Instant hot water systems provide hot water on demand without the need to store heated water in a tank. They heat water as it flows through a heat exchanger or heating element. This allows hot water to be available immediately when a tap is turned on. While instant hot water systems have some advantages, they also have some downsides to consider.
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Higher Energy Costs
One of the main disadvantages of instant hot water systems is that they can use more energy than conventional storage tank water heaters. Because the water is heated instantly rather than stored hot, the system has to apply heat each time water flows through. This frequent heating can lead to greater energy consumption compared to heating and storing a tank of hot water.
According to the U.S. Department of Energy, instant hot water heaters have an energy factor (EF) ranging from 0.82 to 0.94, while storage tank water heaters have an EF between 0.90 and 0.95. The higher the EF, the more efficient the water heater. This indicates instant hot water systems are generally less energy efficient.
The amount of increased energy usage depends on factors like the climate, water usage patterns, and system settings. But energy costs are typically 10-25% higher for tankless water heating. For a household with high hot water usage, this increase in energy bills can add up.
Higher Upfront Cost
Purchasing and installing an instant hot water system also tends to cost more upfront compared to storage tank heaters. Here are some reasons instant systems have higher initial costs:
- The heaters themselves are more expensive. A tankless water heater can cost $1,000 or more compared to $300-$500 for a conventional tank heater.
- Larger or multiple units may be needed to meet household hot water demand.
- More gas pipes or electrical wires need to be installed to provide sufficient power for on-demand heating.
- Venting and water lines may need modifications.
- Installation is more complex, increasing labor costs.
When all these factors are considered, installing an instant hot water system can cost $2,000 or more compared to $500-$1,500 for a storage tank unit. The higher initial investment may take many years to recoup through energy savings.
Possible Loss of Water Pressure
Tankless water heaters provide hot water by sending cold water through a heat exchanger as it flows to the tap. But in some situations, the added resistance of the heat exchanger can lead to a pressure drop that decreases water flow at taps and showers. This loss of pressure and flow is more likely to occur with:
- High demand – When multiple water sources are used simultaneously, the heater has to work harder.
- Large homes – Longer pipe runs to distant bathrooms increase resistance.
- Old plumbing – Mineral deposits in pipes add friction.
- Insufficient gas line or electrical capacity – Underpowered systems can’t heat at full capacity.
Upgrading to larger diameter water lines or a more powerful instant water heater may be needed to maintain proper water pressure throughout the home.
Risk of Scalding
Because there is no water stored at lower temperatures, tankless heaters provide water at a consistently hot temperature when turned on. The typical output temperature of an instant heater is around 120°F. This high output, combined with sudden spikes if the unit is overpowered or cold water line mixing is inadequate, can create a scalding risk.
Proper installation is important to prevent scalding. But even then, anti-scald valves or thermostatic mixing valves are usually installed with tankless systems when the output feeds general plumbing fixtures. These valves mix in cold water to keep temperatures below 120°F. The valves add to installation costs and can fail over time.
Maintenance Requirements
To operate safely and efficiently, tankless water heaters need periodic maintenance. This involves tasks like:
- Inspecting and cleaning the heat exchanger.
- Removing mineral scale buildup.
- Checking and replacing air filters.
- Inspecting gas lines or electrical components.
- Checking and adjusting flow sensors.
- Cleaning and lubricating moving parts.
- Checking anti-scald valves.
Neglecting proper maintenance can lead to problems like scale buildup reducing efficiency, overheating and failure of components, and inability to provide adequate hot water. Most manufacturers recommend professional preventative maintenance annually, adding a recurring cost.
May Not Work Well in Cold Climates
One disadvantage of instant hot water systems is that they function less efficiently in cold weather. Colder inlet water requires more energy to heat. And electronics, sensors, and moving parts can be affected by freezing temperatures and fluctuations.
In addition, tankless heaters shut down when there is no hot water flow. In very cold weather, residual water in pipes and heat exchangers can freeze. This can damage plumbing lines and the heater itself in climates where temperatures frequently dip below freezing.
These cold weather issues reduce efficiency and can lead to breakdowns. Some tankless models offer cold weather packages with insulation, heat trace lines, and recirculation pumps. But this adds cost and maintenance requirements.
More Limited Hot Water Supply
Although properly sized tankless heaters can provide sufficient hot water for typical household needs, they provide a more limited supply compared to storage tank systems. The maximum flow rate for an instant water heater ranges from 3-8 gallons per minute, depending on the unit size and power source. Flow rates are also reduced as inlet water temperature drops.
On the other hand, storage tank systems can support hot water draws of 8-25+ gallons per minute depending on the tank capacity and heat source. Their hot water supply is less affected by usage or inlet water temperature.
For larger homes with high demand, tankless systems may result in occasional shortages of hot water at full pressure. Adding multiple units in parallel could address supply limitations – but this further increases complexity and costs.
Instant Hot Water Use Cases
While instant hot water has some drawbacks in whole-house applications, it can be a good solution in certain scenarios:
- Single point-of-use – For a remote bathroom or other single location
- Supplementary heating – Pair with a conventional tank heater
- Small households – Lower hot water needs
- Warmer climates – Avoid cold weather issues
The disadvantages are most significant when tankless systems are used as the sole water heating method for larger homes.
Comparison of Hot Water Heating Options
Here is a table comparing key factors between conventional tank, instant tankless, and hybrid hot water heaters:
| Factor | Conventional Storage Tank | Instant Tankless | Hybrid Tank + Tankless |
|---|---|---|---|
| Upfront Cost | Low | High | Medium |
| Energy Efficiency | Moderate | Low | High |
| Hot Water Supply | High | Limited | High |
| Complexity | Simple | Complex | Moderate |
| Maintenance | Low | High | Moderate |
| Reliability | Proven | Some concerns | Robust |
Conclusion
In summary, some of the key disadvantages of instant tankless water heaters compared to conventional storage tank systems include:
- Higher energy costs in most homes due to on-demand heating
- Increased upfront purchase and installation cost
- Possible loss of water pressure with high demand
- Increased risk of scalding
- More complex maintenance requirements
- Limitations in colder climates
- More limited hot water supply capacity
These downsides tend to be most problematic when tankless units are installed as the sole water heater for a whole house. Smaller homes and point-of-use applications can avoid some of these disadvantages. Hybrid systems that combine instant and conventional heaters provide another option.
In the end, selecting the right water heating solution depends on factors like household size, hot water usage, climate, budget, and complexity preferences. Consider both the advantages and disadvantages of instant systems compared to alternatives to make the best choice.