This is number 3 in a series on DH. Here’s where to find the first and second posts.
A key way district heating networks go wrong is through poor design. This can come from:
- a lack of good engineering
- a lack of relevant standards and guidance
- a perverse incentive for engineers to be overly conservative in order to protect their PI insurance
- clients not knowing what to ask for from their contractors
In this post, I’ll take a closer look at these problems.
Detailed design of DH systems is often done by mechanical engineers working on behalf of the client. Going back a few years, mechanical and electrical (M&E) engineering was already a tight margin business. The bread and butter work at successful firms required engineers to turn out designs quickly and then scrupulously charge when scope creep required additional work.
Then came the collapse of the construction sector in 2007/8, just when DH was becoming more widespread in new housing projects. Suddenly, M&E companies had to reduce fees, in many cases submitting bids at or below cost in order to win work, while delivering projects with fewer staff.
So during this critical early period of DH development, when engineers should have been improving their designs, they’ve instead been desperate to get projects out the door as quickly and cheaply as possible.
This often means recycling previous plant room and network designs as well as “designing by catalogue” (to quote Bill Orchard), effectively cutting and pasting standard details from manufacturers straight into project designs. In these cases, you could be forgiven for thinking that little real engineering (with original thought and calculations) is actually getting done – even at some of the big reputable M&E firms we all know. To be clear, I’m not claiming that all M&E practices fall into this trap, but I do believe the problem is widespread.
The problem has been compounded by a lack of useful standards and good guidance in the sector. Instead, engineers have had to rely on outdated norms (e.g. flow and return of 82°/71°) and inappropriate or obsolete documents originally intended for more traditional central heating systems. NHBC guidance, the Institute of Plumbing Guide, BS EN 6800 and other standards don’t lead to good DH schemes. In fact the reality can be just the opposite.
At the same time, engineers are under constant pressure to protect their professional indemnity (PI) insurance. This is the insurance that protects a firm if an engineer makes a mistake, and it can get very costly. The number one rule is: don’t get sued.
But there’s a crucial subtlety here: you might get sued if flats don’t get sufficient heat from a DH network, but you’re unlikely to get sued if your design leads to flats getting too much heat. As a result, engineers have an incentive to interpret the few sources of information available to them in the most conservative way possible.
Here’s a brief illustration. I was in a design meeting on a DH project with the lead M&E engineer from one of the big London firms. My colleague, a very experienced director of a DH design practice in Denmark, and I believed that the DH pipework in the engineer’s design was oversized because the engineer had drastically overestimated the peak heat demand. This was adding significant capital cost to the network and would lead to high network losses.
When challenged, the lead engineer pulled out the NHBC guidance on flow rates and said, but what if someone wants to wash dishes at the same time someone else in the flat is taking a shower and a third person is washing their hands? The engineer totted up the huge flow rate in litres per minute that would be required and claimed that we should plan for a peak demand in a one bed flat of 46kW!
Even after a long discussion (with my colleague and me pulling our hair out), the engineer wouldn’t budge. Then we asked, what if the flats had combi boilers instead of HIUs – what size boiler would the engineer specify in the one bed flats? He answered straight away: around 28kW. So why on earth plan for 46kW? Because NHBC guidance makes allowances for combi boilers and everyone is used to them. District heating is new, and so you’ve got to plan for the worst. At which point we went looking for a window to hurl ourselves out of.
While the story so far is pretty grim, things can get worse when the contractor is responsible for detailed DH design. On a design-and-build project, the client provides the contractor with a set of employer’s requirements (ERs), laying out the quality, performance and technical standards that the contractor has to achieve. The contractor puts in the lowest price bid that he can in order to win the work.
Having won the project, the contractor is then free to design, spec and build the scheme however he likes, as long the end result meets the ERs. Therefore, the contractor has one aim: comply with the ERs at the least possible cost.
If the ERs are thorough, clear and specific, contractor design can work very well. And this is achievable where the client knows exactly what they want and understands what to ask for. But for most clients, DH is relatively new. Clients don’t know what to ask for and as a result the ERs tend to be vague and open-ended – or worse, the ERs ask for the wrong things.
In many cases, even before the designer (i.e. the M&E subcontractor) puts pen to paper, it’s already too late. The parameters set by the client have already doomed the system to a short asset life plagued by low efficiency and high operating cost.
This post summarises the causes of bad DH design and it’s all pretty depressing. But don’t lose hope! I’ll go through the steps that I think lead to better networks in a number of future posts. But first, I’ll look at the next way we get DH wrong: installation and commissioning.
Leave a comment