[LazyPup]

Boilers used for home heating fall into one of two categories. 1. Low pressure saturated live steam and 2. Low-pressure Hydronic (circulating hot water) boilers.

Per code;
Saturated live steam boilers are limited to a maximum working pressure of 15psi
Hydronic boilers are limited to a maximum working pressure of 30psi..

While the maximum operating pressure of the boilers will be either 15psi or 30psi it must be remembered that the house potable water supply pressure will be in a range of 35 to 80psi therefore if the house supply water were to feed directly into the boiler it would put the boiler in a very dangerous overpressure condition. One the other hand, the feedwater system does need to be capable of supplying water to the boiler under the full range of operating pressures.

In order to achieve this goal the pressure-reducing valve on the house cold water supply line is set to reduce the house water pressure down to the maximum working pressure of the boiler. After the water passes through the Pressure Reducing Valve (PRV) it is then fed into the boiler by means of an internal “Float control” that regulates the water level in the boiler in basically the same manner as the float control in your toilet tank.

In nature most elements or compounds expand when heated and contract when cooled, but water has a rather unique property. Water reaches its maximum density (62.4lbs/cu.ft) at 39degF. It must also be noted that in all three forms, Solid (ice), liquid (water) or gaseous (steam) water is not compressible.

When water freezes and expands in a pipe or vessel it will literally rupture the pipe or vessel, which explains why we find frozen water pipes that are ruptured or why we must put anti-freeze in our automobile engines.

When water is heated above 39degF it will expand until it reaches a maximum expansion of approximately 10% by volume, then it will flash to steam. When water flashes to steam it expands by 1728 times by volume, thus one cubic inch of water will produce one cubic foot of steam.

In a steam boiler the water level at cold startup is at about ¾ the volume of the boiler vessel. The upper ¼ is then called the steam header. There is a pressure switch in the steam header. As the water is heated the water expands upward into the steam header portion until the water reaches maximum expansion. Once it reaches maximum expansion the water will start flashing to steam, which then fills the remaining portion of the steam header. There is a pressure switch in the steam header that senses the pressure and turns the burner off when the steam reaches a preset pressure limit.

In a Hydronic boiler the water vessel is completely full of water and the working pressure is actually controlled by a thermostat that senses the internal water temperature. We must then install an external “Expansion Tank” which has an air chamber inside. As the water expands it enters the expansion tank and the trapped air inside compresses to absorb the expansion.

To answer the original question. Why is the pressure different on your two boilers?

Answer: The actual pressure in your boilers is a direct result of the temperature of the water in the boilers and has absolutely nothing to do with the PRV. The difference in pressures between the two boilers is directly proportional to the individual boilers rate of cooling between fire cycles.

CAUTION..Due to the sensitive nature of boiler controls and the potential hazards of improperly setting them, no one other than a trained professional should ever touch the boiler controls.





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