How to choose (and install) a Stiebel Eltron tankless water heater without making the mistakes I made

There is no single 'best' Stiebel Eltron model. It depends on your specific situation.

I've been handling hot water system orders for a wholesaler for about 8 years now. I've personally made (and documented) enough mistakes to fill a small filing cabinet—totaling roughly $12,000 in wasted budget between incorrect orders, rushed re-specs, and unnecessary panic shipments. Now, I maintain our team's internal checklist and I help contractors avoid my old errors.

When someone asks, "What's the best Stiebel Eltron tankless water heater?" I know they're looking for a simple answer. But the honest truth is: it depends on how you're using it, where you're installing it, and what your actual water usage looks like. This isn't a marketing cop-out. It's a reality that cost me real money to learn.

Here are the three most common scenarios, what worked (and didn't), and how to figure out which one you're in.

Three scenarios, three different answers

Scenario 1: The whole-house replacement (3-4 bathroom home)

My first big mistake: In my first year (2017), I recommended a Stiebel Eltron Tempra 24 Plus for a 4-bathroom home with two teenagers. It looked fine on paper—24 kW, good flow rate. But the incoming water temperature in that area (Northern Michigan) dipped to 38°F in winter.

The result came back: lukewarm showers, family complaints, an angry contractor, and a $900 redo. The unit we should have recommended was the Tempra 36 Plus (36 kW) for that cold inlet temperature. The difference? About $450 more on the unit cost, but it actually worked.

The fix for this scenario: For a whole-house replacement in a standard 2-3 bathroom home with moderate climate (incoming water above 50°F), the Tempra 24 Plus is usually enough. But if you're in a colder region or have more than 3 bathrooms? Seriously consider the Tempra 36 Plus. I learned to check the incoming water temperature before making a recommendation, not just the flow rate.
(Source: Stiebel Eltron spec sheets; verify current models and pricing at stiebel-eltron-usa.com)

Scenario 2: The point-of-use unit (single sink, small apartment, or remote bath)

This is the scenario where I see contractors make the opposite mistake: they undersize. They see a small unit and think, "This is for a hand-wash sink, it'll be fine." And then it isn't.

I once ordered 12 Stiebel Eltron Mini 3 units for a small office renovation. Each unit was for a single sink. My thought was: small sink, small unit, simple math. But the client had low-flow faucets with a flow restrictor, and the Mini 3 didn't kick on because the flow rate was too low. Every single unit had the issue. 12 items, about $400 in product cost, plus the labor to swap them out. That's when I learned that the Mini series requires a minimum flow rate of about 0.5 GPM to activate. If you have ultra-low-flow fixtures, you need the Mini 6 or Mini 8, which have a slightly lower activation threshold in practice.

The fix: For a single sink in a bathroom, the Mini 3 is usually fine—if the faucet flow rate is standard. For a kitchen sink or a utility sink where you need more hot water faster? Go with the Mini 6 or Mini 8. And if you're not sure about the faucet specs, just go up one size. The price difference is usually less than $100, and the headache saved is way bigger than that.

Scenario 3: The retrofit where space is tight (replacing an old unit in a tight utility closet)

This one is more about installation than model selection, but it's where I see the most frustration. The Stiebel Eltron Tempra Plus series is bigger than the old electric tank units. I've had contractors call me panicked because the new unit didn't fit in the same footprint as the old one.

The most frustrating part of this situation: the specs are all online. You can get the dimensions before you order. But contractors rarely check them—they assume the new tankless will fit where the old tank was. They'd think checking dimensions is common sense, but when you're on a tight deadline and the install is already scheduled, you just assume. Then the unit arrives, and it's 2 inches too wide.

The fix: Before you order anything, measure the space. Ensure you have at least 2 inches of clearance around the unit for wiring and plumbing. If space is critically tight, consider the Stiebel Eltron DHC-E series (electric tankless, but more compact than the Tempra). It's been a lifesaver for retrofits.

How to diagnose your own scenario (without making my mistakes)

Here's a quick checklist I use for every quote now. Answer these three questions:

1. What's the incoming water temperature in winter? (If you don't know, check historical data for your zip code. It matters.)
2. How many fixtures will run simultaneously? (Not just total bathrooms, but actual simultaneous use: two showers + dishwasher? Or just one sink at a time?)
3. What's the available electrical service? (A 36 kW unit requires 150A breaker. A lot of homes don't have that spare capacity.)

If you answer these honestly, you'll land in one of the three scenarios above. If you're still not sure, err on the side of going up one size. It's cheaper than upgrading later.

One last thing: I hit 'confirm' on a big order for 15 Tempra 36 units and immediately thought, 'Did I spec the right models for all of them?' For two weeks I was nervous. Then the contractor called to say the install was smooth, and everyone had hot water. Didn't relax until that call came in. So I get the hesitation. But a little upfront thinking saves a ton of regret.

Prices as of January 2025; verify current pricing and specifications at stiebel-eltron-usa.com.