Last reviewed: September 2026 · By the HVAC Brief Editorial Team · Our sourcing and editorial standards
A new gas hot water boiler must reach at least 84% AFUE and an oil hot water boiler 86%, under 10 CFR 430.32(e). Those floors have applied to units manufactured on and after January 15, 2021, and they come with design requirements: no constant-burning pilot on gas, and automatic water temperature control on most classes.
The bigger decision is condensing or not, because that changes the venting, the condensate drain and the water temperature the system must run at to deliver the efficiency on the label.
Federal minimums by boiler type
The standards apply by product class, and the design requirements attached to them matter as much as the efficiency figure.
| Boiler class | Minimum AFUE | Design requirements |
|---|---|---|
| Gas-fired hot water boiler | 84% | Constant-burning pilot not permitted; automatic means for adjusting water temperature required |
| Gas-fired steam boiler | 82% | Constant-burning pilot not permitted |
| Oil-fired hot water boiler | 86% | Automatic means for adjusting temperature required |
| Oil-fired steam boiler | 85% | None |
| Electric hot water boiler | No AFUE minimum | Automatic means for adjusting temperature required |
| Electric steam boiler | No AFUE minimum | None |
Standby and off-mode power are also capped, at 9 watts for gas hot water boilers and 11 watts for oil. The standards attach to the date the boiler was manufactured, not to when it is installed.
Condensing or non-condensing
This is the choice that drives the rest of the job. A condensing boiler extracts heat from the flue gases until water vapour condenses, which is where the extra efficiency comes from.
| Factor | Non-condensing | Condensing |
|---|---|---|
| Typical efficiency | At or near the federal floor | Above it, up to the mid 90s |
| Venting | Chimney or flue | Plastic pipe, often through a side wall |
| Condensate | None | Acidic condensate needs a drain and often neutralising |
| Return water temperature | Not critical | Must stay low enough to condense, or the efficiency is not realised |
| Best suited to | Existing high temperature radiators | Radiant floors and low temperature emitters |
The last row is the one that gets missed. A condensing boiler connected to old cast iron radiators run at high water temperature will not condense much of the time, so it will not deliver its rated efficiency. Our page on radiant floor heating covers the low temperature case where condensing boilers do their best work.
If the old boiler shared a chimney with an atmospheric water heater, replacing it with a sidewall-vented condensing boiler leaves that water heater venting alone into an oversized, now cold chimney. That can cause poor draft and condensation inside the flue. It has to be dealt with in the same job, usually by relining the chimney or changing the water heater. The same problem arrives with the 95% AFUE furnace standard in 2028.
Sizing, and why bigger is worse
Boilers are commonly replaced like for like, which repeats whatever oversizing the original install had. An oversized boiler short cycles: it satisfies the thermostat quickly, shuts down, and starts again, which wastes fuel and wears the burner.
A heat loss calculation for the building is the correct basis, the boiler equivalent of a Manual J load calculation. Ask for it, and ask what the radiator or emitter output is at the water temperature the new boiler will actually run.
What it costs to run
At 2025/26 heating season average prices and federal minimum efficiencies, heat from an 84% gas boiler cost about $17.92 per million Btu delivered, against $33.68 from an 86% oil boiler.
Fuel price dominates that comparison, not the few points of efficiency between classes. The full table, including heat pumps and electric resistance, is in what heat costs by fuel. For how boiler systems distribute that heat, see boilers and baseboard heating.
What to ask before signing
- What heat loss calculation was done, and what output did it give?
- Condensing or not, and if condensing, what return water temperature will the system run at?
- Where does the flue terminate, and what happens to anything else on the old chimney?
- Where does the condensate drain, and does it need neutralising?
- What is the AFUE of the specific model, and what is its standby power?
Our contractor checklist covers the licence, permit and warranty side of the same conversation.
Frequently asked questions
What AFUE does a new boiler need?
At least 84% for a gas-fired hot water boiler, 82% for a gas steam boiler, 86% for an oil-fired hot water boiler and 85% for an oil steam boiler, for units manufactured on and after January 15, 2021, under 10 CFR 430.32(e). Electric boilers have no AFUE minimum.
Is a condensing boiler worth it?
Only if the system can run at a low enough return water temperature to condense. Paired with radiant floors or generously sized low temperature emitters it delivers its rated efficiency; connected to old high temperature radiators it often will not. It also needs plastic venting and an acidic condensate drain.
What size boiler do I need?
Size it on a heat loss calculation for the building, not on the output of the boiler being replaced. Like-for-like replacement repeats any oversizing in the original installation, and an oversized boiler short cycles, which wastes fuel and wears the burner.
How much does a boiler cost to run?
At 2025/26 season average fuel prices and federal minimum efficiency, about $17.92 per million Btu of delivered heat from a gas hot water boiler and $33.68 from an oil one. Fuel price matters far more than the efficiency difference between classes.
Methodology and limitations
Efficiency and design requirements are quoted from 10 CFR 430.32(e)(2). Household counts are EIA RECS 2020 table HC6.1. Running costs come from our fuel cost page, which uses EIA season average prices and federal minimum efficiencies.
- Standards apply to manufacture date, not installation date.
- We publish no installed prices, because no federal series measures them.
- Heat loss calculation procedures are private industry standards, which we name but do not reproduce.
Sources
- Code of Federal Regulations, 10 CFR 430.32(e)(2), residential boiler standards.
- US Energy Information Administration, Residential Energy Consumption Survey 2020, table HC6.1.
The HVAC Brief is an independent trade publication. We do not sell equipment, take manufacturer advertising, or accept payment for placement in our research. Corrections welcome.
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