TP-14: Boilers

Overview

TP-14 models gas and LPG boilers, covering both regular (system) boilers and combi boilers. The calculation determines the fuel consumed to meet space heating and hot water demands at each timestep, accounting for part-load operation, on/off cycling, standby losses, and location-dependent case losses.

The efficiency methodology is based on the Energy Balance Validation (EBV) method described in STP09/B02 and the hourly approach from SAP 10 Appendix D2.2 (S10TP-12). Tested full-load and part-load gross efficiencies are corrected for high values, converted to an EBV efficiency curve parameterised by return water temperature, and then adjusted at each timestep for cycling and boiler location. Combi boilers carry additional losses from keep-hot facilities, storage, and rejected energy during draw-off.

Inputs

Boiler parameters

Electrical consumption parameters

Combi boiler parameters (hot water service only)

Calculation

Net-to-gross conversion

Tested efficiencies are reported on a gross calorific value basis. The net-to-gross conversion factor $f_{ng}$ accounts for the difference between gross and net calorific values of the fuel:

Net efficiencies are obtained by dividing the gross efficiency by $f_{ng}$:

${\eta }_{net}=\frac{{\eta }_{gross}}{f_{ng}}$

High-value correction

Manufacturer-declared efficiencies at high values can be unreliable. Two empirical corrections cap the net efficiencies before they enter the EBV curve calculation.

Full-load correction

${\eta }_{fl,net,corr}=\min \left({\eta }_{fl,net}-0.673\cdot ({\eta }_{fl,net}-0.955),0.98\right)$

Part-load correction

${\eta }_{pl,net,corr}=\min \left({\eta }_{pl,net}-0.213\cdot ({\eta }_{pl,net}-0.966),{\eta }_{pl,max}\right)$

where ${\eta }_{pl,max}$ depends on fuel type:

The corrected values are converted back to gross:

${\eta }_{fl,gross,corr}={\eta }_{fl,net,corr}\times f_{ng}$${\eta }_{pl,gross,corr}={\eta }_{pl,net,corr}\times f_{ng}$

Efficiency vs return temperature (EBV curve)

The theoretical gross efficiency as a function of return water temperature $T_r$ (°C) is a piecewise function. Below the flue gas dew point, latent heat recovery increases efficiency; above it, only sensible heat is recovered.

Mains gas (dew point 52.2 °C)

${\eta }_{theo}(T_r)=\{\begin{array}{ll}-0.00007T_r^2+0.0017T_r+0.979& T_r<52.2\\ -0.0006T_r+0.9129& T_r\ge 52.2\end{array}$

LPG (dew point 48.3 °C)

${\eta }_{theo}(T_r)=\{\begin{array}{ll}-0.00006T_r^2+0.0013T_r+0.9859& T_r<48.3\\ -0.0006T_r+0.933& T_r\ge 48.3\end{array}$

EBV offset calibration

The EBV curve is shifted vertically so that the average of its values at the two test return temperatures matches the average of the corrected measured efficiencies. The offset $\Delta$ is:

$\Delta =\frac{{\eta }_{theo}(30)+{\eta }_{theo}(60)}{2}-\frac{{\eta }_{pl,gross,corr}+{\eta }_{fl,gross,corr}}{2}$

The calibrated efficiency at any return temperature is then:

${\eta }_{EBV}(T_r)={\eta }_{theo}(T_r)-\Delta$

Part-load ratio and cycling

At each timestep the boiler must meet the energy demand $Q_{dem}$ (kWh) within the available time $t_{avail}$ (hours). The energy output is capped by the rated power:

$Q_{out}=\min (Q_{dem},P_n\cdot t_{avail})$

The current operating power is the larger of the instantaneous load and the minimum modulation power:

$P_{curr}=\max \left(\frac{Q_{out}}{t_{avail}},P_n\cdot {\beta }_{min}\right)$

When the demand is below the minimum modulation power, the boiler cycles on and off. The proportion of the timestep spent firing at the minimum rate is:

${\varphi }_{min}=\min \left(\frac{Q_{dem}}{P_n\cdot {\beta }_{min}\cdot t_{avail}},1.0\right)$

When $0<{\varphi }_{min}<1$, the boiler is cycling. The ratio of off-time to on-time is:

$r_{on/off}=\frac{1-{\varphi }_{min}}{{\varphi }_{min}}$

Standing loss

The default standby heat loss as a fraction of current boiler power follows EN 15316-4-1, equation 5:

$f_{gen}=\frac{c_5\cdot P_{curr}^{c_6}}{100}$

where $c_5=4.0$ and $c_6=-0.4$.

Cycling adjustment

When the boiler cycles ($0<{\varphi }_{min}<1$), each off-period incurs additional case loss proportional to the temperature difference between return water and the environment at the boiler location. The cycling adjustment is:

$\Delta {\eta }_{cyc}=f_{gen}\cdot r_{on/off}\cdot {\left(\frac{T_r-T_{loc}}{\Delta T_{sby}}\right)}^{n_{sby}}$

where:

• $T_{loc}$ is the temperature at the boiler location (°C): room temperature for internal, outside air temperature for external
• $\Delta T_{sby}=30$ K, the nominal temperature difference during the standby loss test (EN 15502-1 or EN 15034)
• $n_{sby}=1.25$, the standby heat loss power-law index

The cycling adjustment is not applied to combi boiler hot water services, because combi draw-offs are inherently intermittent and the test regime already accounts for start/stop behaviour.

Location adjustment

When the boiler is installed externally, the increased case loss is captured by comparing the loss at the assumed internal room temperature ($T_{room}=19.5$ °C) with the loss at the actual external temperature:

$\Delta {\eta }_{loc}=\max \left(f_{gen}\cdot \left[(T_r-T_{room}{)}^{n_{sby}}-(T_r-T_{loc}{)}^{n_{sby}}\right],0\right)$

For internally located boilers, $\Delta {\eta }_{loc}=0$.

Final boiler efficiency

The base efficiency comes from the EBV curve. If the boiler is cycling ($\Delta {\eta }_{cyc}>0$), the corrected full-load gross efficiency ${\eta }_{fl,gross,corr}$ is used as the base instead, because the boiler fires at its minimum rate during on-periods rather than modulating down the EBV curve:

${\eta }_{base}=\{\begin{array}{ll}{\eta }_{fl,gross,corr}& \text{if }\Delta {\eta }_{cyc}>0\\ {\eta }_{EBV}(T_r)& \text{otherwise}\end{array}$

The combined cyclic and location adjustment is applied as an additive reciprocal correction:

${\eta }_{final}=\frac{1}{\frac{1}{{\eta }_{base}}+\Delta {\eta }_{cyc}+\Delta {\eta }_{loc}}$

Fuel demand

Once the final efficiency is known, the fuel energy consumed for a service is:

$Q_{fuel}=\frac{Q_{out}}{{\eta }_{final}}$

For space heating services, the energy output requirement is aggregated across all zones before the efficiency is evaluated. This avoids calculating efficiency separately for each zone at different load fractions when a single boiler circuit serves multiple zones.

Combi boiler losses

Combi boilers incur additional hot water losses from rejected energy during draw-off, internal storage or keep-hot facility losses, and volume-dependent correction. The loss calculation depends on the type of separate DHW test performed.

Daily hot water usage factor

A usage factor $f_u$ scales the draw-off losses when daily consumption is below 100 litres:

$f_u=\{\begin{array}{ll}\frac{V_{HW,day}}{100}& V_{HW,day}<100\\ 1.0& V_{HW,day}\ge 100\end{array}$

Volume factor

The daily volume factor $\Delta V$ adjusts for the difference between actual daily usage and the reference tapping profiles. Three standard EN 13203-2 tapping profiles define reference volumes (at 60 °C equivalent): S = 36.0 litres, M = 100.2 litres, L = 199.8 litres.

The baseline value is $\Delta V=V_M-V_{HW,day}$, with overrides depending on the test type and usage:

Combi loss by test type

For M&L or M&S tests:

$Q_{combi}=Q_{HW}\cdot (f_1+\Delta V\cdot f_3)\cdot f_u+f_2\cdot \frac{\Delta t}{24}$

For M-only tests:

$Q_{combi}=Q_{HW}\cdot f_1\cdot f_u+f_2\cdot \frac{\Delta t}{24}$

When no additional DHW tests have been performed, a default annual loss of 600 kWh is applied:

$Q_{combi}=\frac{600}{365}\cdot \frac{\Delta t}{24}$

where:

• $Q_{HW}$ is the hot water energy demand for the timestep (kWh)
• $\Delta t$ is the timestep length (hours)

The combi loss is added to the hot water energy demand before the boiler efficiency calculation. A fraction (0.25) of the combi loss contributes to internal heat gains.

Auxiliary electrical consumption

The boiler draws electrical power for the circulation pump, flue fan, and standby electronics. The total auxiliary energy per timestep is:

$E_{aux}=P_{circ}\cdot t_{run}+P_{el,sby}\cdot t_{sby}+E_{flue}$

where:

• $t_{run}$ is the total time the boiler fires during the timestep (hours)
• $t_{sby}=\Delta t-t_{run}$ is the time in standby (hours)

Flue fan energy

For modulating boilers (${\beta }_{min}<1$), the flue fan power is linearly interpolated between the part-load and full-load electrical powers based on the modulation ratio:

$P_{flue}(\beta )=P_{el,pl}+\frac{\beta -{\beta }_{min}}{1-{\beta }_{min}}\cdot (P_{el,fl}-P_{el,pl})$

where $\beta =\min (P_{curr}/P_n,1)$ is the current modulation ratio. The flue fan energy for the service is:

$E_{flue}=P_{flue}(\beta )\cdot t_{run,svc}$

For on/off boilers (${\beta }_{min}=1$), the flue fan runs at full-load power whenever the boiler fires:

$E_{flue}=P_{el,fl}\cdot t_{run}$

Time available

The time available for a given service accounts for time already spent on other services within the timestep and any fractional start time:

$t_{avail}=(\Delta t-t_{run,prev})\cdot \left(1-\frac{t_{start}}{\Delta t}\right)$

where $t_{run,prev}$ is the cumulative running time of prior services in the current timestep and $t_{start}$ is the start time offset within the timestep. In hybrid systems where the boiler operates alongside a heat pump, the heat pump elapsed time replaces $t_{run,prev}$.

Outputs

Assumptions

• The EBV efficiency curve assumes that the full-load test is conducted at 60 °C return temperature and the part-load test at 30 °C return temperature.
• Room temperature at an internally located boiler is fixed at 19.5 °C. This value does not vary with the zone temperature calculation.
• The standby loss reference temperature difference is 30 K, matching the EN 15502-1/EN 15034 test condition.
• The standby loss power-law index is 1.25, an empirical value from the EBV method.
• Standing loss follows EN 15316-4-1, equation 5 with coefficients $c_5=4.0$, $c_6=-0.4$.
• Combi boilers use a fixed return temperature of 60 °C for hot water services.
• When no separate DHW test data is available, an annual default combi loss of 600 kWh is distributed uniformly across the year.
• The cycling adjustment is not applied to combi hot water services.
• Flue fan electrical power varies linearly with modulation ratio for modulating boilers.
• Time spent on prior services is assumed to be evenly distributed across the timestep, so the time reduction for a later service is proportional rather than a simple subtraction.

Cross-references

• TP-01: Overview and Climate Data -- simulation timestep length used in energy and time calculations
• TP-03: External Conditions -- outside air temperature for external boiler location adjustment
• TP-04: Space Heating Demand -- space heating demand that the boiler must satisfy
• TP-09: Hot Water Demand -- hot water energy demand and draw-off volumes for combi loss calculation
• TP-10: Pipework and Ductwork Losses -- distribution losses added to the heat demand seen by the boiler
• TP-11: Hot Water Storage -- cylinder losses and return temperatures for regular boilers
• TP-12: Heat Pumps -- hybrid system interaction where a heat pump and boiler share time within a timestep
• TP-16: Heat Emitters -- flow and return temperatures that determine boiler efficiency
• TP-17: Controls -- time and temperature controls that determine when the boiler fires