This section describes the methods for modelling the heating systems for the co- heating test (section 5.4.1) and the HT1 (section 5.4.2).
5.4.1 Modelling the heating system for the co-heating test
The co-heating test (see section 3.3.6) was modelled using electric convectors with 100% efficiency in every zone of the LMP1930 building envelope model (except the unheated loft (attic) and subfloor zones). The average air temperature measured during the co-heating test in each zone was used as the set-point temperature of that zone in the model (Table 5-9).
126 Table 5-9: Measured average air temperature in different zones during the co-
heating test; theses temperatures were used as the set-point temperature of each zone in the DTM when modelling the co-heating test
Zone House 1 Set-point temperature (°C) House 2 Set-point temperature (°C) Living room 24.32 24.40 Dining room 24.64 24.10 Kitchen 25.15 24.29
Hallway ground floor 24.00 24.16
Hallway first floor 24.67 24.48
Bedroom 1 24.62 24.82
Bedroom 2 24.67 23.35
Unoccupied bedroom 24.83 24.43
Bathroom 23.53 24.20
Volumetric weighted Average for the whole
house
24.50 24.82
Electricity used by circulation fans during the co-heating test was considered to end up as heat in the zone, thus there was no need to model these separately.
5.4.2 Modelling the heating systems for the space heating trials
The gas powered central heating systems were modelled to simulate the HT1: one with CC and the other one with ZC. Each heating system consisted of a gas fired condensing combination boiler and 7 radiators as described in section 3.3.4 (Table 3-3). They were modelled for each house using DesignBuilder’s detailed HVAC option.
The condensing combination boilers were modelled with nominal heat output of 30 kW and mean efficiency of 84.2% and 85.7% as measured during the HT1 (see section 4.5). The normalized boiler efficiency curve of condensing combination boilers was selected from DesignBuilder’s template library. The circulating hot water flow temperature was set to maximum during the HT1 which is 88°C according to the
127 manufacturer’s data (Worcester Bosch Group, 2009). In DesignBuilder, the hot water flow temperature in a wet heating system is controlled via a set-point manager which controls the hot water flow temperature according to a schedule. This was set to be always 88ºC.
Radiators were modelled using the water baseboard heater model of EnergyPlus enabling both convection and radiation heat transfer. The water mass flow rate of each radiator supplied from the primary system is calculated at each time step by determining the impact of radiator on surrounding air via convection and to the surfaces by radiation (US Department of Energy, 2012).
There will be water flow rate and therefore heat transfer from the radiator when all of the three following criteria are met: firstly, the radiator unit is “on” at that time step; secondly, there is any heat requirement remaining in the zone to be met according to the zone’s set-point temperature and finally the boiler is “on” according to its
schedule.
The water baseboard heater model requires a number of inputs: rated average water temperature (°C), rated water mass flow rate (kg/s) and rated capacity (W).
According to the radiators’ manufacturer data: rated average water temperature was 70°C and the rated water mass flow rate (kg/s) of each radiator was calculated using equation ( 5-6):
𝑚𝑚 = 𝐻𝐻/(𝐶𝐶𝑝𝑝∗ (𝐼𝐼𝑖𝑖− 𝐼𝐼𝑟𝑟)) ( 5-6)
Where:
𝑚𝑚= rated water mass flow rate (kg/s)
𝐻𝐻= rated capacity of radiator (W) selected from Table 3-3 according to the manufacturer’s data
𝐶𝐶𝑝𝑝= specific heat capacity of water and was approximated as 4187 J/kg.°C for the
128 𝐼𝐼𝑖𝑖 = standard water flow temperature (°C) = 75°C
𝐼𝐼𝑟𝑟 = standard water return temperature (°C) = 65°C
The radiant fraction of the radiators is the portion of the power input transferred to the occupants and surfaces as radiant heat and was considered to be 0.3 for all the radiators according to Oughton & Hodkinson (2008).
A constant speed pump was modelled for the circulating hot water supply loop of each house with a maximum loop flow rate of 0.00034 𝑚𝑚3/𝑠𝑠 and minimum loop flow rate of zero and a rated pump head of 6000 pa according to the specifications of the central heating pumps in the houses. The control type of the pump was selected as intermittent control. This enabled the modelled pump to shut down when no heating was required. When there was heat demand, the pump selected a flow rate
somewhere between the maximum and minimum user defined flow rates in order to meet the heating requirements. Rated energy consumption of the pumps was left as “autosize” and default value of 0.9 was selected for the motor efficiency of the pumps as the electricity consumption of the houses was not studied in this research.
All the pipes in the system were assumed to be adiabatic. There was no information available regarding the pipe run in the houses and obtaining more information required removing a large amount of the floor boards on the ground and first floors which was not possible to do in this work.
The Programmable Room Thermostat (PRT) (see section 4.3) was modelled using the boiler operation availability schedule of DesignBuilder’s circulating hot water loop data. The radiators availability schedules were set to be always “on”.
The default control strategy of a wet heating system in a multi zone building model in EnergyPlus and DesignBuilder is that each zone has its own room thermostat which could be scheduled to assign set-point and set-back temperatures throughout a day. However, this control strategy of the heating system is inherently different from the control strategy in houses with either CC or ZC where boiler operation was controlled by a PRT located in the hallway and set-point and set-back temperatures (only in ZC) for each room are applied by TRVs (in CC) or PTRVs (in ZC). Currently, there is no solution in DesignBuilder in order to better represent the control strategy in multi
129 zone houses with a PRT control over the boiler and overcome the problem
discussed. However, Energy Management System (EMS) which is an advanced feature of EnergyPlus enables one to write custom programmes to describe specific control algorithms in a language called EnergyPlus Runtime Language (ERL) (US Department of Energy, 2013a). Such code could be added directly to the
EnergyPlus’s IDF file to override the existing default control. An ERL code was initially written for this purpose which could be found in appendix A.2. The code was written in order to shut down the hot water supply from the boiler at any time step when the air temperature in the ground floor hallway (where PRT was located) increased above its set-point temperature of 21 °C. However, it was found that adding such code to better represent the control strategy requires accurate
predictions of the air temperature. As it will be discussed in sections 6.3 and 6.4, it was not possible to accurately predict the hallway ground floor air temperature due to complexities involved with modelling the air flow between the ground floor and first floor hallways. Therefore, after running a number of simulations and compare the predictions with the default control strategy, it was decided not to use the ERL code as it could not increase the accuracy in this case when the air temperatures could not be accurately predicted.