This is the sixth post in a series on district heating. Here’s where to find 1, 2, 3, 4 and 5.
We’ve looked at how district heating (DH) can go wrong. Now let’s look at ways to help make sure it goes right. First: design.
I suspect that many DH engineers don’t recognise that their work is hugely important and that it has far reaching consequences. Decisions made at design stage greatly influence system longevity and efficiency and so help determine the price people will pay for their heat for decades to come. Good engineering matters!
So the first thing engineers can do to get district heating right is to recognise their own importance. Fight the pressure to rush a design and get it out the door. Don’t just copy and paste and hope it’ll all be alright. Do good engineering! After all, that’s why you went into this field in the first place, right?
And for bluffers who tell their clients they know what they’re doing but don’t: please stop! When you do your job poorly, you are very literally taking money out of people’s pockets, many of whom are the least able to afford it. Don’t design another thing. Go get help from a more experienced colleague. Go read up on best practice and fill in the gaps in your knowledge. But please stop designing crap DH networks. You’re the reason many networks are performing poorly and people are paying too much for their heat.
Luckily, where three or four years ago there were few sources of good guidance for engineers, now better information has started to emerge. Recent publications by CIBSE and BSRIA, among others, recognise the importance of better design principles in order to produce more efficient networks.
One of the most important steps forward is CIBSE’s endorsement of the Danish standard DS 439 for the diversity factor to apply when calculating peak demand on the network. This standard is a huge leap forward from the catastrophic BS 6700.
Why does it matter? Here’s an example. If you take a hundred home scheme and calculate the peak hot water demand using BS 6700, you’ll get a load of around 22kW per flat and a pipe diameter of 105mm. Use the Danish standard to do the same calculation and you’ll get a load of 3.24kW per flat and a pipe diameter of 40mm.
In the above example, using the Danish standard means there’s 85% less water sitting in the pipe, losing heat to the ground and corridors. What’s more, digging out a Logstor quote, the reduction in pipe size brings more than 60% savings on pipework cost per linear meter!
But endorsing DS 439 is just one example of recent progress on the DH front. In BSRIA’s guide to pumping systems, BG12, they emphasise the importance of using variable volume systems with differential pressure sensors at the far end of the network. They highlight that a good variable volume system can cut pumping energy by 82% compared with constant flow systems. We’re seeing the results of bad pumping strategy on sites where residents are paying more than £200 a year just for pump electricity – this is real money!
Another good resource, CIBSE AM12 brings together many DH design best practice principles together in one place. In particular, take a look at section 9.16 for the following guidance:
- Design for 70/40 flow and return temperatures
- Only use variable volume pumping
- Use HIUs with instantaneous hot water, not cylinders, to reduce return temperature and improve efficiency
- Max 25 degree return temp from DHW plates
- Use DS 439 for diversity calcs
All this guidance allows engineers to design good networks, confident that they’re not putting their PI cover at risk. Rather than giving in to the pressure to be overly conservative, engineers can design good systems and, if challenged, refer to CIBSE, BSRIA, BRE and others to back themselves up.
Clients have an essential role to play as well. Clients need to learn enough about best practice design so they are asking their consultants and contractors for the right things. Employer’s Requirements must be detailed enough so that clients can make a like-for-like comparison of tender returns and the winning bidder is then obliged to deliver an efficient system. If your ERs aren’t working, improve them.
By becoming better educated, clients are also more likely to recognise when a specification item is also a long term commercial decision. For example, clients should know whether more than one billing provider can use a given metering system or whether they will be tied to a single provider forever.
Finally, clients can improve project results by letting the consultants and contractors know at design stage that you will carefully check the resulting system to ensure it delivers on design promises. Consultants and contractors must know from the outset that they are accountable for the results of their work. More on this in the next post.
I hope this post has been more positive than the previous “getting DH wrong” posts. It’s certainly been nicer to write! The next post will be about getting DH right during install and commissioning.
Hey Casey
Really useful discussion!
Just wondering how you converted the BS 6700 to a load? Looking at diversity factors across sizing methods but can’t get my head around that one!
Thanks
Michael
Hi Michael. I took it straight from a CIBSE symposium, the doc for which used to be available on the link. Looks like CIBSE have taken it down at some point.
Ah well! Thanks very much anyway 🙂 These articles have been a big help!