What a truth table proves when you commission a heating system

Commissioning means proving it works before handover

COMMISSIONING is the last job on a new or changed system. It is not “it fired, so it works”. It is proof that every control does its own job: the programmer’s times, each thermostat, each valve, and the interlock between them.

A TRUTH TABLE makes that proof complete. It lists each state the controls can be in, and what each part must do in that state. You set each state in turn and watch. When every row behaves as the table says, the controls are commissioned.

Why bother, when the boiler fires? Because a system can fire and still be wrong. A valve that opens when nothing asked for it, or a boiler that runs with nothing calling, wastes fuel every day it is left. The customer will not notice until the bills come, and by then you have left. The Benchmark commissioning checklist that comes with many boilers asks about the controls too, the boiler interlock among them.

Polarity and earth continuity come first

Before any row, make the dead tests, with the circuit isolated, locked off and proved dead:

• Earth continuity to every part with an earth, and to every parked earth core. A loose earth core can end up touching a live screw.
• Polarity, by continuity: every fuse and single-pole switch in the live, never in the neutral.
• Insulation resistance where the job calls for it, between the conductors and to earth. Disconnect electronic controls first, as their makers’ instructions say: the test voltage can damage them.

Then, with the supply on, confirm polarity at the spur, at the wiring centre and at a load. Live to neutral reads about 230 volts (230 V). Live to earth reads about 230 V. Neutral to earth reads close to 0 V.

Why first? A system with live and neutral swapped can seem to work perfectly. But the fuse then sits in the neutral, and the controls switch the neutral. A part its control has switched off is still joined to the live, and the next person to open it can get a shock.

The rows of an S-Plan’s truth table

An S-Plan has a heating valve and a hot water valve, each opened by its own thermostat. Its table has these rows:

Set one row at a time, and give the valves time to move. A motorised valve takes seconds to open, and the boiler waits for it.

A Y-Plan’s table has the same idea with one valve. Heating only sends the valve to the heating port. Hot water only leaves it at rest on the hot water port. Both together hold it at the mid position. See what white, grey and orange do.

The interlock, row by row

The BOILER INTERLOCK means that when nothing calls for heating or hot water, the boiler and the pump are switched off. On an S-Plan only the valves’ orange cores feed the boiler’s switched live, through their end switches. So the boiler fires only once a valve is fully open.

The rows prove it piece by piece:

• Heating only: watch the order. The valve opens first, then the boiler fires. A boiler that fires before the valve has moved is fed from somewhere other than an end switch.
• Hot water only: coming from the heating-only row, the valves swap over. The boiler stops for a moment as the heating valve shuts, then fires again once the hot water valve’s end switch closes.
• Both satisfied: this is the row that proves the interlock. The programmer still allows both, but neither thermostat wants heat. Both valves shut, and the boiler stops.

Prove that last row with the meter as well as your eyes. On the wiring centre, the block where the orange cores meet reads about 230 V to neutral in the heating-only row, and 0 V in the both-satisfied row. The boiler’s switched live is dead, so the boiler cannot fire.

In England, Approved Document L (2021 edition) asks for exactly this: controls wired so that, with no demand for space heating or hot water, the boiler and pump are switched off. Work on a system with a boiler should leave it with an interlock. Thermostatic radiator valves (TRVs) alone do not provide one: they shut radiators but cannot switch the boiler off. Since April 2018 the Boiler Plus standards, now in the same document, have also asked for time and temperature controls when a boiler is fitted in an existing home. Wales, Scotland and Northern Ireland have their own. The truth table is how you show yours works.

When every row is proved

Only when every row behaves as the table says is the system commissioned. Then show the customer the controls: how to set the times, what each thermostat does, what the system should do when all is well. Fill in the boiler’s commissioning checklist.

When a row fails, the table has already done half the fault finding. It tells you which demand is wrong. Set that row again and trace its chain with the meter: the fault lies between the last point with 230 V and the first without. See last 230 V, first 0 V.

When a row works that should not, such as a valve opening in a row where nothing asked for it, something is feeding that part from the wrong place. Follow its wire back to where it lands.