Stiebel-Eltron Water Heater Not Working? What a 9 PM Emergency Call Taught Me

At 9:14 PM on a Saturday in February, I got the kind of call that keeps me up on weekends. A property manager I've worked with for years had a tenant in unit 4B with no hot water. The unit uses a Stiebel-Eltron DHC instantaneous water heater mounted under the bathroom sink. It had worked fine for two years. Then nothing.

In my role coordinating emergency service for multifamily buildings, I've handled more than 200 after-hours repair calls in the last six years. That night felt routine. I packed my bag, grabbed a few spare parts, and drove over. I expected a bad breaker or a failed unit. I didn't expect to pass 30 minutes in that apartment only to end up in the attic.

The Call: A Stiebel-Eltron Water Heater Not Working

When I'm triaging a rush call like this, I always check the cheap things first. The expensive replacements can wait. The tenant said the faucet ran cold, but the unit's display was still on. The breaker didn't trip. I ran hot water at the faucet, but nothing heated.

So I checked the cold-water inlet strainer. That was it. A small piece of mineral debris had clogged the mesh. I cleaned it, put everything back together, and the unit started heating in about 20 seconds.

Thinking about that call reminds me of the classic rookie mistake I made years ago. I replaced a heating element on a tankless unit before checking the strainer. Cost me $240 and a second truck roll. Now the strainer is the first thing I check on any Stiebel-Eltron water heater not working call.

But the water heater wasn't the only problem that night. Far from it.

The Second Problem: A Honeywell Home Thermostat

After the water was running hot, the property manager said, 'Since you're here, unit 4B is still cold.' I walked over, looked at the Honeywell Home thermostat on the wall. It said Heat On, but the air from the register felt cool. And the outdoor unit was running with warm air blowing off it. That's backwards for a heat pump in heating mode.

For a heat pump vs air conditioner explanation, this is the key. A heat pump and an air conditioner share the same refrigerant circuit. The difference is a reversing valve. In cooling mode, the valve sends hot refrigerant to the outdoor coil. In heating mode, it sends hot refrigerant to the indoor coil. Warm air off the outdoor unit meant the system was in cooling mode while the thermostat called for heat.

I opened the installer setup menu on the Honeywell Home thermostat. The O/B terminal setting was wrong. On most Honeywell Home thermostats, the O/B setting tells the thermostat whether the reversing valve should be energized in cool mode or heat mode. This thermostat had it backwards. Flipped one setting, left the thermostat for five minutes, and the heat pump was blowing warm air.

The Attic Fan That Should Have Been Off

Then the manager said, 'By the way, is the attic fan supposed to run in winter?'

I climbed up. The attic fan was spinning. In February. That's a problem. An attic fan is designed to vent hot attic air in summer, not run year round. Running one in winter creates negative pressure up there. That pulls conditioned air from the living space through ceiling cracks and recessed lights. In that apartment, it was making the heat pump work much harder than it needed to.

We turned it off at the switch. The tenant said the room felt warmer within 20 minutes.

Heat Pump vs Air Conditioner: Same Machine, Different Jobs

Let's settle the heat pump vs air conditioner question while we're here. A heat pump is an air conditioner that can run in reverse. If you live in a climate with moderate winters, a properly configured heat pump is often more efficient than electric resistance heat. If you're in a cold climate, you need a cold-climate model and correct backup heat. But no heat pump will work well if the thermostat is set wrong or the house is losing air through the attic.

According to the U.S. Department of Energy (energy.gov, accessed January 2025), setting your thermostat back 7-10 degrees for 8 hours a day can save as much as 10% a year on heating and cooling. That's the kind of free fix that beats an expensive upgrade.

The Honest Part

Now the honest part. I'm not telling you to replace every water heater with a Stiebel-Eltron DHC instantaneous water heater. It's a point-of-use unit. It's perfect for a bathroom sink, a small kitchen, or maybe a single shower if the incoming water temperature is mild. If you're trying to serve a whole house with multiple fixtures at once, you'll be disappointed. And if your water is hard and you never clean the inlet strainer, any tankless unit will eventually complain.

Same for heat pumps. They are not a magic answer for every home. But if you already have one, don't replace it with an air conditioner just because the thermostat confuses you. In many cases, you're one setting away from comfort.

If you're dealing with a Stiebel-Eltron water heater not working, check these before calling anyone:

• Is the breaker on and the power LED visible?
• Is the water flow strong enough to trigger the flow switch?
• Is the cold-water inlet strainer clean?
• Are any shut-off valves fully open?

Across 200+ emergency calls, I'd say almost two-thirds ended with a part that cost under $50 or a setting change. But I've also seen contractors replace compressors before checking a thermostat. That's not a strategy. That's a gamble.

Even after I recommended the Stiebel-Eltron DHC for that building, I kept second-guessing. What if the water pressure wasn't stable enough? What if the tenant's hard water scaled the element? The two weeks until our follow-up call were stressful. Then the manager called to say no complaints. That's the standard I use now.

That Saturday night wasn't about selling equipment. It was about checking the simple things first. A $2 strainer, a thermostat setting, and an on-off switch fixed more real-world comfort problems than a stack of new parts ever could.