Staying in control

Bob Long, MD Eco Innovate Ltd
Bob Long, MD Eco Innovate Ltd

Heat pump trouble shooter Bob Long continues last month’s theme of bivalent energy

In my last column, I discussed why we need bivalent energy and where best to introduce it into a system. Now I will look at how the bivalent energy can be regulated according to system demand.

When a system is being designed, an operational water temperature will have been decided and the chosen temperature will be led by the type of emitters intended for the particular installation.

There will be many factors to consider when selecting a suitable type of emitter but, when the choice has been made, the water circulation temperature will also be confirmed.

For example, an under floor heating system may require the water temperature to be say 31°C before it can emit enough energy to maintain a stable temperature inside the property when the temperature outdoors is below zero (see MCS guidelines).

Arguably, if the circulating water temperature dropped below 31°C, the indoor temperature would probably also fall, possibly to an unacceptable level. Therefore, in this instance, bivalent energy support should begin here, and maintain the heating water temperature at the designed minimum of 31°C.

The amount of bivalent support will always remain unstable, and difficult to quantify, linked directly to the ever-changing outdoor temperature, and influenced by all potential heat-losses such as ventilation, door opening, or losses through the actual building fabric.

Agreeing that this system in particular needs at least 31°C water temperature to be effective, the heat pump controller is likely to be programmed to start up when the water temperature drops to say 31.5°C, and to cut out when the water temperature increases to perhaps 34°C (Heat pump output & the volume of water in the system should be closely considered when setting differential, to avoid unacceptable stop/starts of the heat pump in quick succession).

During regular operation, above zero ambient temperatures, bivalent energy should not usually be required, and the minimum water temperature is likely to be satisfied entirely by the output from the heat pump.

However, when the heating water temperature reaches 31.4°C (in this instance), we can clearly see that the heat pump is not coping and, unassisted, the water temperature would continue to fall.

Just before the minimum water temperature is reached, say 31.2°C, additional energy from the bivalent source must be added, proportionate to the requirement. Introducing more energy that is actually required would create waste, and elevated operational cost.

How do we control the bivalent energy accurately?
This is where PID controllers, or Proportional-Integral-Derivative to give it its full title, come to the rescue.

PID control can measure and respond to not only the temperature difference, but also to the rate of temperature change, and respond to an energy requirement trend as one begins to form.

PID controllers can turn this information into a modulating electrical output, proportionate to any small energy deficit in the heating systems capacity.

Typically, the output would be 4-20mA or 0-10V and would be used instruct the operation of the bivalent controller. A PID controller with auto-tune will be able to monitor the rate of temperature change over time and eventually anticipate the bivalent energy requirement as and when required.

This relatively complex method of control can be used in all heat-pump powered systems, irrespective of water temperature, made user-friendly by the advent of the auto-tune feature.

Auto-tune is designed to automatically set the control parameters of the device for optimum accuracy. However, the ‘purist’ still has the option to manually tune the device if skill levels to hand exceed the capabilities of auto-tune.

To benefit from PID control, the choice of bivalent source (excluding log burners etc) would therefore be limited to devices that can modulate their output proportionally.

The choice of such devices is currently small but manufacturers respond to market need, and the availability of modulating-output energy sources will likely improve very soon.