As more home owners turn to plumbers to recommend gas water heaters, plumbing connection asked Rheem training manager Jon Palfrey to explain the differences between the three most common technologies and how to decide what’s best for your customers.

Whether on reticulated natural gas or in a dwelling supplied by ‘bottled’ gas, it’s becoming increasingly common that a customer will request your expertise and recommendation on the type of gas water heater that will best suit their needs and expectations.

Selection of a water heater needs care as the wrong decision can lead to years of unsatisfactory supply of heated water or a system that can allow high running costs. This article will explore the basic differences between the three gas-type domestic water heaters available in today’s market – mains pressure gas storage, continuous flow gas and mechanical/instantaneous gas systems – and will explore the differences and benefits of each to provide you with the necessary information to recommend the most suitable gas option for your next water heating job.

Information available in many forms today allows customers to gain some understanding of system choices relating to services in their home. Water heating is often something that’s out of mind as long as there is sufficient delivery during peak demand periods and when hot water is needed quickly and at the desired temperature.

Water heater selection and recommendation then falls into two categories; replacement and new dwelling installations. It’s the replacement installation which often has more constraints than a new dwelling installation and sometimes can be problematic in endeavouring to meet a customer’s needs, expectations and previous water heating experiences. If some alternatives are considered and the customers’ needs factored into the recommended option/s, a better more cost effective outcome can often be found.

The following information is designed to provide the basic details outlining the three domestic gas-type systems available to the market today and to explore the reasons why each system can be considered for the various sized dwellings now found in metropolitan and regional areas of Australia.

Making the recommendation…

In most situations when a water heater installation is needed by a customer, the advice and recommendation of the plumber gasfitter is taken with justified authority and trust.

The customers’ expectations can be accurately met when you, the plumber, are in the position of providing a considered recommendation for your customer. The following key questions when initially discussing a new or replacement gas system will ensure a satisfied customer who has their hot water expectations met:

• Has the old system met your needs, particularly during peak demand periods? If so, a same for same installation can be the most suitable outcome.
• Are you looking to reduce running costs with a more efficient system?
• Does the customer’s usage patterns provide for one or more options?
• What are the customers’ expectations relating to performance, recovery/re-heating, multi point use, delivery time to fixtures, system position etc.?
• Are the household’s usage patterns changing possibly with a growing (or shrinking) family or with teenage children?

Let’s explore the three gas choices for domestic water heating requirements.

Mains pressure gas storage

Mains pressure gas storage systems, as their name suggests, deliver the water pressure that is plumbed to the unit via cold mains supply so they can serve several taps at once without a significant loss of flow. They are very popular when replacing an existing gas storage unit particularly when the customer is seeking the same performance in terms of simultaneous flow to fixtures, and the combination of pressure and temperature.

A gas storage unit holds a quantity of hot water in a thermally-insulated cylinder ready for immediate use. As heated water is drawn from the system, cold water enters to replace it. During operation, reheating (recovery) continues after the flow of hot water has ceased to bring the tank temperature back to a normal setting of 65ºC (75ºC in stainless steel cylinder models).

In a well-designed water heater, hot and cold water will co-exist for a period of time without mixing. This allows the maximum volume of heated water to be drawn off at full temperature, providing maximum time for incoming cold water to be heated.

Referred to as the displacement principal, this design is fundamental to the performance of a mains pressure storage system and is one reason why it can provide more flexibility in performance than continuous flow or instantaneous systems.

All Rheem, Vulcan and Aquamax mains pressure gas storage systems, for example, work on the displacement principal.

With advances in flue design and cylinder insulation, high efficiency, mains pressure delivery as well as quick recovery are all intrinsic benefits of this system type. The best performing gas storage systems (e.g. Rheem Stellar) recover as much as 200L/hour to full temperature – critical in winter to ensure your customer has no shortage of hot water supply.

With no need for electrical supply, labour and material costs in a replacement installation are minimised which is often popular with the customer.  When replacing an old two-star energy rated gas storage system, new four- and five-star models can reduce gas consumption by up to 21% while supplying the same amount of heated water per day.

Compatible with any tapware, mains pressure gas storage systems are a popular choice to meet varied household usage patterns. With homes now being built on a larger footprint and with more fi xtures and distance between points of use, a mains pressure gas system positioned close to the kitchen and the main wet areas is a good choice to provide hot water as quickly as mains pressure water will flow throughout pipework to fixtures.

Gas continuous flow

Continuous flow gas systems have been available in Australia since 2002 and have seen continual design improvements since their introduction. Six-star energy-rated gas continuous flow systems are a popular energy efficient option for the supply of domestic hot water.

Small in size and now with fluing options to allow installation in confined spaces (e.g. balconies) gas continuous flow systems meet most households’ hot water needs where space/footprint is limited.  Internal systems with co-axial fluing are also available providing an option where the internal position has limited fixed ventilation provisions.

When installing a gas continuous flow system, care must be given to correct fitting line sizing and gas meter or gas bottle supply volume due to high mega joule consumption when operating. Electrical power is required for ignition and component operation (i.e. the fan). Both factors can make these systems a more expensive alternative in replacement installations.

Rheem’s continuous flow systems are advanced gas appliances that enable heated water flow and inlet and outlet temperature to be constantly measured to ensure consistent temperature and flow from the unit at all times.

Once the minimum flow rate of around 3L/min is achieved through the system, the unit continuously heats water (hence the term) as it passes through the heat exchanger. Consideration must be given to appropriate tapware as some low flow taps can create resistance to water flow that could cause issues with system performance.

Factoring that most people shower around 40ºC, it can be determined if the number of showers in the dwelling will have sufficient flow from the water heater to meet the household needs and therefore if the system’s flow rate capacity can support the customers expectation of simultaneous fi xture operation.

While a gas storage system holds a volume of heated water at a minimum of 60ºC to be readily supplied at mains pressure, a continuous flow system is measured by its flow rate capability in litres per minute heated at a 25ºC rise from incoming cold water mains supply.

The continuous flow system will if necessary reduce water flow to achieve the required temperature rise so in winter when cold water supply temperature can be below 15ºC, the flow rate can be reduced.

Mechanical/instantaneous gas

Mechanical/instantaneous gas water heaters can be a simple means to provide heated water for small to medium sized domestic applications with minimal fixtures.

Not requiring electrical power to operate, they are popular for ease of installation and reliable basic operation as they lack some of the advanced electronic operating functions of gas continuous flow systems.

While mechanical/instantaneous systems may offer preset hot water temperature options, you will need a suitable tempering valve to be installed where required by AS3500.4. Recent improvements in technology relating to componentry and function mean these systems now have some performance benefits equivalent to continuous flow gas models such as improved temperature control, accessible temperature adjustment, flue terminal design for more consistent operation in high wind areas and multiple water temperature monitoring points within the system.

As an example, the Rheem Pronto instantaneous is such a system and can be considered not only as a replacement option for previous generation models, but also as an alternative to other gas systems with a potential savings in installation due to no power requirement and lower gas consumption compared to some continuous flow models.

These units have energy efficiency rating of 5.8-6-star and a 7% reduction in gas consumption applies per star when replacing older models.

So the quick comparison…

Some system benefits will outweigh others. Balancing your customer’s needs, expectations and installation cost will allow a considered recommendation to be made.

Lower running costs, limited space and footprint and the availability of touch pad temperature control are factors that would point to a customer having a gas continuous flow installed, providing its size and flow capacity meet their delivery performance requirements.

Where mains pressure flow and high temperature to kitchen and laundry outlets are needed, combined with a user adjustable thermostat as well as lower supply and fit costs are key factors, a gas storage system is ‘hard to beat’.

For a smaller dwelling where electrical power is not available where the unit is to be installed, a gas instantaneous model could be considered suitable particularly if it was remotely positioned from the main wet areas of the house, possibly as the second water heater option for an ensuite bathroom.

Often a same-for-same gas system replacement can be the most suitable recommendation and easiest/lowest cost option.

Please remember to remind your customers once that you have installed the new system that all gas water heater options require servicing to guarantee consistent, efficient and durable operation. Service requirements relating to how often and what needs checking are typically all listed in installation instructions/owners’ guides.

Rheem training…

For more technical detail regarding the Rheem gas range, Rheem E-learning training modules are now available on the Rheem website.

Training modules currently on Rheem gas continuous flow systems will shortly be followed by modules on Rheem gas storage, instantaneous, heavy duty and eventually commercial systems.

Completion of each training module (currently there are six) accrues one CPD point in those states where ongoing training is necessary. A certificate of completion is available from the associated training module site.