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energy PUNK The Energy Files Stage One Thermal Foundations Module One Energy, Power & the Balance Understand Energy. Master Heating. STAGE 1 Version 1.0 “The job isn't managing the water. It never was. The job is managing the energy.”
2 | P a g e Contents Energy: what the system is actually managing .................................................................................................... 3 Power, why the rate of delivery matters ................................................................................................................ 4 Heat loss: the demand the system is working against ...................................................................................... 4 Fabric heat loss .................................................................................................................................................................. 5 Ventilation and infiltration loss .................................................................................................................................. 5 Delta T temperature difference as the driver ...................................................................................................... 6 The first scenario .............................................................................................................................................................. 7 The second scenario ........................................................................................................................................................ 8 The challenge property ............................................................................................................................................... 10 Changing energy requirements ............................................................................................................................... 10 Glossary ............................................................................................................................................................................. 13 Changing energy requirements: solution ............................................................................................................ 14
3 | P a g e Energy: what the system is actually managing A heating system is, at its most fundamental, an energy management system. Not a water system. Not a boiler system. An energy system. The components: boiler, pump, pipework, emitters, exist to support the movement and delivery of energy. They are the infrastructure. Energy is the cargo. Energy is the quantity that creates change. In a heating context, it is what raises the temperature of a space, warms the surfaces within it, and creates the conditions we describe as comfort. You cannot see energy directly, but you see its effects everywhere on site in warm surfaces, rising air temperatures, and the thermal comfort of an occupied space. The distinction worth making here is between energy and heat. In everyday use, the two words are often interchangeable. In engineering, they are related but distinct. • Energy is the quantity being managed = the total amount available. • Heat is a description of energy in transit = specifically, energy moving from a warmer object or space to a cooler one as a result of a temperature difference. This matters because it separates the what from the how. When a radiator warms a room, heat transfer is the mechanism. When that warmth escapes through the wall, heat transfer is also the mechanism. Understanding the direction and rate of that transfer is central to understanding system behaviour.
4 | P a g e Power, why the rate of delivery matters Energy alone tells you how much. Power tells you how fast. And on site, fast matters. Power is the rate at which energy is delivered. It is measured in kilowatts (kW). One kilowatt means one kilojoule of energy transferred every second. That is a useful definition, but a more intuitive one might be this: One kilowatt is roughly the electrical input power of a standard microwave. A 10 kW boiler is delivering the equivalent of ten microwaves running simultaneously, continuously, for every second it is operating. This matters because a building does not wait for a system to catch up. It is losing energy continuously. If the system delivers energy at a rate lower than the building is losing it, the internal temperature falls. If the rate matches the loss, temperature holds. If it exceeds the loss, temperature rises. Two systems with similar total energy capacity can feel completely different on site if their power outputs differ. A higher power output system reaches temperature faster and responds more effectively to sudden drops in external temperature. Sounds simple? That’s because it is. Heat loss: the demand the system is working against Every building loses heat. This is not a fault, it is a consequence of physics. Whenever there is a temperature difference between inside and outside, energy moves from the warmer side to the cooler one. The only question is how fast. Heat loss occurs in two ways, and both are always present simultaneously.
5 | P a g e Fabric heat loss This is heat escaping through the physical structure of the building, walls, roof, floor, windows and doors. The rate of loss depends on three things: the area of the surface, the thermal properties of the material (expressed as a U-value, we will cover this in detail in later Modules), and the temperature difference across that surface. Better insulation reduces U-values and slows fabric loss. But it never eliminates it. The loss is always present whenever there is a temperature difference. Ventilation and infiltration loss This is heat leaving with warm air. As warm internal air escapes through gaps, openings, and deliberate ventilation, it is replaced by cooler external air. That replacement air must then be warmed by the system, which costs energy (and cash). Infiltration is uncontrolled air leakage through gaps in the building fabric. In poorly sealed buildings, infiltration losses can be comparable to or greater than fabric losses. They are often underestimated. KEY POINT Heat loss is not a fixed number. It changes with every change in external temperature, wind speed, occupancy, and building use. A building does not have a single heat loss value, it has a range. The system must be capable of meeting the worst-case end of that range. The energy balance The behaviour of every heating system can be described by one relationship: THE ENERGY BALANCE System output (kW) = Building heat loss (kW) When these are equal, internal temperature is stable, and, everybody’s happy.
6 | P a g e This is the central principle. Everything else in Stage 1 builds on it. • If system output equals heat loss: temperature holds steady. • If system output falls below heat loss: temperature drops. • If system output exceeds heat loss: temperature rises. The aim of a well-designed heating system is not to maximise output. It is to match output to demand, across the full range of conditions the building will experience. Delta T temperature difference as the driver The symbol ΔT or delta T ( Δ (delta) is the Greek letter used in science and engineering to mean: "Change in" or "Difference between.") T stands for temperature. appears constantly in heating engineering. It simply means temperature difference. In the context of heat loss: DELTA T — BASIC FORM ΔT = Tin − Tout The greater the difference, the faster the heat loss. This is why a building that copes perfectly on a mild October day may struggle badly on a January night. The building has not changed. The system has not changed. But the delta T has increased, sometimes by a factor of three or more, and with it, the rate of heat loss. Example: Inside 21°C, outside 12°C → ΔT = 9°C (mild day) The same building, the same system but nearly three times the heat loss when outside drops to −3°C (ΔT = 24°C). This is a normal British winter. Think extremes. Every system must be designed with it in mind.
7 | P a g e The first scenario Property type: Semi-detached house, 1970s, uninsulated cavity wall, standard double glazing Design condition: Outside temperature −3°C | Target inside temperature 21°C Delta T: 21 − (−3) = 24°C Existing System: 6 kW output Step 1 : Calculate delta T ΔT = T inside − T outside ΔT = 21°C − (−3°C) = 24°C Step 2: Identify fabric heat loss Given for this property at ΔT24: Fabric heat loss = 6.5 kW Step 3: Identify ventilation and infiltration loss Given for this property (older construction, some draughtiness): Ventilation loss = 1.5 kW Step 4: Calculate total heat loss Total heat loss = Fabric loss + Ventilation loss Total heat loss = 6.5 + 1.5 = 8.0 kW Step 5:Compare to system output System output: 6 kW Total heat loss: 8 kW Deficit = 8 − 6 = 2 kW ■ The system has a 2 kW deficit. On a night like this, it cannot maintain 21°C. It will fall short not because it is broken, but because it was not designed for this demand.
8 | P a g e Notice that the deficit only becomes apparent when the delta T opens up. On a mild evening outside 10°C, delta T of only 11°C the same house might lose as little as 4.5 kW. The 6 kW system copes comfortably. The customer has no complaints. The flaw only surfaces when conditions are demanding. This is why “it works fine most of the time” is not a reassuring statement. It means the system is only being tested within its capability most of the time. Work through this example yourself before reading the solution. Use the same five-step process from the worked example. If you get stuck, re-read Section 2 rather than jumping straight to the answer. The second scenario Property Type: Detached house, 1960s solid brick construction, single glazing, uninsulated loft Design Condition: Outside temperature −1°C | Target inside temperature 21°C Existing System: 10 kW output Given Losses At Design Conditions: Fabric = 8.5 kW | Ventilation = 3.5 kW Your working space Step 1: Delta T: Step 2: Fabric heat loss: Step 3: Ventilation loss: Step 4: Total heat loss: Step 5: Compare to system output / identify deficit or surplus:
9 | P a g e SOLUTION Step 1: Delta T ΔT = 21 − (−1) = 22°C Step 2: Fabric heat loss Given: 8.5 kW Step 3: Ventilation loss Given: 3.5 kW Step 4: Total heat loss Total = 8.5 + 3.5 = 12.0 kW Step 5: Compare to system output System output: 10 kW Total heat loss: 12 kW Deficit = 12 − 10 = 2 kW The calculation indicates a 2 kW deficit at the stated design condition. The poor thermal characteristics of the solid walls, single glazing and uninsulated loft are significant contributors to demand. Before recommending changes to the heating system, the engineer should consider whether reducing building heat loss would be the more effective first intervention. This is an open challenge. No answer is provided in this document. Use what you have learned to work through it. The goal is not a precise number it is a line of reasoning.
10 | P a g e THE CHALLENGE PROPERTY You are called to a 1930s mid-terrace property. The customer reports that the front living room never reaches temperature in winter, despite the boiler running continuously. The rear of the house is warm. The front room has a large bay window original timber frame, single glazed. The front wall is solid brick, no cavity. The loft above has been partially insulated. The installed system output is 8 kW. Design outside temperature in this area is −3°C. The customer’s target temperature is 20°C. Your tasks: 1. Estimate whether the 8 kW system is likely to be sufficient for the whole house. State your reasoning. 2. Identify which element of the heat loss profile is most likely responsible for the front room complaint. 3. Explain why the rear of the house might be warm while the front is cold without assuming a fault. 4. What would you want to know or measure before recommending any changes? Check your reasoning against the written lesson and the core teaching video. There is no single right answer the quality of your thinking matters more than the number Changing energy requirements One final property Let's apply the principle to a different house. A heat-loss assessment has established that this property requires 5.7 kW at its design condition: Working:
11 | P a g e Indoor design temperature: 21°C Outdoor design temperature: −5°C Design ΔT: 26°C We aren't going to calculate the 5.7 kW yet, that's coming later. For now, we're going to use it to explore what happens to demand as outdoor temperature changes. As the outdoor temperature changes, so too does the amount of heat the building loses. A warmer outdoor temperature reduces the temperature difference between inside and outside, meaning less energy escapes through the building fabric. Consequently, the heating system has less energy to replace. In this example, the design temperature difference (ΔT) is: 21°C - (-5°C) = 26°C This allows us to estimate the building's energy requirement for different outdoor temperatures by calculating the energy required for each degree of temperature difference. 5.7 kW ÷ 26°C = 0.219 kW per degree Using this value, we can estimate how the building's heat demand changes as the outdoor temperature increases. The table below demonstrates this relationship. As the outdoor temperature rises, the temperature difference decreases, resulting in a lower heating demand. Give it a try, the first calculation is there as a guide. The solution is at the end of the manual. Key Principle
12 | P a g e A building does not have a fixed heating demand. It only requires enough energy to replace the heat it is losing at that moment. As outdoor temperatures rise, heat loss reduces, and so does the energy required to maintain a comfortable indoor environment.
13 | P a g e Glossary Energy The quantity that enables change. In heating, it is what raises temperature, warms surfaces and creates comfort. Measured in joules (J) or kilowatt-hours (kWh) for larger quantities. Heat Energy in transit specifically energy moving from a warmer body or space to a cooler one as a result of a temperature difference. Heat is a process, not a thing stored. Power The rate at which energy is transferred or delivered. Measured in kilowatts (kW). One kilowatt = one kilojoule per second. Power determines how quickly a system can respond to demand. Kilowatt (kW) The standard unit of power in heating engineering. A 10 kW boiler transfers 10 kilojoules of energy every second it is running. System outputs, heat losses and emitter ratings are all expressed in kilowatts. Heat loss The rate at which a building loses energy to its surroundings. Always expressed in kilowatts because it is a rate, not a total. Comprises fabric loss (through the building structure) and ventilation/infiltration loss (through air movement). Fabric heat loss Heat escaping through the walls, roof, floor, windows and doors of a building. Driven by the thermal properties of each element (U-value), the surface area and the temperature difference across the element. Ventilation loss Heat leaving with warm air that is replaced by cooler external air. Includes both deliberate ventilation and uncontrolled infiltration through gaps and openings. Energy balance At steady state, useful system output matches the building's heat loss and internal temperature remains stable. If output is lower than heat loss, temperature falls; if output is greater, temperature rises.
14 | P a g e Delta T (ΔT) Temperature difference. In the context of heat loss: the difference between internal and external temperature. The greater the ΔT, the faster the heat loss. Deficit The shortfall when a system’s output is less than a building’s heat loss at a given condition. A 2 kW deficit means the system cannot maintain design temperature under those conditions. Changing energy requirements: solution
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