Flow and return temperatures, and what they can tell us

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Flow and return temperatures are not something you want to be watching on a Saturday night (says he writing about them on a Sunday night), however this article covers what they can teach us in the field of district heating.

They are however a treasure of information, that if understood better would rid the world of a lot of real hardship, in the form of inefficient and expensive heating systems that cost a fortune to run. For some, spending three times on your hot water than you need is a significant expense, especially if it only took a trained eye to look at some lines on a screen to spot a problem and do something about it. And there there are all the service calls that could be avoided if data was looked at to check if problems were local or systemic.

But district heating is unregulated, and the pressure to implement efficiency monitoring measures is missing.

Flow Temperatures

The following graph shows 7 days flow temperature feeding an HIU.

It is far from a straight line, as one may initially think. One can see a wealth of information, based on:

  • temperatures
  • the rates of change of temperature
  • patterns and cycles in temperature, and their frequency (and amplitude)
  • the area under the line

As with us humans, temperature tells us a lot about the health of a system, and needs documenting in the same form as medicine would report on medical symptoms.

Rise and Decay

Temperatures fall due to heat loss, and they rise when water flows bringing in hotter water from source.

Flow temperatures only have three stable points:

  • Peak flow rates - when flow rates are high enough to make temperature losses negligible, so we are almost seeing the temperature leaving the plant-room.
  • Ambient temperature - all off and cold.
  • The flow rate where heat loss from supply pipes is exactly equal to the energy brought into the pipework. Achieving this flow by accident for any length of time, or even by choice, is a hard task.

Even a small amount of heated water into a low volume environment will register as a temperature change, making temperatures a more accurate means to monitor low flow rates than most flow sensors. Temperature reflects the physical changes, rather than measuring flow at intervals that will miss small events. With a heat meter for example, the temperature of the flow can tell you more about intermittent flow (and valve activity) than the flow rate reading will.