Heat network at Sønderby – images from linked design guide
When writing a recent post about the low temperature DH network at Lystrup, I contacted the author of the related technical report, Jan Thorsen. In Jan’s response he kindly included a copy of Guidelines for Low-Temperature District Heating (PDF).
This guide is essential reading for designers and operators of DH systems. It shows how DH with flow temperatures of around 55 and return temps of around 25 (also called “fourth generation” or “4GDH”) can be used to serve high efficiency homes as well as buildings on low heat density networks.
At this point you might say, hang on a minute – what are we doing considering 4GDH when we struggle to deliver decent 3rd generation (70/40) networks in the UK? And I’d say you’ve got a point. In fact, I spent a depressingly large chunk of last week trying to help salvage the efficiency of another new network that is horrendously oversized and was probably doomed to low efficiency before it even left the drawing board. So I’m sympathetic with the view that UK engineers need to get our houses in order before moving onto the cutting edge stuff.
But looking ahead to the strategies employed in more advanced, lower temperature systems helps to highlight the design principles that we should be focusing on, even on today’s projects in the UK.
In addition, it’s clear that if DH is to fulfil its promise of meeting up to 20% of UK heat demand by 2030, we’re going to have to get to grips with best practice low temperature networks. Because only by using these 4GDH networks will be achieve the following:
- Highest efficiency of supply
- Increased viability of DH in areas with lower density of heat demand (like the burbs)
- Maximum use of low temperature renewable energy like solar thermal
- Integration of DH and heat pumps while maintaining a decent CoP
- Use of low temperature waste heat – the lower the temperature, the cheaper the waste heat
In other words, new UK networks with flow and return temps of 70/40 are a great start, but they’re only a stepping stone.
So, if you’re an engineer or DH operator, take a look at the guide. It’s concise and pretty engaging all the way through. Here are a few of my favourite bits in no particular order:
- DHW control valves are hugely important. Don’t skimp. This is a view I think the team at Greenwatt Way would echo.
- Ends of branches fitted with thermostatic bypasses (at 35°C!) rather than fixed flow or differential pressure bypasses.
- Given the low flow temps, you can potentially use the return water on an older existing network for the flow water of the 4GDH system. That’s very cool.
- Design for higher flow rate (2m/s) and smaller pipe diameters.
- On the 4GDH network at Sønderby, which serves 75 detached houses, heat losses were 13%.
Happy reading!
“Hang on a minute – what are we doing considering 4GDH when we struggle to deliver decent 3rd generation (70/40) networks in the UK?”
Our jobs as engineers. 😉
(anybody still specifying 70/40, or indeed 2G temperatures, deserves disowning)
Whilst choice of DHW valve is still hugely important, 4G networks aren’t actually as demanding as 3G networks in this respect. You won’t scale a heat exchanger OR scald somebody at 50C, no matter how high your primary flowrate is. (the worst you can do is return excess water to the network)
Controlling those bypasses to keep service pipes warm is more important in my view. Hard to do well mechanically; piece of cake if you have direct connected bathroom zones plumbed for bypass and backup eTRVs for direct supply:supply at the substation.
Good discussion here:
http://www.byg.dtu.dk/~/media/Institutter/Byg/publikationer/byg_rapporter/byg_r250.ashx
Hardest part is finding a DHW valve where you can crank the proportional control (gain) up high enough for low flow temperatures: most place far too much emphasis on stability to avoid the scenario where they scald somebody or scale the exchanger through excess flow on a primary network with 90C+ flow temperatures coupled with unstable operating pressure. Low gain, high damping, crummy hot water performance.
Thanks for the comment, Marko. It’s great to get your feedback.
If we disown the engineers who haven’t moved beyond 70/40, we’ll have about 3 left! It seems that right now most M&E consultants are struggling, even with the basics.
There are more than 3. The world never gets to see their work because it “just works” though.
My understanding* is that it’s been illegal to use 70/40C in Sweden since the 1980s. (SBN 80 called for a maximum 55C flow temperature even way back then – in addition to insulation that put early 2000s UK regs to shame…) Certainly 55/45 through radiators is standard practice there now.
*Google translate and I struggle. Useful search terms:
framledningstemperaturen 55ºC
maximalt 55ºC
I should qualify the initial comment with “engineers designing internal systems for recent build / new build or kW scale distribution at 70/40 deserve disowning.”
70/40 is fine for MW scale distribution and legacy buildings that’ll cost too much to retrofit. (though if you insulate the fabric a little then a system sized for 82/71 should work very happily at 55/45 or 50/30)
Here’s that reference:
rev1 doesn’t have it (1980)
rev2 is clear as day (1983)
pp307:
UPPVÄRMNINGSINSTALLATIONER (HEATING INSTALLATIONS)
“I en byggnad för stadigvarande bruk som innehåller bostads eller arbetsrum och är avsedd för annat ändamål än fritidsändamål skall ett uppvärmningsystem med vatten som värmebärare vara så utformat att framledningstemperaturen vid dimensionerande värmeffektbehov inte överskrider +55°C.”
“In a building for permanent use containing residential or office and is intended for use other than recreational purposes, a heating system with water as the heating medium to be designed so that the flow temperature at the design heating power requirement does not exceed +55°C” [Google translate]
Sweden is sizeable and has been sizing heating systems this way by law for the past 30 years.
Their objective was to make all buildings low-carbon ready: thou shalt ensure that flow temperatures are heat pump ready and absolutely guarantee that your boilers condense in normal operation.
If you want to see where UK Building Regs and design codes might be in 2040, it’s good to read the current Swedish/Danish ones. 😉
I’m fascinated by this. One question I have is dealing with insurance risk. I witnessed a developer talking with a DH provider saying that they could not get any consultant to advise this (I believe based on PI insurance) for 70-40 with instantaneous water heating to give tap water at maybe 50 degrees C. This was due to (perceived) legionella risk.
Have you come across this as an issue? Is it possible to address it in a way that does not create unmanaged risk for anyone?
This is a symptom of the business model: the construction industry is notoriously litigious and some will seek out the slightest excuse not to pay their suppliers. The act of seeking excuses not to pay (going to court) costs the insurer, hence their reluctance to cover against an inevitability.
Want paying? Then design the rubbish non-condensing 2G heat networks that the developer asks for. The efficiency penalty will fall on the householders via the network operating company and is “not your problem” as far as the consultant is concerned. Nor is legionella – actually a problem in overheated cold water systems due to heat losses in risers and communal areas rather than the hot water system – because the developer/architect specified that bit not you so it’s their liability.
You don’t get the same problem if you fix the business model: a heat utility provider whose scope of service extends right up to the ‘meter cabinet’ and is contracted to provide space heat/hot potable water not ‘a heating system’ is free to specify an optimal solution without fear of not being paid.
You will not be sued over legionella:
1) science says it won’t be a problem
2) 10 million combi boilers delivering at 45C say its not a problem
3) you can demonstrate due diligence by referencing established continental design guides. (45C, high flow velocities, <3L volume etc)
There is now no unmanaged risk. You need not cut consultants out altogether: there is still room for them even in the Danish operating model. It is time to get developers specifying the service rather than how it is delivered though: just like gas and electricity. The better ones are already asking for full service right up to the meter point with consultants working for the ESCo rather than the developer.
I do worry that industry attempts at self regulation – which prescribe HOW to achieve things rather than WHAT to achieve – might improve the quality of the WORST work out there but will become a pain in the neck for any engineers attempting to do GOOD work.
Hi Marko,
I agree with much of your comment, but disagree about the route to better systems. Most schemes aren’t going to bring in third party heat providers from the start to help them get the system right.
I think the root cause of contractors delivering poor schemes is that they’ve got no requirement to do so (as you point out). But the solution to this is to change their success criteria. At the moment, the only measure of success appears to be: is the furthest flat getting plenty of heat. That leads to horrendous networks and pumping regimes.
Instead, clients need to get much better at asking for the right thing. The ERs should include strict performance requirements that are then allowed for in every tenderer’s price. The spec should support the ERs. And the contractor’s feet must be held to the fire to achieve the strict criteria, or else no practical completion. Without a change to ERs and specs, we’re not going to see better networks.
Hi Casey,
“I agree with much of your comment, but disagree about the route to better systems… …clients need to get much better at asking for the right thing.”
I think you’re right in how it will work going forward, but think it worth pointing out how it should have worked.
The way it should work on paper is as follows:
PLANNING
Clever people in the planning department work out what the best strategy is for use of space, transportation, energy and so forth. They impose this through planning requirements. GLA falls into this category; providing the scale/expertise that local planners might not have.
BUILDING REGULATIONS
Clever people work out what the most appropriate standards to build to are, from the point of view of economics, security of supply, environmental factors. They write the spec for developers to build to. (If they’re not doing this what’s the point in Part L?)
CUSTOMERS
The process assumes that customers are not experts in architecture or building services engineering. They’re right. The Building regulations people have already done their homework for them in terms of what is the most economic/secure/environmentally sound solution for keeping their buildings warm is. The customers can ask a developer to “build me a home” and the building regulations plus enforcement thereof will ensure that it is safe, warm, and economic to operate.
Unfortunately BUILDING REGULATIONS have totally failed the customers.
1) SAP is inherently the wrong tool for the job. SAP only looks at “cost of energy” not “cost of heat” so you score better for something that costs £1,000/year to maintain and uses £10 of energy than for something that costs £100/year to maintain and uses £100 of energy. Huge mistake. Blame DCLG for specifying/accepting the wrong metric to be optimising for. If you used “cost of heat” rather than “cost of energy” then gas boilers would already be obsolete.
2) SAP monumentally over-estimates the efficiency of communal heating systems. Individual boilers it gets about right, but by default assumes far lower losses in distribution systems and far higher plant operating efficiency than is observed in practice. Blame BRE for using the wrong numbers and DCLG for accepting this flawed data. If the default assumptions were a worst-case-scenario (or even industry-average for the past 3 years) then there would be an incentive to design better heating systems.
3) Building Regulations are never enforced anyway. I have yet to see post-build evaluation of a building or an enforcement notice requiring the as-built to match as-designed from an energy efficiency standpoint. If your building’s habitation certificate were revoked/never issued if it didn’t meet the spec, or better yet it were bulldozed if it didn’t perform, there might then be a reason to care.
This is not a reflection on the individuals involved – given their resources/remit they are quite sensibly focusing on Part A to make sure that the buildings don’t fall on your head – but more the reality of the situation they find themselves working in.
AS A CONSEQUENCE of this failing, the PLANNING folks and the CUSTOMERS have put pressure on the industry to get it’s house in order in spite of how it should work.
We’ve got the (draft) code of practice by CIBSE/CHPA, the Independent Heat Customer Protection Scheme, and far more interest from both developers (can I sell the house afterwards?) and social landlords (what is the lifetime cost of this building?) that is being translated into stricter ER. Behind the schemes there’s the GLA and DECC waving some pretty big sticks if progress isn’t made.
This is great. It’s not the way it should work though. We shouldn’t ever have had this problem if those in charge of building regulations (DCLG) had done their job or their monopoly supplier (BRE) had used vaguely realistic numbers or the professional institutions (CIBSE) had refused to deliver lousy schemes even where the client had specified them.
Hi Tim,
Thanks for the comment. I’m baffled that 70/40 is seen as a PI risk. For one thing, consultants can point straight to CIBSE AM12 to get themselves off the hook.
And 70/40 is conservative compared to the networks in the Low Temp DH Guide! These 4th generation networks are running at 55/25! I can imagine that a (decent) UK consultant might balk at that and need some convincing. But 70/40 should be mainstream by now.
But that’s a great indication of:
1) how far behind UK consultants (they’re not even reading guidance from their own Chartered Institute!)
2) how paranoid they are about their PI and
3) how they’ve got no incentive to deliver efficient networks because clients don’t ask for the right things.
Marko and Casey, thanks for responding. I’m afraid that 70-40 is often regarded as adventurous. We have a lot of work to do!
Marko, I take your point on standards. The industry is concerned about poor design, construction and operation but very much wants to advocate best practice and innovation. The standards work will, I hope develop with time and avoid the prescriptive issues you flag. It is a learning process and we have had some very experienced people help develop this but very much open to thinking how we can take it forward to ensure it delivers.
These things are not easy and it is great to have people like you both advocating doing the very maximum of efficiency.
Can we draw up a list of where the pressure for high temperatures is coming from? What are the latent “needs” that drive UK custom and practice that the Danes/Germans/Swedes don’t have?
Why is local hot water storage, and storing potable water rather than process water, such a big deal here?
-Is a symptom of “my freehold home my freehold castle” mentality?
-Is it because she who must be obeyed wants an airing cupboard?
-Is it dissatisfaction with the service level available with individual home heating systems (perhaps 99.5% availability if you’re lucky but typically only possible to guarantee 95% within any one month) and a desire to have backup options? (immersion heater)
-Is it because our incoming cold mains water dynamic pressure is so godawful (service level guarantee is 0.7bar@9L/minute at the stop tap) that we need a break tank and boost pump just to get a decent shower?
Why do we insist on such high hot water delivery temperatures?
-Building Regs effectively cap bathrooms to 48C; why do we want higher elsewhere?
-Scalding hot water doesn’t kill germs when hand washing or remove any more than cold water does. Are there people who don’t realise this?
-New builds invariably have dishwashers which are awesome at high temperature cleaning to remove fats and burned on deposits then sterilise the dishes all whilst using less water than hand washing. Do some people assume that everybody still washes up by hand? (apart from SAP, but we already know this to be a lost cause…)
-Are the specifications perhaps written by men of an age/income that means they’ve never washed dishes? Do they assume that people wash dishes with no detergent under a scalding hot running tap? (and therefore require 55C to soften fats) Perhaps they don’t realise that soaking for 2 minutes @ 37C with some Fairy liquid will remove far more. (and if it doesn’t you’ll need some cream cleaner and a pan scourer) Of course nobody has a kettle they can boil and tip into a particularly stubborn baking tray, or a hob they can pop it back onto to ensure that it’s absolutely scalding hot.
-Perhaps once upon a time somebody designed a secondary circulation system for a giant hospital that recirculated water for days on end through filthy rusty steel pipework at the ideal breeding temperature for bacteria then aerosolised it over immuno-compromised patients and killed a few. In the resulting over reaction they specified one temperature to avoid it, the next person in the chain added a little for luck and then the professional institution gold-plated it. This was before the first oil crisis and in a time of electricity too cheap to meter. Later it got picked up by domestic designers searching for a standard to work to for their all-plastic systems flushed daily with high velocities, little surface roughness to promote biofilm adherence, and minimal volumes. Finding none in the domestic sector, and it’s never been revisited since.
-Maybe the evidence in the form of 10 million homes quite happy with gas combi boilers @ 45C and the majority of domestic hot water tanks kept @ 50C isn’t adequate?
I’m being deliberately provocative here. Somewhere though, there are a combination of factors that are driving the UK to supplying domestic hot water at 55C or above, and preferably from a local store. This is a great inconvenience.
What’s the laundry list and how do we address their needs to improve efficiency?
Does a supplementary 3kW under-sink unit in the kitchen address all concerns? Is it even a value-add feature if we made it the boiling water tap? (if you could find a product that’s energy efficient and long lived: none on the market current are)