For years, I told customers the choice between a furnace and a boiler comes down to budget and existing ductwork. That's true—but it's not the whole story. I'm a heating installer handling boiler and furnace installs for 11 years. In that time, I've personally made and documented 18 significant mistakes, totaling roughly $12,000 in wasted budget. Now I maintain our team's startup checklist. If you're an installer or specifier, this article is for you.

This is a furnace vs boiler comparison, but it's not the one-size-fits-all 'radiators are always better' manifesto. It's built from what I actually see on the job: Vaillant boilers, expansion vessel failures, F22 fault codes, and more than a few furnaces that were installed 'perfectly' and still failed early.

Furnace vs Boiler: What We're Actually Comparing

Furnaces heat air and push it through ducts with a blower. Boilers (like the Vaillant ecoTEC line I install most often) heat water and pump it through radiators or underfloor pipes. The difference sounds simple, but it changes everything: how the house feels, how much maintenance is involved, and how you think about faults.

I'm going to compare them across four dimensions: comfort and efficiency, installation complexity, maintenance and faults, and total cost. In each one, I'll give you a clear conclusion—and one might surprise you.

Comfort and Efficiency: Boilers Win, But Not For the Reason You Think

The physics hasn't changed. Moving heat through water gives a more even, gentle warmth. Forced air heats up fast but can also create drafts and dry air. That's not a myth.

But I didn't believe it until I made the mistake. I only believed comfort differences after ignoring it and recommending a high-efficiency furnace to a customer who wanted a boiler. The furnace heated the house quickly. It also produced a week of dry-sinus complaints and cold spots in every room. The boiler would have been a better fit. Not because the furnace was bad—because the building had plaster walls, no ductwork, and the homeowner's priority was even heat, not speed.

When it comes to efficiency, the old rules have shifted. What was best practice in 2020 may not apply in 2025. Modern condensing boilers modulate their burner output, so they don't blast at full power for ten minutes at a time. Furnaces have improved too, and some can hit similar AFUE numbers. The fundamentals haven't changed, but the execution has transformed. Efficiency alone shouldn't decide this comparison.

Installation Complexity: It's Never Just the Appliance

Here's where I see the most costly mistakes.

A furnace installation needs a gas line, electrical wiring, venting, a condensate drain (if high-efficiency), and ductwork. If the ducts are already in place, it can be a one-day swap. A boiler installation needs all of that plus radiators or underfloor loops, a pump, and an expansion vessel. In the UK, according to the Gas Safe Register, boiler installation must be done by a Gas Safe registered engineer. Furnace installs also require licensed techs in most jurisdictions.

On a Vaillant ecoTEC Pro 28, the expansion vessel is built in. Many installers skip the startup check. That's a mistake. If the vessel pre-charge is wrong—too high or too low—the vessel can't absorb expanded water properly. Pressure spikes, the pressure relief valve opens, water is lost, and then you get F22 when the system cools. The most common is the Vaillant boiler F22 fault, which means system pressure has dropped below the minimum threshold—usually around 0.8 bar according to the technical manual. Same part, two entirely different failure modes.

A senior engineer once told me: 'Check the pressure before you touch the pump.' I thought he was being dramatic. After the F22 callbacks, I understood.

I once told an apprentice to 'check the expansion vessel.' He heard 'test the pump.' Two hours and a new pump later, the boiler still threw F22. We were using the same words but meaning different things. We discovered it when the customer called back and asked why the pressure was still low. Communication failure, not a mechanical failure. Now I write specific tasks on the checklist: 'confirm pre-charge with a pressure gauge,' not 'check vessel.'

I don't tell this story to embarrass anyone. I tell it because the same type of miscommunication happens on every trade. The contractor says 'wire the thermostat to the correct terminals.' The apprentice hears 'wire it however you think.' The result is a callback and a room that doesn't heat.

Another thing: boiler installs can be physically awkward. I've spent more than one July afternoon in an unventilated mechanical room with a Ryobi fan pointed at my face while soldering pipe joints. Furnace installs are usually more airy, but they come with their own enemies—old crawlspaces, sharp ductwork, and clearance issues.

Maintenance and Faults: The Vaillant Boiler F22 Fault Is Not What You Think

Boilers are often described as more complicated than furnaces. In my experience, the complexity isn't the problem. It's the water.

What does the Vaillant boiler F22 fault mean? It means the system pressure has dropped below the minimum operating threshold. Homeowners see F22 and panic. But in most cases, it's simple: a small leak, a stuck pressure relief valve, or an expansion vessel that has lost its gas charge. The Vaillant ecoTEC Pro 28 expansion vessel is an 8-liter unit, pre-charged to about 1.0 bar (I might be slightly off for your exact model, so check the manual). If the vessel loses pressure, every heating cycle pushes water through the PRV, and then the system pressure falls enough to trigger F22.

If you get F22 on a Vaillant boiler, first job is to pressurize the system to around 1.0 bar and check if the pressure holds. If it drops while the boiler is cold, you have a leak. If it only drops when the boiler heats up, suspect the expansion vessel or PRV. That simple workflow solves most F22 calls.

Furnaces don't have water pressure. But they have blower motors, filters, igniters, heat exchangers, and condensate drains. I've replaced furnace blower bearings more often than I've seen a genuine expansion vessel failure. The difference is that boiler fault codes look intimidating, while furnace parts just wear out quietly.

That said, boilers do require annual servicing. Don't skip it. A dirty heat exchanger on a condensing boiler can cost you real efficiency and can lead to premature pump failure.

Total Cost and Lifespan: The Short-Term Trap

In most North American markets, a furnace install costs less upfront—often $2,500 to $4,000 less if the ductwork exists. In the UK and Europe, a boiler is the norm, and existing radiator networks usually stay, so the cost gap is smaller. But the price tag isn't the whole picture.

That upfront gap also ignores the cost of changing distribution. Replacing a furnace with another furnace is a swap. Changing to a boiler can mean draining pipes, fitting radiators, and chasing floorboards. I've quoted jobs that looked like $6,000 and turned into $11,000 once we opened the walls. You need to account for that before you promise a client anything.

I've seen clients choose the cheaper option and then pay for it in fuel costs for a badly sized system. According to the U.S. Department of Energy's Energy Saver guide, properly maintained heaters can last 15 to 30 years. But that lifetime depends on the installation. A cheap furnace with undersized ducts will run longer and eat its own bearings. A boiler with a dead expansion vessel will short-cycle and keep tripping F22 until someone fixes it.

The upside of recommending a boiler in a house with no pipework was better comfort. The risk was opening walls and blowing the renovation budget. I kept asking myself: is comfort worth potentially wrecking the client relationship? In the end, I presented both options and let the client decide. He chose the furnace. I still think it was the right call for his budget.

Furnace vs Boiler: Which Should You Choose?

I'm not going to give you an absolute answer, because the right answer depends on the building and the owner's tolerance for disruption. But after years of doing both, here's my rule of thumb.

Choose a boiler (Vaillant or similar) when:

  • You want even, quiet heat and are happy with radiators or underfloor heating.
  • You already have hydronic pipework and can keep it.
  • You're in a region where gas boiler installation and service expertise is common.
  • You're willing to invest in better comfort and don't mind annual maintenance.

Choose a furnace when:

  • You already have ductwork and a tight budget.
  • You need fast warm-up after a setback (for example, in a cold climate with time-of-use electricity rates).
  • You're in North America where furnace parts are cheap and contractors are everywhere.
  • You want a simpler heating-only system with no water in the living space.

The question isn't 'boiler vs furnace, which is better?' It's 'which system fits this house and this owner?'

Conclusion: The Best Installer Is the One Who Checks the Basics

Looking back, I should have standardized a startup checklist years earlier. At the time, I thought I could trust memory. I couldn't. And if you're in front of a customer who says 'just make it cheaper,' slow down. Cheaper is not always lower cost.

The F22 fault on a Vaillant boiler taught me more than any training class. Expansion vessel pressure, water fill pressure, air purge, PRV test—those are the real boiler installation fundamentals. Furnaces have their own list. The tools vary, but the principle is the same: every component matters, and 'good enough' is not a setting.

I still use a Ryobi fan on hot jobs, and I still check the expansion vessel before I fill the system. You don't need to know everything before you start. You just need to know which mistakes are waiting to happen.