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发表于 2017-1-11 13:07
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在网上看到的对比：Continuous Flow Instant Hot Water
vs
Traditional Hot Water Tanks
When it's time to get a new hot water system, two of the most asked questions are:
1. “Should I get a traditional hot water tank or a tankless continuous flow system?”
2. "What size hot water system do I need?"
Both types of heaters supply hot water but they do it in different ways.
Hot water heater sizing is best determined by peak demand.
We review the two hot water system types and compare the pros and cons of both, as well as the prices, to help you make an informed decision...
Running Out of Hot Water
running out of hot waterTraditional hot water tank heaters can only heat a certain number of litres at a time.
Residential hot water heaters are typically in the 135L to 170L range.
Off-peak electric units need to have a much larger capacity, as they have to last through the entire day.
Once all of the hot water in the tank is used up, it takes time to heat up the fresh water entering the tank.
If it runs out right in the middle of a shower, you have an uncomfortable problem.
You can finish with a cold shower or just stay soapy while you wait for the water to reheat. Neither is a good choice.
With a continuous flow instant hot water system, the water is heated as you use it.
The unit can maintain the flow rate indefinitely.
So, you basically never run out of hot water, no matter how many people get to the shower before you.
Energy Consumption
Continuous flow gas hot water systems only needs to run when the hot water is actually required and wasteful pilot lights are a thing of the past.
On the other hand, with a hot water tank, you have to use energy to maintain the temperature of the stored water 24/7.
The technology and energy saving designs are also very different.
Energy Star Ratings
Star rating labelModern tankless continuous flow models typically have a 6 Star energy rating.
There are even 7 Star units now available.
Tankless systems are available in both natural gas and LPG models.
In contrast, most tank hot water heaters are only rated at 4 or 5 Stars.
This means both the energy consumption and the running costs will be higher.
Tank heaters are available in electric, natural gas and LPG models.
So, while we'd love to sell you more gas, we'd rather have you saving money and energy with the more efficient models.
They are also the best choice for the environment.
Size and Aesthetics
Tankless hot waterStorage tank hot water heaters are big, especially the electric off-peak models.
Even when installed outdoors, the size can get in the way, particularly with today’s smaller lot sizes.
Having this large object next to your house also does nothing for the appearance of your home.
At about the size of a small suitcase, tankless water heaters are much smaller than a traditional storage tank heater.
Mounted flush against the wall, the smaller tankless units take up less space and are also more aesthetically pleasing, as they don’t dominate the exterior appearance of your home.
CO2 Emissions
Greenhouse gas emissions are reduced through the use of tankless water heaters.
Tankless gas hot water systems produce about 75% less CO2 greenhouse gas emissions, when compared to a storage tank hot water systems powered by coal-fired electricty.
Even when compared to gas storage tank units, tankless units are more eco-friendly.
The 6 to 7 Star tankless models consume less gas and create less CO2.
Life Span
Traditional storage tank water heaters have a life span of around 10 to 12 years.
Warranties for storage tank units are typically 5 to 10 years.
Tankless hot water systems can last up to 20 years.
Warranties for tankless units are typically 10 to 12 years.
Water Quality
Water from a storage tank water heater may pick up bits of rust and scale that build up within the tank.
Since there's no tank to rust and no water stored with a tankless system, the water stays cleaner and fresher.
Speed
When you first turn on the tap, there is typically a delay in getting hot water, as the water in the pipes leading from the water heater to the tap will be cold.
Because it is kept hot, tank systems can be slightly faster in delivering the initial flow of hot water, as the tankless system takes a second or two for the unit to get up to temperature.
However, now there is a new system to eliminate the delay entirely.
The Rinnai Smartstart® Water Saver was developed to reduce water wastage whenever a hot water is turned on.
It pre-heats the water in the pipes before you turn on the hot tap.
It is part of the Rinnai Infinity range of tankless systems.
Initial Cost
In the past, there was a substantial difference in the price between tankless and storage tank hot water units.
However, the days of super cheap 3 Star storage tank units are long gone, as are their higher operating costs.
The difference has now shrunken to the point where there is almost price parity between comparably sized units.
This is especially true if you are comparing 5 Star tank units to the 6 or 7 Star tankless models.
Health & Safety
Storage tank hot water heaterWith storage tank water heaters, there is the potential for bacteria growth within the tank.
The World Health Organisation -- WHO -- recommends that stored hot water be kept at a minimum of 60°C.
Unless a minimum of 60°C is maintained, hot water tanks can be a breeding ground for Legionnaires’ disease, or Legionellosis, that is caused by a bacterium, Legionella pneumophila.
It is a respiratory disease that can cause severe pneumonia and is sometimes fatal.
However, Canadian studies have shown, even when the thermostat is set at 60°C, a high percentage -- approximately 40% -- of electric water heaters remain contaminated.
This is because of the lower temperature, of about 30°C to 40°C, at the bottom of the tank.
With tankless systems, the prevention of bacterial growth is not an issue, as there is no stored water in which bacteria can breed.
Tankless systems can also be set at 50ºC, so the risk of scalding is greatly reduced without the need of a tempering valve.
How a Tankless Water Heater Works
Schematic of continuous flow hot water heaterIn summary:
• Cold water flows into the water heater when a hot water tap is opened inside your home.
• The water flow is detected by a sensor that ignites the gas burner, to heat the water in the heat exchanger.
• The water typically follows a serpentine pattern through the heat exchanger, absorbing as much heat as possible.
• An electronic control unit modulates the gas burner to maintain a set water temperature.
• Available in LPG and natural gas models
See all the details on:
How a Tankless Hot Water Heater Works
Hot Water System Sizing?
Don't Calculate by Number of People or Bathrooms
The sizing information available is very confusing because manufacturers, and others, are trying to provide a simplified answer to a complicated question.
Some suggest that you calculate by the number of bathrooms in your home while others say to go by the number of people in your family.
Also, in many cases, their recommendations are based on the worst case scenarios, assuming higher consumption than your actual water use.
Buying Too Large Wastes Money
The result is that many people end up buying a larger system than they actually need.
This costs them extra money when they buy the hot water system and will increase the ongoing operating costs, in many instances.
Factors to Really Consider
Simultaneous use is one of the fundamental determining factors.
The number of people in your home combined with how and when they use hot water is the key.
Do they all take showers at the same general time of day?
Do they take showers simultaneously in different bathrooms?
Do the showers have WELS 3 Star Rated Showerheads (maximum 9L/Min) or standard 18 to 25L/Min showerheads?
Do some prefer baths, which use more hot water? Is the bath or spa oversized, requiring even more hot water?
Do you run the washing machine, using hot or warm water, while people are showering?
Is your dishwasher connected to the cold or hot water? If hot, do you use it while hot water is being used elsewhere?
Showers Are the Key
Showers use the most hot water in a home. People generally shower for 5 to 10 minutes.
Assuming you have a 3 Star shower head and you mix in some cold water with the hot, the typical hot water consumption would probably be in the 7 litres per minute range.
If you also assume an 8 minute shower, you will consume about 56 litres of hot water per shower or a family of four would consume 224 litres.
The use of WELS (Water Efficiency Labelling and Standards) 3 star rated showerheads is an important factor in saving water and energy whilst reducing the size and cost of the hot water system required.
Hot Water Tank ExampleRinnai HOTFLO 170 gas hot water system
In the above example, if all four people take their showers in the morning, you would only need a storage hot water system with 224 litre first hour capacity, regardless of how many take showers at exactly the same time.
This all presumes that you are not using any other hot water devices at the time.
The use of additional hot water, after the shower period, would depend on the hot water system’s recovery time required to achieve full temperature.
So, with a larger family or house guests, a bit of scheduling may be required to avoid the need for a larger unit.
Off Peak Electric Issues
Off peak electric storage units are a completely different story.
Inasmuch as they only heat water at night, they need to be quite large to make sure you don’t run out of water during the day.
So, some of the electric savings is offset by the fact that you need to heat a larger tank.
The bigger tank would also cost more to purchase.
Tankless Hot Water Example
Bosch 21e tankless gas hot water systemFor a tankless continuous flow unit, it would depend on how many showers are being used simultaneously.
Using the example above, if it’s two showers, then you only need a unit with a minimum flow rate of 14L/min.
If you have 3 people showering at exactly the same time, you would need a minimum 21L/min.
This does assume that you are not using other hot water devices simultaneously.
Recovery time is not an issue with continuous flow units, so even a large family with guests would not run out of hot water.
Compare Gas Hot Water Heater Systems Prices
Now you can compare prices for all the major brands of gas hot water systems including Rinnai, Bosch, Kelvinator, Rheem, Dux and AquaMAX.
Click on your category of interest to compare prices:
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