Badger HomeownerWisconsin home guide

Radiators and Boilers

Many older Madison-area houses still heat with a boiler and cast iron radiators. Hot-water and steam systems look alike but work differently, and each has a short list of things an owner can check. This is drawn from the manufacturers' own manuals.

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Hot Water or Steam

  • A boiler heats water and sends either hot water or steam to the radiators.(DOE)
  • Steam boilers have a water-level gauge glass, a pressure gauge, a pressure limit, a low-water cutoff and a safety valve, and run about three-quarters full of water.(HUD) One-pipe steam radiators each have an air vent; two-pipe radiators have a steam trap on the return.(HUD)(Hoffman Specialty (Xylem))
  • Hot water systems have a circulating pump, an expansion tank and a pressure gauge that usually reads 12 to 22 psi; radiators have a small bleed valve near the top.(HUD)
  • Steam runs hotter and is inherently less efficient than hot water.(DOE) HUD says steam is seldom installed in houses now but still common in older ones, and needs more upkeep.(HUD)
  • Old cast iron radiators are extremely durable and normally reusable.(HUD)

Hot-Water Systems

  • Pressure. Typical cold-fill pressure for a house is 12 psi, and the relief valve limits it to 30 psi.(Weil-McLain)
  • The expansion tank lets heated water expand against an air cushion. A waterlogged tank makes the relief valve dump water each time the system heats up; HUD says recharging it is a contractor job.(HUD) Weil-McLain says repeated dumping and refilling can eventually crack boiler sections.(Weil-McLain)
  • Uneven heat on hot water usually means air in the radiators. DOE also suggests adjusting radiator valves for an 18 to 20°F drop from inlet to outlet.(HUD)(DOE)

Bleeding Radiators

  1. Bleed hot-water radiators at the start of each heating season.(DOE)
  2. Start on the lowest floor and work up, one radiator at a time.(Weil-McLain)
  3. Open the bleed valve and close it when water squirts out.(Weil-McLain) Have a rag and cup ready.
  4. Check the boiler's pressure gauge afterward. HUD says the makeup valve adds water when pressure falls below 12 psi.(HUD)

Steam radiators aren't bled. DOE says check their air vents and replace any that are clogged or stuck open.(DOE)

Steam Systems

  • How it works. In one-pipe steam, steam rises to each radiator, pushes air out the vent, condenses, and drains back down the same pipe. The pipes must slope back toward the boiler.(HUD) Every radiator and the end of every main needs an air vent.(Hoffman Specialty (Xylem))
  • Low pressure. U.S. Boiler says the pressure control should normally cut out at 2 psi or less and cut in at 1/2 psi or less.(U.S. Boiler) Peerless sets 2 psi with a 1.5 psi differential.(Peerless) If the pressure runs above a vent's rating, the vents can stay shut and heat gets uneven.(Bell & Gossett (Xylem))
  • Water level. Normal is halfway up the gauge glass.(Weil-McLain) A full glass means a flooded boiler; an empty one means too little water.(HUD) U.S. Boiler says even with an automatic feeder, nothing replaces the owner checking the level.(U.S. Boiler)
  • Low-water cutoff. It shuts the burner off when the water gets low. McDonnell & Miller's float-type No. 67 should be blown down weekly during the heating season, inspected yearly and replaced every 10 years.(McDonnell & Miller) Newer probe-type cutoffs aren't blown down; the service tech tests them.(Weil-McLain) Follow the manual for the control your boiler has.
  • Banging pipes near a radiator can often be fixed by tipping the radiator slightly so water drains back.(HUD)
  • Insulate steam pipes; U.S. Boiler says mains need at least 1 inch of insulation.(U.S. Boiler) Old pipe covering may contain asbestos: see asbestos rules before touching it.(EPA)

Safety and Yearly Service

  • CPSC urges a yearly professional inspection of every fuel-burning heating system, and CO alarms on every level and outside sleeping areas.(CPSC)(CPSC) Boiler makers say the same: service by a qualified technician at least once a year.(Weil-McLain)(Peerless)
  • If the relief valve discharges water, shut off gas to the boiler and call for service.(Peerless) HUD warns an old relief valve may stick open when tested, so leave the test to a technician with a spare on hand.(HUD)
  • CO symptoms: headache, dizziness, weakness, nausea, vomiting, sleepiness, confusion. If you suspect CO, get outside and call 911.(CPSC) CO alarm rules covers what Wisconsin requires.

Replacing a Boiler

  • Service life. HUD's rehab guide: about 20 years for most steel boilers and 30 for cast iron. Old coal boilers converted to gas or oil may last longer but are usually quite inefficient.(HUD)
  • Efficiency. Old natural-draft boilers with pilot lights run 56 to 70% AFUE; condensing, sealed-combustion boilers 90 to 98.5%.(DOE) The federal minimum for boilers made since January 15, 2021 is 84% for gas hot water and 82% for gas steam, with no standing pilot.(10 CFR 430.32)
  • Venting. High-efficiency boilers produce acidic exhaust that an old unlined chimney can't handle, so they need new venting or a liner.(DOE)
  • Sizing. DOE says improve insulation and windows first, then have the contractor size the boiler.(DOE) For steam, U.S. Boiler says size from the radiators, not by matching the old boiler.(U.S. Boiler)
  • Converting. HUD says two-pipe steam can be converted to hot water, usually with new, larger return piping.(HUD) Heat pump vs gas compares running costs.

Rebates in 2026

  • Focus on Energy takes an instant discount off natural gas boilers through the contractor's invoice: $300 at 90% AFUE, $350 at 95%, and $450 for a 95% combination boiler, or $500, $550 and $650 for income-qualified households.(Focus on Energy)
  • Equipment must be new, replace existing equipment, and be bought and installed in 2026.(Focus on Energy)
  • The federal 25C credit doesn't apply to equipment placed in service after December 31, 2025.(IRS) Energy rebates has the full 2026 list.

Permits and Contractors

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Questions

What pressure should my hot-water boiler be at?

Weil-McLain gives 12 psi as the typical cold-fill pressure for a house, and its relief valve limits pressure to 30 psi. HUD's rehab guide says 12 to 22 psi is normal. If water keeps coming out of the relief valve, shut off the boiler and call for service.

How do I bleed a radiator?

Hot-water radiators have a small bleed valve near the top. Open it until water comes out, then close it. Weil-McLain says to work from the lowest floor up. DOE says to bleed at the start of each heating season. Steam radiators aren't bled; they have an automatic air vent instead.

What pressure should a home steam boiler run at?

Low. U.S. Boiler says the pressure control should normally cut out at 2 psi or less, and Peerless sets its limit at 2 psi with a 1.5 psi differential. Follow your own boiler's manual.

Is there a rebate for a new boiler in 2026?

Focus on Energy takes $300 to $650 off qualifying natural gas boilers at 90% AFUE or higher, more for income-qualified households, through the contractor's invoice. The federal 25C tax credit ended for equipment installed after 2025.