Heat interface units do more
Heat interface units play a vital role in communal heating systems by enabling low-carbon heat to be utilised effectively. Ken Sharpe finds out how one company has advanced their technology.
Building-services technology is notoriously slow to change — one example, according to Neil Parry, specification manager with Altecnic, being heat-interface units used in communal heating systems. HIUs, as they are commonly known, mimic the function of gas-fired combi boilers but with hot water from central plant as their ‘energy’ supply. This approach enables the central plant to integrate low- and zero-carbon energy sources such as CHP, biomass, solar, heat pumps etc., which would not be possible on the scale of individual dwellings.
Neil Parry says, ‘For many years, there has been very little development in HIU design, with the majority of products changing little over the last 10 years, and technical innovation has stagnated.’
So when news filtered back that readers had latched on to Altecnic from our Ecobuild preview back in March and targeted the company’s stand to find out more, we decided to take a closer look for ourselves by visiting the company at Stafford.
Neil Parry himself has long-term experience with heat interface units, particularly working with consulting engineers, having spent many years dealing with central plant and district heating systems both in the UK and across Europe before joining Altecnic.
Altecnic itself is a UK company that is 90% owned by Caleffi in Italy. Caleffi has offered a mechanical HIU for many years, initially in Europe and then in the UK. Early last year, the company started to develop a new unit based on electronic valves with enhanced capabilities and energy efficiency, and Neil Parry had input into the design. There are direct (secondary heating circuit linked to the primary circuit) and indirect versions (secondary heating circuit isolated from the primary circuit by a plate heat exchanger).
He explains, ‘The main objectives were to reduce weight and size and to increase performance and efficiency. A key element was switching from mechanical to electronic controls to reduce weight and dramatically improve responsiveness to the requirements of the heating and hot-water systems.’
Much of the reduction in weight to 19 kg compared to about 35 kg for competitor units is due to the electronic valves. These valves combine the functions of modulating the flow of water and also shutting it off, which normally requires two mechanical valves. In addition, unlike conventional on/off valves, utilising the plate control valve does not require a DPCV (differential-pressure control valve) to protect them against constantly changing flow and pressure from a variable-speed pump. The overall benefit is one valve replacing three — a major saving in weight.
On the indirect version, both functions, heating and hot water, have their own separate plate heat exchanger as the interface to the primary hot-water circuit.
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| Advances in controls used in Altecnic’s heat interface unit nearly halve the weight compared to competitor units and make possible significant energy savings. |
The heating plate heat exchanger is only supplied with hot water from the central plant when there is a demand for heat. On the other hand, the primary supply to the DHW plate heat exchanger is controlled to keep it warm, so that hot water is instantly available. Conventional HIUs continue to bypass exchanger even when the plates are up to temperature. The Altecnic HIU stops this energy wastage by isolating the exchanger plates as soon as they are up to temperature. The overall benefit of controlling draw off from the primary circuit in this way is to reduce the energy consumption of the central plant.
Neil Parry tells us that the normal mode of operation for a communal system is variable flow in the primary circuit. Electronic valves are quick to sense the effect of such variable flow and make adjustments to the secondary circuit to maintain a stable temperature to heat emitters and hot-water outlets. The temperature of DHW can be maintained very accurately.
That same precise control capability is also put to good effect in controlling the heating system. The preferred ∆T between the flow and return temperatures can be preset. When the return temperature starts to increase, the flow temperature is automatically lowered, thereby achieving a similar effect to weather compensation and reducing the output from the heat emitters to save energy. This type of control is not possible using mechanical valves.
The result of smoothly modulating down the heat output is a more balanced room temperature, a comfortable environment for the tenant and lower energy usage.
The steadier delivery of heat is also assisted by not all the heat output of a unit being diverted to produce domestic hot water when a tap is opened. A unit might, for example, have a heat output of 15 kW and 50 kW DHW output, but the heat available for producing hot water is controlled according to the rate of draw-off and the amount of hot water priority that’ the user has pre-set into the HIU.
Another feature is a pump bypass that only opens when it is needed — when all TRVs are closed. This feature enables every radiator to have a TRV, rather than one being left open to ensure a minimum flow at all times.
Heat interface units naturally require heat to be metered for charging. Altecnic uses ultrasonic flow measurement for its reliability. Temperature is monitored by Pt 100 sensors. This approach to metering is MID and RHI approved. Heat consumption can be monitored remotely, and a card reader can be provided for prepayment.
Neil Parry says that the cost of these HIUs is comparable with mechanical units and they give a better controlled environment.
During the course of this year, consultants have specified them on numerous projects, amounting to around 2400 units. Example projects include an apartment with over 70 units and a care home with 30 units.
In reducing the carbon emissions associated with supplying heating and hot water to appropriate buildings, the capabilities of heat interface units to maximise the low-carbon capabilities of central plant have a key role to play. The efficient use of low-carbon heat is vital.
