Putting gas boilers out in the cold

In Australia, New Zealand, Japan and, to a slightly lesser extent, the USA and Canada, along with almost every other technologically savvy country other than the UK, the practice of installing gas boilers and water heaters inside a building is considered rather unwise and certainly uneconomical.
In fact it is estimated that worldwide over three million external water heaters are installed every year. Safety considerations are sited as one reason; other countries see a high-output heating device with integral high temperature burners and a gas input supply as presenting an undesirable internal risk to property when it is not mounted outdoors.

When it’s put like that, the subject perhaps requires a little further investigation. But with modern hi-tec built-in product safeguards, and perhaps supplementary gas and smoke detectors, in the UK we see those risks as minimal, after all the use of gas cookers in kitchens is widespread and we don’t see many objections to that practice!

There is another aspect to the argument, which in these days of carbon footprint awareness is not so easily ignored and that is the clear cost saving advantage to mounting boilers and water heaters externally, derived primarily from the space that is saved by not having the equipment installed indoors. Even when the compact size of most heating units is taken into account the figures still add up. And with internal space costs spiralling, the savings can be considerable.

The solution lies with the principle of continuous flow water heating which has been around in the UK for many years and has already been applied successfully in many commercial applications. The gas, tankless water heater concept is simple; heat the water required just when it’s needed, only for as long as it’s needed, and when the water is turned off, the heater goes off. Of course, this means that neither a cold water header/expansion tank is required, nor a copper hot water cylinder, nor any of the associated pipe work.

A continuous flow water heater simply needs a cold water feed and a gas supply employing a close control system that is used to obtain very accurate temperature control of the hot water. This is achieved using microprocessors together with fast response sensors alongside fully modulating control of the combustion process. In combination with modern manufacturing techniques, this has enabled very powerful systems capable of continuously providing over 1000 litres of hot water per hour to be packaged into extremely small wall mounted units.

To see the complete picture, consider a continuous flow gas water heater mounted externally to supply a large commercial installation, like a hotel or leisure complex. First of all, with the manufacturing costs of such boilers at around 20% less, the equipment is significantly cheaper than those that are generally offered. Then there are the savings in floor space, and it now becomes much easier to site the water heater closer to the required point of delivery, with no need for a flue and generally less labour required to fit and commission. This can mean, for instance, in a hotel where pipe runs can be considerable and guests justifiably unimpressed with long waiting times for water to run hot, energy wasteful solutions like trace heating or pumped circulation become unnecessary. Moreover, for hot water supply, this technology even removes the need for storage tanks with their associated standby losses and their own considerable capital cost and raw material content. In some large building such as hotels, it is not unusual to place the tank on top of a building by crane (and this may necessitate closing the adjacent road) and then building the boiler room around the tank.

Most specifiers and installers might worry however that the external mounting of devices containing water has a number of possible drawbacks, notably freezing in winter. In practice, this never occurs due to a novel little device called a frost-stat! The energy consumption of the anti-frost heaters is minimal in comparison to the standby losses of a tank storage hot water system.

As with all new ideas, though, it takes time to change established, tried and trusted procedures. Manufacturers subjecting their equipment to rigorous weather testing at levels far in excess of anything that is likely to be experienced in practice generally provide the necessary assurance, like a simulated hurricane test carried out using salt-water spray, the result being that no reduction of operating efficiency can be recorded at all. In the US externally mounted heaters have been working successfully for years, despite regularly being subjected to minus 30ºC for three months at a time. Considering that 30% of Rinnai’s UK market is for external water heaters, to date there have been no problems with vandalism or theft at all, but in any event, guards, inset wall boxes and anti theft brackets are available. In effect there is an added benefit in that externally mounted units can be more easily accessed for servicing, with little or no disruption to normal business activities within the premises

Given that the advantages do seem to warrant pursuing these apparent benefits, how does the installer go about practically fitting such a system? Well, for new builds, the preferred method of externally mounting the smaller models of such heating equipment is by using a specially designed external mounting box, presenting a neat finish similar to that of an external utility meter box. This ensures that the heater and its water, gas and electrical connections are neatly and safely enclosed inside.

The enclosure does not contravene building regulations, and the fire rating of the wall will not be affected either, as long as it is not a boundary wall. Gas appliances cannot exhaust over a boundary and must be accessible for service, so mounting on a boundary wall is not an option in any event. This clearly would preclude its being fitted anywhere outside the perimeter of the premises.

Over the next few years, if all the new systems planned were switched to this method of water heating, then it would make an incredible contribution to the reduction of the final construction bill, even before any consideration was made for actual savings resulting from lower energy consumption. Employing this technology clearly provides a variety of ways to minimise the carbon footprint of the construction activity required. An external water heater in these circumstances replaces a tank full of hot water, in effect returning the dedicated space needed to house the tank to be used for other purposes, never mind the space normally taken up by a boiler and the associated internal pipework. And without a hot water storage tank, there is immediately no need to maintain its stored water at 60ºC (to be proof against Legionella etc) and hence all associated standby losses are eliminated. By reducing the cost of the provision of an everyday essential that we cannot live without – hot water – a real contribution can be made to the struggle against global warming. So, perhaps now is the time for a bit of thinking outside the box.

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