TP-10: Pipework and Ductwork Losses

Technical reference for the HEM pipework and ductwork loss methodology, covering distribution system heat losses and gains.

Overview

Heat is lost from distribution pipework and ventilation ductwork whenever the fluid temperature inside differs from the surrounding temperature. TP-10 quantifies these losses and determines how they are allocated: heat lost from pipes and ducts inside the thermal envelope is partially recovered as internal gains, whilst heat lost from external runs is wasted entirely.

Two distinct loss mechanisms are modelled:

Pipework losses from hot water distribution and primary heating circuits, calculated as cool-down losses (energy released when pipe contents cool from delivery temperature to ambient) and steady-state conductive losses during active flow.
Ductwork losses from MVHR (mechanical ventilation with heat recovery) supply, extract, intake, and exhaust ducts, calculated as steady-state radial heat transfer through insulated or uninsulated duct walls.

The pipework methodology follows the 2021 ASHRAE Handbook, Section 4.4.2 for steady-state radial heat flow through cylindrical shells. The ductwork methodology follows ISO 12241:2022 for both circular and rectangular cross-sections.

Inputs

Pipework Parameters

Parameter	Symbol	Unit	Description
Internal diameter	$d_{i}$	m	Internal diameter of the pipe
External diameter	$d_{e}$	m	External diameter of the pipe (wall only, excluding insulation)
Pipe length	$L$	m	Length of the pipe run
Insulation thermal conductivity	$k_{in s}$	W/(mK)	Thermal conductivity of the pipe insulation
Insulation thickness	$t_{in s}$	m	Thickness of the pipe insulation
Surface reflectivity	—	boolean	Whether the outer surface is reflective (low emissivity)
Pipe contents	—	—	Water, 25% glycol/75% water mixture, or air
Location	—	—	Internal (within thermal envelope) or external
Fluid temperature	$T_{f l u i d}$	°C	Temperature of the fluid inside the pipe
Surrounding temperature	$T_{s u r r}$	°C	Air temperature surrounding the pipe

Ductwork Parameters

Parameter	Symbol	Unit	Description
Cross-section shape	—	—	Circular or rectangular
Internal diameter	$d_{i}$	m	Internal diameter (circular ducts only)
External diameter	$d_{e}$	m	External diameter (circular ducts only)
Duct perimeter	$P_{d u c t}$	m	Internal perimeter (rectangular ducts only)
Duct length	$L$	m	Length of the duct run
Insulation thermal conductivity	$k_{in s}$	W/(mK)	Thermal conductivity of the duct insulation
Insulation thickness	$t_{in s}$	m	Thickness of the duct insulation
Surface reflectivity	—	boolean	Whether the outer surface is reflective (low emissivity)
Duct type	—	—	Intake, supply, extract, or exhaust
MVHR location	—	—	Whether the MVHR unit is inside or outside the thermal envelope
MVHR efficiency	$η_{m v h r}$	—	Heat recovery efficiency of the MVHR unit (0 to 1)
Indoor air temperature	$T_{in t}$	°C	Zone air temperature
Outdoor air temperature	$T_{e x t}$	°C	External air temperature

Heat Transfer Coefficients

The internal and external surface heat transfer coefficients are fixed values from CIBSE Guide C:

Coefficient	Symbol	Value	Unit	Source
Internal HTC, air	$h_{i, ai r}$	15.5	W/(m²K)	CIBSE Guide C, Table 3.25 (air flow ~3 m/s)
Internal HTC, water	$h_{i, w a t er}$	1500	W/(m²K)	CIBSE Guide C, Table 3.32
Internal HTC, glycol (25%)	$h_{i, g l y co l}$	1500	W/(m²K)	Assumed equal to water
External HTC, reflective	$h_{e, r e f l}$	5.7	W/(m²K)	CIBSE Guide C, Table 3.25
External HTC, non-reflective	$h_{e, n r e f l}$	10.0	W/(m²K)	CIBSE Guide C, Table 3.25

Material Properties

Property	Water	Glycol (25%)	Air	Unit
Density	1.0	1.0	0.001204	kg/litre
Specific heat capacity	4184	3757	1006	J/(kgK)
Volumetric heat capacity	4184	3757	1.211	J/(litreK)

Calculation

Pipework Thermal Resistance Model

The steady-state heat loss from a pipe is modelled as radial heat flow through three resistive layers: the internal fluid film, the pipe wall plus insulation, and the external surface film. All thermal resistances are expressed per unit length of pipe, in Km/W.

Internal Surface Resistance

$R_{i} = \frac{1}{h _{i} π d _{i}}$

Where:

$h_{i}$ is the internal heat transfer coefficient for the pipe contents W/(m²K)
$d_{i}$ is the internal diameter of the pipe m

Insulation Resistance

$R_{in s} = \frac{l n ( D _{in s} / d _{i} )}{2 π k _{in s}}$

Where:

$D_{in s} = d_{e} + 2 t_{in s}$ is the outer diameter including insulation m
$d_{i}$ is the internal diameter of the pipe m
$k_{in s}$ is the thermal conductivity of the insulation W/(mK)

External Surface Resistance

$R_{e} = \frac{1}{h _{e} π D _{in s}}$

Where:

$h_{e}$ is the external surface heat transfer coefficient W/(m²K), selected as $h_{e, r e f l}$ or $h_{e, n r e f l}$ depending on surface reflectivity
$D_{in s}$ is the outer diameter including insulation m

Total Resistance and Heat Loss

The total thermal resistance per unit length is:

$R_{t o t a l} = R_{i} + R_{in s} + R_{e}$

The steady-state heat loss from the pipe is then:

$\dot{Q}_{p i p e} = \frac{T _{f l u i d} - T _{s u r r}}{R _{t o t a l}} \times L [W]$

Where:

$T_{f l u i d}$ is the fluid temperature inside the pipe °C
$T_{s u r r}$ is the surrounding air temperature °C
$L$ is the pipe length m

Pipe Volume and Cool-Down Loss

The volume of fluid contained in the pipe is:

$V_{p i p e} = π (\frac{d _{i}}{2})^{2} L \times 1000 [litres]$

When flow stops, the pipe contents cool from the delivery temperature to the surrounding temperature. The total cool-down energy loss is:

$Q_{coo l d o w n} = c_{v} (T_{f l u i d} - T_{s u r r}) \times V_{p i p e} [J]$

Where $c_{v}$ is the volumetric heat capacity of the fluid J/(litreK).

Converted to kWh:

$Q_{coo l d o w n} = \frac{c _{v} ( T _{f l u i d} - T _{s u r r} ) \times V _{p i p e}}{3 , 600 , 000} [kWh]$

Temperature Drop Over a Timestep

For a one-hour timestep, the temperature drop of the pipe contents is derived from the energy balance $Q = c_{v} V Δ T$ :

$Δ T_{p i p e} = \frac{Q ˙ _{p i p e} \times 3600}{c _{v} \times V _{p i p e}} [K]$

This is clamped so the fluid temperature cannot fall below the surrounding temperature:

$Δ T_{p i p e} = min (Δ T_{p i p e}, T_{f l u i d} - T_{s u r r})$

Distribution Pipework Losses (Hot Water)

Distribution pipework connects the hot water source to each tapping point (showers, baths, other outlets). The pipe length is divided equally among all tapping points:

$L_{a v g} = \frac{L _{t o t a l}}{N _{t a p s}}$

Where $N_{t a p s}$ is the total number of hot water tapping points.

Distribution pipework uses the simplified cool-down model (no thermal resistance network; only the volumetric energy content of the pipe contents is considered). Each draw-off event flushes hot water into the pipe; after the event, the water left in the pipe cools to ambient. The loss per event per pipe section is $Q_{coo l d o w n}$ . The total distribution pipework loss for a timestep is:

$Q_{d i s t} = N_{e v e n t s} \times \sum_{j} Q_{coo l d o w n, j} [kWh]$

Where $N_{e v e n t s}$ is the number of hot water draw-off events in the timestep and $j$ indexes each distribution pipe section.

Losses are separated by location:

Internal losses ( $Q_{d i s t, in t}$ ): pipe sections inside the thermal envelope, where $T_{s u r r} = T_{in t}$
External losses ( $Q_{d i s t, e x t}$ ): pipe sections outside the thermal envelope, where $T_{s u r r} = T_{e x t}$

Primary Pipework Losses (Heating Circuit)

Primary pipework connects the heat source (boiler, heat pump) to the hot water storage cylinder. These pipes carry water at flow temperature and experience three loss phases within each heating cycle:

Start of heating event: Cool-down loss from the previous cycle, calculated as the energy released when the pipe contents cool from flow temperature to surrounding temperature. This is a one-off loss at the start.
During heating event: Steady-state heat loss $\dot{Q}_{p i p e}$ W calculated from the thermal resistance model, converted to energy over the timestep:

$Q_{p r ima r y, s t e a d y} = \frac{Q ˙ _{p i p e} \times Δ t _{h}}{1000} [kWh]$

Where $Δ t_{h}$ is the timestep duration in hours.

End of heating event: Cool-down loss from internal pipe sections is credited as an internal gain (the heat enters the dwelling).

The total primary pipework loss is added to the energy demand on the heat source. Heat lost from internal pipe runs is recovered as an internal gain; heat lost from external runs is wasted.

Ductwork Thermal Resistance Model

Ductwork for MVHR systems uses the same three-resistance approach as pipework, adapted for the duct geometry.

Circular Ducts

The resistances per unit length are identical in form to the pipework model:

$R_{i} = \frac{1}{h _{i, ai r} π d _{i}}$

$R_{in s} = \frac{l n ( D _{in s} / d _{i} )}{2 π k _{in s}}$

$R_{e} = \frac{1}{h _{e} π D _{in s}}$

Where $D_{in s} = d_{e} + 2 t_{in s}$ .

Rectangular Ducts

For rectangular (or square) ducts, the resistances per unit length are calculated using the duct perimeter per ISO 12241:2022:

$P_{e x t} = P_{d u c t} + 8 t_{in s}$

$R_{i} = \frac{1}{h _{i, ai r} P _{d u c t}}$

$R_{in s} = \frac{2 t _{in s}}{k _{in s} ( P _{d u c t} + P _{e x t} )}$

$R_{e} = \frac{1}{h _{e} P _{e x t}}$

Where:

$P_{d u c t}$ is the internal perimeter of the duct m
$P_{e x t}$ is the external perimeter including insulation m
The factor of 8 in the external perimeter formula is specified in ISO 12241:2022

Duct Heat Loss

The heat loss from any duct is:

$\dot{Q}_{d u c t} = \frac{T _{ai r, in} - T _{ai r, o u t}}{R _{t o t a l}} \times L [W]$

Where $T_{ai r, in}$ is the air temperature inside the duct and $T_{ai r, o u t}$ is the air temperature outside the duct.

Ductwork Loss Allocation by MVHR Location

The allocation of ductwork losses depends on whether the MVHR unit is located inside or outside the thermal envelope, and on the duct type. There are four duct types in an MVHR system:

Intake: brings outdoor air into the MVHR unit
Supply: delivers warmed air from the MVHR unit to the dwelling
Extract: carries indoor air from the dwelling to the MVHR unit
Exhaust: expels stale air from the MVHR unit to outside

MVHR Unit Located Outside the Thermal Envelope

When the MVHR unit sits outside the dwelling, the supply and extract ducts run between the unit (outside) and the dwelling (inside). Heat lost from these ducts is lost to the external environment.

Duct type	Air temperature inside duct	Loss calculation
Intake	$T_{e x t}$	Zero loss (duct and surroundings at same temperature)
Supply	$T_{e x t} + η_{m v h r} (T_{in t} - T_{e x t})$	$\dot{Q}_{d u c t} (T_{s u ppl y}, T_{e x t})$
Extract	$T_{in t}$	$\dot{Q}_{d u c t} (T_{in t}, T_{e x t}) \times η_{m v h r}$
Exhaust	$T_{e x t}$	Zero loss (duct and surroundings at same temperature)

The supply duct temperature accounts for heat recovery: the MVHR unit warms the incoming air by a fraction $η_{m v h r}$ of the indoor-outdoor temperature difference. The extract duct loss is multiplied by $η_{m v h r}$ because only the recoverable fraction of the extracted heat matters; heat lost from the extract duct before reaching the heat exchanger reduces the recovered energy.

MVHR Unit Located Inside the Thermal Envelope

When the MVHR unit sits inside the dwelling, the intake and exhaust ducts run between outside and the unit (inside). Heat exchange between these ducts and the dwelling interior affects the zone heat balance.

Duct type	Air temperature inside duct	Loss calculation
Intake	$T_{e x t}$	$\dot{Q}_{d u c t} (T_{e x t}, T_{in t}) \times η_{m v h r}$
Supply	—	Zero loss (duct entirely within conditioned space)
Extract	—	Zero loss (duct entirely within conditioned space)
Exhaust	$T_{e x t} + (1 - η_{m v h r}) (T_{in t} - T_{e x t})$	$\dot{Q}_{d u c t} (T_{e x ha u s t}, T_{in t})$

The intake duct carries cold outdoor air through the dwelling; heat gained from the zone is multiplied by $η_{m v h r}$ because the benefit is proportional to recovery efficiency. The exhaust duct temperature reflects the air after heat recovery: it retains only the unrecovered fraction of the indoor-outdoor difference.

Gains Allocation

Distribution Pipework Internal Gains

Internal distribution pipework losses are recovered as internal gains to the zone. The total internal gain from hot water use is:

$\dot{Q}_{g ain s, d h w} = \frac{Q _{d i s t, in t} + Q _{g ain s, u se}}{Δ t _{h}} \times 1000 [W]$

Where $Q_{g ain s, u se}$ is the fraction of delivered hot water energy that becomes an internal gain:

$Q_{g ain s, u se} = 0.25 \times c_{w} V_{t a p} (T_{h w} - T_{in t}) [kWh]$

Where:

$0.25$ is the fixed fraction of DHW energy assumed to become an internal gain
$c_{w}$ is the volumetric energy content of water kWh/litre
$V_{t a p}$ is the volume of hot water delivered at tapping points litres
$T_{h w}$ is the hot water delivery temperature °C
$T_{in t}$ is the internal air temperature °C

External distribution pipework losses ( $Q_{d i s t, e x t}$ ) are not recovered.

Ductwork Internal Gains

Ductwork gains are distributed across zones in proportion to zone volume:

$\dot{Q}_{d u c t, z o n e} = \dot{Q}_{d u c t, t o t a l} \times \frac{V _{z o n e}}{V _{t o t a l}} [W]$

Where:

$V_{z o n e}$ is the volume of the zone m³
$V_{t o t a l}$ is the total dwelling volume m³

The sign convention for ductwork gains distinguishes between losses to the zone and losses to outside:

Intake and exhaust ducts: heat exchanged with the zone is added to internal gains (may be negative, representing heat drawn from the zone into cold duct air)
Supply and extract ducts: heat lost to the external environment is subtracted from internal gains

Outputs

Quantity	Symbol	Unit	Description
Pipe steady-state heat loss	$\dot{Q}_{p i p e}$	W	Instantaneous heat loss rate from a pipe
Pipe cool-down loss	$Q_{coo l d o w n}$	kWh	Total energy released when pipe contents cool to ambient
Pipe temperature drop	$Δ T_{p i p e}$	K	Temperature reduction of pipe contents over one timestep
Distribution pipework loss (internal)	$Q_{d i s t, in t}$	kWh	Cool-down losses from internal distribution pipes per timestep
Distribution pipework loss (external)	$Q_{d i s t, e x t}$	kWh	Cool-down losses from external distribution pipes per timestep
Primary pipework loss	$Q_{p r ima r y}$	kWh	Total primary circuit pipework loss per timestep
Duct heat loss	$\dot{Q}_{d u c t}$	W	Heat loss rate from a single duct
Total ductwork gains	$\dot{Q}_{d u c t, t o t a l}$	W	Net heat gain to the dwelling from all MVHR ducts
Internal gains from DHW	$\dot{Q}_{g ain s, d h w}$	W	Internal gains from pipework losses and hot water use

Assumptions

The internal heat transfer coefficient for water (1500 W/(m²K)) is adopted for the 25% glycol/75% water mixture. Given the high value relative to insulation and external resistances, this has negligible effect on the total resistance.
Distribution pipework uses the simplified cool-down model only (no steady-state loss during flow). Each draw-off event results in one complete cool-down of the pipe contents.
The number of cool-down events per timestep equals the number of hot water draw-off events. Overlapping events that share pipework are not discounted.
The pipe length for distribution losses is divided equally among all tapping points.
External surface heat transfer coefficients are fixed constants (5.7 or 10.0 W/(m²K)) regardless of wind speed or orientation.
The fraction of DHW energy that becomes an internal gain is fixed at 0.25.
MVHR ductwork is assumed to operate continuously (100% of each timestep).
The temperature of pipe contents cannot fall below the surrounding air temperature.
For rectangular duct external perimeter, the constant factor of 8 applied to insulation thickness follows ISO 12241:2022 without further geometric correction.

Cross-references

TP-03: External Conditions: external air temperature used as surrounding temperature for external pipe and duct runs
TP-04: Space Heating Demand: pipework and ductwork internal gains feed into the zone heat balance
TP-06: Ventilation and Infiltration: MVHR system definition, ductwork configuration, and heat recovery efficiency
TP-09: Hot Water Demand: hot water draw-off events, tapping point volumes, and delivery temperatures that drive distribution pipework losses
TP-11: Hot Water Storage: primary pipework connects the heat source to the storage cylinder; primary losses are added to the heat source demand
TP-12: Heat Pumps: flow temperature used in primary pipework loss calculations
TP-14: Boilers: flow temperature used in primary pipework loss calculations

TP-09: Hot Water Demand

TP-11: Hot Water Storage