Rinnai has solution for limited gas supplies

Rinnai, DHW
Continuous-flow water heaters can be used where the gas supply is limited with the help of a small buffer vessel to smooth out short-term peaks in demand.

The benefits of Rinnai’s continuous-flow water heaters to deliver domestic hot water can be brought to refurbishment projects with a restricted gas supply by using the company’s pack system to add a buffer tank to a bank of heaters to meet peak loads. The concept is especially applicable to hotels, where there can be very short spikes in demand during peak periods, and could reduce the size of the required gas supply by over a third.

Ian Jenkins, technical support manager in the UK, suggests that the capacity of the buffer tank added to a bank of water heaters should be 15% of the peak-hour load to meet short, second-by-second, demands placed on the system. He backs up this suggestion with a detailed analysis of hot-water requirements for a hotel.

The peak-hour load for a 3-star hotel is 35 l per person, rising to 45 l/person for 4- and 5-star establishments.

Continuing the analysis for a 3-star hotel with 100 bedrooms, each occupied by two people, gives a peak-hour load of 7000 l.

‘But,’ asks Ian Jenkins, ‘how likely is it that the hotel will have 100% occupancy? Virtually never. Most of the time it will be between 75 and 80%. This is called the diversification factor, which gives a peak-hour load of 5250 l with a diversification factor of 75%.’

Five Rinnai HDC 1500 condensing units will deliver 4800 l/h at a temperature rise of 50 K to give an outgoing temperature of 60°C and a return temperature in a recirculation system of 55°C to satisfy legionella-control legislation. The combined maximum flow of these water heaters is 1.33 l/s, which may not be enough to meet the second-by-second demand — hence the need for a buffer tank. When demand falls or there is no demand, the cylinder temperature will be topped up by the heaters if required.

More water heaters is another possibility, but Ian Jenkins suggests that eight would be needed, requiring a 60% larger gas supply — which may not be available for a refurbishment project.

Rinnai pack systems can comprise the required number of heavy-duty water heaters with flue kits (for internal installation) or pipe cover boxes (for external models) and sufficient valve packs. Also included is a hot-water storage vessel of 300, 500 or 800 l, as required, with the necessary valves and primary pump with valves and an unvented kit.

A pack is completed with a 50 or 80 l expansion vessel, two drain cocks, tundish, temperature and pressure relief valve, control thermostat and overheat thermostat.

For more information on this story, click here: March 2012, 80
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