Many of my clients are fortunate to live in big houses, however it will come as no surprise that these have their own specific problems when it comes to home energy use. One issue that is almost inevitable in a larger property is finding an appropriate solution for getting hot water from the cylinder to the taps in less time than it takes Donald Fagen to release a new album.
In smaller homes, hot water is typically supplied either direct from a combi boiler or from a hot water cylinder straight to the tap or shower head – there will be a small delay whilst the hot water travels along the length of pipework but this is usually quite manageable.
Once a house reaches a certain size, the time taken for hot water to reach the tap can start to become an issue, with an increasing amount of water wasted in the run-off (a particular concern for those on water meters) and heat often wasted in the pipework once the tap is turned off.
The traditional solution to this problem is a design called secondary pumped circulation – this arrangement involves replacing the “one-way street” with a separately pumped secondary circuit from the cylinder around the house, with spurs to each of the tap points and the shower heads. (This is not to be confused with the primary circuit which supplies heat from the boiler or heat pump to the cylinder.)
A pumped secondary circuit ensures that hot water is quickly available on demand however it can come with a significant downside – energy efficiency. We usually find that two major issues which this design, the first being the timing of the pump. More often than not, the pump is set to run continuously day and night, regardless of the demand for hot water in the household. As a result, hot water can be pumped over large distances 24/7, resulting in unnecessary energy consumption from the pump and heat loss from the pipework. When the cylinder is not being topped up from the boiler or heat pump then the supplied water will get progressively colder through the day.
The second compounding issue is a lack of insulation on the secondary pipework. This is very common, especially in older properties and can effectively act like an uncontrolled underfloor heating system – not particularly helpful at any time, but especially not where there is no call for space heating in the warmer months. In the worst cases it can make a considerable contribution to overheating and discomfort.
It can be challenging to put a number on the additional energy consumption caused by poorly-managed hot water, however in one case where we asked the homeowner to monitor their gas consumption over a few summer days it was apparent that their constant hot water circulation was more or less doubling their annual gas bills, with only a small fraction of that being useful hot water draw-off.
The solutions are pretty straightforward. The secondary pump can easily be fitted with a timer that can be programmed to run during periods of typical hot water draw-off, for example early morning and evening. Hot water will remain available at all times, however it will take longer to arrive at the tap outside of the pumped periods – if desired, this delay could be mitigated by fitting an in-line instantaneous water heater under bathroom sinks and using a boiling water tap in the kitchen.
An alternative is to fit a smart circulation pump such as the Grundfos Comfort Autoadapt which learns patterns of draw-off over time and regulates accordingly. Grundfos claims a two-thirds reduction in heat loss compared to a continuously circulating pump.
The final step is to insulate as much of the secondary circuit pipework as possible. In many cases this will be more challenging as it will be buried under floorboards, however pipework is sometimes routed through accessible eaves or loft voids. When and where renovations are taking place and pipework can be accessed in voids, the opportunity should be taken to insulate the pipe runs.