How to Bleed Radiators: A Guide for Older Twin Cities Homes with Boilers

If you live in one of the older homes around the Twin Cities, maybe a 1920s bungalow, a Tudor near 50th and France or a Victorian in Anoka, there's a good chance you heat with a boiler and radiators. They're comfortable, quiet and durable, but they have one chore forced-air homes don't: bleeding the air out of the radiators.

Here's how to tell if your radiators need it, how to do it safely, and what to watch on the boiler afterward. First, though, you need to know which kind of system you have.

First: hot water or steam?

Bleeding is for hot water systems. Steam systems work differently, and you should not try to bleed a steam radiator with a key.

• Hot water systems have a pressure gauge on the boiler, often combined with a temperature gauge, reading in psi. Radiators usually have a small square or slotted bleed valve near the top of one end, and two pipes connected to each radiator. Many have an expansion tank near the boiler.
• Steam systems have a sight glass on the boiler showing a water level, and the gauge reads very low pressure. Radiators often have a single pipe and a small vent, shaped like a bullet or cylinder, partway up one end. That vent hisses as steam pushes air out. If your vents are spitting water or not hissing, that's a service call, not a bleeding job.

If you're not sure, call us before you open anything. Hot water and steam systems both show up in older Twin Cities homes, and some houses have had their original systems converted over the years.

Signs your radiators need bleeding

• The radiator is warm at the bottom but cool at the top.
• You hear gurgling, trickling or bubbling in the radiators or pipes.
• One radiator, often on the top floor, stays cold while the others heat.
• The system was recently drained, refilled or repaired.
• It's the start of the heating season and the system has sat idle all summer.

What you need

• A radiator key that fits your bleed valves, available at most hardware stores. Some valves take a flat screwdriver instead.
• A rag or small cup to catch water.
• A towel to protect the floor.

How to bleed a hot water radiator

• Turn the thermostat down so the boiler and circulator pump stop, and let the radiators cool for an hour or so. Hot water under pressure can scald, and bleeding with the pump running can pull more air in.
• Start with the radiators on the top floor. Air rises, so it collects at the highest points in the system.
• Hold the rag under the bleed valve and turn the key slowly counterclockwise, about a quarter to half turn. You should hear air hissing out.
• When a steady stream of water comes out with no sputtering, close the valve. Don't force it or over-tighten it.
• Move to the next radiator and repeat, working your way down through the house.
• Check the boiler's pressure gauge when you're done, then turn the heat back up and feel the radiators after they warm.

Check the boiler pressure afterward

Every bit of air you let out is replaced by water from the boiler's fill valve, or it's simply lost from the system. That's why the pressure gauge matters. Most residential hot water systems run at roughly 12 to 15 psi when cold, rising somewhat as the water heats. Your boiler's manual or the label on the boiler gives the right range for your system.

If the pressure is low after bleeding, many systems have an automatic fill valve that tops up the water on its own. Some older systems need water added manually. If you're not comfortable doing that, or you don't know where the fill valve is, call us. Too little pressure means the top-floor radiators won't fill and air gets drawn back in. Too much can make the relief valve on the boiler open and drip.

When to bleed, and how often

The best time is early in the heating season, after the boiler has run for a day or two and pushed any air that collected over the summer to the radiators. Most homes only need it once a year. Do it again after any repair that involved draining or refilling the system. If one room still runs cooler after bleeding, the radiator's own valve may be partly closed or stuck. Older radiator valves can be stiff, so turn them gently, and don't force one that won't move.

When bleeding isn't the fix

Bleeding once a season is normal. Bleeding every week isn't. If air keeps coming back, or you keep needing to add water, something else is going on.

• A small leak somewhere in the system lets air in and water out. Look for drips at radiator valves, pipe joints and the boiler.
• A waterlogged expansion tank can cause big pressure swings and a dripping relief valve.
• A failing automatic air vent or air separator near the boiler can let air build up.
• A circulator pump that's weak or failing can leave far radiators cold even with no air in the system.
• Sludge buildup in very old systems can block flow. Flushing or treating the system may help.

Any of those calls for a technician. So does water on the floor around the boiler, a relief valve that's dripping or spraying, or pressure that climbs past the top of the normal range.

Radiant floors and newer boilers

If your home has radiant floor heating, the tubing is purged through a manifold rather than bled at a radiator. That's a job for a technician. The same goes for many modern high-efficiency boilers, which have automatic air elimination built into the system. If one of those zones isn't heating, call rather than open fittings.

Keeping an older boiler system healthy

An annual boiler service checks the burner, venting and safety controls, the expansion tank, the relief valve and the circulator, and it's the best time to bleed and balance the system. Boiler maintenance can be added to our maintenance plans. If you're wondering whether to keep your boiler or switch to forced air, our guide to boilers vs. forced air in older Twin Cities homes walks through the tradeoffs.

We repair and replace boilers and radiant systems across the Twin Cities. If your radiators won't heat after bleeding, or you'd rather have us do it, call Heating & Cooling Two at 763-428-3677. After-hours emergency service is available.