60-16 Heat Networks
This article is based on our BESA VWART Calculations:
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16°C Return Temperatures
The BESA test shows how an HIU can achieve an annual average return temperature of 16°C. This was with keep-warm turned off.
The question is then can a network as a whole perform to this level?
Without a keep-warm, and no by-passes on the network, the return temperature to plantroom will be relative to the heat gain on the return. Note the use of the term heat gain and not loss. 16°C is lower than ambient temperatures in a building, and may be close to ground temperature. At these temperature differences, the heat losses are minuscule, and it is proposed that return pipework within buildings only demands the thinnest of insulation.
Bypasses are however required to maintain the flow temperature high enough for hot water production within the allocated time-frame.
For a 60-16 network, one requires that no keep warm circulation is fed into the return. This can be achieved using a separate re-circulation pipe inserted down the main network flow pipe, or typically 10mm to 15mm. A small recirculated volume back to the central thermal store results in a flow pipe maintained at the desired temperatures, without losing heat (entropy) back into the return pipework.
So it is possible.
Less than 30°C Return Temperatures
Maybe less of a leap into the new age would be to introduce some keep-warm through a 50°C thermostatic bypass at the top of risers. How this effects system VWART will be a function of the heat loss on the network, and the flow required to overcome this, compared to overall loads.
Or one can always turn on HIU keep warm. If this is done all the time across a network then the BESA tests show an expected return of 29.2°C. In reality, only a portion of HIUs will be set this way, with others on timed functions, so the expected return will be between 20°C and 30°C.
Flow Temperatures
Contrary to what some may expect, increasing flow temperatures reduces return temperatures. The BESA tests confirm this, with a slightly higher average return temperatures 28°C as opposed to 29.2°C, when a flow is at 70°C rather than 60°C, with keep warm on.
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However, increasing flow temperature does impact on thermal efficiency. What the test shows is that flow temperatures can be varied without negative impact on return temperatures.
Elevated flow temperatures will be needed to satisfy heating on that day when you get -15°C weather most likely. Typically one would size pipework based on 60/20 in Summer, and 80/20 for winter, or even 90/20, for peak -15°C winter load.
How is this Possible
- A partnership between the most experienced HIU manufacturer in the UK, Thermal Integration, and the most experienced HIU manufacturer in the Netherlands, HSF.
- Tried and tested components, sized correctly.
- Clever software.
- The most advanced production facilities in Europe.
