1 Domestic Central Heating Systems

• Domestic central heating systems are installed in domestic dwellings.
• Learners will study old and new central heating systems.
• Learners will look at components that make up systems.
• Learners will be able to describe the type of system at home.
• Central heating is needed, and it is important to understand what gives us heat in homes.
• It is important to make sure the customer understands how to work their central heating.

Legislation

• Building Regulations part L: Conservation of fuel and power.
• BS EN 12828: Design of water based heating systems.
• BS EN 14336: Installation and commissioning of water based heating systems.
• BS EN 442: Manufacture of radiators.
• CHESS: Central Heating Efficiency System Specification (basic and best practice).
• Building Regulations part L covers:
  • L1a: New dwellings.
  • L1b: Existing dwellings.
  • L2a: New, other than dwellings.
  • L2b: Existing, other than dwellings.

Types of Systems

• The main purpose of central heating is to provide thermal comfort within a property.
• An open fire gives heat in one room, but central heating can give heat in every room.
• Thermal comfort exists when the desired heat balance between the body and surroundings is met within the customer's economic constraints.

Full Central Heating

• All rooms in a property are heated simultaneously to a specific temperature set by the customer.
• This comfort temperature must be reached, even when the outside temperature is -1°C.

Selective Central Heating

• Some rooms in a property are heated simultaneously to a specific temperature set by the customer.
• The customer selects which rooms are heated.
• This comfort temperature must be reached, even when the outside temperature is -1°C.

Background Central Heating

• Can be the same as full or selective, but it is installed mainly on cost basis.
• A smaller boiler or heat emitters may be used, which will only take the chill out of rooms.
• If a system cannot achieve the customer's desired comfort temperature when the outside temperature is -1°C, the system would be classed as background heating.
• Over the past 100 years, the open fire in each room has been developed and replaced with:
  • Gravity hot water system
  • Pumped heating with gravity hot water
  • Fully pumped system
• Central heating is now preferred to an open fire, as it heats up the whole property.
• Solid fuel has been replaced with gas, oil, and electricity to heat the system.
• Customers will be cost conscious, not only at the point of installation, but also with running costs.
• Running costs include maintenance, efficiency, performance, and controls.
• Modern domestic central heating systems fall into two categories: low pressure open vented systems and sealed pressurized systems.

Low Pressure, Open Vented Systems

• Fed from a feed and expansion cistern located at a high level.
• Can be fully pumped or gravity systems.

Sealed, Pressurized Systems

• Fed directly from the mains cold water and incorporate an expansion vessel to take up the expansion of the heated water.
• This system is a more modern fully pumped or combination boiler system.

Central Heating System Types

• Wet central heating systems include open-vented (low pressure) systems and sealed (pressurized) systems.
• Open-vented (Low pressure) systems include:
  • Semi-gravity systems
  • Pumped heating only systems
  • Fully pumped systems
• Sealed (pressurized) systems include the following components
  • System boiler option
• Semi-gravity systems include:
  • One-pipe systems
  • Two-pipe systems
• Pumped heating only systems use a system with two two-port motorized valves.
• Fully pumped systems include:
  • System using one three-port mid position valve
  • C-Plan system
• System boiler options include:
  • Systems using external expansion vessel
  • Combination boiler systems

Gravity or Semi Gravity Systems

• Older systems use convection currents to circulate water around the pipework.
• Usually have 28mm primaries associated with solid fuel boilers and the need for convection currents.
• Gravity or semi-gravity systems include:
  • One pipe semi gravity
  • Two pipe semi gravity
  • C plan semi gravity
  • C plan plus semi gravity system
  • Two pipe semi gravity with heat sink

Full Gravity Systems

• These systems are no longer installed but you may come across them in older properties.
• Systems offer background heating and use large diameter pipework that has to be laid at the correct fall to allow convection to work properly.
• No pump exists in the system for either hot water or heating, so the heat-up time was lengthy.
• Systems are old and rarely seen but still in use in some older houses, that are single feed or primatic hot water cylinders.
• There is no F&E to fill the primary circuit; done via the cold feed to the cylinder.
• No inhibitor can be added to this type of heating system.
• It is a gravity only system and cannot be converted to pumped, or the air lock separating the two waters will be lost.

One Pipe Semi-Gravity System

• Older, simpler system includes a one-pipe circuit going from and returning to the boiler.
• The circuit would be pumped, but the flow and return stems to each radiator relied on gravity to heat them up.
• Installation cost was cheaper due to only one ring pipe being installed.
• Drawback: As the return water from each heat emitter entered the main ring, it would cool the flow water down to the following emitter; progressively each radiator was cooler than the one before.

One Pipe Gravity System

• Advantages:
  • Cheap to install.
• Disadvantages:
  • Each radiator progressively cools down
  • Only the main ring is pumped
  • Uncontrolled room temperature
  • Boiler could cycle
  • Not compliant to Building Regulations part L
  • Boilers connected to these systems are low efficiency

Two Pipe Semi Gravity

• The system has gravity circulation to the hot water circuit but pumped circulation to the heating circuit.
• Separate pipes for the flow and return, water was forced into the heat emitters, and the flow and return waters did not mix.
• Allows the heat emitters to achieve the same temperature.
• Control on the heating was sometimes by a room stat, which only switched off the pump.
• The hot water had no such control and was often very hot.
• Advantages:
  • All heat emitters reach the same temperature.
  • Two pipe system is quicker when heating up.
  • Cheaper to run.
• Disadvantages:
  • Uncontrolled heat.
  • Boiler cycling.
  • Not compliant to Building Regulations part L.
  • Boilers connected to these systems are low efficiency.

C Plan Two Pipe Semi Gravity

• An updated version of a two pipe semi gravity system that includes a cylinder thermostat and zone control on the hot water.
• The C plan plus includes a room thermostat, TRVs and zone control on the heating, as well as controls on the hot water.
• The C plan plus is the minimum acceptable compliant system with Building Regulations part L16 and the Domestic Heating Compliance Guide, which states the system has to incorporate a thermo mechanical thermostat.
• Advantages:
  • All heat emitters reach the same temperature.
  • Two pipe system is quicker to heat up.
  • Compliant to Building Regulations L16.
  • Full control on heating and hot water.
• Disadvantages:
  • Not fully pumped.
  • Boilers fitted to this system tend to be lower efficiency.

Two Pipe Semi Gravity With Heat Sink

• Used with solid fuels, which are not as controllable as gas or oil boilers.
• If gravity circulation stops due to the flow and return being the same temperature, the boiler is still producing heat.
• The heat sink can dissipate this without the boiler overheating.
• The heat sink is generally a radiator with two lock shield valves.
• The boiler thermostat on a solid fuel boiler is often as simple as an air damper, rather than an electrical control.
• Advantages:
  • Compliant with Building Regulations part L.
• Disadvantages:
  • Restricted heat control.
  • Only used on open vented systems.
  • Overheating possible.
  • Can be expensive.

Fully Pumped Systems

• The modern systems use a pump to circulate the heated water around the hot water and heating circuits.
• This is controlled by installing a cylinder thermostat, room thermostat and programmer, along with either two zone valves or a three port mid-position valve.
• These systems offer better control, design and boiler type.
• The boiler position no longer needs to be lower than the cylinder.
• Heat-up times are much quicker, so the system is more economical on fuel and operating costs.
• Boilers can be fueled from natural gas, LPG or oil.

Pumped Central Heating

• Older type systems do not have any provision for hot water but serve some heat emitters around the property.
• The cold feed and vent pipe can be taken from the boiler (four tapping boiler) or from the pipework.
• It is generally a two pipe system with a circulator to assist the heat-up time.
• The controls consist of a timer, boiler thermostat and possibly a room thermostat, heating all rooms to a controlled, comfortable temperature.

Fully Pumped Mid-Position Valve – Y Plan System

• The three port mid-position valve controls the flow of water to the hot water cylinder and heating circuit.
• The valve reacts tothe room thermostat and cylinder thermostat.
• Has an automatic bypass valve, which connects to the flow and return pipe.
• The bypass opens if the system pressure increases when circuits close down due to them reaching temperature.
• Allows water to flow through the boiler, stopping lock out, and it also prolongs the circulator life.

Fully Pumped Two, Two Port Valves – S Plan System

• This system uses one zone valve to control the hot water, which is activated by the cylinder thermostat, and a second zone valve, which is activated by the room thermostat.
• The zone valve acts as an isolator, closing the flow of water off.
• If a property is over 150m², an additional heating zone valve should be fitted, allowing independent control of upstairs and downstairs.
• As with the Y plan, an automatic bypass is fitted to the system.

Y Plan vs S Plan

• Y plan and S plan both offer full thermostatic control and building regulations compliance.
• S plan is recommended for larger properties, and it has boiler interlock.
• Y plan and S plan can both be used with sealed systems, system boilers, and zonal systems.

Locking Out

• A protection system within a boiler.
• A thermostat shuts the boiler down if it detects the flow is above 85°C and therefore stops it from overheating.

Aeration

• A big cause of corrosion within a central heating system.
• Air plus iron equals rust: rust is iron oxide that produces magnatite, which is the black sludge.
• Aeration also causes system noise and can burn the circulator out.

Sealed System

• A system is not open to atmosphere and incorporates a sealed expansion vessel, which replaces the feed and expansion cistern of the open system.
• A filling connection needs to be installed, along with a detachable filling loop.
• Allows a point at which the system can be filled under mains pressure.
• A double check valve needs to be installed, to prevent the possibility of back siphonage of central heating water into mains water.
• On both of the filling loop connections, a means of isolation needs to be fitted.
• Water Regulations state that this loop needs to be disconnected after the system has been charged to the correct pressure.
• A pressure relief valve will need to be installed to discharge pressure to a safe location if required.