How to solve water heater oversizing


Over the past two decades, developments in water heater sizing methods have changed the industry significantly. Long-winded, manual calculations have made way for today’s quick and easy online tools. Chris Meir, sales director at Andrews Water Heaters, reflects on how sizing practices have changed over the years and discusses the solutions available today.  

When Andrews Water Heaters launched in 1976, the specification of water heaters was based on the closest match for demand, rather than accurate sizing calculations. Heating engineers would calculate the size needed – from a much more limited product portfolio – based on estimates provided in reference guides. How times have changed.

The first reference points available to engineers and installers to size water heating systems properly came in the form of CIBSE guidelines and the Andrews Water Heaters Sizing Guide. Originally, water heaters were selected based on the demand of the building they would be installed in, with the flow rate calculated based on ‘demand units’ such as basins, showers and baths. There wasn’t as much diversity in system specification as there is today and many systems were oversized. After all, end users were unlikely to complain about having too much hot water, and energy efficiency was not as high on the agenda as it is today.

But oversized systems are problematic, even if end users don’t realise it. The capital outlay is greater on larger plant, more energy is used in operation, and, if the demand for hot water is not there, it will cost the end user more in energy bills. This is all before we consider that larger plant takes up more space, and can therefore cause installation issues in applications where a very limited area has been allocated for housing equipment.

Andrews Water Heaters saw the issues of oversizing in the industry and recognised the need for a handy tool which could accurately size products. So, in 1996, the Size-it tool was launched to the market. It was the first sizing tool of its kind for the UK water heater industry, and remained the market leader. The way it worked was simple – users just inputted their hot water requirements depending on property type to determine appropriate products that would meet their needs.

Since its launch, a handful of other sizing tools have come onto the market as more manufacturers aim to combat the issue of oversizing. However, more still needs to be done to assist engineers with straightforward, accurate sizing calculations while on site. This is why Andrews Water Heaters has released a revamped version of its original Size-it tool. The launch is the latest milestone in the company’s successful history and coincides with its 40th anniversary celebrations.

Size-it calculations are based on the hot-water requirements guidance in CIBSE’s Guide G Public health and plumbing engineering and Andrews Water Heaters’ 40 years of experience in hot water generation. Available for free via the Andrews Water Heaters website and, the improved tool is quicker, personalised and now accessible online on any device.

It also allows users to manage and share a portfolio of projects and tailor the sizing to their exact property type and hot water requirements. The new Size-it links to the most up-to-date information on CAD, BIM and ErP labels when recommending a product to allow for simpler specification, and all information given can be easily downloaded as a summary or detailed report. What’s more, users can earn two efficiency credits, which can be used to increase a project’s BREEAM rating, just by using the tool*.

However sizing tools develop in the future, it’s important that they remain as user-friendly as possible and are accessible to everyone, everywhere. They also need to make complex calculations straightforward so that busy professionals can get the required information, and match the right product to the right job every time. This approach benefits all parties – specifiers, installers and end users – and from a manufacturer’s point of view it ensures that our products are chosen for applications where they can deliver hot water in the most efficient way.

For more information and to try out the new Size-it please visit:

* When assessing the expected performance of water heaters for compliance with Building Regulations Approved Document L an additional 2% heat efficiency credit is given for the use of manufacturers’ water-heater sizing guides.

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