Gas Safe Registration (ACS) Domestic Heating Systems, Controls and Energy Efficiency Flashcards

24 sample cards from the Domestic Heating Systems, Controls and Energy Efficiency deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. What is weather compensation control and how does it improve efficiency?

   Weather compensation uses an outdoor temperature sensor to vary the boiler flow temperature according to outside conditions: lower flow temperatures in mild weather and higher in cold. Running cooler flow temperatures keeps a condensing boiler in condensing mode longer, improving seasonal efficiency and comfort by reducing overshoot.

2. What is load compensation and how does it differ from weather compensation?

   Load compensation modulates the boiler flow temperature based on the room temperature relative to its setpoint (the heating 'load'): as the room approaches setpoint the flow temperature is reduced. Weather compensation uses outdoor temperature instead. Both lower return temperatures to boost condensing efficiency; load comp uses indoor demand, weather comp uses external climate.

3. What is Boiler Plus and when did it come into effect in England?

   Boiler Plus is a 2018 (effective 6 April 2018) Building Regulations Part L standard in England setting minimum performance for domestic gas boiler installations. It mandates a minimum boiler efficiency and, for combi boilers, one additional energy-saving measure, raising overall heating system efficiency.

4. What minimum boiler efficiency does Boiler Plus require for a new gas boiler in England?

   Boiler Plus requires a minimum ErP seasonal space-heating efficiency of $92\%$ (ErP, based on gross/net per the directive) for new gas boilers, equivalent to a high-efficiency condensing boiler. Boilers below this standard cannot be installed.

5. Under Boiler Plus, what additional measure must be fitted when installing a gas combi boiler in England?

   A new gas combi boiler must include at least one of four energy-efficiency measures: (1) flue gas heat recovery, (2) weather compensation, (3) load compensation, or (4) smart controls with automation and optimisation. Time and temperature control is already a baseline requirement for all boiler installs.

6. What baseline controls must every boiler installation include under Boiler Plus, regardless of boiler type?

   All installations must have time control and temperature control as a minimum: a programmer/timer plus a room thermostat (or programmable thermostat), and TRVs where appropriate, so the system can control when and to what temperature each space is heated.

7. Why are condensing boilers more efficient, and what causes condensing to occur?

   Condensing boilers recover latent heat from water vapour in the flue gases by cooling them below the dew point (~$55\ ^\circ\text{C}$), causing the vapour to condense and release additional energy into the return water. This requires low return temperatures; efficiency can exceed $90\%$ (gross). The condensate (mildly acidic) is drained away.

8. What is the difference between a vented and an unvented hot water cylinder?

   A vented cylinder is supplied from a cold water storage cistern (gravity head) and has an open vent pipe over the cistern; it operates at low pressure. An unvented cylinder is fed directly from the cold mains at mains pressure, is sealed, and requires safety devices to handle expansion and overheating — giving balanced mains-pressure hot water at all outlets.

9. List the key safety devices required on an unvented hot water storage system.

   Three-tier protection: (1) thermostat (normal control), (2) a non-self-resetting energy cut-out (high-limit thermostat), and (3) a temperature & pressure relief valve (T&P) set typically to ~$90\text{--}95\ ^\circ\text{C}/7\ \text{bar}$. Plus an expansion vessel or bubble-top air gap, an expansion (pressure) relief valve, a pressure-reducing valve, a check valve, and a tundish with discharge pipe.

10. What is the purpose of the tundish in an unvented hot water system?

    The tundish is a visible air-break fitted in the discharge pipe (D1) from the temperature/pressure relief valves, located within ~$500\ \text{mm}$ of the safety valve. It provides a clear visual indication that a relief valve is discharging and prevents backflow contamination, before water runs to a safe (D2) discharge point.

11. What qualification/competence is legally required to install an unvented hot water storage system over 15 litres?

    Installation of an unvented hot water storage system exceeding $15\ \text{litres}$ is a controlled service under Building Regulations (G3) and must be carried out by someone holding the relevant competence (e.g. an unvented hot water/G3 qualification) and notified, because of the scalding/explosion risk if controls fail.

12. What is a thermal store and how does it differ from a conventional hot water cylinder?

    In a thermal store the stored water is the heated primary/heating water; mains-pressure domestic hot water is heated instantaneously by passing through a coil (or external plate heat exchanger) within the store. A conventional cylinder stores the actual domestic hot water and heats it via a coil. Thermal stores give mains-pressure DHW without unvented safety controls on the store.

13. What is power flushing and what does it achieve?

    Power flushing is a process using a pump to circulate water (often with cleaning chemicals) at high flow but low pressure through the system to dislodge and remove magnetite sludge, scale and corrosion debris from radiators, pipework and the boiler heat exchanger. It restores circulation and efficiency and is recommended before fitting a new boiler to an old system.

14. What is magnetite and why is it a problem in heating systems?

    Magnetite ($\ce{Fe3O4}$) is black iron-oxide sludge formed by internal corrosion of steel radiators in oxygenated water. It accumulates at low points and in radiator bases, causing cold spots, blocked pipes and pump/heat-exchanger damage, reducing efficiency. It is magnetic, which allows removal by magnetic filtration.

15. How does a magnetic (in-line) system filter work and where is it fitted?

    A magnetic filter contains a strong magnet that attracts and traps suspended magnetite particles from the circulating water, while non-magnetic debris settles in the canister. It is fitted on the system return pipe, before the boiler, to protect the heat exchanger. It must be serviced/cleaned periodically (typically annually).

16. What is the recommended sequence when removing an old boiler and fitting a new condensing boiler regarding system cleaning?

    Per BS 7593 / manufacturer guidance: thoroughly clean (power flush or dynamically flush) the existing system to remove sludge and debris, fit a system filter, then refill, dose with a corrosion inhibitor to the correct concentration, and verify inhibitor level. Cleaning protects the new heat exchanger and validates the warranty.

17. What does a central heating inhibitor do, and how often should its concentration be checked?

    A corrosion inhibitor is a chemical dosed into the system water that forms protective films and buffers chemistry to prevent corrosion of steel, scale formation and microbial growth, thereby suppressing magnetite production and protecting the boiler/radiators. Per BS 7593, inhibitor level should be checked at least annually (e.g. at service).

18. How is inhibitor concentration typically tested in the field?

    A sample of system water is drawn (e.g. from a radiator bleed point) and tested using a manufacturer's test kit — either a dip-test strip read against a colour chart, a titration/reagent test, or a laboratory analysis. If the reading is below the recommended level, the system must be re-dosed to the correct concentration.

19. What does BS 7593 cover in relation to heating system water quality?

    BS 7593 is the British Standard 'Code of practice for the preparation, commissioning and maintenance of domestic central heating and cooling water systems.' It specifies cleaning, dosing with inhibitor, filtration, and routine testing/maintenance of system water to control corrosion and scale, including annual inhibitor checks.

20. What are the typical signs of corrosion/sludge problems found during system diagnosis?

    Indications include radiators cold at the bottom but warm at the top (sludge settled in base), frequent need to bleed gas (hydrogen from corrosion), dirty black water when bleeding/draining, noisy boiler (kettling), pump failures, and a fouled magnetic filter. Water testing confirms low inhibitor and high iron content.

21. Why does air/gas repeatedly accumulate at the top of radiators, and how do you distinguish corrosion gas from air ingress?

    Repeated gassing can be hydrogen produced by internal corrosion (electrolytic reaction of metals in untreated water) rather than air. Test: collect the gas and apply a flame — hydrogen burns/pops, whereas air does not. Persistent hydrogen indicates active corrosion needing flushing and inhibitor; air ingress points to a leak, faulty AAV or pump pulling air on its suction side.

22. What causes boiler 'kettling' and how is it diagnosed and remedied?

    Kettling is a banging/rumbling noise caused by limescale and/or sludge deposits on the heat exchanger creating localised overheating that boils water (steam bubbles collapsing). It is diagnosed by noise on firing, dirty water and restricted flow. Remedy: descale/clean or power flush the system, fit a filter, ensure correct pump flow, and dose inhibitor; replace exchanger if badly scaled.

23. How do you diagnose a faulty (failed or air-locked) circulating pump?

    Symptoms: radiators stay cold while the boiler is hot, no temperature difference across the pump, no vibration/hum, or noise without flow. Checks: confirm electrical supply and that the pump is running, feel for warmth either side, bleed the pump (air lock) via the centre bleed screw, check the speed setting and for a seized impeller. A seized or electrically dead pump must be freed or replaced.

24. A system makes gurgling/circulation noise and some radiators are cold — list the likely causes to investigate.

    Investigate: trapped air/air locks (bleed radiators and high points, check AAVs), incorrect pump speed (set too high causing noise, too low causing poor circulation), sludge/magnetite blockage restricting flow, low system pressure drawing in air, an undersized or wrongly positioned pump, and an incorrectly set/failed expansion vessel. Balance the system after rectifying.

What this deck covers

Domestic Heating Systems, Controls and Energy Efficiency flashcards FAQ