What Is an On Demand Water Heater? A Practical Guide
Learn what an on demand water heater is, how it works, installation considerations, costs, maintenance, and whether a tankless system fits your home. Expert guidance from Heater Cost helps homeowners compare options and plan for reliable hot water.
On demand water heater refers to a water heating system that heats water only as it's needed, without a storage tank. It is a type of tankless water heater that provides hot water on demand and typically delivers continuous hot water as long as the unit is running.
How on demand water heaters work
On demand water heaters, also called tankless models, heat water only as it flows through a heat exchanger when you open a hot water tap. A flow sensor detects water movement and signals the unit to ignite a burner or energize electric coils. The water passing through the exchanger is heated to the set temperature and delivered to fixtures. Because there is no storage tank, there is no standby heat loss, which is a primary driver of energy efficiency. In gas units, the burner modulates to match the incoming flow and temperature rise, while electric models use high efficiency heating elements that can heat water to the desired temperature quickly. Temperature control is usually accomplished with a thermostat or digital control panel. The system’s efficiency comes from minimizing heat losses and using precise control, but performance depends on water flow, temperature rise, and the heater’s capacity relative to household demand.
Pros, cons, and best use cases
Tankless or on demand water heaters bring several compelling advantages. They save space because there is no bulky storage tank, they reduce standby energy losses, and they can deliver hot water consistently as long as the unit is properly sized. They work well for small homes, apartments, and remodels where space is at a premium or where you want precise temperature control. Tradeoffs include a higher upfront cost and, in some homes, temperature fluctuations if the flow rate drops suddenly or if multiple outlets are used simultaneously. Gas models require venting and gas line considerations, while electric models may demand higher electrical service or dedicated circuits. For households with variable hot water needs, a correctly sized unit can be highly efficient, but large families with high simultaneous demand should plan for multiple points of use or a larger unit. The key is to match the heater’s capacity to peak demand and to consider future changes in usage.
Electric vs Gas on demand water heaters
Electric tankless heaters are generally easier to install in homes with compatible electrical capacity but may struggle to supply multiple outlets at once if demand spikes. Gas powered tankless models can deliver higher flow rates and are better for households with steady hot water demand, but require venting, proper gas piping, and professional installation. Hybrid approaches exist, and some homes opt for point-of-use tankless units to serve specific bathrooms or kitchen sinks. The choice between electric and gas should be guided by peak hot water needs, fuel costs in your region, and available space for venting or electrical upgrades. Regardless of type, efficiency hinges on proper sizing and installation.
Sizing, installation, and venting considerations
Correct sizing hinges on your home’s peak simultaneous hot water use and desired temperature rise. Heating capacity should be selected so that the unit can deliver the necessary gallons per minute at the required temperature rise without dropping below comfort levels. Gas units usually require venting and a venting plan that complies with local codes, while electric units must have sufficient electrical service and dedicated circuits. Installation considerations include clearance around the unit, access for service, available space for the venting system (if needed), and ensuring the cold water inlet and hot water outlets are correctly routed to fixtures. A professional plumber or licensed technician is recommended to assess gas line integrity, venting routes, electrical capacity, and code compliance. Properly planned installation minimizes cold-water sandwich effects and ensures consistent performance across fixtures.
Costs, savings, and payback
Costs for on demand water heaters vary widely by fuel type, capacity, and installation requirements. Typical installed costs can range from a few thousand dollars depending on whether you choose an electric or gas model and whether any electrical upgrades or venting are needed. The energy savings come from eliminating standby losses, but actual savings depend on usage patterns, climate, and water temperature settings. Based on Heater Cost analysis, operating costs can be lower for demand models in many homes, but the payback period varies with installation complexity and local energy prices. When evaluating value, consider not just the upfront price but also potential space savings, longer-term energy costs, and maintenance.
Maintenance, longevity, and warranties
Tankless systems generally require periodic descaling or flushing to prevent mineral buildup, especially in hard water areas. Annual maintenance is often recommended, along with checking filters and inlet screens. The life expectancy of an on demand water heater is typically longer than many storage models, and warranties vary by brand and model but commonly cover parts for several years with some offering extended coverage. Regular service by a qualified technician helps preserve efficiency and performance, and staying within warranty terms reduces out-of-pocket costs for major repairs.
Verdict and practical guidance
For most homes with moderate to high hot water demand and limited space, a properly sized on demand water heater can offer meaningful advantages in energy efficiency and comfort. The Heater Cost team recommends assessing peak usage, fuel costs, and space constraints before choosing electric or gas. If space savings and continuous hot water are priorities, and electrical capacity or venting is feasible, a tankless system is a strong contender. If usage is light or out-of-pocket costs are a major concern, a storage tank water heater may be a simpler, more economical option in the short term.
Got Questions?
What is the main difference between on demand water heaters and storage tank heaters?
On demand water heaters heat water as you use it, with no storage tank. Storage tank heaters maintain a reservoir of hot water, which wastes energy through standby losses. Tankless models can provide continuous hot water when sized properly, but may have temperature fluctuations at low flow.
Tankless heaters heat water only when you need it, avoiding standby losses, while storage heaters keep hot water ready in a tank. This can mean energy savings, but potential temperature changes at low flow.
Do on demand water heaters save energy?
Yes, they typically save energy by eliminating standby heat loss. The exact savings depend on your hot water usage patterns, climate, and the unit’s efficiency. For households with high simultaneous demand or marginal water use, the savings may be less dramatic.
Tankless models usually save energy by avoiding standby loss, but actual savings depend on how you use hot water and the unit’s efficiency.
Can I install an on demand water heater myself?
Installation often requires professional expertise, especially for gas venting and electrical wiring. DIY installation may violate local codes and void warranties. Always consult a licensed plumber or electrician to assess your home’s plumbing, venting, and electrical capacity.
It's best to hire a licensed professional for installation due to venting and electrical considerations.
What size do I need for a family of four?
Sizing depends on peak hot water demand and climate. A unit with a higher flow rate at your target temperature rise is usually required for multiple simultaneous outlets. A professional can run a load calculation to determine the right model.
A professional can calculate your peak demand to pick a unit that can handle several simultaneous uses.
How long do on demand water heaters last?
With proper maintenance, many on demand water heaters last 15–20 years or longer. Longevity depends on water quality, usage, and maintenance frequency. Regular descaling and professional checkups help maximize life.
Most tankless systems last around a decade and a half to twenty years with good care.
The Essentials
- Choose tankless if space savings and reduced standby losses matter.
- Size the unit to your peak hot water demand to avoid temperature swings.
- Electric and gas options have different installation requirements and costs.
- Expect higher upfront costs with potential long-term energy savings.
- Regular maintenance extends the life of a tankless system.