The HIU Revolution
This article has been inspired by the recent publication of independent test data on a spread of Hydraulic Interface Units, as well as by the ensuing technical discussions.
To us it is fairly clear how a modern heat network should run, and the fact that we "spanked" the test proves we have an idea or two up our sleeves.
This article explains why its time to throw out the existing book, and embrace the revolution.
Accuracy of Data
The data used in this article is extracted from the published test data from recent tests carried out at SP in Sweden under the new UK testing standards.
Key Advances in Technology
At the core of the revolution is the shift from mechanical systems to electronic. Believe it or not, the district heating industry still clings to the concept that mechanical is best and more reliable. This is simply down to having not yet experience an electronic system from a reliable manufacturer. No surprise then that systems costing upwards of a million pounds, and responsible for delivering our most important and costly services, are generally still less intelligent than a wristwatch.
There is however no doubt that the ability of electronic systems to deploy advanced control strategies, using just the one valve per circuit, enables systems to become more advanced without the need to add more and more valves (or valve components).
The introduction of electronics also enables networked communications. It allows remote management of settings, remote fault alarming, and holistic heat network functionality.
The nice thing is, there is usually all the hardware already in place to allow this networking to take place, in the form of a billing system, and we will in part cover the latest advances in this arena.
The real breakthrough that has been made in the HIUs, enabled by the use of electronics, is the way the systems manage the volumes and times that primary heat is drawn, in order to manage the heat network in its entirety. The effects on heat loss and volumes circulated is, as the independent data proves, quite revolutionary.
Efficiency of District Heating
Saving on wasted energy is the key to it all. Paying less for your energy, and pumping less carbon into the atmosphere than we need to. Important stuff in our book.
We believe inefficiencies (or losses) can be split into seven categories:
- Generation efficiency - how efficient the plant room is at generating heat, and typically a function of flow and return temperatures.
- Heat losses from main network flow pipework - these are the distribution pipes that carry heat to homes and need to be maintained at temperature.
- Heat losses from branch flow pipework - these are the pipes in the vicinity of properties, that can drop in temperature (conditions apply) without affecting performance.
- Heat loss from HIUs - unnecessary losses directly paid for by occupants.
- Heat loss from return pipework - this includes both branch and main network pipes and reflects the losses of heat from unused heat returning to plant.
- Pipe over-sizing - unnecessary losses incurred by oversized pipework.
- Sterilisation losses - these are heat loses incurred as a result of Legionella policies that require pipework to run hotter than the minimum required for performance.
Generation efficiency
Many heat sources rely on low temperatures to be efficient, and one should be able to obtain data relating to exactly how efficiency is affected by return temperatures for any particular type of heat generator, or boiler.
A condensing boiler, for example, may experience a 10% gain in efficiency by dropping return temperatures from 55C to 25C.
The biggest generation efficiency gains come from the use of low grade heat sources. Heat pumps benefit greatly from reduced temperatures, as do waste heat recovery systems, including CHP.
This article is not so concerned however with the mix of fuels and which is most efficient, rather to demonstrate how it is possible to ensure the lowest possible return temperatures are provided with our technology.
Weather compensation is a feature enabled by electronic control that allows us to drop district heating temperatures and improve boiler inefficiencies further still. We do weather compensation in our HIUs by tracking the district heating flow temperature and targeting a secondary temperature to radiators 5 degrees lower. DHW generation generally requires heat above 55C at peak loads (pipe/pump/heat exchanger sizing been the limits) so this can be taken as a lower limit. Between, therefore, 55C and 85C one can vary the DH flow temperature and the HIU will adapt accordingly - varying radiator flow between 50C an 80C, while keeping return temperatures as low as possible, and under set limits.
Heat losses from main network flow pipework
There are two means to reduce heat loss from main distribution pipework:
- Insulate - these pipes are hot all the time and are the main source of heat loss.
- Weather compensate as far as DHW performance allows (as described above) - the cooler the pipework, the lower the losses.
Heat losses from branch flow pipework
Pipework leading up to an HIU needs to be warm enough to deliver hot water within seconds of opening a tap. We typically like to aim for 10 seconds to over 40C (too hot for a shower).
The way to save heat loss in branch pipework is to let it drop in temperature as far as you can without sacrificing DHW response times, and this is where some of our more advanced functionality comes into play.
Our trick is to set a keep warm temperature just above ambient temperatures, maintaining a very minimal flow through the branch pipework, in the region of a litre per hour. This maintains a temperature gradient in the pipework, rather than letting it all go cold, or all stay hot.
And for this to work to maximum effect we use another trick. As soon as a tap is opened, if the district heating temperature into the HIU is lower than required, the HIU draws a boost flow of up to 20 litres per minute. This brings heat from the main network pipework that is over 50C into the HIU is seconds.
The test data shows this nicely. We have data from three methods of keep warm. One with a 55C keep-warm at the HIU, one with keep-warm at t 26C, and one with keep-warm turned off. Following the overnight keep-warm tests the HIU went straight into a DHW response test. This is plotted below.
What the graph shows is how a reduced keep warm temperature has almost no affect on delivery times, as well as how no keep warm results in a significant delay.
One can also see from the no keep-warm test (in blue) how the temperature gradient alters twice. This relates to the pipe sizes on the primary side (the heat network) of the test rig, with the larger pipes nearer the boiler loosing heat slower.
As a side, it should be noted that Coheat are able to use the second resolution data sent by the HIU to identify individual lengths of insulation missing from branch pipework. Another beneficial side effect of having electronic HIUs.
Weather compensation will also provide a benefit on reducing heat loss from branch pipework during non-peak heating season.
Heat loss from HIUs
If branch pipework is insulated then the greatest heat loss savings can be had at the HIU. Insulation thicknesses are typically lower than on pipes, but the surface area is considerable.
Even a well insulated HIU will loose in the region of 100W maintaining DHW at 55C keep-warm.
Heat loss from an HIU registers on the end-users heat meter, and they pay directly in terms that a pre-pay meter will gradually run out of credit.
Keeping the keep warm at just above ambient temperature means the heat loss from the HIU is tiny by comparison. The user no longer returns from holiday to find credit has run out. And to top it off they pay less for the energy they do use.
Heat loss from return pipework
With properly managed keep warm, as explained above, the temperature of return pipework should be near ambient temperatures, like the HIU. Removing return heat losses (outside heating season) will have a significant impact on overall system losses.
It raises a question. With no significant heat loss from return pipes, is it more economical to spend budget instead on better flow pipe insulation?
Counter Arguments
Possible the biggest counter argument for reducing keep warm temperatures so far stems from the fear of reducing comfort levels, and introducing delays in DHW production.
Despite the test data that shows it has little effect, one could still argue the test rig itself is not representative of a real life system where larger pipe volumes are involved.
To resolve this we have provided a worked example of pipework temperatures, flow rates, and heat losses for a top floor on a riser. It shows how as the number of HIUs connecting into a branch increases (as you get nearer the main network), the temperature drops reduce significantly. While the flow rate in the pipes increases significantly, the heat loss does not, so the temperature drop reduces significantly.
For any remaining non-believers, we hope to work towards a more comprehensive, ideally interactive, network model based on consensus.
VWART
