Cold radiator may be victim of inaccurate pressure gauge

Inspector's Eye

November 18, 2001|By Dean Uhler

Chris Forsberg of Baltimore read a recent column I wrote on bleeding radiators and had some specific questions about his own system. Forsberg wrote that none of his third-floor radiators seemed to work last winter. He bled all the radiators recently, but when he got to the third floor, no water came out at any of the radiators after the air was bled.

When he ran the heat, the pipes to the third floor were hot, but the radiators there were warm only at the bottom. Recognizing that these could be symptoms that there wasn't enough water in the system, he checked the pressure gauge on the boiler. The boiler pressure gauge read 16 PSI (pounds per square inch), which is within the normal operating range.

He was concerned about adding water since the owner's manual said: "Continual make-up water will reduce boiler life. Minerals can build up in sections, reducing heat transfer, overheating cast iron, and causing section failure."

It sounds as though there's not enough pressure in the system to fill the third-floor radiators with water, since no water would come out of the bleed valves. But the pressure gauge indicated 16 PSI, more than enough pressure to keep most residential heating systems filled to the top. In general, systems are operated at only 14 PSI. Therefore, either the gauge is wrong and the pressure is lower than the gauge says it is, or the house is so tall that 16 PSI is not enough pressure to fill the heating system to the top.

It takes 1 PSI to fill a system with 2.3 feet of water vertically. Sixteen pounds of pressure will support 36.8 feet of water (16 pounds times 2.3 feet per pound). If the third-floor radiators in Chris' house are more than 36.8 feet above the boiler, the system pressure will have to be set higher to fill them to the top.

Since most houses are not that tall, it's more likely that the pressure gauge is inaccurate and that pressure is actually much lower than indicated. The reading on the gauge can be checked by buying a pressure gauge that screws onto the drain valve on the boiler, or a plumbing or heating contractor can check it. An inaccurate gauge should be replaced.

If it turns out the pressure gauge is correct, it may be necessary to add a few pounds of water to the system. It should not hurt the heating system to operate it at a somewhat higher pressure, say 20 PSI, if that's what it takes to fill the third-floor radiators.

When monitoring boiler pressure, bear in mind that it should never reach 30 PSI, which is the pressure at which the pressure-relief valve on the system should begin to vent water. Check this by turning up the heat after you've finished bleeding the radiators.

If the boiler pressure rises more than a few pounds above its normal cold pressure, or if water drips from the relief valve, the expansion tank on the system may be waterlogged. A waterlogged expansion tank is one so full of water (rather than air) that there isn't room for water in the heating system to expand as the system heats up. You can drain a waterlogged expansion tank yourself, or consult a contractor to do so.

The warning in your boiler's owner's manual against the continual addition of makeup water doesn't apply here. It is intended to address a system that is losing water (and, therefore, having fresh water added) on a regular basis. Adding makeup water once each season to increase system pressure will not cause mineral build-up.

Inspector's Eye

Dean Uhler is a home inspector and president of Boswell Building Surveys Inc. of Baltimore.

Questions, with name, address and daytime telephone number, about homes and home inspections can be faxed to 410-783-2517, e-mailed to or mailed to Inspector's Eye, Second Floor, 501 N. Calvert St., Baltimore, Md. 21278-0001.

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