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OFTEC Heat Pump practice questions

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Question 1Heat pump system design and sizing

In a domestic heat pump system, what does the term 'design heat loss' represent?

  1. The maximum electrical input the heat pump compressor can draw
  2. The rate of heat energy that must be supplied to maintain the indoor design temperature on the coldest design day Correct answer.
  3. The total annual energy the building will consume for heating
  4. The quantity of heat the emitter circuit can release at peak flow temperature

Explanation

Design heat loss is the calculated steady-state heat loss of the building at the chosen external design temperature, and it sets the required peak heating capacity. Annual energy use is a separate seasonal calculation, and emitter output is a property of the distribution system rather than the building fabric.

Question 2Heat pump system design and sizing

For which external condition is a heat pump's heating capacity normally matched to the building's design heat loss?

  1. The mean winter external temperature
  2. The lowest temperature ever recorded locally
  3. The external design temperature selected for the site Correct answer.
  4. The summer design external temperature

Explanation

Sizing is carried out against a defined external design temperature so that capacity and load are compared on the same basis. Using the absolute recorded minimum would oversize the unit, while a mean winter temperature would undersize it.

Question 3Heat pump system design and sizing

A dwelling has a calculated design heat loss of 9 kW at the external design temperature. Which heat pump rated capacity is the most appropriate starting point?

  1. 6 kW
  2. 18 kW
  3. 4.5 kW
  4. 9 kW Correct answer.

Explanation

The unit's rated heating output at the design external temperature should be at least equal to the design heat loss. Matching capacity to load avoids both shortfall on cold days and excessive cycling.

Question 4Heat pump system design and sizing

Which quantity is used to describe the ratio of heat delivered to electrical energy consumed by a heat pump at a single specified operating point?

  1. Coefficient of performance Correct answer.
  2. Seasonal performance factor
  3. Flow temperature differential
  4. Specific heat capacity

Explanation

Coefficient of performance (COP) is an instantaneous ratio at a defined set of conditions. The seasonal performance factor averages performance over a whole heating season instead.

Question 5Heat pump system design and sizing

Why does lowering the design flow temperature generally improve heat pump efficiency?

  1. It reduces the temperature lift the compressor must produce Correct answer.
  2. It reduces the compressor's electrical supply voltage
  3. It increases the refrigerant mass flow through the evaporator
  4. It raises the heat output of the compressor at a given speed

Explanation

A smaller difference between source and delivery temperature means less work is required per unit of heat moved. This is why low-temperature emitters such as underfloor heating support higher efficiency.

Question 6Heat pump system design and sizing

Which emitter type is generally best suited to the low flow temperatures favoured by heat pumps?

  1. Small, high-output steel panel radiators
  2. Large-surface area underfloor heating Correct answer.
  3. Towel rails in bathrooms
  4. Fan convectors sized for 80 °C flow

Explanation

Heat output depends on emitter surface area and the temperature difference to the room, so large-area emitters deliver the required output at lower flow temperatures. Compact radiators and units sized for high flow temperatures need higher water temperatures to meet the same load.

Question 7Heat pump system design and sizing

What is the main purpose of a room-by-room heat loss calculation rather than a whole-dwelling total?

  1. To determine the refrigerant charge required
  2. To calculate the building's EPC rating
  3. To size individual emitters and check room temperatures can be achieved Correct answer.
  4. To select the compressor displacement

Explanation

Room-by-room calculations show the load in each space so emitters can be sized to deliver it at the design flow temperature. A whole-dwelling total cannot reveal whether individual rooms will reach their design temperature.

Question 8Heat pump system design and sizing

Which factor most directly increases the design heat loss of a dwelling?

  1. A larger hot water cylinder volume
  2. A higher proportion of glazing in the external walls Correct answer.
  3. A longer primary pipe run to the cylinder
  4. A higher set-back temperature at night

Explanation

Glazing typically has a much higher U-value than insulated wall construction, so increasing glazed area raises fabric heat loss. Cylinder volume and pipe runs affect hot water and distribution losses, not the space heating fabric loss.

Question 9Heat pump system design and sizing

A heat pump has a rated output of 12 kW at A7/W35 but only 9 kW at A-2/W35. The building design heat loss is 10 kW at the external design temperature of -2 °C. What does this indicate?

  1. The unit is oversized and will cycle excessively
  2. The unit is undersized at the design condition and may need supplementary heat or a larger model Correct answer.
  3. The flow temperature should be raised to 45 °C to increase capacity
  4. The unit is correctly sized because its nominal rating exceeds the load

Explanation

Capacity must be assessed at the design external temperature and design flow temperature, not at nominal rating conditions. At -2 °C the unit delivers 9 kW against a 10 kW load, so it cannot meet design conditions alone.

Question 10Heat pump system design and sizing

Why is oversizing a heat pump beyond the design heat loss generally undesirable?

  1. It causes more frequent cycling and poorer part-load efficiency Correct answer.
  2. It prevents the unit from reaching design output
  3. It increases the refrigerant pressure ratio beyond safe limits
  4. It reduces the maximum flow temperature available

Explanation

An oversized unit reaches the required output quickly and then cycles, which reduces efficiency and increases component wear. Sizing close to the design load allows longer, steadier running.

Question 11Heat pump system design and sizing

A dwelling has a design heat loss of 8 kW. Hot water demand is estimated at 2 kW equivalent continuous load. What is the most sensible approach to sizing?

  1. Size the heat pump for 10 kW or apply a suitable hot water allowance or priority control Correct answer.
  2. Size the heat pump for 6 kW because hot water is intermittent
  3. Size the heat pump for 8 kW and ignore hot water
  4. Size the heat pump for 16 kW to provide a safety margin

Explanation

The unit must cover space heating plus the domestic hot water load, either by adding a suitable allowance or by using priority control that interrupts space heating during reheat. Ignoring hot water risks shortfall at peak demand.

Question 12Heat pump system design and sizing

Which statement best describes the relationship between flow temperature and emitter output?

  1. Output is fixed once the emitter is installed
  2. Output depends on the mean water temperature relative to room temperature Correct answer.
  3. Output depends only on the water flow rate
  4. Output is proportional to flow temperature regardless of room temperature

Explanation

Emitter output is driven by the temperature difference between the mean water temperature in the emitter and the room air. Higher flow temperatures or lower room temperatures both increase output.

Question 13Heat pump system design and sizing

A designer calculates the building heat loss using a room temperature of 21 °C but the client intends to keep bedrooms at 18 °C. What is the effect of using 21 °C throughout?

  1. It overestimates the heat loss in the bedrooms Correct answer.
  2. It reduces the required flow temperature
  3. It underestimates the total heat loss
  4. It has no effect on the total heat loss

Explanation

Heat loss rises with the indoor-to-outdoor temperature difference, so assuming a warmer bedroom than intended increases the calculated loss for those rooms. This can lead to oversized emitters and a slightly overstated total.

Question 14Heat pump system design and sizing

Which of the following best explains why a heat pump's declared capacity should be checked against manufacturer data at the actual design flow temperature?

  1. Capacity varies with the temperature lift between source and delivery Correct answer.
  2. Capacity is constant for a given model regardless of conditions
  3. Capacity depends only on the electrical supply rating
  4. Capacity is fixed by the refrigerant charge

Explanation

Heating capacity falls as the temperature lift increases, so the same unit delivers different outputs at different source and flow temperatures. Manufacturer tables give capacity at defined conditions so the designer can interpolate for the design case.

Question 15Heat pump system design and sizing

What is the main reason for including a buffer volume in a heat pump heating circuit?

  1. To stabilise operation and prevent short cycling when system volume is low Correct answer.
  2. To reduce the design heat loss of the building
  3. To raise the maximum flow temperature
  4. To increase the refrigerant charge

Explanation

A buffer adds thermal mass so the compressor has a stable load and does not cycle rapidly, particularly when emitter volume is small or zones are closed. It does not change the building's heat loss or the achievable flow temperature.

Question 16Heat pump system design and sizing

A designer is comparing two emitters for a room with a 2 kW load. Emitter A delivers 2 kW at 45 °C flow, emitter B delivers 2 kW at 55 °C flow. Which choice best supports good heat pump performance?

  1. Emitter A, because it meets the load at a lower flow temperature Correct answer.
  2. Emitter B, because it will reduce compressor cycling
  3. Emitter B, because higher flow temperatures improve comfort
  4. Either, because output is the same

Explanation

Meeting the load at a lower flow temperature reduces the temperature lift and improves COP. Emitter selection should therefore be based on achieving the required output at the lowest practical flow temperature.

Question 17Heat pump system design and sizing

What does a seasonal performance factor (SPF) account for that a single-point COP does not?

  1. The refrigerant's global warming potential
  2. The emitter surface area
  3. Performance across varying outdoor temperatures and operating modes over a season Correct answer.
  4. The electrical supply voltage fluctuations

Explanation

SPF integrates performance over the whole season, including part-load operation and changing source temperatures. A single COP figure describes only one steady operating point.

Question 18Heat pump system design and sizing

A dwelling's design heat loss is 7 kW and the selected heat pump delivers 7.5 kW at design conditions. The calculated system water volume is 12 litres. What is the most appropriate design response?

  1. Reduce the emitter sizes to lower system volume
  2. Add buffer volume or review control strategy to prevent short cycling Correct answer.
  3. Raise the design flow temperature to 55 °C
  4. Increase the heat pump size to 10 kW

Explanation

A close capacity match is good, but low system water volume can cause rapid cycling, so adding buffer volume or adjusting controls is the appropriate response. Increasing capacity would worsen cycling and reduce efficiency.

Question 19Heat pump system design and sizing

Which of the following is the most appropriate way to determine the domestic hot water load for sizing purposes?

  1. Use the same figure as the space heating design load
  2. Estimate from occupancy, cylinder volume and reheat time requirements Correct answer.
  3. Assume a fixed 5 kW for all dwellings
  4. Ignore hot water because it is intermittent

Explanation

Hot water demand depends on occupancy and the required reheat performance, so it should be estimated from these factors and the cylinder specification. A fixed assumption or ignoring the load can lead to inadequate hot water.

Question 20Heat pump system design and sizing

A heat pump is being sized for a dwelling where the calculated design heat loss is 11 kW. Manufacturer data shows 11.5 kW at A-2/W45 and 9.8 kW at A-2/W55. Which conclusion is correct if the design flow temperature is 55 °C?

  1. The unit is oversized and should be downrated
  2. The flow temperature should be reduced to 45 °C to increase capacity
  3. The unit falls short at design conditions and supplementary heat or a larger model is needed Correct answer.
  4. The unit meets the load at design conditions

Explanation

Capacity must be read at the design flow temperature. At W55 the unit delivers 9.8 kW against an 11 kW load, so it cannot meet design conditions alone. The higher figure at W45 is not applicable if the system is designed for 55 °C.

Question 21Heat pump system design and sizing

A designer proposes sizing the heat pump to the design heat loss with no margin, in a dwelling with poor fabric and a low system water volume. Which combination of risks is most relevant?

  1. Increased emitter output and reduced design heat loss
  2. Shortfall on colder-than-design days and rapid compressor cycling Correct answer.
  3. Excess refrigerant charge and high flow temperatures
  4. Reduced hot water volume and lower COP

Explanation

Sizing exactly to the design load leaves no reserve if conditions are more severe than the design assumption, while low system volume encourages rapid cycling. Both risks should be addressed through design review and appropriate buffering or control.

Question 22Heat pump system design and sizing

A dwelling has a design heat loss of 6 kW at -3 °C. The selected unit's capacity at -3 °C and the design flow temperature is 6.2 kW, but its output falls to 4.5 kW at -7 °C. Which design consideration is most important?

  1. The unit is oversized and should be downrated
  2. The unit is clearly undersized and must be replaced
  3. The flow temperature should be raised to compensate
  4. The unit is adequate at design but the effect of colder-than-design periods should be assessed Correct answer.

Explanation

The unit meets the declared design condition, but performance at lower temperatures should be considered, for example through a bivalent point with supplementary heat or an immersion backup. This is a design review point rather than an automatic rejection.

Question 23Heat pump system design and sizing

Which of the following best describes the purpose of a bivalent point in a heat pump system design?

  1. The temperature at which the compressor is switched off for defrost
  2. The maximum flow temperature the emitters can accept
  3. The temperature at which the heat pump alone meets the full building load
  4. The outdoor temperature at which supplementary heating is brought into operation Correct answer.

Explanation

The bivalent point is the outdoor temperature at which the heat pump's capacity equals the building load and below which a supplementary heat source assists. It allows a smaller heat pump to be specified without compromising comfort.

Question 24Heat pump system design and sizing

A designer is comparing a monovalent heat pump system with a bivalent system for a poorly insulated dwelling. Which statement best captures the trade-off?

  1. Monovalent systems cannot meet design conditions
  2. Monovalent systems always require a larger heat pump and no supplementary heat
  3. Bivalent systems always have higher seasonal efficiency
  4. Bivalent systems allow a smaller heat pump but rely on a supplementary heat source at low temperatures Correct answer.

Explanation

Bivalent design permits a smaller heat pump by using supplementary heat during the coldest periods, which can reduce capital cost but introduces a second heat source. Monovalent design relies on the heat pump alone and therefore requires capacity to match the full design load.

Question 25Heat pump system design and sizing

Why should the design flow temperature be established before finalising emitter selection and heat pump capacity?

  1. Because it sets the building's design heat loss
  2. Because it links emitter output, heat pump capacity and efficiency in a single design decision Correct answer.
  3. Because it fixes the electrical supply requirements
  4. Because flow temperature determines the refrigerant type

Explanation

Flow temperature affects how much output each emitter delivers, the capacity available from the heat pump, and the resulting efficiency, so it must be fixed early to allow consistent sizing. It does not change the building's heat loss.

Question 26Heat pump system design and sizing

In heat pump system design, what does the term 'design heat loss' of a dwelling represent?

  1. The heat given off by occupants and appliances within the dwelling
  2. The total annual energy consumed by the heating system over a full year
  3. The rate of heat transfer from the building to the outside under the selected design external temperature and internal temperatures Correct answer.
  4. The maximum heat output the heat pump can deliver at the warmest expected outdoor temperature

Explanation

Design heat loss is the calculated steady-state rate at which heat escapes the fabric and via ventilation at the chosen design conditions, and it sets the peak load the heat pump must meet. It is a power value at specific temperatures, not an annual energy figure.

Question 27Heat pump system design and sizing

Which outdoor design temperature is normally used as the basis for sizing a heat pump in a UK dwelling?

  1. The summer design temperature used for cooling calculations
  2. The lowest temperature ever recorded at the nearest weather station
  3. The average outdoor temperature across the whole heating season
  4. A cold but not extreme winter external temperature selected for the location, often around -2 to -3 °C Correct answer.

Explanation

Design external temperatures are chosen to represent a cold but statistically representative winter condition, so the system meets load for the vast majority of the year without being sized for a rare extreme. Sizing to record lows would grossly oversize the unit.

Question 28Heat pump system design and sizing

Why is the fabric heat loss of a dwelling usually expressed per degree of temperature difference (W/K) during design?

  1. Because ventilation losses are excluded from the calculation
  2. Because it allows the loss to be scaled to any chosen internal and external temperature pair Correct answer.
  3. Because heat pumps are always rated in watts per kelvin
  4. Because it removes the need to know the floor area of the dwelling

Explanation

Expressing fabric transmission as a heat transfer coefficient in W/K lets the designer multiply by any internal-to-external temperature difference to obtain the loss at those conditions. It is a convenient intermediate value, not a requirement of heat pump ratings.

Question 29Heat pump system design and sizing

What does the term 'flow temperature' mean in a heat pump heating circuit?

  1. The temperature of the outside air used as the heat source
  2. The temperature of the domestic hot water stored in the cylinder
  3. The temperature of the water leaving the heat pump and entering the distribution circuit Correct answer.
  4. The temperature of the water returning from the emitters to the heat pump

Explanation

Flow temperature is measured at the outlet of the heat pump where heated water enters the distribution pipework toward the emitters. Return temperature is measured after the water has given up heat in the emitters.

Question 30Heat pump system design and sizing

Which quantity is used to express how much heat an emitter can release at a given flow temperature and room temperature?

  1. The emitter's output in watts at a stated temperature difference between mean water and room air Correct answer.
  2. The emitter's pressure drop at design flow rate
  3. The emitter's water content in litres
  4. The emitter's physical surface area alone

Explanation

Emitter output is quoted as a heat output at a defined temperature difference between the mean water temperature and the room air, because output rises as that difference increases. Water content and pressure drop affect dynamics and pumping but not the rated heat output directly.

Question 31Heat pump system design and sizing

What is the primary purpose of a heat pump system's design flow rate through the emitters?

  1. To keep the compressor operating at its maximum speed at all times
  2. To reduce the volume of water in the system to zero
  3. To ensure the emitters can deliver the required heat output at the chosen flow and return temperature difference Correct answer.
  4. To maximise the temperature difference between flow and return

Explanation

Flow rate, together with the flow-to-return temperature difference, determines the heat carried to the emitters; it must be adequate for the required output. Excessively high temperature differences reduce emitter mean temperature and output.

Question 32Heat pump system design and sizing

Which of the following is a legitimate reason to select a heat pump with a rated capacity slightly above the calculated design heat loss?

  1. To allow the flow temperature to be raised to 70 °C in winter
  2. To allow for capacity lost during defrost cycles and for real-world installation differences Correct answer.
  3. To eliminate the need for a domestic hot water cylinder
  4. To guarantee the compressor never modulates below full speed

Explanation

A modest allowance covers defrost derating, pipework losses, and small uncertainties in the heat loss calculation so the building still meets load at design conditions. Excessive margin is avoided because it harms part-load efficiency and cycling.

Question 33Heat pump system design and sizing

Which of these best describes the difference between a heat pump's rated heating capacity and its electrical input power?

  1. Heating capacity is the heat delivered to the water, while input power is the electricity consumed by the compressor and fans Correct answer.
  2. Heating capacity is measured in kilowatts and input power in kilowatt-hours
  3. They are always numerically equal at design conditions
  4. Input power is always larger than heating capacity

Explanation

Heating capacity is the thermal output delivered to the heating circuit, whereas input power is the electrical energy rate drawn by the unit. Their ratio gives the coefficient of performance, which is greater than one for a heat pump.

Question 34Heat pump system design and sizing

A room has a design heat loss of 1.8 kW. The selected radiator is rated at 2.4 kW at a mean water-to-room temperature difference of 50 K. If the design mean water temperature is 35 °C and the room is 20 °C, what approximate output should be expected?

  1. About 1.2 kW Correct answer.
  2. About 2.4 kW
  3. About 3.0 kW
  4. About 1.8 kW

Explanation

Emitter output varies approximately with the temperature difference raised to a power near 1.3. At a 15 K difference versus a 50 K rating, output falls to roughly 2.4 × (15/50)^1.3, which is around 1.2 kW, well below the 1.8 kW load.

Question 35Heat pump system design and sizing

A designer is sizing a heat pump for a dwelling with a design heat loss of 9 kW and a hot water cylinder that requires reheat. Which approach best accounts for hot water in the sizing decision?

  1. Size the heat pump to twice the space heating load to cover hot water
  2. Size to the space heating load and confirm the unit can also meet hot water reheat within an acceptable recovery time, using a cylinder with sufficient volume Correct answer.
  3. Ignore hot water entirely because it is not part of the heat loss calculation
  4. Add the full cylinder reheat capacity to the heat loss and size the heat pump to the sum

Explanation

Hot water reheat is intermittent and can often be scheduled or allowed a longer recovery, so the unit is normally sized on the space heating peak with a check that reheat is achievable. Adding full reheat to the peak load leads to unnecessary oversizing.

Question 36Heat pump system design and sizing

A dwelling has a design heat loss of 10 kW at -3 °C. The chosen heat pump delivers 10.5 kW at -3 °C with a 45 °C flow. The calculated hot water reheat adds a 1.5 kW continuous equivalent. What is the most appropriate conclusion?

  1. The unit may be marginal if hot water reheat coincides with peak heating, so reheat timing or a larger unit should be reviewed Correct answer.
  2. The unit is grossly oversized and should be replaced with a smaller one
  3. The unit is adequate because it exceeds the space heating load
  4. The hot water load should be ignored because cylinders store heat

Explanation

The unit covers the space heating load with a small margin, but if reheat overlaps with peak heating the combined demand exceeds capacity. The designer should assess whether reheat can be scheduled outside peak periods or whether additional capacity is needed.

Question 37Heat pump system design and sizing

Why does the required flow temperature influence the choice of emitter size in a heat pump system?

  1. Lower flow temperatures require smaller emitters because water moves faster
  2. Lower flow temperatures require larger emitters to deliver the same heat output Correct answer.
  3. Higher flow temperatures always improve heat pump efficiency
  4. Emitter size is independent of flow temperature

Explanation

Emitter output depends on the mean water temperature relative to the room, so a lower flow temperature reduces output per unit of emitter and must be compensated with more emitter area. This is why low-temperature systems need generously sized radiators or underfloor heating.

Question 38Heat pump system design and sizing

During a design review, a designer finds that the selected heat pump's capacity at the design external temperature and design flow temperature is below the building heat loss. What is the most appropriate first action?

  1. Reduce the design internal temperature by 5 °C to lower the load
  2. Increase the flow temperature to 65 °C to boost capacity
  3. Re-examine the heat loss assumptions, emitter sizing and achievable flow temperature before changing the unit Correct answer.
  4. Accept the shortfall because heat pumps are always oversized in practice

Explanation

A capacity shortfall can often be resolved by improving fabric assumptions, increasing emitter area to allow a lower flow temperature, or correcting an overestimated loss. These should be checked before jumping to a larger unit, which may not be necessary.

Question 39Heat pump system design and sizing

Which of the following best explains why the heat distribution circuit is often designed with a lower flow temperature than a traditional boiler system?

  1. Because emitters cannot withstand temperatures above 40 °C
  2. Because heat pumps operate more efficiently when the temperature lift from source to flow is smaller Correct answer.
  3. Because lower flow temperatures increase the compressor's maximum output
  4. Because lower flow temperatures reduce the risk of legionella in cylinders

Explanation

Heat pump efficiency depends on the temperature lift the compressor must provide; a lower flow temperature reduces that lift and raises COP. This is why low-temperature emitters are central to good heat pump design.

Question 40Heat pump system design and sizing

A designer is deciding between a 5 kW and a 7 kW heat pump for a dwelling with a design heat loss of 6.2 kW. The 5 kW unit is cheaper. What is the correct conclusion?

  1. Choose the 5 kW unit and raise the flow temperature to compensate
  2. Choose the 5 kW unit because it is closer to the load
  3. Choose neither and install a boiler instead
  4. Choose the 7 kW unit because it can meet the load at design conditions Correct answer.

Explanation

The selected unit must meet or exceed the design heat loss at the design conditions; a 5 kW unit cannot deliver 6.2 kW regardless of flow temperature in most cases. The 7 kW unit provides a small, acceptable margin.

Question 41Heat pump system design and sizing

What is the significance of the 'bivalent temperature' in a bivalent heat pump system?

  1. It is the temperature at which the heat pump switches to cooling mode
  2. It is the temperature at which the compressor is guaranteed to stop
  3. It is the maximum flow temperature the heat pump can produce
  4. It is the outdoor temperature at which the heat pump's capacity equals the building load and the back-up heater begins to assist Correct answer.

Explanation

The bivalent point is where heat pump output matches the building demand; below it, a supplementary heat source makes up the balance. This allows a smaller heat pump to be selected without losing coverage on the coldest days.

Question 42Heat pump system design and sizing

A poorly insulated dwelling has a design heat loss of 14 kW. The designer considers a monovalent heat pump versus a bivalent system with a smaller heat pump plus an electric back-up. Which statement is most accurate?

  1. The bivalent system is always more efficient than a monovalent system
  2. The bivalent system can allow a smaller heat pump and reduce capital cost, but back-up running costs and efficiency at low temperatures must be considered Correct answer.
  3. The monovalent system is always cheaper to install because it needs no back-up
  4. The bivalent system eliminates the need for a heat loss calculation

Explanation

A bivalent arrangement lets the heat pump be sized below the peak load, with the back-up covering the coldest periods; this can reduce capital cost but the back-up may be less efficient and more expensive to run. The trade-off should be evaluated against the dwelling's load profile.

Question 43Heat pump system design and sizing

Which of the following best explains why a heat pump's declared capacity at A7/W35 differs from its capacity at A2/W45?

  1. The A7/W35 rating applies only to cooling operation
  2. Capacity is fixed by the compressor and does not change with temperature
  3. The difference is due only to the electrical supply voltage
  4. Capacity depends on both the source air temperature and the flow temperature, so changing either alters output Correct answer.

Explanation

Heat pump capacity varies with the evaporating and condensing conditions, which are set by source and flow temperatures. A colder source or a higher flow temperature reduces capacity, so ratings must be read at the actual design conditions.

Question 44Heat pump system design and sizing

A dwelling has a heat loss of 8 kW at -3 °C. The heat pump delivers 8.5 kW at -3 °C but only 6.5 kW at -7 °C. The local design external temperature is -3 °C. How should the designer interpret this?

  1. The unit is adequate because it meets the load at the local design temperature of -3 °C Correct answer.
  2. The design external temperature should be changed to -7 °C
  3. The unit is undersized because it cannot meet the load at -7 °C
  4. The unit must be replaced with one that delivers 8.5 kW at -7 °C

Explanation

Sizing is based on the local design external temperature, so capacity at -3 °C is the relevant figure. Capacity at -7 °C is informative for colder spells but is not the design basis for this location.

Question 45Heat pump system design and sizing

Why is it important to establish the design flow temperature before finalising emitter selection?

  1. Because flow temperature has no effect on emitter output
  2. Because emitter output at the design flow temperature must meet the room load, and this determines the emitter size Correct answer.
  3. Because the flow temperature determines the refrigerant charge in the heat pump
  4. Because emitters cannot be changed after the flow temperature is set

Explanation

Emitter output depends on mean water temperature, so the design flow temperature must be known to size emitters that meet the room loads. Selecting emitters first and then setting flow temperature risks a mismatch between output and load.

Question 46Heat pump system design and sizing

A designer is comparing a heat pump rated 9 kW at A2/W35 with the same unit rated 7.5 kW at A2/W45. The building requires 8 kW at a 45 °C flow. What is the correct conclusion?

  1. The unit is inadequate at 45 °C flow because it delivers only 7.5 kW Correct answer.
  2. The unit is adequate because it delivers 9 kW
  3. The unit should be run at 35 °C flow to deliver 9 kW, accepting lower room temperatures
  4. The unit is adequate because flow temperature does not affect capacity

Explanation

Capacity must be checked at the actual design flow temperature. At 45 °C the unit delivers 7.5 kW, which is below the 8 kW load, so it is inadequate for that design unless the flow temperature can be reduced.

Question 47Heat pump system design and sizing

Why is the heat loss calculation for a heat pump sometimes carried out with a lower design internal temperature in bedrooms than in living rooms?

  1. Because bedrooms have no heat loss
  2. Because bedroom radiators are always smaller
  3. Because bedrooms are typically kept cooler, and using a lower internal temperature reduces the calculated loss and avoids oversizing Correct answer.
  4. Because heat pumps cannot serve bedrooms

Explanation

Design internal temperatures should reflect actual intended use; using a lower bedroom temperature reduces the calculated loss for those rooms and prevents unnecessary oversizing. This is a common refinement in room-by-room calculations.

Question 48Heat pump system design and sizing

A dwelling's design heat loss is 12 kW at -2 °C. The heat pump delivers 12.5 kW at -2 °C with a 40 °C flow, but the existing radiators are sized for a 55 °C flow. What is the most significant design issue?

  1. The heat pump is undersized
  2. The design heat loss is incorrect
  3. The radiators will not deliver the required output at 40 °C, so they must be upsized or the flow temperature raised Correct answer.
  4. The heat pump should be replaced with a larger unit

Explanation

Radiators sized for a 55 °C flow produce much less heat at 40 °C, so the emitters would be unable to meet the room loads even though the heat pump has adequate capacity. The emitters must be upsized or the flow temperature reassessed.

Question 49Heat pump system design and sizing

A designer is evaluating whether a buffer vessel is needed for a heat pump serving a small, low-volume circuit with several zone valves. Which reasoning is most sound?

  1. A buffer is unnecessary because zone valves always maintain adequate flow
  2. A buffer is only needed when underfloor heating is installed
  3. A buffer should be avoided because it reduces heat pump capacity
  4. A buffer is likely beneficial because the low water volume and zoning can cause short cycling and defrost issues Correct answer.

Explanation

Small water volumes combined with zoning can lead to insufficient system volume, causing short cycling and problems during defrost. A buffer adds thermal mass and hydraulic stability, which is why it is often recommended in such cases.

Question 50Heat pump system design and sizing

A dwelling has a design heat loss of 15 kW. The available heat pump delivers 13 kW at design conditions with a 45 °C flow, and the emitters can achieve a 40 °C flow if upsized. Which approach is most appropriate?

  1. Increase the flow temperature to 55 °C to increase capacity
  2. Upsize the emitters to allow a 40 °C flow, which increases heat pump capacity, and recheck the capacity against the load Correct answer.
  3. Install the 13 kW unit as-is and accept a shortfall on the coldest days
  4. Reduce the design internal temperature to lower the load

Explanation

Lowering the flow temperature increases heat pump capacity and reduces the temperature lift, so upsizing emitters can close the gap while improving efficiency. The capacity must then be rechecked at the new flow temperature against the design load.