13 October 2023
By Martin Lowe
The surge of interest in Heat Pumps as a possible replacement technology for new and existing heating water distribution systems will involve a more focused approach on design and installation or retrofit. With the current technology based on current refrigerants, no longer will there be a reliance on the compensation currently a orded by the high temperatures of gas boilers for poor design, poor selection, poor installation, and out of balance systems. The CIBSE guides will eventually have to be revised and updated to accommodate the changes.
The current focus of discussions is on replacement of the heat source. Existing Terminal Units pipework and balancing valves must be sized to the new fl ow rates. Heat Pump designs must be sufficiently sophisticated to remove the need for ultra-low fl ow rates that are impossible to measure and only give on/o control. Sizing of control valves for Heat Pump systems is a job made easier with the introduction of Pressure Independent Characterised Control Valves (PICCV), provided the right control valve characteristic as specified. An equal percentage characteristic is preferred for lower Delta Ts between output and return, which is a marked change from the more linear characteristic valves used for 20°C to 40°C Delta Ts that are achievable on boiler systems. Ideally the Delta T to each terminal unit should be measured and monitored.
A more favoured solution
Three Port Control valves, if installed to Terminal Units, will cause return temperature problems, and should not be installed. A more favoured solution would be to choose a temperature-controlled end of line bypass. Measuring and monitoring bypass temperature should also be a consideration as open bypasses will affect the Heat Pumps performance. Heat Pumps loose performance if the temperature difference between flow and return temperatures is not achieved. To achieve the removal of dirt from the system remains a major objective and the BSRIA BG29 2021 gives good advice on designing system to ensure good flushing velocities. Heating water flow rate tolerances will require revision, possibly to something like those of the Chilled water tolerances as Heat pumps work in the reverse way to Chillers.
In contrast to the use of new designs within new builds, replacing existing boilers with heat pumps is not as straightforward as you would imagine. Existing buildings suffer from legacy designs and they can be from 30 years to ones which have only just been completed. The designs can vary between Constant volume 3 port systems with belt driven pumps to variable volume ‘state of the art’ Inverter driver intelligent pumps with Smart actuators to each Terminal device. These new systems would be much easier than the older system as design information, drawings and commissioning data will be more readily available.
As all the buildings already exist and have working systems with boilers, any move away from fossil fuel boilers will have to address the issues of suitability. Buildings that are going to be gutted and refurbished can be considered as a new build. However, what about most buildings that are currently occupied, with no plan to refurbish, that have remaining life not only in the boiler, but in the pipework and terminal units?
The challenge of converting any existing system from one heat source to another requires a completely different approach and a lot more data focused on what is happening at the Terminal unit. This information is currently not available as the devices needed to collect that data are either not installed or if installed are not being utilised. Existing boilers could be used to simulate lower temperatures like those given by Heat Pumps. But without collecting data on each individual terminal unit at the lower temperatures, how can the change be evaluated. In addition, lowering the temperature without increasing the flow rate to each terminal unit will not give a reasonable evaluation of using a lower temperature heat source. Even if the flow rate could be increased to each unit, the existing pump that has been sized to a much lower design flow rate, is unlikely to be big enough for the increase required. This would mean either the installation of a larger pump or introducing diversity into the system.
All is not lost
The solution is the installation of Smart actuators, initially to some or all the Terminal units. These Smart actuators, with the ability to measure flow and return temperatures while simultaneously working out the best stem position to ensure the best return temperature, would be beneficial for both the existing boilers and for any change to a Heat Pump system. Working with BACnet, Modbus or the Internet of Things (IoT). Data can be collected on flow rates, flow and return temperatures to each Terminal Unit.
The introduction of Smart actuators does not necessarily mean a change in the PICCV valves, but if the heating has no PICCV valve the addition of a Smart actuator with a PICCV valve on the ends of circuits on floors, would give the eyes to what is happening at initial start-up and then through the day’s activities. As data is collected from the Smart actuators for the current design flow rate using boilers, the information of virtual flow and actual flow and return temperatures can be logged and evaluated.
Once the information regarding performance at the 80°C supply temperature is understood, the supply water temperature at the boiler could be dropped from a design of 80°C to a new design of 45°C. The change in temperature can be accommodated by a change in flow rate, this can be done through the Smart actuator configuration programme that allows the valve position to be changed without physically visiting the valve. The monitoring and logging of information for each Terminal unit can be evaluated at the new lower temperature. This information would give an insight to those who need to understand the full economic impact of changing from a boiler to a Heat Pump before any work is undertaken.
The initially installation of a PICCV valve that can work with the Smart actuator would require some alterations to the Terminal unit. But after that the fixing of the actuator and modular wiring can all be done by Commissioning Engineers or the onsite maintenance team. Each Smart actuator using modular digital cable can be connected to other Smart actuators and the modular Input and Output (I/O) cable for the temperature clip-on measurement sensors can be installed on the flow and return pipework. The configuration tool is simple and straightforward to use, and information is in real time. The control of the actuator and the Data collected can be programmed in by the Controls Engineer.
Hands on courses
Changing to Heat pumps and the introduction of new Smart actuators should not be undertaken lightly as some understanding of PICCV valves, Smart actuators and flow measurement devices is required. Practical ‘hands on’ courses have been developed to give consultants, contractors, commissioning and service engineers and maintenance engineers the understanding and skills to understand all the disciplines involved.
In conclusion, developing a strategy for the implementation of Heat Pumps or any other alternative energy source for the buildings relies on detailed data. Understanding that any change will impact on the complete system is key to developing that strategy. However, with the use of new innovative Smart actuators the installation of Heat Pump systems need not be the nightmare described in the current popular press.
Also, remember that there is new technology involving Helium gas as the refrigerant instead of fluorinated compounds being developed in Norway and the US. This could then easily replace gas boilers without any changes in output temperatures. However, helium gas is rare and so this may be an expensive option. But, CO2 is also being considered as a refrigerant so lack of Helium may not be a stumbling block for long.