carbon limited

This is number 7 in a series on district heating. Here’s where to find  1, 2, 3, 4, 5 and 6.

Your district heating network might be perfectly designed, but if it’s not installed and commissioned right it is doomed to fail, potentially costing you and your tenants a huge amount of money. In this post, we’ll talk about how to get DH install and commissioning right.

Installation

If your network is well designed and your Employer’s Requirements are sound, the best thing you can do is make sure the contractor delivers on their commitments. Here are some aspects of install that deserve particular focus:

Make sure insulation is fitted to the specified standard and covers all possible components on the network. This includes insulating valves, pumps and anything else that would otherwise radiate heat.

Check that the bulk heat meters are installed at the right locations and have been wired up to the datalogging system that will be used to read the heat meters in dwellings.

Despite the ubiquitous employer’s requirement that networks should be flushed properly, they rarely are. Dirty water can bring a network down and strainers and other key components gum up and prevent water from flowing. The ongoing maintenance cost (not to mention reputational damage) can be huge, so make 100% absolutely sure the contractor has thoroughly flushed the system.

Commissioning

In general, clients need to take a sceptical view of commissioning certificates. Contractors are incentivised to get the job signed off and handed over, while the client will have to live with the results of the contractors work for years to come. For this reason, the client must have a suitably qualified technical employee or agent present at commissioning of key systems, and to carry out spot checks on other elements (e.g. HIUs).

To check that commissioning has been done right, the client already has a hugely powerful weapon in their arsenal: the heat metering system. Each heat meter collects information on flow and return temperatures and flow rates at that location on the network. This means each flat already has its very own performance monitoring system. Add one or more bulk heat meters at key locations on the network and you have a complete picture of how the DH system is operating.

Without a working metering and data collection system, it’s almost impossible to tell whether the DH system is working well or abominably (and it will be impossible to meter and bill residents for consumption after occupation). For this reason, DH commissioning can only complete once the heat meters and the M-Bus network that connects them to a central datalogger are installed and functioning correctly. It sounds like a small thing, and it is – but this aspect is often ignored at huge cost to the client.

Once up and running, the metering and data collection system should be used to verify commissioning results and spot problems, for example HIUs that are plumbed in back to front or meter sensors that haven’t been connected.

As noted in a previous post, in order to achieve high efficiencies on a DH network it’s necessary to properly commission heating systems internal to flats. As with flushing, this is routinely promised and rarely done. Instead subcontractors (often traditional plumbers used to 82/71) set up the internal systems to provide plenty of heat without giving any thought to the resulting return temperatures, which should be no more than 40° C, even at part load.

To confirm internal commissioning has been done right, you can do spot checks of random flats. Or even better, you can use the heat meter in each flat (read remotely!) to help verify those commissioning certificates.

At commissioning of central plant, the contractor must demonstrate that the flow and return temperatures on the network are in line with design requirements (e.g. 70/40), especially at part load, and the pumping strategy is working as intended. If not, commissioning can’t complete. Again, the central heat meters are invaluable for verification purposes. Using these meters, you will see very quickly if, for example, bypasses have been left open after flushing, creating a short circuit in the DH system.

In summary the best way to ensure install and commissioning of DH systems are done properly is for the client to pay proper attention and hold the contractors to their commitments. By 1) assigning technical staff or good employer’s agents to engage with the project at key stages and 2) using monitoring systems to verify results, the client is much more likely to end up with an efficient system ready to enter into many decades of service.