March 14, 2023
Benefits of sub-metering when installing a heat pump
We visit many buildings during our decarbonisation work, and one of the key factors is understanding energy use – how much, where and when energy, particularly fossil fuels such as gas and oil, is used. This is important when calculating energy, cost and carbon savings and accurately sizing replacement low-carbon heating (and cooling) systems. We regularly come across multi-building sites with a single gas meter measuring gas consumption across the entire site, including heating, hot water, multiple plant rooms, direct gas-fired heating, catering and gas used for laboratories and workshops. This is usually provided in a monthly gas bill form. Although it’s possible to use this data to estimate heating and hot water energy usage, going beyond this into system design means making many assumptions and consequently guestimating the peak demand and heat pump size.
Ideally, each plant room will be metered for gas/oil consumption or heat generation. And the data will be provided on a half-hourly basis for at least one year. This speeds up the subsequent calculations and ensures an accurate system design.
It has been said that ‘you cannot improve what you don’t measure’, which is undoubtedly the case with energy. An accurate baseline of how buildings operate and perform at the start of any project is essential to check that a new system is delivering the operational performance, cost and carbon savings stated during the design phase.
But what if you don’t have meters in every plant room or only receive monthly bills?
That is where sub-metering comes in.
Sub-meters provide a clear and accurate picture of how, when and where energy is used across a site. Imagine a site with eight plant rooms delivering gas-fired heating and hot water to various buildings; which buildings have the highest consumption? Are all buildings operating as you expect? Are boilers running through the night when not needed, costing you money?
Used primarily on large multi-building locations, such as University campuses, retail or leisure complexes, sub-meters enable these questions to be answered by providing an accurate account of how and where energy is used.
How do sub-meters work?
Many buildings have a single gas or electric meter, which calculates the gas and electric usage across the site. They do not provide a breakdown as to where and when you are using the most energy.
Sub-meters are installed at different locations across a set of buildings to collect data and show how much energy is used in specific locations, points and times. Multiple sub-meters can be placed across a system to isolate readings for particular buildings or rooms. This provides an up-to-date and accurate picture of how and where energy is used site-wide. Unlike most single-use meters, which provide monthly reports, they can also allow data to be collected daily, hourly or even minute-by-minute.
Sub-meters can be installed in two different ways:
Temporary meters are clamped to the gas, heating and electric incomers, which then relay information to a data collector over an hour, week or month. The data collected is then compared and analysed to highlight trends and anomalies in energy usage. A non-invasive method, it isn’t as precise as fixed options but does provide a cost-effective solution to access the data needed to make an accurate analysis of energy usage.
Fixed meters need to be directly installed within pipework or distribution boards, so they involve permanent changes to the system. This makes the readings highly accurate, allowing the most detailed data to be captured, but it will increase the initial installation costs.
How is the data collected?
As with meters, two different types of data collectors can be used. Some are inbuilt into the meter, so you simply download the data as and when required using a USB attached to the data collection point.
Others use a data collector installed at the plant, which sends the information via Wi-Fi or 5G to a portal. This allows the system user to access the data from anywhere in the world.
How can sub-metering help when installing a heat pump?
To accurately calculate the size of a heat pump required, it is crucial to understand precisely how your buildings are working, where your peak energy usages are and what level of heat needs to be generated by your heat pump.
For example, let’s return to the eight gas-fired plant rooms described above. The heat pump solution for this site may consist of two heat pump energy centres delivering heat via a heat network to each plant room. Without sub-metering in each plant room, assumptions on total gas usage have to be made on sizing each heat pump, the heat main and possibly heat exchangers in each plant room. With sub-metering in each plant room, definite decisions about plant sizing can be made, which can then be baselined and measured during operation.
Furthermore, a sub-meter assessment can reveal potential savings without needing to install heat pumps. For example, when working on a recent system at Gloucestershire College, analysis of the half-hourly sub-meter data showed that the gas boiler was unexpectedly triggering the heating in the early hours of the morning, costing thousands of pounds and producing an 800kW heat demand spike when the College was closed. By analysing the data, the College could fix the problem and reduce their energy consumption as they knew where the extra heat demand was coming from. Without this data, we could also have been calculating for a much higher heat pump demand which just wasn’t needed.
Understanding how you use energy across your site and the heat demands, spikes, and seasonal changes mean that the most significant efficiencies can be built into your system.
In the same way, it’s good practice to fit a sub-meter after your heat pump is installed to check for efficiencies and verify the COP calculation of the system. The Coefficient of Performance (COP) of a heat pump calculates how much heat is generated compared to how much is consumed to determine the heat pump’s efficiency. Electricity and heat meters on the heat pump itself are strongly recommended to ensure the unit operates as designed.
Sub-metering for cooling systems
The same principle applies to cooling systems as heating systems. Sub-metering can help businesses to understand the cold (coolth) being generated to determine the correct chiller size required. Without calculating the usage this way, you are relying on static cooling calculations, which will be less accurate and, ultimately, more costly. Rather than gas meters, cooling meters on the cold water pipework will be required to understand chiller output; electrical meters could also be deployed on the chiller itself to understand unit performance.
How do I know if it is suitable for my building?
When considering installing sub-meters, the first thing to do is understand how your current meter works. Look at the baseline of what you have; how often is the meter reporting, how many meters do you currently have across your buildings, and is this giving you enough information to understand how your site or campus is working?
Sub-meters aren’t always necessary, but it is important to get all the data you need to make an informed decision. For some, it can be a cost-effective and straightforward way to understand what is happening across your site and ensure you are getting the best out of your existing systems.
Still trying to figure it out?
Are you looking for more information about how sub-metering could help with your project? Looking to install a heat pump and want to size your system accurately? Laura at Infinitas Design is an expert heat network designer who can help you understand what the right option is for you.
With 12 years of experience, we give you honest, straightforward advice to help drive maximum efficiencies out of your project.
Get in touch today to learn how sub-metering could support your project.