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Article Date: 25th January 2017

Spirax Sarco - Hot Water Buffer Vessels - It’s not a Binary Choice

Hot Water Buffer Vessels - Buffer Vessels - Steam Plant

Spirax Sarco - Hot Water Buffer Vessel

The debate over whether commercial buildings need a buffer vessel or not continues to divide the heating industry, with one side putting up a robust defence and the other adamant that they should be consigned to history. Paul Hardman, Regional Business Development Manager at Spirax Sarco, argues that the use of buffer vessels is not a binary choice.

For a piece of equipment that is, essentially, a large water storage tank, the arguments for and against the use of buffer vessels in commercial buildings continue to be hotly debated.

The decline in the use of buffer vessels in recent years, much like the decline in domestic hot water tanks as the British public have opted to use the space for a walk-in shower, has led many to believe that they can be consigned to history.

Whilst I agree that there are fewer and fewer applications that require a buffer vessel, I am not so quick to give them the last rites.

Context is everything, and it is important to understand the sort of applications in which buffer vessels have traditionally been used, and their limitations.

From a cold condition, with a low heat input, the water in a buffer vessel can be heated in circa one hour, which has the advantage of not requiring much in the way of boiler or steam power.

However, inevitably there are going to be applications that need hot water much quicker than that, but in order to do this, a greater heat input is required.

Years ago, this led to the development of the load-leveller combination of storage vessel with a higher heat input that is capable of recharging the vessel in 15 minutes, but is also capable of providing some instantaneous water. The higher the heat input delivered by a higher boiler power, in turn delivers more steam.

Sizing the system for this potential instantaneous hot water demand can be problematic. In a hospital, for example, the heat input could range from 30-40kW up to 400-500 kW. Often the hot water demand could be perceived to be very high but, in practice, the actual demand is much, much lower.

This differential occurs because the guidance that is available is, I would argue, not specific enough to healthcare premises. For example, BS EN 806: Specifications for installations inside building conveying water for human consumption, is often used by design engineers to calculate water demand in buildings based on a loading units approach. However, for health facilities and similar this presents figures which are very high and the reality is often different.

For example, a 28 bed standard ward in a hospital built over 20 year ago would traditionally have two bathrooms available and maybe eight or so wash hand basins, but many modern hospital wards of a similar size now have upward of 20 en-suite facilities and numerous wash hand basins. Yet the amount of hot water used on these modern wards has generally stayed the same but, because guidance is based on an outlet point’s count, a calculation for today’s ward gives a much higher anticipated demand.

Spirax Sarco - Hot Water Buffer VesselThe difficulty of determining hot water demand in a modern commercial building, such as a hospital, is one of the biggest headaches for consultants. The traditional buffer vessel is no answer to this problem, as they are unable to deliver the right amount of instantaneous hot water, but equally they may lead at times to a large volume of stored water with limited turnover. Also, stored water vessels require much more maintenance in line with 1. "L8: Legionnaires disease - The control of legionella bacteria in water systems" (as well as the associated HSG274 part 2.) and 2. "Hospital Technical Memorandum (HTM) 04-01: safe water in healthcare premises".

There are however other downsides to buffer vessels. Firstly, they take up a lot of space which, in a commercial building, is expensive real estate that could be utilised in other ways rather than as an area for a very large water tank. Secondly, they are very heavy, which adds costs during construction and they can also be difficult to maintain, with a requirement for separate vessels to ensure the continuous availability of hot water during any maintenance procedure. Maintenance is important, because the relevant guidance as detailed previously, says that buffer vessels have to be cleaned regularly.

Nevertheless, the biggest risk factor is unquestionably the potential for legionella, which I believe is probably the major reason why buffer vessels have fallen increasingly out of favour.

Buffer vessels have a perceived legionella risk, particularly those with a horizontal design that risk leaving a stagnant water area at the bottom of the tank, a concept highlighted in L8. For the buffer vessel to be safe, all the hydraulics of the system have to be correct, in order to ensure that the water is maintained at the correct temperature throughout the system as detailed in both L8 and HTM 04-01.

So, are buffer vessels a bad idea and should nobody use them? In my opinion buffer vessels do have a role to play in specific applications e.g. in a factory or food processing environment that requires a regular end-of-day wash down of the facilities. Here the demand for hot water is easily calculated and the slow heating of water over an estimated one to two hour period can be easily and efficiently accommodated.

My advice is to take a pragmatic approach and do without a buffer vessel if at all possible, but there are bound to be certain applications in which high volumes of hot water are needed on a regular basis. In this situation, a buffer vessel can perform a very valuable role, offering an efficient way of storing the hot water that has been slowly and efficiently generated.

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