Introduction
Domestic hot water flow rates have always been problematic to calculate, because of issues with simultaneous usage of hot water tapping points. Full load conditions (all showers in use at the same time) will result in very large flow rates and oversized pipes. Many calculation methods for simultaneous flow rates are known and used for domestic hot water piping. Most of these methods only account for simultaneous flow rates, and not for simultaneous power, because domestic hot water networks are mostly operated as single pipe / fixed temperature systems.
When carrying over these norms to the central heating system (satellite boilers or heat exchangers), the propagation of 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.
Example
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, 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:
m = a * M^b + c
with a = ..., b = ... and c = ...
Hysopt then computes a simulateous factor f = m_DHW/M_DHW. We then compensate for the central heating volume flow and power on units not in DHW mode, by computing the combined mass flow
m = f * M_DHW + (1-f) * M_CH
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.