Boiler vs Water Heater: When a Stiebel-Eltron Tankless Water Heater Makes Sense

Boiler vs Water Heater: There’s No Single Winner

I get asked about the “boiler vs water heater” comparison a lot. It sounds like a simple contest. It isn’t. A boiler and a water heater do two different jobs. Honestly, asking which one is better is kind of like comparing an electric leaf blower to a Vornado fan.

An electric leaf blower pushes a focused stream of air. A Vornado fan circulates a room. Both move air. But if you swapped them, you’d end up with a mess. The same is true for boilers and water heaters.

When I first started in the trades, I assumed a boiler was just a bigger, stronger water heater. Then I was called to a four-story apartment building where the boiler was working perfectly, and every tenant was complaining about no hot water. The boiler was doing its job—it was pushing heat through the radiators. It was never designed to supply the showers. That call changed the way I triage these requests. (It also reminded me not to assume.)

Scenario 1: You Need Space Heating

If you’re working on a building with radiators, baseboard, or radiant floors, you’re looking at a boiler. A boiler heats water—or makes steam—and sends it through a hydronic loop to warm the space. In HVAC terms, ASHRAE makes a clear distinction: a boiler produces hot water or steam for space heating, while a water heater produces potable hot water for domestic use.

A residential water heater cannot do the boiler’s job. It doesn’t have the output or the pressure rating for a closed heating loop. So if the building needs radiators, start with a boiler.

What if you also need domestic hot water for sinks and showers? You have two decent routes:

• Install a boiler and add an indirect storage tank for domestic hot water.
• Use the boiler for space heating and a separate water heater or tankless unit for domestic hot water.

I’ve gone back and forth on this for actual projects. The indirect tank approach can be efficient because a well-designed boiler can run at higher water temperatures. But it also means running the boiler in summer just for hot water—unless your controls are set up right. After seeing too many summer fuel bills, I often lean toward separate systems. It’s not the right answer for every building. But it’s worth putting on the table.

Look, I’m not saying boilers are bad. They’re not. They just need to be paired with the right domestic hot water setup.

Scenario 2: You Need Domestic Hot Water Only

This is where the question starts to make sense. For hot water at sinks, showers, and commercial kitchens, you’re talking about water heaters—not boilers. And the best type depends on the building.

If someone searches for a Stiebel Eltron instantaneous water heater, the first unit I usually recommend is the Stiebel-Eltron Tempra 24 Plus. It’s a 24 kW electric tankless water heater that heats water on demand. No storage tank means no standby losses, and you can size it for multiple draws in the right conditions.

But the Tempra 24 Plus has a catch: it needs real electrical capacity. It’s a 240-volt unit, so it requires roughly 100 amps of electrical service. If the panel can’t handle that, you’ll need a smaller unit or a service upgrade. That’s one of the first things I check before recommending it.

The key measurement for any tankless heater is temperature rise. If incoming groundwater is 50°F and you need 110°F at the tap, that’s a 60°F rise. Flow rate drops as required temperature rise goes up. That’s why you don’t size tankless heaters by square footage alone. You size them by hot water demand and incoming water temperature.

“The question isn’t ‘Which appliance is better?’ The question is ‘Which appliance is doing which job?’”

Scenario 3: You Need High Efficiency and Don’t Have Gas

Since the U.S. Department of Energy’s 2023 water heater efficiency standards took effect, the all-electric conversation has shifted. Many 40- and 50-gallon electric tank units now use heat pump technology. A heat pump water heater can be two to three times more efficient than a conventional electric resistance tank. That’s a big deal for operating cost.

Here’s the counterintuitive part: even though tankless heaters like the Tempra 24 Plus are popular, a heat pump tank unit can sometimes beat them on energy cost. The tradeoff is space and air. A heat pump water heater needs airflow, a condensate drain, and usually a taller room clearance. In a tight mechanical closet, that can be a deal-breaker.

So if you ask me “boiler vs water heater” in an all-electric building, I’ll ask you three things: How much hot water do you need? How much electrical capacity do you have? And how much floor or closet space is available? The answers will point you toward a tankless unit, a heat pump tank, or sometimes a combination of both.

What About a Cold Room or Garage?

Not every comfort complaint is a water heating problem. If you have a cold workshop, a spare room, or a garage that needs heat only a few months a year, you might not need a boiler or a big water heater at all. A Stiebel-Eltron wall heater could be the practical fix. Mount it, wire it, and you’re done. That’s the kind of right-sized thinking that saves clients time and money.

How to Decide: A Quick Triage

When I’m triaging a call, I use the same logic every time:

1. Define the job. Is this for space heating, domestic hot water, or both?
2. Check the fuel. Natural gas, propane, electric, or a hydronic loop already in place?
3. Confirm the space. Can you fit a tank? Can you fit a heat pump unit? Is there enough electrical capacity?
4. Think about summer. Do you want to run a boiler all summer just for hot water? If not, separate systems deserve a closer look.

In my role coordinating emergency replacements, I’ve handled 40+ urgent calls in the last eight years. Last March, a dental office called at 4:30 on a Friday after their storage tank ruptured. We expedited a Stiebel-Eltron Tempra 24 Plus, paid extra for overnight freight, and had hot water back by Saturday afternoon. The alternative was closing on Monday. I tell that story to make one point: the right system, installed by people who know what they’re doing, can turn a potential disaster into a footnote.

Boiler vs Water Heater: It’s a Both/And World

Here’s the thing. In many commercial buildings, you won’t choose one over the other. You’ll have a boiler for space heating and a water heater—possibly a tankless electric unit—for domestic hot water. They work together.

So if someone tries to sell you on a universal “boiler vs water heater” winner, be careful. It’s an electric leaf blower vs Vornado fan question all over again. The right answer depends on the job.

Bottom line: boilers heat buildings. Water heaters heat water for taps and showers. The Stiebel-Eltron Tempra 24 Plus is a great tankless option when the electrical conditions are right. And sometimes the smartest move is neither—a simple wall heater fixes the actual problem.

Take it from someone who has stood in a cold parking lot at 6 a.m. waiting for a part: define the real need first. The right equipment follows.