Hot Water Made To Order
By ROB WATERSGreen Technology Heat Pumps Heating Hydronics Plumbing Solar
At one time, selecting a water heater was a fairly simple process. There were not a lot of options to choose from; you got a tank type water heater, which was either gas or oil fired, or electric. The water heater efficiency and space requirements were not usually considerations.
Fast-forward 30 years and the story is much different. Now there are many water heater options available, and new efficiency standards are rapidly changing the dynamics of the industry. Customers are demanding much higher efficiencies, multiple venting options, smaller space requirements and remote controls. New suppliers have entered the market from Asia and Europe, challenging the traditional North American suppliers.
DIFFERENT DOMESTIC WATER HEATING OPTIONS
Tank type water heaters are still the dominant player in the marketplace. These water heaters are a simple technology everyone has been used to for many years and they are available in multiple fuels. They are typically the least expensive of any water heating option and are readily available. Tank type heaters are fast to install, easy to service and retrofit, and have lots of size options to meet any project’s requirements. Due to the stored volume of heated water in the tank, satisfying large DHW loads is rarely a problem and short cycling of the burner is not an issue.
On the flipside, tank type water heaters require a large amount of space, and have lower efficiencies and a shorter lifespan than other options. With space being at a premium in most new construction projects, size can often be an issue. Efficiency is quite low with non-condensing units having Energy Factors (EF) in the 0.65 range.
Storing a volume of hot water in the tank all the time, even when it is not being used, results in high stand-by heat losses, which results in low efficiency and higher fuel bills. There are currently some high efficiency condensing tank type water heaters on the market with EFs up to 0.96, however they are not as widely used due to their high cost.
The efficiency requirements of non-condensing water heaters is about to change. New water heater efficiency standards coming down the pipe in the U.S. and Canada will significantly change the way tank type water heaters will have to be constructed. When the new water heater standards come into play, the tank type water heater will gain in the efficiency, however it will not be as inexpensive an option as it has been in the past.
Time will tell how much of an impact the new efficiency standards will have on the dominant market share that tank type water heaters currently hold.
Tankless instantaneous water heaters from Asia and Europe started appearing on the market about 30 years ago. They have been slowly gaining popularity ever since. There are now numerous manufacturers offering tankless water heaters in an array of sizes and features. Their main appeal is the small amount of space they take up in the mechanical room, and the lower fuel consumption compared to tank type water heaters.
Floor space is at a premium in many new homes and condos, making tankless heaters a great choice for many architects and builders. Tankless water heaters are efficient and virtually eliminate stand-by heat losses. They are available in both non-condensing with typical EFs around 0.80, and condensing versions with EFs around 0.95. The heaters produce water on-demand, so if the unit is properly sized, the customer should never run out of hot water. There are multiple venting options available with most offering direct sidewall vent with intake air connection for sealed combustion. Some tankless models are now offering Wi-Fi connected controls that provide a new level of customer interface.
In spite of their appeal, there are some drawbacks to tankless water heaters. They are typically much more expensive than traditional tank type heaters. In addition, if a tank type heater is being replaced with a tankless, installation costs can skyrocket due to the bigger gas line that is needed and vent pipe requirements.
A tankless heater has a limited output capacity (gpm flow rate) which depends on its size, and if not sized properly, the customer may not be happy with the hot water output rate when there are multiple draws of water. This has given the heaters a poor reputation with some. Whereas a traditional tank type heater is a familiar and simple technology to most contractors, a tankless heater may be more challenging to service. It has many more working components packed in a very small box.
While scale buildup can be a problem for any water heater, it can be especially problematic for a tankless. Any buildup of scale in the small passageways in the heat exchanger can wreak havoc on the heater’s performance and longevity. Being an instantaneous water heater, a tankless can be susceptible to short cycling of the burner when there are multiple short draws of DHW. This constant ON/OFF can lead to more wear and tear on all the components and the increased possibility of breakdowns. The constant flow created in a DHW recirculation loop will often cause frequent ON/OFF cycling in a tankless. Some tankless manufacturers have introduced hybrid models that use a tankless heater built on to a storage tank to try and avoid these cycling issues.
In Europe where hydronic heating is king, indirect water heaters are very popular. Here in North America however indirect water heaters are not nearly as widely used. The exception would be when hydronic heating is being used in the building. Indirect water heaters do not have a combustion chamber or element, but instead rely on an internal heat exchanger coil to transfer heat from the heating boiler to the DHW water in the tank.
Indirect water heaters typically have very long lifespans, have minimal service requirements, and do not require a separate vent pipe or fuel connection. It is not uncommon to have a high quality stainless steel indirect water heater last 20 years or longer. The efficiency of heating water with an indirect is very high, especially when combined with a gas-fired modulating condensing boiler. Because there is no vent pipe through the centre of the tank, the stand-by heat loss from an indirect is extremely low. Any type of fuel fire boiler (gas, oil, electric) can be used.
The main drawback of indirect heaters is that they are only applicable to homes that use a hydronic boiler. This limits their use to only a small percentage of Canadian homes. They are more expensive to purchase than standard direct fired water heaters, and they do require specialized hydronic piping skills for installation.
Solar water heaters use roof mounted flat-plate or vacuum tube solar collectors to collect energy from the sun and transfer it to the storage tank. The biggest appeal of this type of technology is that the source of energy is free from the sun. This reduces the usage of fossil fuel and results in lower greenhouse gas emissions. Solar water heaters can also be expanded to work with hydronic heating systems to supplement the space-heating load.
These heaters have an expensive upfront cost. They also still require a separate auxiliary water heater to provide backup, as solar cannot provide 100 per cent of the year round load. Solar panels must be mounted on the roof in a suitable location and this may not be applicable to every site. Installation and service do require specialized hydronic skills.
Heat pump water heaters use a small top mounted air-to-water heat pump to heat the water in the storage tank. They also use electric elements to supplement the heating of the domestic hot water. The biggest advantage of a heat pump water heater is definitely its efficiency, with EFs up to 3.0. Standard electricity supply is used, with no vent pipe required. They can be utilized in homes with a renewable electricity supply such as a solar PV, making them a very green water heating option. Remote control with Wi-Fi is also an option for some units.
These water heaters are more expensive when compared to other electric water heating tanks. They are more complicated to service and will require refrigeration skills if a compressor were to fail. One of the biggest drawbacks when these tanks are used in Canada is that in the winter months they use heated air from the basement as the heat source. This can cause the area around the tank to become cool, and this air must be reheated by the heating system. In the summer months this is not a problem as this cooling effect becomes a benefit.
Hydronic heating boilers that have a separate domestic water heat exchanger built in are known as combination boilers. Both space heating and domestic water heating can be provided from the same device. These types of devices have been widely used in Europe and Asia for many years, but are now starting to become more common in Canada. The appeal of combi-boilers is their potential to save space in the mechanical room, with one small appliance providing both space and water heating. Combi-boilers offer the ultimate flexibility with their hydronic heating capabilities, and small footprint. With a condensing boiler the water heating efficiency is high.
The drawbacks of these units are similar to those seen in tankless water heaters and indirect heaters. They can be susceptible to ON/OFF short cycling and scaling damage due to their small heat exchangers. Most service technicians will be unfamiliar with these units and service will be more involved than some other options. They may also be limited in their domestic water output capabilities and in their ability to provide a domestic water recirculation loop. Since they will be primarily used in buildings using hydronic heating, this does somewhat limit their application.
So there you have some of the many equipment options available for domestic hot water today. Choosing which option is best for your project will depend on what the customer’s priorities are: upfront cost, fuel costs, space requirements, fuel options, maintenance and lifespan. Availability of some of the new technologies can also be an issue, as wholesalers are often reluctant to stock new technologies.
While selecting a water heating appliance is not as simple as it used to be, it is nice to have choices.
Robert Waters is president of Solar Water Services Inc., which provides training, education and support services to the hydronic industry. He is a mechanical engineering technologist graduate of Humber College and has over 30 years experience in the hydronic and solar water heating industry.