This is the fourth post in the series. If you haven’t yet, check out the first, second and third posts first.
Poor installation and commissioning can bring even the best designed network to its knees. In this post, I’ll look at some of the ways these crucial phases go wrong. First, installation:
Inadequate insulation is a common major problem on DH installs. Many contractors feel it’s reasonable to insulate only straight pipe runs, and leave everything else uncovered (or barely covered). You’ll commonly see valves, strainers, pumps and branches with little or no insulation. Similarly, hydraulic interface units (HIUs) are often uninsulated. All this bare pipework quickly adds up, and all of it will leak valuable heat.
How much heat is being lost from the network? On many schemes it’s impossible to know, mainly because the landlord doesn’t have the means to measure it. Unfortunately, on many schemes heat meters are missing at the plant room boundary and on key network branches. Without this relatively inexpensive kit, it’s impossible to monitor network performance.*
Once the network is in, the whole system must be flushed. Often flushing is only partially done, or not done at all. As we’ll talk about in the upcoming post on operating networks, water quality is the single biggest factor determining the longevity of your network. Without thorough flushing, you’ll never get decent water quality.
During flushing, to avoid forcing dirt through delicate equipment, such as the fine heat exchangers in HIUs, bypasses are usually installed in key locations. However, it’s extremely common for bypasses to be left open, creating a short circuit on the network, with serious knock on effects for efficiency as described below.
With the system installed and flushed, it’s time for commissioning:
It’s essential at commissioning to ensure that the plant and network are set running in line with the original specification and design intent. This is a job for the client’s M&E engineer, but unfortunately there’s often no one competent at commissioning to fight the client’s corner. As a result, the client will get a system that’s only been part commissioned and may not meet the requirements set out in the design.
One key area that is often ignored at commissioning is the DH control regime and operating conditions, like flow and return temperatures. A common attitude among contractors and their engineers is: if the flats are getting sufficient heat, the system must be working fine. This is absolutely not true. Instead engineers need to make sure the control philosophy in the spec has been implemented and the operating conditions reflect this.
As mentioned above, it’s common for bypasses to be left open. Unless these open bypasses are caught at commissioning, the large distribution pumps in the plant room will run at full pelt – forever! This is because pumps are often controlled on differential pressure. With bypasses open, the pumps are chasing a target they can never catch; but that won’t stop them from trying! They will move huge volumes of hot water around the network unnecessarily, with catastrophic consequences for efficiency.**
Poor commissioning can be especially devastating to a network because too often it’s the only time for several years that anyone touches key kit. And as a result, it’s only several years later, once the heat provider realises just how much money they’re haemorrhaging, that anyone gives it a second thought.
In the next post, we’ll take a look at how poor operation practices can affect system efficiency and longevity. And then after all this doom and gloom we’ll finally get onto the much happier topic of how to get DH right!
* There’s been a recent spate of check meter retrofits on plant rooms and branches as heat providers try to better understand their network performance. But it’s much costlier to retrofit heat metering instead of putting it in at the start.
**I’ve assumed that the system is variable volume (which is absolutely the best design philosophy for DH) rather than fixed volume. The system might be fixed volume by design, in which case efficiencies will be extremely low – but this is a problem with design rather than install or commissioning.
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