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Hot Water Heating Continuous Flow Systems shows dramatic savings – says independent report

A recent independent report emphatically concluded that continuous flow water heating systems have a clear advantage in terms of initial capital costs as well as life cycle costs over 20 years, when compared with traditional stored hot water systems*.

The study** shows that continuous flow water heating systems can be up to 7% more economical than equivalent ‘traditional’ stored hot water systems but that capital costs can be 41% less than indirect systems and a whopping 49% less than direct systems.

“In terms of maintenance you need to be realistic and not theoretical,” says Rinnai managing director Tony Gittings. “If a site has direct or an indirect system then it has to effectively shut down for maintenance. There’s no way around this problem. Not so with Rinnai continuous flow units as we cascade them together so that the demand for hot water can be met without closing down the whole site.”

AECOM, the world’s biggest building services consultants, was commissioned to carry out a totally independent life cycle study on continuous flow water heating systems, and to provide a comparison with conventional storage systems. Two case studies were provided on projects using continuous flow water heating systems.

AECOM then determined an equivalent storage based system for each case study and compared operational and capital costs.

For the life cycle comparison, the analysis period was 20 years, based on the expected service life. The net present value (NPV) calculation is based on a discount rate of 3.5% (The GREEN BOOK – HM Treasury), an inflation rate of 2% for servicing costs and projected retail fuel costs from DECC1.

In both case studies the continuous flow system showed a clear advantage in terms of initial capital costs, as well as life cycle costs over 20 years, with the continuous flow system consistently lower in energy use than the two storage systems.

Case study 1

This was a system for a typical, small, pre-fabricated fast food restaurant. It is assumed the system only serves the hot water demand.

The life cycle analysis includes the initial capital costs, projected annual fuel costs, and estimated annual servicing costs which are assumed to increase by 2% each year. The annual fuel cost assumes the same daily fuel consumption over the whole year.

The servicing cost is based on £130/year per boiler, and £160/year for a boiler and cylinder. The resulting net present values of the three showed that the continuous flow system is 6 – 7% lower than the two storage systems based on that 20-year analysis period.

Case study 2

This was a system for a shower block in a holiday camp, with six showers and four basin taps.

For the continuous flow option, the analysis is based on a configuration of 4 continuous flow water heaters that each have a nominal output of 48 kW, which, for this particular application, because there is a very short run of pipe work between the water heaters and the fixtures, it is possible for the water heaters to generate water at 40°C. The risk of legionella is overcome by the regular turnover of the system.

For the energy required to deal with the instantaneous hot water demand the analysis assumes that the bulk of the hot water demand would be due to the showers. Each shower would typically have a flow rate of around 9 litres per minute, which if mixed to 40ºC would equate to an instantaneous load of around 19 kW each, so even if there was only one shower running, the load on a single continuous flow water heater would equate to an efficiency of 95% based on the performance curve. As the hot water demand increases, the modular nature of this particular configuration would allow the 95% efficiency to be maintained for the bulk of the demand.

The difference in daily fuel costs between the three systems is around 7%. The usage profile for this case study is particularly suited to continuous flow systems (i.e. high but infrequent demand over the day), which is why there is an advantage in fuel consumption over the storage systems.

Systems employed in the study

* Indirect fired storage systems

This consists of separate heat source and storage cylinders, where for this study the heat source will be gas fired boilers running on 80ºC flow and 60ºC return with gross efficiency of 89%. As the hot water is stored at 60ºC in insulated cylinders there will be standing heat losses. The standby electricity is assumed to be 15W per boiler, and 60W when operating to cover fan and controls.

Direct fired storage systems

This is where the hot water storage cylinder has an integral gas burner to directly heat the water, and the key difference with indirect fired storage systems is that they are designed to operate in condensing mode and generally achieve gross efficiency up to 96%. A disadvantage of direct fired storage systems is that the standing losses are around 3 times higher than indirect storage cylinders.

Continuous flow systems

This is essentially a gas boiler that is designed to instantaneously heat mains water for supply directly to water fixtures without any storage. The key advantages with this type of hot water heating, is firstly saving in space needed for hot water cylinders and the associated standing heat losses. The challenge is that instantaneous hot water demand can vary hugely for most systems and therefore the continuous flow heating system needs to have a wide modulation range as well as maintaining efficiency over that range.

For more information on the RINNAI product range of high efficiency condensing continuous flow hot water heating units and systems visit www.rinnaiuk.com.
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Hybrid hot water solution tops the green bill

Rinnai’s Chris Goggin explains why hybrid technology is an attractive option for reliable and cost effective hot water provision as it becomes abundantly clear that renewables, on their own, just aren’t delivering.

Energy efficient continuous flow condensing gas fired water heaters clearly have their part to play as we head into ‘greener’ country. This ‘tankless’ form of hot water delivery is growing in popularity and offers huge ongoing financial benefits to building owners and tenants even before the advanced energy saving technologies of the units themselves are taken into account.

This makes the argument for their inclusion as a component in a hybrid renewables/gas boosted hot water system even more compelling. It is becoming abundantly clear that there is not one single method to achieve energy efficiencies as renewables on their own just aren’t delivering. There is no point in waiting for the perfect renewable solution to come along we can utilise a very effective mix of existing renewables and traditional technologies.

In effect we need to find ways of using current renewables technology, be it solar thermal, solar PV or heat pump technology, in a practical manner maximised by traditional fuel sources – in this instance, gas, whether Natural or LPG.

Modern renewables-ready heavy-duty condensing water heater units guarantee the highest efficiencies and therefore the lowest running costs of any commercial water heater. In short, a HE unit used as a gas booster for solar or ASHP systems maximises solar gain, as it only consumes energy when more hot water than the panels can cope with is needed. The rest of the time the unit is inactive.

The use of evacuated tube or flat plate solar collectors can make a big difference when paired with condensing continuous flow water heaters as a boost. Doesn’t it make sense to store the free energy collected by an array of solar panels within in a solar thermal store which can be heated to the required temperature on demand by a gas fired water heater or the water to the outlet can be easily boosted, taking advantage of the expansive range of modulation associated with Rinnai technology.

A gas fired continuous flow water heater is also the only booster that ensures never-ending hot water – no matter how fierce the demand. Fossil fuel is only used when the renewable store is below its set-point, thus reducing the dependency on fossil fuel and reducing CO2 emissions.

Easily ‘retro-fitable’ with current renewable technology, these units are a better environmental option than an electric booster. By using gas, a direct energy source, you can significantly reduce greenhouse gases.

Renewables ready Rinnai Infinity condensing HDC1200 internal and external models show an impressive energy performance of 107 per cent net efficiency while the larger Rinnai HDC1500 models turn in 105 per cent net efficiency*.

For large commercial, operations Rinnai has just launched the Infinity HDC1600i, its premium condensing water heater which has been tested to below 20 ppm NOx, producing the lowest emissions on the market, with 107 per cent gross efficiencies*.

Infinity HDC 1600 renewables-ready, retro-fitable low NOx series is the first heater of its kind to utilise pre-mix burner technology. The newly developed down-firing burner allows increased efficiency and reduces NOx gases. The patented burner also reduces the possibility of flame lift, which has previously been a problem with premix down firing burners. It has a 13-1 turn down ratio – the largest on the market – of 58.4kw-4.5kw and is ultra quiet in operation – so quiet that you wouldn’t know it was on unless you glance at the number on the display.

The Infinity 1600i low NOx series is available with an output of 58.4 kW producing 16.2 litres of temperature controlled hot water per minute (or 9.73 per hour) raised at 50°C. That is .2 litres a minute more than the existing Rinnai Infinity 1500 HDC.

*Tested and certificated by Technigas to EN 26.

For more details on RINNAI products visit www.rinnaiuk.com.

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