OFTEC Solid Fuel flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Stoichiometric air for kerosene

Approximately 12.5 kg of air is needed to completely burn 1 kg of kerosene; flue CO2 near 12% indicates near-stoichiometric combustion with modest excess air.

Smoke number interpretation

Smoke number 0-1 indicates clean combustion, 2 is acceptable, and 3 or above indicates poor atomisation, incorrect air setting or a fouled appliance needing attention.

Nozzle flow versus pressure

Nozzle flow rate varies with the square root of pressure; doubling the pressure increases flow by a factor of about 1.41, not by two.

Nozzle rating pressure

Oil nozzle flow markings (such as 0.60) are US gallons per hour at the standard rating pressure of 100 psi, approximately 7 bar.

Oil line vacuum limit

Vacuum at the burner should not exceed about 0.4 bar (roughly 12 in Hg); higher values suggest a blocked filter, restricted line or faulty foot valve.

Cad cell resistance meaning

A photocell (cad cell) shows low resistance, typically under 1-2 kOhm, when it sees a good flame; high resistance indicates weak flame or a dirty cell.

One-pipe versus two-pipe oil

One-pipe lift systems with a de-aerator suit tanks below the burner; two-pipe gravity systems suit tanks above the burner where oil flows by gravity.

Condensing boiler flue temperature

For condensing operation the flue gas temperature should be only about 20-40 C above the boiler return temperature, keeping the appliance in condensing mode.

Solid fuel combustion air

Solid fuel appliances need a permanent, unobstructed combustion air supply, with a ventilator sized to the appliance's rated input as required by building regulations.

Chimney draught principle

Draught is created by the temperature difference between hot flue gases and outside air; a cold flue produces weak draught and poor appliance performance.

Wood fuel moisture target

Wood should be seasoned to around 20% moisture or lower; wetter fuel causes creosote and tar deposition and reduces net efficiency.

Smoke control area fuels

In smoke control areas only authorised smokeless fuels such as anthracite may be burned; bituminous coal is generally prohibited unless an exempt appliance is used.

Open vent on solid fuel

A solid fuel boiler must be arranged as an open-vented system with a cold feed and open vent; a closed unvented arrangement is unsafe.

Solid fuel service tasks

Servicing includes sweeping the flue, checking baffles, door gaskets and seals, and verifying the draught stabiliser operates correctly.

Over-firing versus under-firing

Over-firing from excessive primary air shortens appliance life; under-firing causes incomplete combustion, soot and creosote build-up in the flue.

Flue termination requirement

A solid fuel flue should terminate above the roof line and clear adjacent windows and openings in accordance with building regulations.

Draught stabiliser function

A draught stabiliser admits air to the flue to limit excessive draught, protecting the appliance from over-firing and improving combustion stability.

RCD protection for heating

Domestic final circuits are protected by 30 mA RCDs; a 6 A MCB commonly protects a 1.0-1.5 mm2 control or lighting circuit.

Cable derating factors

Cable current-carrying capacity must be derated for grouping, ambient temperature and thermal insulation using the factors given in BS 7671.

Switched live versus permanent live

The permanent live supplies power for pump overrun and frost protection, while the switched live comes from the programmer or room thermostat to call for heat.

Earth loop impedance

Earth loop impedance must be low enough for the protective device to disconnect within the required time at the prospective fault current.

SELV control separation

SELV controls such as 24 V or 0-10 V signals must be kept physically separate from mains wiring, with barriers and correct cable colours.

Three-port valve mid-position

A three-port mid-position valve can supply heating and hot water simultaneously when in its mid-position, unlike a two-port zone valve.

Appliance isolation requirement

Each heating appliance needs a means of functional switching and a double-pole isolator for safe maintenance isolation.

Extraneous conductive part bonding

Metallic pipework that is an extraneous conductive part must be main bonded; supplementary bonding may be needed in bathroom locations.

Heat pump COP formula

COP equals heat output in kW divided by electrical input in kW; the seasonal performance factor is the same ratio measured over a full season.

Carnot COP limit

The theoretical maximum COP equals T_hot divided by (T_hot minus T_cold), with both temperatures expressed in kelvin.

Flow temperature and COP

Lower flow temperatures, around 35-45 C, improve heat pump COP; underfloor heating suits these temperatures better than high-temperature radiators.

Heat pump sizing basis

A heat pump must be sized to the building's heat loss at the design external temperature, not simply to the rating of the existing boiler.

Ground source collector choice

Ground-source systems use horizontal collectors where land is available or boreholes where land is limited; loop length depends on ground thermal conductivity.

Solar thermal orientation

In the UK, solar thermal collectors are typically mounted facing south at a tilt of about 30-45 degrees to maximise annual yield.

Thermal store purpose

A thermal store buffers heat between generation and demand, helping heat pumps and biomass boilers run efficiently without short cycling.