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Re: Pressuretrol Location

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Posted by HeatPro on December 06, 2007 at 10:52:06:

In Reply to: Pressuretrol Location posted by Boston on December 06, 2007 at 08:34:03:

: The boiler runs until the thermostat tells the system to shut off… the pressure switch never shuts the system down during the firing cycle. This means the boiler is running for 45min or more (depending on the temp in the house it could run for over an hour)… for obvious reasons this is using allot of oil. Is it normal that the system is firing for that long?
++ Yes. When the weather is near as cold as it will be, the burner might fore for that long. It is heating the house when needed. As heat loss is BTUH, the "H" being the hour, you need to fire it for near an hour when the heat loss takes an hour with the correct input rate.

If you want it to go on and off more frequently, then oversize the unit. Some folks will go to the 'reductio ad absurdum:' wanting the burner 'not to work so hard' they might but a boiler in 50 times larger than necessary, so it only runs one minute out of the hour.

Does the pressuretrol register the pressure of the expanding water or the steam?
++ By the laws of physics the pressure inside the boiler will be the same in the steam drum above the water and below the water line "(excepting a small difference due to weight of water at that point.) The pressuretrol and gauge read the steam pressure, not the hydraulic pressure of expanding heated water.

Shouldn’t the pressuretrol be located above the boiler water line?
++ Yes. That is one reason for the pigtail, to give a water seal so the innards of the gauges aren't subjected to the steam temperature.

However, we will hope that the reason the burner runs the whole hour is that the gauges are still reading the correct pressure, and that pressure is below the 2 psig maximum needed while the burner is running, or the pressuretrol isn't working.

To find the most likely location, look at the drawings of the new boilers that are available at Burnham, Weil McLain and other boiler manufacturer's instruction sheets on the web. As an example, look at the location of the pressuretrol on this ad page. It is above the middle of the gauge glass, about as high as the top of the top of the glass.

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