November 17, 2025

Whole-House Heating with a Tankless Water Heater: Can It Be Done?

Many Seattle homeowners love the idea of squeezing more value out of their tankless water heater. After all, tankless water heaters are efficient, compact, and provide that endless hot water everyone gets hooked on. So it is natural to wonder whether the same unit that keeps your showers warm can also power whole-house heating. When you hear people talk about combining tankless heaters with radiant systems, baseboard heat, or even air handlers, it becomes an interesting question worth exploring.

At Fox Plumbing, Heating & Cooling, we hear this question all the time during tankless water heater installation consultations. Some homeowners are trying to replace older furnaces, some want to move toward a cleaner energy setup, and others simply want to lower hot water costs while keeping their home warm. The truth is that it can be done, but only under very specific conditions. A tankless heater is powerful, but it is not the same as a dedicated heating appliance. Whether it works depends on sizing, the type of heat distribution, the design of the home, and how well the entire system links together.

Understanding How Tankless Water Heaters Produce Heat

Before deciding if whole-house heating is realistic, it helps to understand how tankless water heaters actually make heat. Unlike a traditional hot water heater with a storage tank, tankless units fire up only when water flows. As soon as a tap or appliance calls for hot water, the burner or heating element kicks in. The unit heats water as it passes through, usually delivering a specific temperature rise at a set gallons per minute rate.

This means tankless water heaters are built for constant movement, not the steady recirculation that many heating systems use. In a classic radiant or hydronic setup, water flows through PEX tubing or baseboards even when it is not being heated at full strength. That gentle looping keeps the structure warm without pushing the heater to peak output every moment. Tankless systems are more like sprinters than marathon runners. They perform incredibly well for short, strong bursts, but they do not always enjoy long, gradual cycles.

This difference is important when choosing your tankless water heater. A unit that easily supports daily DHW demand may not be ready for the sustained heating loads needed on a freezing morning. Tankless heater EF, flow capacity, and temperature rise become key factors when determining whether one can successfully double as a whole-home heat source.

When a Tankless Water Heater Can Heat the Whole Home

Even though tankless water heaters were not originally designed for space heating, there are situations where they perform surprisingly well. These setups usually involve smaller, well-insulated homes where the heating load is modest. Think townhouses, accessory dwelling units, homes with updated windows, or structures that already use radiant systems.

Most successful installations share these traits:

The home has very low heat losses
Heating zones are limited and well-balanced
The unit is a high-BTU model, such as certain Rinnai or Navien systems
The distribution system uses low-temperature heat, like radiant floors
The tankless heater is part of a primary loop that keeps water moving

Radiant systems tend to pair best with tankless heaters because they require lower overall water temperatures and spread heat slowly and evenly. With a properly sized primary loop and mixing valves, the heater can cycle on when necessary without being overwhelmed. That said, it is essential to understand that tankless water heater replacement or installation for space heating requires specialized skills. The system design matters as much as the unit itself.

Even if the tankless heater can theoretically produce enough BTUs for the load, the overall balance between flow rate, temperature rise, and distribution efficiency determines real performance.

Where Tankless Systems Struggle with Whole-House Heating

Not every home is a good match for a tankless heating setup. Many homes around Seattle were built in a time when insulation was minimal and drafts were common. If a home loses heat faster than a tankless unit can produce it, comfort will suffer. Tankless heaters also do not like extremely low flow situations. Many hydronic systems run water slowly through radiant loops, and if flow drops too low, the heater might not even detect demand.

There are several red flags that indicate a tankless heater might not be a good fit:

The home requires high-temperature heating to stay warm
The system uses long piping runs that reduce water movement
The heater short-cycles due to fluctuating demand
Heat pumps or furnaces would easily outperform the load

Not all radiant systems run at low temps. Some need higher water temperatures that tankless units can technically reach, but struggle to maintain consistently across long cycles. If the system causes the heater to constantly switch on and off, the lifespan of the tankless unit can be shortened dramatically.

Another challenge is simultaneous demand. If someone showers while the heater is supplying whole-house heating, the DHW side and heating side may compete with each other. Without proper design, the home can cool off, or the shower can run cooler than expected. For families, that can turn into a daily frustration.

Hydronic and Radiant Systems: The Best Match for Tankless Heating

Hydronic systems use hot water instead of air to heat the home. This includes radiant floors, radiant wall panels, and baseboard systems. These systems excel at comfort because they rely on gentle, even warmth. Radiant floors, especially those using PEX tubing embedded in concrete or underfloor panels, are one of the most efficient forms of heat when paired with the right equipment.

Tankless heaters can work particularly well when part of a dedicated hydronic system that is engineered specifically for low-temperature operation. Instead of producing piping hot water like a boiler, a tankless unit might serve a primary loop that distributes warm water between 90 and 130 degrees. With mixing valves, expansion tanks, and circ pumps working together, the tankless heater warms the loop as needed without operating at maximum output constantly.

Some things to consider with hydronic systems and tankless units:

Lower water temperature requirements allow longer, gentler cycles
Radiant floors reduce the total heating load due to even coverage
Properly designed primary loops help avoid flow problems
A backup heat source may still be recommended in very cold weather

Homes that already use radiant heating often ask whether converting the boiler to a tankless water heater makes sense. Sometimes it does, but the decision requires careful calculation of heat losses and sizing the tankless unit to the real load. What works beautifully in a compact home may fail in a larger, leakier space.

Energy Efficiency and Operating Costs

Tankless water heaters are known for excellent efficiency ratings. High-tier models often use condensing technology to extract more heat from exhaust gases. The tankless heater EF reflects how well the unit converts fuel into usable heat. For DHW, this usually means lower hot water costs and less energy waste.

When applied to whole-house heating, the picture gets a bit more complicated. While the heater can be efficient on paper, the real-world performance depends on whether the system is designed well. If the heater short-cycles, pushes water at higher temperatures, or struggles with long distribution loops, efficiency can drop. In some homes, heat pumps or hybrid systems outperform tankless heating setups significantly.

Still, there are homeowners who combine a tankless unit with solar panels to offset fuel or electricity usage. This can substantially reduce operating costs. Homes that are very tight and well-insulated also benefit more from a tankless heating setup because the unit does not need to run as often.

Reasons some homeowners still choose tankless units for heating:

They already want endless hot water and prefer a single appliance
They live in smaller or highly insulated homes
They prefer a compact system instead of a full-size furnace or boiler
They want a system that pairs well with low-temperature radiant heat

Sizing Considerations and System Design

If you are seriously thinking about using a tankless water heater for whole-house heating, sizing becomes the most crucial step. Most tankless water heater installations are sized for hot water usage only and cannot support space heating without significant upgrades. Choosing your tankless water heater for heating means looking at BTU output, temperature rise, and gallons per minute flow rates with far more scrutiny.

Proper system design includes:

Calculating heat losses room by room
Matching BTU output to actual heating load
Designing a stable primary loop for consistent water movement
Choosing appropriate pumps and mixing valves
Ensuring the DHW side will not conflict with heating loads

Some tankless units, including certain models from Rinnai, are specifically designed to work with hydronic heating applications. These models sometimes include internal controls or remote control options that help manage dual use. Even so, professional design remains essential. Improper installation can result in poor heating, lukewarm showers, or equipment failure.

If the math shows that the tankless heater will be running near maximum capacity throughout winter, then a dedicated heating appliance or a heat pump may be the better investment.

Call Fox Today for Any Tankless Water Heater Upgrades in SeaTac or the Greater Seattle Area!

If you are considering a tankless water heater installation, replacement, or new system design that includes radiant heating or hydronic loops, our team at Fox Plumbing, Heating & Cooling is here to help. We have worked with homeowners across the Seattle region for more than five decades and understand how to evaluate whether a tankless heater can realistically support whole-house heating in your specific home.

Whether your project involves improving DHW performance, upgrading radiant systems, or planning bathroom plumbing remodeling along with a tankless system upgrade, our technicians arrive fully prepared. We bring the training, the tools, and the experience to make sure your installation is done safely and correctly. If you want honest guidance, clear explanations, and workmanship that keeps your home comfortable year-round, give our SeaTac team a call, and we will help you find the best path forward.