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Achieving a measure of success with heat meters

You only really know what a system is doing by measuring it. Georgina Orr explains the main features of heat meters and outlines where and how they should be placed
The renewable heating sector has entered a new era since the launch of the Renewable Heat Incentive (RHI) by Government last year. Some may say the industry is going from strength to strength, others may argue that all of these government backed financial incentive programmes are creating uncertainty and further red tape for an industry that was already successful.

The RHI is a financial programme designed to encourage the heating of buildings, water and processes by renewable heat sources. At present, the scheme only covers non-domestic properties. If you fulfil the eligibility requirements, you can be paid for every kWh of heat you generate using a renewable technology.

RHI uptake lower than expected
So far, the uptake of the RHI has been lower than expected and the significant stumbling blocks seem to be the application process and the correct positioning of heat meters. To claim the RHI, heat meters must be installed to measure the renewable heat generated and delivered. Advice is provided by Ofgem in the substantial volumes of the RHI Guidance notes which state that:

In simple installations, only the eligible heat source need be metered.

In complex installations, heat generation from eligible and non-eligible plants, plus heat use of eligible purposes must be metered.

All heat meters must be MID (Measuring Instruments Directive) Class 2 or better and certificates of conformity must be provided by the applicant during the application process. Complex sites all require an independent metering report (visit http://www.rhiapply.co.uk to access help with this) and during this process the provenance of the meters need to be verified.

Heat meters can be costly and sensitive. Install them incorrectly and they will provide nothing except inaccurate and meaningless numbers, but getting it right requires real attention to detail and it is important that several key factors are considered:

Fit the right size meter for the application in which it is being installed. Heat meters are sized according to pipe size and flow rate and should be correctly matched to the installation. Smaller meters are of course cheaper, but if they are undersized they become an obstruction, preventing proper operation of the system.

Meters must be fitted the right way round. This may sound trivial, but even competent and well briefed engineers can put them in backwards. Check the arrow on the back, the arrow must point in the direction of flow unless stated otherwise by the manufacturer.

Ensure that the temperature sensors working in partnership with the meter are a calibrated pair and are installed in the correct pipes; the warm sensor (often identified using the colour red) must go in the hot pipe (flow).

The cool sensor (often identified using the colour blue) must go in the cooler side of the system (return). This may also sound trivial, but these sensors provide the temperature difference which in turn will calculate the heat measurement for the meter.

Usually, temperature sensors come with pockets allowing easy access post installation without the need for system drain down. It is very important that the temperature sensors are fully inserted into these pockets and that the pocket actually sits within the water flow. For large pipework this may mean long pockets, but the sensors must read the water temperature and not a 'dead leg' 6 inches from the pipework.

Ensure that heat meters and associated components are not installed too close to high temperature objects and stores. Heat can be transmitted through pipework from storage vessels such as buffer tanks and low loss headers. This heat transfer can have a pronounced effect on the temperature sensors and give inaccurate readings, especially if it means the return temperature is warmer than it otherwise would be. This automatically reduces the temperature difference and potentially reduces the amount of RHI funding received by the site.

Heat meter requirements vary
Straight sections of pipe are also important in the installation of heat meters to remove turbulence and ensure linear flow through the flow meter. The requirements for this vary depending on the meter, but should always be checked before installation. Air can also be very problematic; heat meters should not sit in pipework where air may collect.

Heat meter positioning to comply with the RHI regulations can seem taxing. While money should not be paid to systems that are incorrectly installed or where heat from fossil fuel could potentially be claimed under the RHI, the positioning of these meters is still a contentious issue. An example recently given by a biomass boiler company was the requirement of a heat meter at each end of a 30m underground pipe between a boiler and a building (see the figure left). One might think that a meter at point of heat delivery would be sufficient? This would account for losses and mean that RHI payment is only given for the 'useful' heat delivered.

This issue has been addressed in the recent guidance update issued by Ofgem; however, the aspect of positioning meters when supplementary boilers are installed is still on-going. When a back-up fossil-fuelled appliance is included in the system, there are some very uncertain issues regarding the positioning of heat meters.

When this fossil fuelled back-up directly feeds a common vessel such as a buffer tank or accumulator, then the heat provided by this fossil fuelled boiler should be metered.

By contrast, if the fossil fuelled boiler is downstream of the renewable source, either after a heat exchanger or after all the required heat meters for the renewable system, one really has to ask whether this heat needs to be accounted for?

If the renewable heat is being recorded completely, in other words at point of generation and at point of use, does it matter that an oil boiler then acts as a backup after this?

The official answer to this is unclear, but we believe it would be strongly argued by installers, site owners, and consultants alike, that adding a meter to measure a seemingly unused number when considering the payment of the RHI is somewhat overly burdensome.

What about domestic systems? The schedule for introducing domestic RHI has changed more than once and the current timetable is now set for summer 2013. As yet it is unclear whether metering will be required at this scale.

Perhaps this is a topic for the future. However, you only really know what a system is doing by measuring it. Surely it is better to fit meters when the renewable technology is installed and only issue payments to those utilising renewable heating where it is suitable to do so?

The author is energy and environmental consultant Kiwa GASTEC at CRE
6 August 2012

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