Ambient loop heat networks - how REHAU can help

There is always a debate about 4th vs 5th generation district heating, and which is the best approach but there is no one size fits all solution. We typically assume 5th generation being a direct progression of the 4th but there are in fact some very significant differences and these can even be used together.

28.07.2025

There is always a debate about 4th vs 5th generation district heating, and which is the best approach but there is no one size fits all solution. We typically assume 5th generation being a direct progression of the 4th. In fact, it is a very different kind of system that heavily depends on both local suitable sources, careful operational monitoring and sometimes also reversible heat pumps that can make the system cover both heating and cooling demands.

REHAU are very active in 4th generation heat networks across the UK, typically with large, centralised heat pumps running at 60-70°C flow temperatures, and pre-insulated PU-foamed polymer pipes, such as RAUVIPEX to distribute the heat.

In this month's CIBSE Journal, there is also an article on "Generation 4.5" heat networks by Adam Selvey at Jacobs where the network operates at 45C flow temperature, which is an interesting concept, trying to get the best of both worlds.

As one of the market leaders in polymer district heating pipe, we often get asked by designers and contractors about pipework for 5th generation heat networks, also known as ambient loops. For these designs vary and we’ve come across specifiers using both insulated and un-insulated pipework. Designers opt for a pre-insulated pipe for a variety of reasons including:

Temperatures could reach 20-40°C, so there would be heat lost to the ground (at ca. 10-12°C), albeit this would have less heat loss than a 4th generation network
Additional physical protection for the carrier pipes to prevent damage
Possible thermal transmission between flow and return pipes

If the pipes are to be insulated, a non-bonded foam PEX insulation could also be an option to give more flexibility on site for the contractor, and it also typically has a lower cost per metre.

If the specifier does not want the pipes to be insulated, REHAU can still offer various uninsulated options for direct burial in the ground:

PE-Xa pipes up to 160mm (joined using the EVERLOC compression sleeve system or our FUSAPEX electrofusion system)
PP-R pipes SDR 11 up to 400mm (a butt-welded system)
PP-R pipes SDR 17.6 up to 630mm (a butt-welded system)

One significant factor to be aware of is that 5th generation scheme often has a much smaller delta T (difference in flow / return temperatures) which makes the pipe sizes far larger than an equivalent heat load in a 4th generation scheme. A well-designed 4th generation heat network would operate at 25-30°C delta T, whereas an ambient loop might only have a 5 degree delta T. This is shown by the example below:

Flow / return temperature

Heat Load

Recommended pipe size

Pressure loss (Pa/m)

4th generation scheme at

65/35°C

1,500kW

125mm (DN100)

163.1

5th generation scheme at

15/10°C

1,500kW

250mm (DN200)

155.7

4th and 5th generation networks do not always need to be considered as opponents, the two can also be combined. Thus, some of the main pipework array can be in ambient loop, feeding local district/cluster heat pumps which, in their turn, would then boost the network to 4th generation systems avoiding heat pumps at every connection. In this case an optimum heat-efficient distribution in uninsulated or partially insulated pipework meets heat delivered directly to an HIU with a flexible, low heat loss pre-insulated pipe.

Here at REHAU we can work with you to find the best pipework solution for your 4th or 5th generation heat network.

Read more about district heating and polymer heat networks here