The following is based upon our experience and site observations over many years. It is not a based upon a formal scientific study and we would welcome any thoughts and comments on the following text.

Direct flow single wall evacuated tubes ( DFSWEVT) have been fitted to solar thermal systems for many years.

Examples of these are: Thermomax DF 100, DF 400, Viessmann 200 T.

Direct flow evacuated tubes have the advantage of being able to be fitted at any angle, including horizontally. They also, if set up correctly and not allowed to overheat, are exceedingly efficient and effective at heating water – even in very low ambient temperature conditions.

The working principal

There is effectively a flow and return pipe inside the manifold. Each of the tubes has an internal tube approximately 6mm diameter and external tube approximately 12mm diameter.

A short video explaining how direct flow systems work inside the manifold

The solar transfer medium (normally propylene glycol eg: Tyfocor, Fernox, Solaris and others) is  is pumped down the anulus of external tube and up the internal tube. The proportion of flow through each tube is approximately Total Flow / Number of tubes.

This system can work well for many years. However, this is conditional on the following:

  • There is an adequate flow through each of the tubes in order to extract all of the solar heat.
  • The system never overheats.
  • Either all of the energy is used or a heat dump is fitted.
  • There is not solid material present in the system.

Our experience with direct flow evacuated tube panels

In our experience these systems:

  • Often overheat.
  • The glycol breaks down, leaving a mildly acidic solution. This solution in turn corrodes the system and particularly viscous deposits are formed, and when dry turn into a solid, insoluble material.
  • The deposits and degraded glycol can block up pipework and non-return valves and seize circulation pumps.
  • When blockages or partial blockages occur, there is no longer an even flow rate through individual tubes.
  • Localised overheating occurs, which increases blockages and exacerbates the problem.
  • Though a good flow rate may be experienced through the entire panel, there may not be any flow rate through a number of tubes, rendering there heating input to zero.
  • Where the flow rate is impeded, high temperatures and pressures are generated that are difficult to dissipate.
  • Though a large amount of heat is generated by the whole collector, the actual output is very low.
  • The flexible pipe that connects the tube into the manifold is prone to failure

Cleaning solar thermal systems

We have attempted on a number of occasions to use proprietary solar cleaning chemicals such as Sentinel R200 https://www.sentinelprotects.com/uk/products/sentinel-r200-solar-cleaner and Fernox Solar Cleaner https://fernox.com/product/solar-cleaner/ but we have not found them to be effective.

Engineering Solutions for solar thermal systems

If the tubes are still in reasonable condition and not yet blocked, modifying the system to ensure that overheating can be avoided is a possibility. This may include fitting a Heat Dump and / or an uninterruptible power supply (UPS).

Viessman TM 300 collector

Alternatively, the collector would can be changed. If the collectors are pitched, there are a number of options available using “Heat pipe” collectors that are far more tolerant to overheating.

If the collector needs to be mounted horizontally, there is only one collector that we know of that works on the heat pipe principal, but can be mounted flat: the Viessmann TM – 300.

Though this is an excellent product it is a costly solution.

Conclusion on direct flow pipes

For all of the above reason we do not consider direct flow, single wall evacuated tubes suitable for mass market solar thermal systems.

Direct flow single wall evacuated tubes can effectively be used in certain specialist applications, if installed by an engineering team that fully understands the working principals and is confident that the system will never overheat.

However, there have over the last 40+ years been many of these systems installed. Some are still functioning well and in these cases we would recommend not making any changes.

We are often contacted to repair these systems and have experienced many of the problems above. Whilst we feel that we are competent to attempt this work, we would like to make it very clear that some systems, even if less than a year old, may never be repairable.

As a business we made the decision many years ago to try to avoid the installation of DFSWEVT, which has proven to have been a wise decision.