Boiler vs furnace

How to read those two dials

Both are federal minimums on the same scale, annual fuel utilization efficiency. Today the bar is higher for a boiler: a gas hot-water boiler must reach 84% AFUE, while a non-weatherized gas furnace may still be sold at 80%. That flips for furnaces made from 2028-12-18, which must reach 95%, a condensing furnace in practice. At the top of the market both condense: ENERGY STAR asks 90% of a gas boiler and 95% (South) or 97% (North) of a gas furnace.

AFUE measures the appliance, not the delivery. Duct losses in an attic or crawlspace and heat lost from pipes in an unheated basement both come after the rating. The AFUE guide explains the test, and the AFUE fuel cost calculator turns any rating into dollars at your gas price.

Sources: 10 CFR 430.32(e)(1)(iii) (read 2026-09-04) · EPA ENERGY STAR, Boilers (read 2026-10-18) · EPA ENERGY STAR, Furnaces Key Product Criteria (read 2026-10-18)

The comparison

A residential hot-water boiler runs at no more than 160 psig and 250 °F, and a low-pressure steam boiler at no more than 15 psig. Boilers also carry two federal design rules furnaces do not: a gas boiler may not have a standing pilot, and a hot-water boiler needs an automatic means of adjusting its water temperature, so it can run cooler when the house needs less heat. Boilers with a tankless domestic coil are exempt from the second rule.

Sources: 10 CFR 430.2, definition of Low pressure steam or hot water boiler (read 2026-10-18) · 10 CFR 430.32(e)(2)(iii)(A) (read 2026-10-18)

Cooling is the deciding question

A furnace already has a blower, a coil cabinet and ducts, so central air is an outdoor unit and a coil away. A boiler house has none of that. Its cooling options are ductless heads, a small-duct high-velocity system, or new ductwork. If the house has a boiler and needs cooling, read ductless vs central air and size the heads with the mini-split sizing calculator.

Air quality and comfort

A furnace moves the whole house's air through one filter over and over, which is an asset with the right filter (see MERV 8 vs MERV 13) and a dust pump with the wrong one. A boiler moves no air at all, so it stirs up no dust and adds no drafts. It also filters nothing and ventilates nothing, so a tight boiler house still needs mechanical ventilation (see ERV vs HRV).

Replacing one

Replace like with like and the distribution system is reused. That is the biggest cost lever. Switching carriers is a renovation: boiler to furnace means building ducts, and furnace to boiler means piping and emitters in every room. Size the replacement to a load calculation, never the old nameplate. The furnace size calculator works for either, because both are sized to the same heating load.

Service

A furnace's routine is filters, the flame sensor, the heat exchanger and the venting. The furnace troubleshooting guide covers the common faults. A boiler's routine is system pressure, the expansion tank, air removal, the circulator, combustion and, on a condensing unit, the condensate neutralizer. On steam, the water level and the low-water cutoff come first. Both belong on a yearly visit; the maintenance guide has the list.