Old Boilers: Keep, Fix, or Replace
When a converted coal boiler or an old gravity system should be replaced, what to replace it with, why a natural-draft boiler and new air sealing can be a dangerous pair, and how to keep the old radiators with a new boiler.
- 3 min
- 5 steps
- 2 questions
- Lesson 84 of 111
In this lesson
- Gravity and converted coal boilers
- When to replace
- Venting and air sealing
- Keep the radiators
- Try it
Picking up where you left off.
Gravity and converted coal boilers
Many old houses still run original gravity boilers, which have no pump and rely on hot water rising. They rarely break down, but they are “incredibly inefficient”: many run at no more than about 60 percent AFUE (annual fuel utilization efficiency), against 80 percent and above for today’s standard boilers, so replacing one is especially cost-effective 1. Coal boilers converted to oil or gas are the same story. The National Park Service’s brief on heating old buildings shows one converted coal boiler still at work 2.
Quick check
That’s why replacing one is especially cost-effective.
When to replace
Minnesota’s Center for Energy and Environment recommends replacing a boiler at 25 years or older, before it fails, so there’s time to choose well rather than taking whatever is available on a cold day 1. Replace it with a condensing boiler of at least 90 percent AFUE; new high-efficiency boilers cost about $11,000 to $12,500, and gas utilities often offer rebates 1.
Venting and air sealing
Most 80-percent boilers are natural-draft: their exhaust rises up the chimney on its own and can spill back into the house. Boilers of 90 percent and above use sealed combustion, pushing exhaust out through a closed vent. This matters most when you insulate and air seal (module 2): a tighter house can make a natural-draft boiler spill, and the best fix is a sealed-combustion boiler 1. Have the draft tested after any air sealing, and keep carbon monoxide alarms on every level (module 5).
Quick check
Sealed-combustion boilers vent their exhaust in a closed pipe and can’t spill.
Keep the radiators
The radiators can stay. The Park Service recommends reusing radiator systems with new boilers where possible; new boilers and circulating pumps can serve old hot-water radiators 2. For steam, a properly sized boiler matters as much as the vents: an oversized or poorly controlled boiler makes loud vents and wet steam 3.
Try it
Find the boiler’s nameplate and write down its make, model, input rating, and any date. Look up its AFUE, or note that it’s a gravity or converted coal unit. That page is what a contractor needs to quote a replacement.
Lesson complete
Nice work.
Sources for this lesson
- 1Clara Higgins. Replacing Your Boiler. Center for Energy and Environment (Minnesota). 2025. verifiedReplace a boiler at 25 years or older, before it fails. Upgrade to a condensing boiler of at least 90% AFUE; these have sealed combustion venting, while most 80% boilers are natural-draft and can spill exhaust gases into the house, a concern when insulating and air sealing. Original gravity boilers are simple but 'incredibly inefficient'; many do not operate above 60% AFUE, so replacing them is especially cost-effective. New high-efficiency boilers about $11,000-12,500; check utility rebates.
- 2Sharon C. Park. Preservation Brief 24: Heating, Ventilating, and Cooling Historic Buildings. National Park Service, Technical Preservation Services. 1991. verifiedBy the late 1800s steam and low-pressure hot-water radiator systems were common, often fired from coal; hydronic radiators can be kept and served by new boilers and circulating pumps. Recommends reusing radiator systems with new boilers where possible and keeping historic grilles and radiators; cover photo of a coal boiler converted to oil and still in use.
- 3Steam System Balancing for Multifamily Existing Buildings. Building America Solution Center, Pacific Northwest National Laboratory (U.S. DOE). verifiedAir left in steam pipes after a cycle must be pushed out before steam can reach radiators; air trapped in mains and risers ('air binding') leaves far rooms underheated. 'Most steam systems have vents that are too small'; install very-high-capacity master vents at the ends of mains and tops of risers. 'Air binding exemplifies coal's legacy': coal fires ran slowly all day, while oil and gas fire hard and cycle repeatedly, so air must vent fast. Oversized or poorly controlled boilers make loud vents and wet steam.