The Correct PSI Reading for a Hot Water Boiler System
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The Correct PSI Reading for a Hot Water Boiler System

Understand what the psi pressure reading means on a hot water boiler.

The normal pounds per square inch (psi) reading of a hot water boiler are important for the proper operation of the unit. The unit will not function properly if the pressure is too high or too low. Before you place your boiler in service, you need to learn about the pressure reading and some other facts about maintaining the boiler.


Hot water boilers need to maintain a safe psi level. Ideally, the acceptable reading for the boiler is 12 to 15 psi, but some may go as high as 20 psi. Generally, when the pressure is below 12 psi, the boiler needs water or it could be a more serious problem such as deposits in the system lines.


Hot water boilers must maintain a water temperature of 140 to 180 degrees Fahrenheit. If the temperature on the combination pressure/temperature gauge goes above 200 degrees Fahrenheit, the system is not functioning correctly. If the temperature of the water is high, the pressure reading will be above 15 to 20 psi.

Type of Water

When you fill your boiler or have an automatic feeder, you must use water with less than 200 parts per million (ppm) chlorine and a pH of between 7.0 and 8.5 ppm. Generally, household tap water has only about 5 ppm chlorine and ideal pH levels. If you are unsure of the acidity of the water, you can call your local water company to request a water test or have your well water tested if you have well water. Never use water from a water softener or the system or corrosion can result.

Amount of Water

The sediment in boilers must be drained from the unit every two weeks to get the correct water level reading in the boiler. The minimum water level in the sight glass on the boiler is half full. The maximum level is three-quarters full. If you don’t drain out the sediment in the bottom of the water tank, you won’t get a factual reading. If there are inches of sediment on the bottom of the tank, you will have a water level reading in the sight glass that includes the sediment because the water sits on top of the sediment. This will cause a low psi because the unit doesn't have enough water.

Automatic Water Feeder

Boilers that have automatic water feeders still need to be checked regularly. The feeders are for emergencies only. For stance, if you are gone and the water level drops below the halfway mark, the feeder adds water to the boiler and the water pressure (psi) on the boiler remains at a safe level. However, this is only a safety feature and not a replacement for inspecting the sight glass weekly to check the water level.

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Comments (2)

Great and informative share.thanks for posting

Great presentation, keep it up!