ventilation heat loss, heat gains, emitter selection from a manufacturer's table with the correct correction factor, pipe sizing on flow rate and pressure drop, pump duty, and expansion vessel sizing for a sealed system.

Underfloor heating. Its own section twice over: how it is designed (floor construction, pipe spacing, flow temperature, screed) and how it is installed and balanced.

Electrical work. Wiring diagrams, safe isolation, circuit testing, and connecting the controls. This is the part that most surprises people arriving from Level 2 — you are expected to read and produce a wiring diagram, not just connect what is on it.

Commissioning and handover. Filling, venting, flushing, inhibitor, balancing, setting to work, and the customer conversation that ends the job.

Fault diagnosis. Twelve lessons of it, which tells you how heavily it is weighted. Symptom to cause to test to remedy, on hydraulic faults, control faults and electrical faults.

What actually makes it hard

The calculation chain has to be done in order and it has to be done with the right figures. A radiator table gives output at a stated mean water temperature; using it at a different flow temperature without the correction factor gives a radiator that is the wrong size and a candidate who did the arithmetic correctly and still lost the mark.

Controls questions are really Building Regulations questions. Zoning, time control, temperature control, boiler interlock. Knowing what a two port valve does is Level 2. Knowing why a house over a certain floor area needs two space heating zones is this unit.

Wiring diagrams are read across, not memorised. S-plan and Y-plan look similar on the page and behave differently at the cylinder stat. The examiner will give you a scenario and ask what happens, not what the diagram is called.

Fault diagnosis rewards elimination. One cold radiator, a whole cold circuit, and a system that heats but will not satisfy the room stat are three different problems with almost no overlap in their causes. The mark is for saying which one you are looking at.

Where it connects to the rest of the course

Unit 332's cylinder is on the other side of most of these wiring diagrams. Unit 335's heat pumps change the flow temperature and therefore every emitter size in the chain, which is why the low-temperature design work sits in both. And Unit 336 is where the programme, the cost and the variation notice around a heating installation are assessed.

Every article on Unit 333

One article per section of the unit. Each is a complete answer on its own, and each ends with a self-test drawn from the course question bank.

Types of central heating system One-Pipe, Two-Pipe, Microbore, Open Vented and Sealed How the four layouts differ, the design temperatures and velocity limits, the neutral point on an open vented system, and where a sealed vessel goes. Controls and zoning Heating Controls and Zoning: S-Plan, Y-Plan, W-Plan and the Interlock Programmers against timers, zone valves and their wiring, S-plan and Y-plan compared, the 150 square metre zoning rule, and what boiler interlock really is. Underfloor heating Underfloor Heating: Floor Temperatures, Pipework and the Manifold Why a heated floor is capped at 29 °C, the pipe patterns and insulation figures, pressure testing before the screed, and how a manifold blends and protects. Multiple boilers and headers Low Loss Headers: When to Separate, and When Not To Why the boiler side and the system side disagree, what a low loss header actually fixes, and the order of preference from no separation to a four-port buffer. Factors and sources for the design Where a Heating Design Starts: the Brief, the Building and the Documents What customer needs mean on a design sheet, how the building and fabric-first change the order of work, fuel and efficiency, and which document wins. Heat loss and heat gain Heat Loss Worked Properly: Fabric, Ventilation and the Gains You Ignore U-values and the 0.33 ventilation factor, the design temperatures inside and out, a room worked end to end, and why solar and appliance gains are left out. Calculating and selecting components From Watts to Millimetres: Emitters, Boiler Size, Pipes, Pump and Vessel Correcting catalogue outputs to real temperatures, sizing the boiler and hot water load, flow rate and pipe size, and the index circuit and pump duty. Positioning and fixing components Where Everything Goes: Boiler, Cylinder, Pump, Valves, Radiators and Pipe Siting a boiler and cylinder and whose rules win, where a pump works best, the automatic bypass, radiator fixing heights, and clip spacing and insulation. Underfloor heating installation Installing Underfloor Heating: the Three Constructions and the Pour Solid, timber and floating floors compared, emission plates and output deductions, the layers before the screed, laying the pipe, and the test and the cure. Sealed systems and filling Sealed Systems: the Vessel, the Safety Valve and the Filling Loop Where an expansion vessel goes and what charge pressure it needs, the safety valve rules, backflow protection on a filling loop, and setting the cold fill. Supplying and connecting the controls The Electrical Side: Spurs, Certificates, Earthing, Cable and Terminations Why heating controls are fused at 3 A, checking the existing circuit under 132.16, minor works against an EIC, earthing and bonding sizes, and terminations. Wiring diagrams and circuit testing Reading Heating Wiring Diagrams, and the Tests That Prove Them S-plan, Y-plan and S-plan plus read off a diagram, the wiring centre and pump overrun, the dead test order, and what polarity and earth continuity each prove. Safe isolation and pre-installation checks Safe Isolation: the Seven Steps, Proving Dead, and Damaged Cable The seven-step isolation procedure and the step most often dropped, GS38 instruments and the three readings, and checking an existing installation. Wiring, filling and testing the controls Wiring, Filling and Proving a Heating Control System Supplying and isolating the controls, running sensor cables, filling with the pump isolated, the visual inspection, the dead tests and the interlock test. Inspection, filling, testing and flushing Filling, Soundness Testing and Flushing a Heating System The five conditions before commissioning, filling and venting in stages, the soundness test pressures and the plastics tests, and the BS 7593 cleaning order. Setting to work and handover Setting to Work: Operational Checks, Balancing and Handover Checking the vessel charge and proving the stats one room at a time, setting the flow temperature, balancing radiators and manifolds, and the handover. Finding the fault Finding a Heating Fault: Cold Spots, Pumping Over, Pressure Loss and Pumps Questioning the customer, reading fault codes and flow charts, radiator cold spots, pumping over, pump and valve faults, pressure loss and heat pump patterns. Putting the fault right Putting a Heating Fault Right: the Seven Steps From Diagnosis to Handback Notifying the client, temporary decommissioning and the compulsory isolation step, replacing like for like, restoring commissioned settings and handing back.

🔢 The numbers worth memorising

The chain
heat loss → emitter → flow rate → pipe size → pump duty → boiler output
Emitter output
quoted at a stated mean water temperature — apply the correction factor for yours
Design flow and return
traditionally 82/71 °C; a heat pump system is far lower, which changes every emitter
Sealed system
expansion vessel sized from system volume and temperature rise, pre-charge to the fill pressure
Boiler interlock
a Building Regulations requirement, not a preference
Fault diagnosis
12 of the unit’s 95 lessons — which is how heavily it is weighted

⚠️ Where people go wrong

  • Reading a radiator output straight off the table at a different flow temperature. The correction factor is where the mark is.
  • Sizing pipework on flow rate alone. Pressure drop per metre is the other half, and it is what decides between two sizes.
  • Confusing S-plan and Y-plan behaviour at the cylinder stat. They are examined on what happens, not on the name.
  • Treating balancing as optional. An unbalanced system meets its heat loss on paper and does not in the house.
  • Answering a “one cold radiator” question with causes that would make the whole circuit cold. Narrow it first.
Want to work through this properly? Unit 333 on PlumbMate is 95 lessons with quizzes, flashcards, key facts, AI-marked short answers and interactive tasks — written against the City & Guilds standards.
Open Unit 333 →

📝 10-Question Self-Test

Straight from the Level 3 course question bank. Click an option to see whether you got it right — the explanation appears instantly, and there is nothing to submit.

Your score: 0 / 10
Question 1 of 10
An expansion vessel forms part of which of these systems?
Question 2 of 10
In an S-plan heating system, which control operates the DHW two-port zone valve directly?
Question 3 of 10
In a Y-plan heating system, the boiler and pump are switched directly by which component?
Question 4 of 10
Once the call for heat is satisfied, what drives a two-port motorised valve back to its closed position?
Question 5 of 10
Fitting thermostatic radiator valves throughout a system is likely to bring down which of the following?
Question 6 of 10
At what height above the floor do the recommendations say a room thermostat should be mounted?
Question 7 of 10
In a Y-plan heating system, where does the switched live that feeds the boiler and pump come from directly?
Question 8 of 10
Where on a hot water storage cylinder should the cylinder thermostat be positioned?
Question 9 of 10
Guidance on the kind of central heating system to be fitted in a new domestic dwelling is found in which Building Regulations document?
Question 10 of 10
A new dwelling has a floor area over 150 m2. Which control arrangement satisfies the Building Regulations?
Start the series →One-Pipe, Two-Pipe, Microbore, Open Vented and Sealed

Going further: the Unit 333 lessons

These articles are the public answer. Unit 333 of the Level 3 course takes the same ground to the depth the exam and the synoptic assignment ask for, in 95 lessons across 18 sections, each with its own quizzes, flashcards, key facts, an AI-marked short answer and interactive tasks.

  • Types of central heating system
  • Controls and zoning
  • Underfloor heating
  • Multiple boilers and headers
  • Factors and sources for the design
  • Heat loss and heat gain
  • Calculating and selecting components
  • Positioning and fixing components
  • Underfloor heating installation
  • Sealed systems and filling
  • Supplying and connecting the controls
  • Wiring diagrams and circuit testing
  • Safe isolation and pre-installation checks
  • Wiring, filling and testing the controls
  • Inspection, filling, testing and flushing
  • Setting to work and handover
  • Finding the fault
  • Putting the fault right
Met a word you did not know? Every term in the Level 3 Diploma, explained in plain English and free to read. The dotted words above link straight into it.
Open the glossary →