carbon limited

This is the fifth post in a series on district heating. Here’s where to find  1, 2, 3 and 4.

So far we’ve looked at how poor design, installation or commissioning can doom a DH network to poor efficiency. In this post, I’ll briefly outline why it’s important to monitor and look after a DH network throughout its life, and what can go wrong if you don’t.

Most DH systems are commissioned and then ignored. It may be many months or even several years before anyone revisits the scheme to look closely at how it’s operating, usually prompted by something going badly wrong. The first casualty of network neglect is efficiency.

To monitor efficiency, you need data. These days, just about every residential DH scheme has heat meters installed in line with the Domestic Building Services Compliance Guide. Less common, but still necessary, are check meters at the plant room boundary and key points on the heat network, like major branches. Without these relatively inexpensive check meters, it’s impossible to locate problems in the system; you’ll know something’s wrong but you won’t know where.

But just having meters isn’t enough. You need a quick and easy way to get the data, such as automatic meter reading (AMR) using an open protocol like M-bus. This allows you or any of a number of service providers to retrieve heat consumption data without having to access the flats.

Thankfully, problems with M-bus networks connected to domestic heat meters are usually caught during commissioning or when you bring in a billing service provider.

The check meters in the plant room and on the network can present a bigger problem as they’re either non-existent or aren’t hooked up to the AMR system. If they do exist, they’re often wired to a BMS that no one can get access to, no matter what the controls specialist promised at design stage.

Occasionally, it’s only after commissioning that the client discovers they have a proprietary AMR system that only one company can read or interact with. The subcontractor, in fulfilling the employer’s requirements, chose inexpensive kit that actually had a long term commercial relationship attached to it. Fine if the client made an informed choice, but a massive headache where they didn’t – and the supplier now has the client over a barrel. Not good.

With meters in flats, you know how much heat is being consumed. With meters at the plant room and key points of the network you know how much heat is being produced and lost in transit. To get the full picture of system performance, the last piece of the data jigsaw is input fuel. You need to know how much gas, biomass or other fuel is going into the system. The ideal is to meter and measure fuel input on the same AMR system as your heat meters, but this simple step is rarely taken. Instead the data can only be found on hard copy invoices, which usually lie buried in some obscure filing cabinet in the basement. Despite their huge importance, these invoices can take many months to get hold of, all the while you may be haemorrhaging cash from a poorly operating DH system.

Finally, all this information is useless if there’s no one to understand it and take action. While external consultants might help, there’s no substitute for someone within the client organisation taking ownership and having the responsibility to monitor how schemes are performing.

The next casualty of network neglect is water quality. It’s incredibly boring (and given that you’ve read this far I’m assuming you’ve got a very high boredom threshold) but ridiculously important. After install, water is the primary determining factor of DH asset life; poor water quality can reduce the life of the network and key DH components by years or even decades. That’s all I’ll say about that until the how to get DH right posts.

The final casualties of network neglect are the missed opportunities. Chief among these are the potential improvements that come from operating a network seasonally. In the summer, your heat loads drop and so can your flow temperatures – at least to a point. A rule of thumb I picked up from a Danish colleague is this: a flow temp reduction of 1° can equate to drop in losses of 0.9%. Those savings are way too good to pass up.

Instead the opposite tends to happen. On a neglected network, pump sets typically run flat out through the summer because they haven’t been commissioned correctly or because there’s no proper control regime in place. Or, god forbid, it’s a fixed volume system.* With pumps running flat out, flow temps stay high and the deltaT (the difference between flow and return temps) shrinks. The DH system does an awesome job of pumping heat into the ground and around already overheated corridors. The electricity bill for the pumping energy goes through the roof. The system efficiency goes through the floor. And somebody ends up paying for it. Somebody always does.

Hopefully the last few posts have been a useful cautionary tale. A number of people have got in touch to tell me that the scenarios I’ve described in these posts are all too familiar to them. I’ve been very sorry to hear it but not shocked.

But I’m done dwelling on the negatives! In the next few posts I’ll come onto the much more positive topic of things you can do to make DH work well.

*On a recent job we flagged up to the client that their primary pumps were running flat out 24/7, with the associated electricity bill adding more than £200 per flat per year to each resident’s heat bill and summer system efficiency in the mid twenties. When presented with the pump electricity consumption figures, the contractor’s engineer looked at them and said, yeah that’s about what I’d expect from a pump running flat out. Looks fine to me. I’m actually writing this blog post from prison as I immediately assaulted the engineer with a monkey wrench.