Introduction
...
When carrying over these norms to the central heating system (satellite boilers or heat exchangers), the propagation of simulateous simultaneous power also becomes important! This effect is amplified by the fact that central heating often operates at lower power but higher flow rates / smaller temperature delta and domestic hot water heat exchangers operate at higher power but lower flow rates.
Hysopt incorporates an extension of the DIN 1988-300 standard into the Hysopt software. We have extended the calculation to cope with simultaneous central heating and domestic hot water usage, and with combination of power needed in mixed systems.
...
Consider a system of n appartments, each having a satellite heat exchanger for instantaneous domestic hot water production. To keep the example simple, we assume each appartment to have a DHW heat load of 40kW with a temperature regime of 70°C / 30°C. Suppose we want to combine these with a 50kW , and a 15kW central heating heat load, with a temperature regime of 70°C / 60°C. This results in following flow rates in relation to n. We use M for total mass flow.
| Q CH (kW) | M CH (kg/s) | Q DHW (kW) | M DHW (kg/s) | |
|---|---|---|---|---|
| 1 | 15 | 0.36 | 40 | 0.24 |
| 2 | 30 | 0.72 | 80 | 0.48 |
| 3 | 45 | 1.08 | 120 | 0.72 |
| 4 | 60 | 1.43 | 160 | 0.96 |
| 5 | 75 | 1.79 | 200 | 1.19 |
| 6 | 90 | 2.15 | 240 | 1.43 |
| 7 | 105 | 2.51 | 280 | 1.67 |
| 8 | 120 | 2.87 | 320 | 1.91 |
| 9 | 135 | 3.23 | 360 | 2.15 |
| 10 | 150 | 3.58 | 400 | 2.39 |
Because all domestic hot water (DHW) units will never be active at the same point in time, we use a simultaneous flow rate for DHW, using the following formula:
...
Simply taking the maximum of M_CH and f * M_DHW would result in a flow rate which is too low. For component selection and pipe sizing, the maximum of m and M_CH is used.
To give an indication of the regime and power associated with these simulataneous flow rates, we compute
Q = f * Q_DHW + (1-f) * Q_CH and Q sizing = MAX(Q_CH, Q)
| Q CH (kW) | M CH (kg/s) | Q DHW (kW) | M DHW (kg/s) | m DHW (kg/s) | f (-) | m total (kg/s) | Q total (kW) | m sizing (kg/s) | Q sizing (kW) | T return (°C) | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 15 | 0.36 | 40 | 0.24 | 0.17 | 0.72 | 0.27 | 33 | 0.36 | 33 | 48.0 |
| 2 | 30 | 0.72 | 80 | 0.48 | 0.33 | 0.69 | 0.55 | 64 | 0.72 | 64 | 48.5 |
| 3 | 45 | 1.08 | 120 | 0.72 | 0.43 | 0.60 | 0.86 | 90 | 1.08 | 90 | 50.0 |
| 4 | 60 | 1.43 | 160 | 0.96 | 0.51 | 0.53 | 1.18 | 113 | 1.43 | 113 | 51.1 |
| 5 | 75 | 1.79 | 200 | 1.19 | 0.57 | 0.48 | 1.51 | 135 | 1.79 | 135 | 52.0 |
| 6 | 90 | 2.15 | 240 | 1.43 | 0.62 | 0.43 | 1.84 | 155 | 2.15 | 155 | 52.8 |
| 7 | 105 | 2.51 | 280 | 1.67 | 0.67 | 0.40 | 2.17 | 175 | 2.51 | 175 | 53.3 |
| 8 | 120 | 2.87 | 320 | 1.91 | 0.71 | 0.37 | 2.51 | 194 | 2.87 | 194 | 53.8 |
| 9 | 135 | 3.23 | 360 | 2.15 | 0.75 | 0.35 | 2.85 | 213 | 3.23 | 213 | 54.2 |
| 10 | 150 | 3.58 | 400 | 2.39 | 0.78 | 0.33 | 3.19 | 232 | 3.58 |
...
| 232 | 54.5 |
Graphs
To clarify things further, we include some graphs for the combined flow rates and power. These graphs are for increasing domestic hot water power / flow rates at 70° / 30°C with a fixed power of 50kW heating at 70° / 60°C.