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Condensing boilers (not)

The exhaust from a natural gas appliance contains about 0.15 litres of water per kWh of gas input, and about a tenth of the thermal output is lost because that water is emitted as vapour. Condensing boilers are a good idea in theory because they can condense the vapour and recover latent heat from the products of combustion, boosting output by around a tenth.

In practice, too few condensing boilers achieve their potential because they cannot cool the flue gas below its dew point (around 59C ). Result: plumes of vapour outside. This one resembles what you’d see boiling a 2-3 kW kettle in the open air, and that’s a measure of how much energy is being wasted.

The truth is that so-called condensing boilers need to be installed in heating systems with low return water temperatures. Underfloor heating, or systems with oversized radiators for example. Only then will they get sufficiently-low temperatures in their heat exchangers to get the exhaust vapour to condense.


Nice try, but…

A recent issue of the CIBSE Journal, which one would have thought ought to have high editorial standards, recently published an article which was basically a puff piece for a certain boiler water additive. It contained some fairly odd assertions, such as that the water in the system would heat up faster but somehow cool down more slowly. Leaving aside the fact that large systems in fact operate at steady water temperatures, this would be magic indeed. The author suggested that the additive reduced the insulating effect of  steam bubbles on the heat-exchanger surface, and thus improved heat transfer. He may have been taking the word ‘boiler’ too literally because of course steam bubbles don’t normally occur in a low or medium-temperature hot water boiler, and if they did, I defy him to explain how they would interfere with heat transfer in the heat emitters.

But for me the best bit was a chart relating to an evaluation of the product in situ. A scatter diagram compared the before-and-after relationships between fuel consumption and degree days (a proxy for heating load). This is good: it is the sort of analysis one might expect to see,

The chart looked like this, and I can’t argue that performance is better after than before. The problem is that this chart does not tell quite the story they wanted. The claim for the additive is that it improves heat transfer; the reduction in fuel consumption should therefore be proportional to load, and the ‘after’ line ought really to have a shallower gradient as well as a lower intercept. If the intercept reduces but the gradient stays the same, as happened here, it is because some fixed load (such as boiler standing losses) has disappeared. One cannot help wondering whether they had idle boilers in circuit before the system was dosed, but not afterwards.

The analysis illustrated here is among the useful techniques people learn on my energy monitoring and targeting courses.


We’ve had some very enthusiastic feedback from delegates at MAVCON17, the third National Measurement and Verification Conference,  which we held on 16 November.

Delegates wrestle with the thorny issue of non-routine adjustments

Adam Graveley of Value Retail for example described it as “a very informative and well-organised conference that provided a great deal of practical insight” .

The event consistently attracts around 70 M&V practitioners who value not only the networking opportunity but also what they call the ‘geek element’ (expert technical papers with extended question-and-answer sessions), group exercises, and a no-holds-barred expert panel discussion for which this year’s theme was “when M&V goes wrong”.

(l. to r.)  Chairman Richard Hipkiss, keynote speaker Denis Tanguay and organiser Vilnis Vesma

Our keynote speaker was Denis Tanguay, Executive Director of the Efficiency Valuation Organisation, the body responsible for the International Performance Measurement and Verification Protocol (IPMVP). We are planning to run MAVCON again in early November 2018 and are open for offers of technical papers and ideas for group exercises.

We are grateful to our other speakers Dave Worthington, Hilary Wood, Colin Grenville, Steve Barker and  Emma Hutchinson and our expert panelists Sandeep Nair, Ellen Salazar and Quinten Babcock. You can read more about them at the conference web site

We should also acknowledge the venue, the Priory Rooms, for the quality of their service including excellent catering which also drew much favourable comment.


Kinetic plates

When this “kinetic plate” was installed in 2009, the Guardian published an article which suggested that it would harvest up to “30 kWh per hour” of “green energy” from the traffic passing over it. Rubbish, of course. Firstly (as was acknowledged in a muted disclaimer at the foot of the article) it wasn’t free energy; it was contributed by the passing drivers. But what about the 30 kWh per hour claim? That’s the equivalent of harnessing the output from engine of this Peugeot and running it flat out for 15 minutes in the hour.

Really? We can do some quick sums on this. Say the car, with its driver, weighs about 1,400 kg. Suppose that it depresses the plate 10mm (0.01m). If we take gravitational constant as 9.8 N/kg, the energy imparted by the car as it drives onto the plate is 1,400 x 0.01 x 9.8 = 137.2 joules (watt-seconds). That is only 0.000038 kWh. In other words, you’d need getting on for eight hundred thousand cars an hour to achieve 30 kW output, even if the mechanism were 100% efficient, which it won’t be.

Along similar lines the IMechE published an article about a kinetic pavement in 2015. This related to a system for capturing energy from pedestrians, and rather usefully it included some statistics: that 54,267 steps had generated 217,028 watt-seconds. I hope all my readers can confirm for themselves that this equates to a mere 0.06 kWh.

Daylight-linked consumption

When monitoring consumption in outside lighting circuits with photocell control, it is reasonable to expect weekly consumption to vary according to how many hours of darkness there were. And that’s exactly what we can see here in this Spanish car park:

It is a textbook example: with the exception of two weeks, it shows the tighest correlation that I have ever seen in any energy-consuming system.

The negative intercept is interesting, and a glance at the daily demand profile (viewed as a heatmap) shows how it comes about:

Moving left to right we see from January to March the duration of daylight (zero consumption in blue) increases. High consumption starts at dusk and finishes at dawn, but from about 10 p.m. to 5 a.m. it drops back to a low level. It is this “missing” consumption for about seven hours in the night which creates the negative intercept. If they kept all the lights on from dusk to dawn the line would go through the origin.

For weekly and monthly tabulations of hours of darkness (suitable for England and other countries on similar latitudes)  click here.


Unethical sales of so-called energy saving product

The BBC’s Watchdog programme (series 38, episode 1) did an excellent job exposing the activities of a company called Energysave, which was caught training its salesmen to use high-pressure sales techniques on vulnerable customers. They were claiming, outrageously, that their product, a water-repellent coating for masonry, cut heat loss by a third. (Although their website says conductivity “decreases enormously with dampness”. Whoops)

Warning sign: an iIlliterate promotional video

You can catch up with the episode on BBC iPlayer. The relevant material is in two parts at 16’04” and 32’53” with the final confrontation scene at 51’52”, but the episode also includes stuff on defective smart meter installations.

Thanks to newsletter reader Istvan Sereg for the tip-off.

Smartphones and energy surveys

In a recent newsletter I listed six things that a smartphone could be used for in connection with energy surveys:

  1. as a camera, obviously;
  2. as a stopwatch: for example timing a 1-bar drop in air pressure when you turn off your compressors, to estimate leakage;
  3. as a means of reading inaccessible rating plates or meter dials;
  4. to co-ordinate ‘drop tests’ where one person does spot meter readings while another turns major loads on and off;
  5. as a hand-held meter-reading terminal using a web service like MeterPad;
  6. just as a browser to gather information on unfamiliar plant and equipment.

Readers were not slow coming forward with more ideas. Daniel Lash  was first to respond: apparently if you point the camera at fluorescent lights, you can see the flicker in those which do not have high-frequency ballasts. He also mentioned using the phone as a compass; downloading an app that estimates heights; and adding plug-in infra-red camera attachments… And it’s a torch.

Peter Henderson was next in, with news of the Flir CAT S60, a rugged mobile phone with integrated thermal imaging.  Paul Spencer unearthed ‘Spike’, a clamp-on laser rangefinder that connects through Bluetooth.  Alan Turner mentioned the KSB Sonolyzer app, which gives a rough idea of motor speed from the fan noise frequency. Asynchronous induction motors slow down (‘slip’) in proportion to mechanical load, and as their full-load speed is stated on the rating plate, their speed can be used to estimate their load factor.

Not everything lives up to its promise. Howard Ward reported testing a lux meter app on three different phones and getting inconsistent results when compared with a real instrument.

Finally if you want a peek at the ultimate in smartphone deployment, have a look here at what Dr Russell Layberry has been doing at Oxford with cheap second-hand phones

Energy Savings Opportunity Scheme

ESOS is the UK government’s scheme for mandatory energy assessments which must be reviewed and signed off by assessors who are on one of the approved registers. We are now in Phase 2 with a submission deadline in December 2019, but the Environment Agency is trying to get participants to act now.

I run a closed LinkedIn group for people actively engaged with ESOS; it provides a useful forum with lots of high-quality discussion.

Background reading

Useful resources

These are documents which I have developed to support the ESOS assessment process. I used them for my assignments during the first phase and have since revised them in the light of experience:

Energy jargon buster

Absorption chiller: Cooling apparatus driven by heat alone

Accuracy: Degree to which a measurement reflects actual reality

AHU: See Air handling unit

Air curtain: Sheet of heated air projected across an open doorway to prevent discomfort from draughts; usually in retail premises

Air handling unit: Air handling unit:  assembly of fan, filters, etc  for supplying air to or extracting air from a ducted distribution system

Ammeter: Instrument for measuring electric current

amp (A): Unit of measurement of electric current

AMR: Automatic meter reading

Anemometer: Device to measure air velocity

Audit, energy: Systematic review of energy-using systems and associated procedures with a view to identifying opportunities for energy saving

For training in energy management topics see

Ballast: Component of electrical control gear in a fluorescent light fitting.

BEMS: Building energy management system:  computerised control and monitoring equipment for regulating time and temperature schedules etc

BEMS: See Building energy management system

Blending valve: See Mixing valve

Blowdown: Removal of a fraction of boiler water to enable removal of sludge and dilution of dissolved solids

BMS: See Building energy management system

Building energy management system: Computer system which controls the operation of heating, ventilation, air conditioning, lights and other energy services in buildings

For training in energy management topics see

Calorific value: Energy content of fuel per unit mass or volume

Capacity charge: Rental paid for an electrical supply connection capable of carrying a certain current

CFL: Compact fluorescent lamp:  plug-in substitute for a light bulb.

Chiller: Machine for cooling air, water, etc

CHP: See Combines heat and power

CNG: Compressed natural gas

Code 5: In UK context, a meter for registering half-hourly electricity consumption for loads above 100 kW

Coefficient of performance: In refrigeration systems, the ratio of output cooling power to input electrical power.

Cogeneration: See Combined heat and power

Colour rendering: How effective a given light source is at allowing discrimination between colours

Colour temperature: Numerical value describing a light source in terms of how ‘warm’ or ‘cool’ it appears

Combined heat and power: Electricity generation in which part of the waste heat is put to use

Compensator: Control device to regulate circulating water temperature in a heating system, reducing temperature when heat demand is low and vice versa

Condensate: Water resulting from the cooling of steam

Condenser: In a refrigeration circuit, the component through which heat is rejected.

Conduction losses: Heat losses through the walls, roof, floors, doors, windows and other solid elements of the building envelope.

Constant temperature: In heating system, regime in which circulating water is maintained at a fixed temperature and control of heat output is effected by regulating flow to heat emitters

Convection losses: Heat lost in air leaving the building through draughts and deliberate ventilation

Convector, fan assisted: Heat emitter on a central heating system in which a fan blows room air across a heat exchanger

Convector, natural: Heat emitter on a central heating system, usually enclosed in a cabinet with inlet and outlet vents, which warms room air without the assistance of a fan

Cooling tower: Device in which water used for cooling something gives up some of its heat to the air, enabling it to be recirculated in a closed loop. May be ‘dry’, employing a sealed heat exchanger, or ‘wet’ in which case evaporation of the water increases the cooling effect

CoP: See Coefficient of performance

Coriolis meter: Technology for measuring the flow rate of for example dust-laden gases; the flow rate affecting the resonant frequency of a U-shaped section of vibrating pipe

Correction factor: In the context of natural gas, the factor by which its metered volume must be multiplied to account for its pressure and temperature being other than that assumed as standard

CT: In heating system:  see Constant temperature;  in metering see Current transformer

Current: Rate of flow of electrical energy

Current transformer: Device placed around one conductor of an electrical supply cable to convert the current flowing in it into a safely-measurable signal for input to a meter

Cusum: Cumulative sum of deviance

CV: See Calorific value

For training in energy management topics see

Damper: Flap used to control air flow in a duct

Data logger: Device for recording data from energy meters, temperature probes and other instruments

Dead band: Switching differential between for example the activation of heating and cooling in a space; sometimes in a thermostatic control the spread between temperatures that trigger changes of state each way between on and off.

Degree days: Measure of how hot or cold the weather was over a given interval, typically a week or month, in a given location or region. Used in an analogous fashion to production output as the driving factor for heating opr air-conditioning energy consumption.

Deliquescent: Dessicant material which dissolves in the water it absorbs

Delta: Connection method for three-phase devices where the load is connected from phase to phase without a neutral (cf star)

Demand, maximum: See Maximum demand

Dessicant: Material which absorbs water vapour; used for example in compressed-air dryers

Deviance : Difference between actual and expected consumption

Dew point: Air temperature at which moisture will begin to condense; also known as saturation temperature

Dichroic: Attribute of a filament spotlamp whereby the reflector allows heat to escape through the back

Direct-fired heater: Heater in which fuel is used directly, as distinct from a heat emitter on a hot-water or steam circuit with centralised combustion.

Discounted cash flow: Method of accounting for future expenditure and receipts which explicitly recognises that postponing a cash flow diminishes its value in present terms

Driving factor: Recurrent and measurable determinant of energy consumption, such as production output or degree-day value.

Dry cycling: Excessive starting and stopping of boilers, especially when supporting each other’s standing losses

For training in energy management topics see

Economiser: Heat recovery unit specifically for preheating boiler water from heat in exhaust gases

Efficiency: The ratio of useful output to energy input

Electronically-commutated motor: machine, typically for DC or single-phase AC supplies, which turns a permanent-magnet rotor by synthesising of a rotating magnetic field

Embedded generation: Electricity generator owned and operated by the organisation which uses the output

EMS: See Building energy management system

Energy: Electricity, gas, oil, steam, compressed air or other like medium

Energy performance coefficient: ratio of actual to expected consumption; a numerical index of energy performance much less susceptible to distortion than specific energy ratio

Enthalpy: Total energy content of a fluid, representing both thermal and mechanical energy that could be extracted from it

Enthalpy control: Control regime in air-conditioning systems to optimise the energy requirement for regulating both humidity and temperature

Evaporator: In a refrigeration circuit, the component through which heat is absorbed

Expected consumption: rational estimate, either based on the known previous relationship between consumption and one or more relevant driving factors , or calculated from first principles

For training in energy management topics see

Far infra-red: ill-defined term most commonly associated with fake energy-saving products; usually denoting heat radiated from surfaces at no more than about 80°C

Firetube: Boiler design in which combustion gases pass through submerged tubes

Flash steam: Steam resulting when hot condensate is dropped to a lower pressure

Flue: Duct through which combustion products pass en route to the chimney

Flue gas: Combustion products

Fluidised bed: Combustion not in an open flame but within a bed of loose powder

Forced draught: Fans which drive combustion air into a boiler (cf Induced draught)

Free cooling: Cooling effect achieved by drawing in cold fresh air rather than chilling recirculated air

Frost protection: Automatic application of heat to prevent freezing damage out of hours

For training in energy management topics see

GCV: See Gross calorific value

Geothermal energy: Heat drawn from deep underground at sufficiently high temperature to be directly useable (cf Ground-source heat pump)

GLS: General lighting service:  conventional filament lamp

Gross calorific value: Total chemical energy content of a fuel, including what would be recovered by condensing the water vapour from the products of combustion. Also called higher calorific value. Cf Nett calorific value.

Ground-source heat pump: Reverse refrigeration cycle which cools the ground in order to provide a heating effect from its condenser (cf Geothermal energy)

GSHP: See Ground-source heat pump

For training in energy management topics see

HCV: See Gross calorific value

Heat: Thermal energy which can raise something’s temperature, or melt or boil it.

Heat emitter: Radiator, convector, or other device delivering heat in a heating system

Heat exchanger: Device within which a hot fluid stream gives up heat to a cold stream while maintaining separation between the two

Heat map: Graphical display of profile data in which for example demand levels are depicted as colour contours on a matrix in which each column represents one day, midnight to midnight.

Heat pipe: Closed length of tube containing a small charge of volatile liquid and a wick; transfers heat end to end through fluid boiling at one end and condensing at the other, returning via the wick

Heat pump: Refrigeration unit operated in reverse, providing heat at moderate temperature by cooling either the outside air or the ground nearby.

Heat wheel: Form of regenerative heat recovery in which the heat storage matrix is in the form of a wheel rotating continuously between the hot and cold streams.

hertz (Hz): Unit of measurement of frequency:  cycles per second

Higher calorific value: See Gross calorific value

Historical baseline performance characteristic: Performance characteristic which applied at the outset of the energy management campaign

For training in energy management topics see

Incidental gains: Heat gains in a building arising from lights, equipment, uninsulated hot surfaces, occupants, sunshine etc..

Induced draught: Fans which draw exhaust gases from a boiler (cf Forced draught)

Inhibitor: Chemical additive in boiler water to reduce corrosion

Insulation: Material which reduces the conduction of heat

Inverter: Electrical device for converting direct to alternating current; component of a variable-speed drive

joule (J): Unit of energy: one watt-second

For training in energy management topics see

kelvin: (K) unit of temperature difference

Landfill gas: Methane emanating from waste-disposal sites and collected as a fuel

Latent heat: Heat required to melt or vaporise a substance; or heat released when a vapour condenses or a liquid solidifies. Notwithstanding the release or absorption of heat, the change of state between liquid and solid or vapour occurs without a change of temperature.

Load factor: Ratio between actual output or input and the maximum theoretically possible with continuous operation at full output or input.

Logger: See Data logger

LPG: Liquefied petroleum gas

lumen: Unit of light output power

Luminaire: Light fitting including reflector, lamp holder, control gear and lens etc

lux: Measure of the light incident on a surface per unit area

LV: Low voltage

For training in energy management topics see

M&T: See Monitoring and targeting

Maximum demand: In UK context, the peak electrical power drawn over any half-hour period in a month

Mercury discharge: Type of fluorescent lamp typically used for floodlighting

Metal halide: Type of filament lamp

Mixing valve: In a Variable temperature heating circuit, the valve which regulates flow temperature by blending water from the boiler(s) with cooler water returning from the heating system. Also called a three-port valve as it has two inputs and one output connection.

Monitoring and targeting: Systematic assessment of actual against expected consumption by means of weekly (usually) overspend league table augmented by analysis tools to assist in target-setting and diagnosis of abnormal performance.

Motorised valve: Valve actuated by an electric motor under the dictates of a control system

MV: See motorised valve

For training in energy management topics see

Night blind: Insulating blind on chilled display cabinet to reduce cooling requirement out of hours,

NOx: Oxides of nitrogen generated as a by-product of combustion

NTP: Normal temperature and pressure:  reference used for correcting volumes and densities (0°C and 1.013 bar); cf STP, RTP

For training in energy management topics see

Optimum start: Time-control regime which postpones startup, usually of heating boilers, to the latest possible time commensurate with achieving desired internal conditions at the required time.

Orifice plate: Restriction placed in a gas, air, or steam line to create a measurable pressure drop from which flow arte can be inferred

OSC: Optimum start control

Overspend league table: Weekly (usually) or monthly report in which deviations from expected consumption are ranked in descending order of excess cost.

For training in energy management topics see

Passive infra-red: Technique for detecting the presence of people in a space

Performance characteristic: Mathematical relationship between energy consumption and one or more driving factors

Phase: Alternating-current electrical supplies are either single-phase (delivered through one pair of wires) or three-phase (delivered through three wires, the current waveform in each being one-third of a cycle behind or ahead of the others).

PIR: See Passive infra-red

Polyphase: arrangement of electrical supply in which (most commonly) three lines are energised by alternating current one-third of a cycle apart

Power:  Rate of flow of energy

Power factor: In alternating-current electrical supplies, the ratio of useful power delivered to the theoretical maximum possible for the given current and voltage

Precision: Degree of resolution in a measurement

Profile: Pattern of demand over a day, week, or other chosen interval.

For training in energy management topics see

Radiant: Heat transfer without physical contact

Receiver: Pressure vessel for storing compressed air.

Recuperator: Heat-recovery device

Reflective: property of material e.g. aluminium foil which impedes the flow of radiated energy across a transparent medium

Refractory: High-temperature insulation found in furnaces and kilns

Regenerator: In heat recovery, a pair (usually) of heat stores which take it in turns to collect heat from a hot stream and return it to a cold stream

Regression line: Best-fit line through points on a scatter diagram

Relative humidity: The ratio between the amount of water vapour present in the air and the theoretical maximum at the prevailing temperature

Repeatability: Degree to which measurements vary consistently irrespective of accuracy

Rewind: Repair of electric motor by replacement of burned-out windings

RTP: Reference temperature and pressure used for correcting volumes and densities (25°C and 1.013 bar) cf NTP, STP

Run-around coil: Split heat-recovery system in which heat is recovered to an intermediate fluid circuit, allowing heat collection and delivery to be in different places

For training in energy management topics see

Sankey diagram: Diagrammatic representation of energy flows through a process or organisation in which the magnitudes of flows are represented by the widths of pathways

Saturation temperature: See Dew point

SCADA: See Supervisory, control and data acquisition

Scatter diagram: Chart showing the relationship between one value and another:  commonly showing weekly energy use (vertical or x axis) against driving factor (horizontal or y axis)

Sensible heat: Heat which when added to or removed from a body, alters its temperature

Sequencing: Usually of boilers but also applicable to chillers, compressors, and other ganged utility equipment:  a control regime which regulates how many units are enabled so as to match the load.

Smart meter: Consumption meter which is capable of recording data at frequent intervals for onward transmission, normally with two-way communication to facilitate remote disconnection of the user, reporting, etc

Smoke pump: Device to measure the level of soot present in flue gases when testing oil and coal-fired appliances

SON: Sodium discharge lamp giving a pure yellow light (commonly used for street lighting)

Specific energy ratio: simple ratio of energy input to (usually) product output. Supposedly an indicator of energy performance but highly susceptible to distortion.

Specific heat: Property of a material expressing the amount of heat required to raise its temperature by one degree

Stack: Chimney

Stand-alone control: Time or temperature control device that operates independently (as distinct from an Outstation)

Standing loss: Incidental heat loss from equipment incurred regardless of demand

Star: Connection method for three-phase devices where the load is connected between each phase and neutral (cf delta)

STP: Standard temperature and pressure:  reference used for correcting volumes and densities (15.5°C and 1.013 bar); cf NTP, RTP

Sun pipe: Internally-reflective tube used to conduct daylight into an internal space where conventional rooflights cannot be used.

Supervisory, control and data acquisition: Computerised control and monitoring equipment for industrial process plant

Survey, energy: Review of energy-using systems with a view to identifying opportunities for energy saving

Switching differential: See Dead band

Synchronous motor: AC machine with permanent-magnet rotor which rotates exactly at supply frequency or submultiple thereof; absence of rotor winding giving reduced losses

For training in energy management topics see

Target performance characteristic: Performance characteristic representing the best achievable consumption relative to appropriate driving factors

Tariff: Table of charges; typically understood as applying to supplies other than those negotiated under contract

TDS: See Total dissolved solids

Temperature: Measure of how hot something is

Thermocouple: Temperature-measuring device exploiting the small voltage developed when a junction between dissimilar metals is heated

Thermostatic radiator valve: Direct-acting device for regulating heat output from a radiator

Three-port valve: See Mixing valve and diverting valve

Total dissolved solids: Measure of the concentration of dissolved salts in boiler water

Trace heating: Heating applied to pipes in order to prevent the contents solidifying

Transformer: Device for converting alternating-current electricity from one voltage to another.

TRV: See Thermostatic radiator valve

Turbine: Rotating device for converting (typically) steam into mechanical power; also found on a small scale as a fluid-metering technology

Two-port valve: Straight-through valve giving on/off or regulated flow

For training in energy management topics see

U-value: Property of an element of the building envelope expressing how easily heat flows through it per square metre of surface area

Vapour-compression chiller: Cooling apparatus driven by mechanical power

Variable air volume: Regime for ventilation air distribution where the supply of cooling is regulated by changing the volume of air distributed

Variable refrigerant volume: Regime for regulating cooling power in an air conditioning system

Variable temperature: In heating system, regime in which the circulating water temperature is varied to limit heat output according to likely demand (see also Compensator)

Variable-speed drive: Electronic device which alters the mains frequency fed to an electric motor, causing it to rotate at a different speed

VAV: See Variable air volume

Venting: Removal of air from, for example, steam circuits

Venturi: Tapering constriction in pipework used as a means of inferring flow rate from the pressure drop in the throat

Viscosity: Property of a fluid which determines its resistance to flow

volt (V): Unit of measurement of electric potential or driving force

Vortex meter: Gas, air or steam meter in which flow rate is inferred from the frequency of eddies shed by the fluid passing over a bluff body

VRV: See Variable refrigerant volume

VSD: See Variable-speed drive

VT: See Variable temperature

watt (W): Unit of power

HVAC interlocks

It’s like a curse. Waking up last Wednesday to a view of the full moon reflected off the Adriatic in the pale light of dawn, I opened the sliding door to the hotel balcony to take a snapshot. As I did so I heard the air conditioning fan stop and there in the pale light of dawn I saw a magnetic reed switch on the door frame, evidently linked to the fan-coil unit control. “Brilliant” I thought: “a picture of that will be perfect for my session at our conference on energy in hotels”.

To be fair to myself I did photograph the moonlit Adriatic first and the interlock switch later.