By Derim “Mike” Vinca, Owner of A-1 Milwaukee Heating & Cooling · 17+ years in the trade · Updated October 2026 · More HVAC guides
A lot of Milwaukee homes still heat with boilers: in Bay View, Wauwatosa, West Allis, Shorewood, the East Side and much of the South Side we work on hot-water and steam radiator systems every week. When the boiler fires but the house stays cold, the problem is almost always in how the heat is being moved, not in the boiler itself. Here is how we sort it out.
First, which kind of system do you have?
Look at a radiator. If it has a small air vent on one end that hisses when the heat comes up and the pipe comes into one side only, it is a steam system. If there are two pipes to each radiator and a bleeder screw at the top, it is hot water. A sight glass with a water level on the boiler also means steam.
Hot-water systems: common causes
- Air in the system. Upstairs radiators that are cold at the top and warm at the bottom, or that gurgle, have air in them. Bleeding them with a radiator key often fixes it. If you have to keep bleeding, there is a leak or a bad air eliminator.
- Low system pressure. The gauge on the boiler should usually read 12 to 15 psi cold on a two-story house. If it is near zero, water is not reaching the upper floors. The fill valve or expansion tank may have failed.
- Failed circulator pump. Boiler hot, pipes near it hot, everything else cold. Sometimes you can hear a hum with no flow. Usually a same-day replacement.
- Stuck zone valve or zone pump. One zone cold, the others fine. The valve motor or end switch has failed.
- Mineral-clogged piping. Older systems with years of fresh water added can scale up, especially where someone has been topping off a leak.
Steam systems: common causes
- Clogged or failed radiator vents. Steam cannot enter a radiator until the air gets out. A vent stuck shut means a cold radiator; one stuck open spits water. Cheap to replace, and the most common fix.
- Failed main vents. If the radiators farthest from the boiler are always last and coldest, the main line vents in the basement are likely done.
- Water level too low or too high. Check the sight glass. Too low and the low-water cutoff shuts the burner off; too high and the boiler surges and bangs.
- Radiator pitch. A one-pipe radiator must slope slightly toward the supply valve. If it has settled the other way it fills with water and hammers.
- Partially closed supply valves. On one-pipe steam, the valve must be fully open or fully closed. Halfway causes banging and poor heat.
What you can safely do yourself
Bleed hot-water radiators, check the pressure gauge, check the steam sight glass and make sure radiator valves are fully open. Do not add large amounts of water to a steam boiler that is hot, and do not try to adjust the pressuretrol, aquastat or gas valve. If the boiler is leaking, short-cycling on the low-water cutoff, or you smell gas, shut it down and call.
Why call A-1 for a boiler
Boilers are a different trade from furnaces, and a lot of HVAC companies send a furnace tech who guesses. We service, repair and replace hot-water and steam systems in Milwaukee’s older housing every week. You get a diagnosis with readings, a flat-rate price before work and an honest answer on whether the boiler itself has years left, which it usually does. More on our boiler repair and replacement, and on ductless mini-splits if you want air conditioning without ductwork. Based in South Milwaukee, working throughout Milwaukee and Waukesha counties. Call or text 414-595-5175.
Frequently asked questions
My radiators bang loudly when the heat comes up. Is that dangerous?
Water hammer in a steam system is not usually dangerous, but it means water is pooling where it should not, from pitch, a bad vent or a water-level problem. It is fixable and worth fixing before it damages a valve.
Should I replace my 40-year-old boiler?
Not just because of age. Cast-iron boilers routinely run 40 to 60 years. We inspect the sections for leaks and corrosion and test combustion, then tell you honestly where it stands.
Can I add air conditioning to a radiator house?
Yes. Ductless mini-splits are the usual answer and avoid tearing into plaster for ductwork.