Fine tuning for heat pumps

There has been a general movement in recent years to make the energy that consumers use more apparent to them, in the belief that this awareness will lead to behavioural change. Energy companies have been subsidising the use of electricity monitoring devices and the government is also incentivising heat meters as part of the RHI. All the evidence suggests that giving consumers a visual representation of their consumption does lead to greater care in energy expenditure. Stiebel Eltron have responded to this movement by providing an Internet Service Gateway service for their heat pumps. This allows customers to use their computers and mobile devices to see how much energy their heat pumps are harvesting from the environment and using - and adjust the performance of the pump accordingly.

With a Stiebel Eltron heat pump, you have a far more advanced set of options than your average boiler. They have the ability to have their performance fine tuned to customise it for the individual location, property and owner. In this article I want to go through some of the basic and more advanced controls that can be used to maximise efficiency. This information can be passed on to consumers who are confident with technology and are interested in their systems – and alternatively provides an opportunity for installers to add value through service contracts with routine monitoring and adjustment. It isn’t necessary to have the Internet Service Gateway to adjust the performance of a heat pump by using these controls, although it does provide a very easy to use interface and I would recommend it for any installation.

Most heat pumps are, of course, set by engineers at commissioning in a “set and forget” fashion. This is entirely standard, and provided manufacturers’ recommended guidelines are followed, a heat pump will function perfectly well and economically without further intervention. In situations such as social housing developments, where many units are installed, this may well be set as policy, to prevent costly service calls caused by inexpert modifications. However, even in this situation, the remote monitoring and adjustment made possible by Stiebel Etron’s Internet Service Gateway allows contractors to offer an added value service by routine review of performance to continually improve efficiency on behalf of residents. There are of course also installations where the clients are themselves interested in the operation of a system – either as individual residents or because there is an in-house property management team who can take on this task.

So let’s first of all tackle why you might need to change initial settings.

The first point is that although a responsible installer will have discussed heating patterns and response times with a client before commissioning the system, it is never possible to exactly match the performance of a pump to a property until all the factors of the immediate environment, performance of chosen emitters and performance of the building itself have been fully brought into the equation. This means essentially that until you have the data from a system that has been running for weeks, months and even years, you won’t have a full picture on which to base decisions. The second big point is that people’s lifestyles change with time. For example, I know from my own experience how hot water and heating consumption rises dramatically when children arrive on the scene! In social housing situations the residents of properties may change; a family may replace an older resident moving into sheltered accommodation for example.

Basic performance tuning for heat pumps

Of course it is as easy to change the timers and programme settings on heat pumps as it is for a boiler. However there are a few more advanced controls to consider here. We start of course with the heat pump mantra that, “the lower the temperature of the water going through the system, the more efficient it will be”.

This is borne out by the excerpt below from the DECC heat emitter guide which shows how seasonal performance (SPF) increases as flow temperature decrease:

Heat emitter guide

The two basic settings on a heat pump that can be adjusted to change this balance are:

1. The heat curve (weather compensation). This sets the water temperature for heating depending on the outside air temperature.

The angle of this curve changes as you set the curve for your system. The curve below is 0.6, which is suitable for underfloor heating. Curves greater than 1 give you a much flatter curve suitable for radiators or other heat emitters.

2.the room temperature setting. This raises or lowers the water temperature to give you the room temperature you require.

Lowering the water temperature by adjusting the curve and the room temperatures has a major impact on efficiency, but at the cost of heating response times. This is where a balance with the client’s lifestyle requirements must be made.

3. I would also always advise the use of a set-back temperature for those times when a customer is out or in bed. Ideally in the UK this should be about 3-4 degrees lower than the occupied daytime temperature. Rather than turning the heating off altogether during these times it is better with heat pump systems to reduce room temperatures to a level a level which can be quickly recovered. This needs to be monitored to ensure the right balance of customer comfort and efficiency.

You can find useful videos on how to programme Stiebel Eltron pumps with these settings on our YouTube channel.

Some settings for advanced fine tuning of heat pumps

Beyond these more commonly adjusted controls are a number of additional refinements to consider:

1.Kelvin Minutes
This setting governs how hard a heat pump works. It looks at the temperature of the water in your heating system and compares the set temperature (decided by the heat curve and outside temperature) with the actual water temperature. If the temperature is close to the set temperature it waits a relatively long time before recalculating and turning the pump up to speed heat up or down if the set temperature has been reached (to see how little effort is required to maintain the set temperature). If the differential is large it waits a short time before recalculating and adjusting the effort up or down. So for example if the temperature of the water is rising 2 degrees every minute, the controller might add 2 minutes to the time before recalculating the effort required. As the water comes closer to the set point the intervals between recalculations get longer and longer. There is a manual setting from 0-500 that allows you to influence the intervals between recalculations – so if we take our earlier example we might slow the response so that the system adds 4 minutes instead of 2 to its thinking time when the temperature is rising at 2 degree every minute Slowing the response like this will have a beneficial effect on efficiency as the heat pump will work less hard – but of course this can be at the expense of response time. This is where buildings with a large thermal mass (think those with thick solid stone walls) can benefit. Because they have a high thermal mass they will cool down relatively slowly and you can afford to be more conservative about how responsive you are to fluctuations in outside temperature, without risking sudden drops in the internal temperature. For example, these buildings may well manage to retain heat during a short cloudy or showery spell , compared to a lightweight timber-framed construction in which the temperature drop may be felt as unacceptable.

2. Summer mode
This setting takes an average temperature over a given period of time to decide if the weather is warm enough to switch off the heating altogether and provide only hot water. If this setting is enabled the heat pump won’t bother to respond to short periods of cooler temperature – perhaps in the early evening or morning - when the overall perception is that the weather has warmed up. I often see installations where this setting has been left turned off and I would always advise setting it up to take advantage of spells of fine weather in spring and autumn and avoiding turning it on during short spells of cooler weather in summer . A client contacted me not long ago to ask why his buffer tank was warm in summer and the reason was simply that without this mode set “on” his pump had responded to some short cooler spells. My advice is to set it to monitor a 24 or 48 hour period and enable the mode when the temperature has not fallen below an average of 18 during this period.

3.Pump cycles
The pump that circulates the hot water in the system can simply run continuously when the thermostat calls for the heat to be turned on. However, it is more efficient to set a pump to “pulse”, running for a few minutes at a time, a certain number of times an hour. This both saves energy and can extend the life of your system. The potential risk in this approach is that farther apart the pulses, the greater the risk the building suffers unacceptable fluctuations in temperature. Gradually testing how far apart pulses can be timed without a noticeable impact on comfort is a key part of an advanced fine-tune of a heat pump’s performance.

So in summary there are a multitude of variables that affect the efficiency of a heat pump – as there are for any heating system. And in fact there are even more settings than I have shared here, which can be used to adapt them to their specific task. If you want to know more then join us on our free workshops for installers. With a Stiebel Eltron heat pump you have a great deal of flexibility and control to monitor performance and make a difference to the efficiency of a client’s system over time. Whether you do this for a client – either as a chargeable service or as a service differentiator to win future business – or pass on the information to them to help them manage their own system is of course up to you.