Why Your Tankless Water Heater Might Be the Wrong Solution (And What We Miss Instead)

If you’re researching stiebel eltron tankless water heaters, I bet you’re asking the same question everyone asks: “What size do I need?” It’s a fair question. Most buyers focus on flow rate (GPM) and total kw rating—like the popular Stiebel Eltron Tempra Plus 36 kw—and assume that’s the whole story. But the question they should be asking is very different. And the answer might surprise you.

I’m a quality compliance manager for a company that specifies a lot of different heating and cooling equipment. I review hundreds of specs and installations every year. Over the years, I’ve seen the same mistake happen again and again, and it’s rarely about the brand or the sticker price. It’s about a fundamental misunderstanding of the problem you’re trying to solve.

The Surface Problem: Which Tankless Heater?

Typically, the conversation starts like this:

“I need a tankless water heater. I hear they’re more efficient. Should I look at Stiebel Eltron? What about a propane heater vs. electric?”

And that is a valid starting point. You’ve got a product category in mind, and you’re sizing up the options. But in my experience, this is where most people stop their analysis. They compare specs on a PDF, maybe see that the Tempra Plus is a solid unit, and then immediately pivot to price comparisons or installation costs.

That’s a mistake. Not because those are wrong—but because they’re surface issues. What’s really driving your decision might be something you haven’t even considered.

The Deeper Problem: ‘Water Heater vs Boiler.’ Are We Even Talking About the Same Thing?

Here’s the blind spot that catches most buyers: The real decision isn’t Stiebel Eltron vs. Rheem. The real decision is water heater vs boiler. And I don’t mean that in a brand comparison way.

Most people use the terms interchangeably. They say “water heater” when they might actually need a “boiler.” I’ll admit, even within the industry, the lines get blurry—or rather, the boundaries are clear, but the marketing makes them fuzzy.

• Water Heater (Tankless): Typically heats potable (drinkable) water for your taps, showers, and sinks. Electric units like the Tempra Plus do this very well.
• Boiler: Heats water for a hydronic (radiator/radiant) heating system. Then, often, it has a separate coil or tank for domestic hot water.

Now, a lot of people think a high-kw tankless can replace a boiler for whole-home heating. “I’ll just use the hot water from this unit to run my baseboard radiators.” That’s a classic misconception—a legacy thinking from an era when some systems did that. Today, the engineering is very different. Most electric tankless units are not rated or certified for closed-loop hydronic heating. You can ruin the unit, or worse, create a dangerous pressure situation.

So the first question you should ask yourself is: “Am I trying to heat my floors, or am I trying to wash my dishes?” The answer determines whether you should even be looking at a Stiebel Eltron tankless or should be calling a boiler installer. Seriously, I’ve seen a $22,000 redo because a buyer bought a Stiebel Eltron Tempra Plus 36 kw for an application that needed a 50 kW condensing boiler.

The Cost of Getting It Wrong

Let me give you a specific example. In my Q1 2024 quality audit, we reviewed an installation for a small commercial shop. The owner had seen the efficiency numbers for tankless and decided to go electric. They chose a Stiebel Eltron Tempra Plus 36 kw because it had great reviews. But their need was for a small radiant floor heat system in a 1,200 sq ft space plus one restroom sink.

Actual, their electric service couldn’t handle the 36 kw demand. They either needed a massive service upgrade (think: new panel, new meter, thousands of dollars) or had to switch to a different fuel. They went with a propane heater instead, a high-efficiency tankless gas unit.

I said “tankless.” They heard “electric tankless.” Because I didn’t clarify, and they didn’t ask. The mismatch cost two extra weeks and $2,500 in change orders. We were using the same word but meaning different things. Discovered this when the electrician said there was no room in the panel.

The Tempra Plus is a fantastic unit—for its intended purpose. But if you don’t ask the right questions upfront, you can end up buying a $1,200 paperweight. The Tempra Plus doesn't run a whole house on 30A service.

The Real Solution: Define Your ‘Problem’ Before Your ‘Product’

So, what’s the solution? It’s not “buy a Stiebel Eltron.” It’s not “buy a propane heater.” It’s not “buy a boiler.” The solution is to define the boundary of your problem before you even look at products.

Here’s my advice, based on reviewing a ton of failed specs:

1. Map the load: List every single thing that needs hot water. Showers? Radiant floors? The dishwasher?
2. Check your utility: Can your existing electric service handle a 36 kW draw? If not, you’re looking at gas (propane or natural gas) or a heat pump unit.
3. Define the system type: Is it potable water? Or a closed-loop heating system? If it’s a closed loop, you probably aren’t looking for a Stiebel Eltron Tempra Plus.

If you need a tankless for your domestic hot water, the Stiebel Eltron Tempra Plus 36 kw is one of the best electric units on the market. It’s super reliable, and the build quality is excellent. But if you need whole-home heat, you might be looking at a propane heater or a boiler, and that’s a different conversation altogether.

The mistake isn’t picking the wrong brand. The mistake is skipping the first step—which is to admit you don’t yet know which category of equipment you actually need. A good specialist will tell you that. A generalist will just sell you a water heater.