Water Heater vs Boiler: A Procurement Checklist for Stiebel Eltron Electric Heaters, Heat Pumps, and Small Parts

Here's the thing: I don't buy heating equipment because I love spec sheets. I buy it because it has to work, and it has to not blow up the operating budget. Over the past six years of tracking invoices for a mid-sized facilities company, I've built a procurement checklist for heating and hot water projects. It's saved us from embarrassing redos and at least one change order that would've hurt.

I'm not an HVAC engineer, so I can't speak to heat-loss calculations or refrigerant circuit design. What I can tell you from a procurement perspective is how to structure the decision so you don't overpay.

Who should use this checklist

Use this when you're replacing or upgrading hot water equipment, comparing a water heater vs boiler, or evaluating a heat pump for a multi-unit building. It also applies if your project includes basic ventilation and controls—bathroom fans, pressure sensors, and similar items that get lost in a big bid.

This checklist is based on about 40 projects in commercial and multifamily buildings. If you're buying for a single-family home, some steps still work, but your experience may differ.

There are seven steps. Go through them in order.

Step 1: Start with the function — water heater vs boiler

Before you compare brands or models, define what the system has to do. A water heater heats tap water. A boiler heats water for space heating, and often for domestic hot water too. If you only need hot water at a sink, a Stiebel Eltron electric heater is almost always simpler than a boiler. If you need radiators or underfloor heat, you need a boiler or a heat pump.

The phrase "water heater vs boiler" matters because contractors quote completely different systems depending on how the request is written. I've seen quotes for a 400,000 BTU boiler when the building only needed a point-of-use electric heater. The boiler estimate was six times higher. That's not a "better product" problem; it's a scope problem.

Step 2: Build a total installed cost, not a price list

The biggest mistake is comparing unit prices. A Stiebel Eltron electric heater may have a lower purchase price than an air-to-water heat pump. But total installed cost includes delivery, electrical work, mounting, ventilation, controls, commissioning, and disposal of old equipment. An electric resistance heater has lower upfront cost and higher running cost per unit of heat. A heat pump has higher upfront cost and lower running cost in many climates. Which is better depends on the building's annual operating hours and local energy rates.

One option I've evaluated is the Stiebel Eltron air to water heat pump. It can replace both a boiler and a separate water heater in certain buildings, but the upfront cost and installation requirements need to be modeled before you commit.

I use a simple TCO spreadsheet. It lists every line item in the same order for every vendor. That spreadsheet caught a $1,200 difference in startup labor that none of the sales folks had mentioned. One vendor included it; the others planned to bill it as an extra.

Step 3: Compare efficiency by operating hours, not brochure numbers

For an electric resistance heater, efficiency is not the real variable; you get roughly one unit of heat per unit of electricity. For an air-to-water heat pump, the important numbers are COP for steady conditions and SCOP for seasonal performance. A heat pump can move three or four units of heat for one unit of electricity, but that assumes the right climate, water temperature, and controls.

I ask for efficiency data at my operating conditions. Per FTC advertising guidelines (ftc.gov), energy-saving claims need to be truthful, not misleading, and backed by evidence. So when a salesperson says "guaranteed energy savings," I ask for the engineering analysis. If they can't produce it, I don't include the savings in my payback calculation.

This gets into engineering territory, which isn't my expertise. I'd recommend consulting a mechanical engineer for exact load calculations. From a cost-control side, I ask for best-case, design-case, and worst-case efficiency scenarios. If the payback only works in the best case, I walk away.

Step 4: Verify the installation site before you issue a PO

This is where hidden costs live. Does the electrical panel have capacity for a Stiebel Eltron electric heater or heat pump? Is there a condensate drain? Is there room for a buffer tank? Is the location protected from freezing? What is the distance between the outdoor unit and the indoor hydronic manifold?

If you skip this, you get change orders. I'd rather spend an hour at the site than approve a $2,000 change order later. I dodged a bullet once when I checked the mechanical room before signing: the quote didn't include a condensate pump, and the existing floor drain was on the other side of a firewall. That would've meant a wall core, a pump, and a lot of time. The vendor wasn't trying to hide it; it just wasn't on their walkthrough list.

Step 5: Don't forget bathroom fans, oil pressure sensors, and other small-line items

This is the reason I include the "small stuff" in a heating procurement checklist. During a 2023 upgrade, the main package included the boilers and hot water heaters, but the quote missed the bathroom fans in the unit baths and the oil pressure sensor on the existing backup boiler. Both were small-line items. They still held up the certificate of occupancy inspection.

A bathroom fan isn't a luxury item. In a building with new windows and tighter doors, stale air and humidity have to go somewhere. If the exhaust fan is undersized or too loud, people disable it, and then you have moisture problems.

And an oil pressure sensor is a simple safety device that shuts off the burner if oil pressure is lost. If you're comparing water heater vs boiler and keeping an old oil-fired boiler as a backup, check whether the sensor matches the burner controller. The replacement sensor cost, I want to say, was around $60—but don't quote me on that exact number. The emergency service call after it failed was well over $600.

My rule: in every bid, ask for a line item for bathroom fans, pressure sensors, circulator pumps, expansion tanks, backflow preventers, and controls. If they're not listed, ask why.

Step 6: Get three quotes with the same scope

Our procurement policy requires three quotes minimum. But quotes are meaningless if the scope differs. I built a TCO spreadsheet that maps every quote to the same line items. It catches differences in delivery, commissioning, controls, and disposal.

Why does this matter? Because contractors don't quote the same things. One includes the circulator pump; another doesn't. One includes a condensate neutralizer; another treats it as an extra. The lowest number is not the cheapest system. When a vendor recommends a STIEBEL-ELTRON model, I put it into the same TCO sheet as the other proposals.

In one case, the lowest initial quote was the highest after I added accessories and startup labor. The difference was about 14 percent of the original equipment price. So, yes, the spreadsheet takes time. It pays for itself.

Step 7: Plan for downtime, spare parts, and a service partner

Time is also a cost. I had two hours once to decide on a temporary heater for a critical room before a facility reopened. Normally I'd compare TCO over a week. There was no time. We went with a vendor we trusted and paid a fair price. It worked, but only because we already had a relationship with the supplier. Build those relationships before you need them.

If you're installing a Stiebel Eltron air to water heat pump, confirm who handles warranty service in your region. If you manage multiple buildings, keep a few common controls and sensors in stock. A $60 part can turn into a $600 emergency call when you don't have it.

A few mistakes to avoid

• Choosing "water heater vs boiler" based on the first sales pitch. Define the system boundaries first.
• Ignoring electrical and ventilation prerequisites. That's where the budget dies.
• Comparing a Stiebel Eltron electric heater to an air-to-water heat pump without annual operating hours and energy prices.
• Forgetting bathroom fans and oil pressure sensors can be part of the same project and save labor time.
• Trusting a quote that lists "miscellaneous" without a definition.

Price is what you pay at the beginning; total cost is what the system asks for over its life. Do the checklist before you sign, and you'll keep both numbers in the same neighborhood.