“What is the actual difference, beyond the price?”
A furnace burns fuel
A furnace burns gas to heat a metal heat exchanger, and a blower pushes house air across it. Warm air goes out through the supply ducts, cooler air returns through the returns.
Because it burns fuel, it has combustion and venting to get right, and both should be tested — not just at installation but at maintenance. It needs ductwork, and it heats the house in comparatively short, hot bursts.
A heat pump moves heat
A heat pump does not create heat, it relocates it. Refrigerant absorbs heat from outdoor air — there is usable heat in cold air — and releases it indoors. Reverse the cycle in summer and the same machine is an air conditioner.
Because it moves heat rather than making it, it can deliver more heat energy than the electrical energy it consumes. It produces a gentler, longer supply of warm air than a furnace, which people often read as 'not as hot' even when the house holds temperature better.
Cold-climate inverter models are specifically engineered to keep useful capacity at low outdoor temperatures. How a given model behaves in an Illinois January is a question about that model's data, not about heat pumps as a category.
A boiler heats water
A boiler heats water and circulates it to radiators, baseboard or in-floor loops. There is no ductwork and no blower, so there is no forced air — which means no duct losses and no air being moved around, and also no path for central air conditioning or duct-based filtration.
Plenty of homes heat this way, and they need a different skill set: circulator pumps, expansion tanks, zone valves, air trapped in the loop, system pressure. Combustion testing still matters, for the same reasons it does on a furnace.
What actually decides it
- What the house already has — ducts, or pipes and radiators
- Whether you want cooling from the same system
- What the ductwork can actually carry, measured rather than assumed
- A load calculation for this house as it is now, not a size matched to the old equipment
- How the house is used: gentle continuous heat suits some households, fast recovery suits others
The sizing point, again
Whichever of the three, size it from a load calculation. Oversized equipment short-cycles, controls humidity poorly in summer, and wears faster — and 'the old one was this size' is not a calculation, it is an inherited guess.