Boiler Not Heating Radiators in Philadelphia? What to Check First
If your boiler is not heating radiators in your Philadelphia home, the usual causes are trapped air, low water or pressure, a stuck valve or vent, a circulator pump that quit, or a burner that isn't firing. Some of those you can check in five minutes. Others need a tech. Here's how to tell the difference.
I've been working on commercial and residential heating and cooling since 2005. Every fall, the first week people turn the heat on, the same calls come in: the boiler's running, but half the radiators are cold. Most of the time it's a fixable problem, not a dead boiler.
First: hot water or steam?
This matters, because the fixes are different. A lot of Philly rowhomes and older homes in Wynnewood, Narberth and Bala Cynwyd run on one or the other.
- Hot water (hydronic): the boiler has a pressure gauge, usually reading around 12 to 15 psi when it's cold, and radiators or baseboards have small bleed valves you open with a key or screwdriver.
- Steam: the boiler has a sight glass showing the water level, and each radiator has a metal air vent on the side that hisses when the heat comes up.
Not sure which you have? Take a photo of the boiler and the side of one radiator. That tells us a lot.
5 reasons your boiler isn't heating the radiators
1. Air trapped in the system
This is the big one in hot water systems. Air rises to the top floor radiators and blocks the hot water. The classic sign: the radiator is warm at the bottom and cold at the top. Bleeding it lets the air out.
2. Low water or low pressure
On a hot water system, if the gauge reads way below where it normally sits, there isn't enough water pushing through to reach every radiator. On a steam system, low water in the sight glass can shut the burner off completely. Either way, the system is telling you something is leaking or not refilling right.
3. A stuck valve or a bad air vent
A radiator valve that got closed over the summer, or one that's seized, stops heat at that one radiator. On steam, a clogged or painted-over air vent can't let air out, so the steam never fills the radiator.
4. The circulator pump stopped
Hot water systems use a pump to move water through the pipes. If the boiler is hot but the pipes leaving it are cool, and the radiators upstairs stay cold, the pump may be stuck or its motor may be done.
5. The boiler isn't really firing
Sometimes the problem isn't the radiators at all. A thermostat calling for heat that never reaches the boiler, a bad ignition part, or a safety switch that tripped will all leave every radiator cold. If nothing is warm anywhere, start here.
What you can safely check yourself
- Thermostat: set it a few degrees above room temp, set to Heat, and swap the batteries if it has them. A dead Honeywell battery fools a lot of people.
- Radiator valves: make sure they're open all the way.
- Bleed a hot water radiator: with the heat off and the radiator cool, crack the bleed valve with a towel underneath. Close it when water comes out steady with no air. Then check the boiler pressure gauge again.
- Look, don't touch, on steam: check the sight glass. If it's empty or very low, shut the boiler off and call. Don't add water to a hot steam boiler.
- Power: check the boiler's service switch and the breaker.
If you ever smell gas, get out of the house and call your gas utility first. Don't flip switches on the way out.
When it's time to call a tech
Call when bleeding didn't help, the pressure keeps dropping, the pump is hot or humming but not moving water, the burner won't light, or water is showing up on the floor. Those aren't guess-and-check jobs.
When we come out, we find the cause first, show you what we found, and tell you straight what it needs. We work on what Philly basements actually have, including Crown boilers and Honeywell thermostats. Every job comes with our 90-day guarantee on all work.
Keep it from happening next fall
Most cold radiator calls trace back to a system nobody looked at since last winter. A yearly tune-up catches low pressure, weak parts and air problems before the first cold night. Ours is $76: we clean it, vacuum it, wipe it down, take readings and tell you what's going bad.
And if your boiler is past saving, you'll hear that straight too. Free in-home estimates for qualified replacement projects.
Quick answers
Why is my boiler on but the radiators are cold?
Usually air in the lines, low pressure, a closed or stuck valve, or a circulator pump that stopped moving water. If the boiler itself is cold, the burner or the thermostat signal is the problem.
Why is only one radiator not heating?
One cold radiator almost always means a problem at that radiator: trapped air, a closed valve, or a bad air vent on steam.
Why are my upstairs radiators cold but downstairs is hot?
Air collects at the highest point, so top floor radiators go cold first. Low pressure in a hot water system does the same thing, because the water can't climb high enough.
Can I bleed radiators myself?
On a hot water system, yes, with the heat off and a towel ready. Steam radiators don't get bled with a key. If a steam vent is spitting water or not hissing at all, it needs to be replaced.
More: Boiler repair · Thermostat repair · $76 tune-up · Furnace tune-up checklist · All blog posts