Boiler Pressure and Temp range

Old 01-16-09, 08:00 PM
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Boiler Pressure and Temp range

I have a 3 story house with old galvanized heating piping and hotwater heat system. Aquastat just replaced by pumber, also new bladder expansion tank set at 15psi.

Today I checked the guages and the pressure was 20. Only change I had made was to increase the temp so that the boiler cycled between 190 and 160 degrees F, since we are having a cold snap and the house wasn't getting warm enough. I drained off some water through the over pressure release valve and now the pressure fluctuates with the temp. At its lowest it is 10 and highest is 16.

There is an adjustable valve on the water intake that the plumber said he adjusted. Another plumber from the same firm said I should turn off the water intake all together to get a stable pressure since the intake valves don't work all that well.

Questions: With the new bladder tank should the pressure fluctuate at all or should it stay at the 15 mark since that is what the bladder tank is supposed to be at?
Does pressure normally fluctuate as described with the temperature?
Should the water intake be off all together or should that darn intake valve pressure controller work better.
Thanks for your help,
Old 01-16-09, 08:19 PM
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Luke, there was nothing wrong with 20 PSI... perfectly acceptable. Should not have drained the water.

Pressure WILL fluctuate... HAS to ! When you heat the water it expands. Expansion tank takes up the excess expanded water. When it cools, it shrinks again.

ALL systems pressure will fluctuate. How MUCH it fluctuates is a function of how much water is in the system, how much of a temperature swing it sees, and the size of the expansion tank.

MINIMUM pressure is generally accepted as 12 PSI for a number of reasons that I won't explain just now...

WAIT... I just re-read yer message... if you have a THREE STORY house with the boiler in a basement, that means that your MINIMUM pressure is going to be at LEAST 17 PSI ... if you let it drop to 10 PSI, you will start having problems with heat on the upper levels and air in the system.

What is the HEIGHT from the bottom of the boiler to the tippy top of the highest radiator or baseboard in the system?

The proper pressure of a COLD! system is going to be :

(HEIGHT in FEET X .433) + 4 ... if you are looking at say 30 feet, that's 17 PSI .

When it's HOT, expect to see at LEAST 3 PSI higher, and it could be as much as 10 PSI higher without any trouble or problem. Anything higher than 27 PSI needs to be looked into.

The pressure in the expansion tank is supposed to be set to match the system pressure when the system is COLD.

Sure, get the auto feed valve fixed up, but you probably don't wanna do that in the middle of a cold snap... if you watch the pressure on the boiler and don't let it drop too low, you should be fine until spring.
Old 01-17-09, 12:28 PM
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Thanks very much for the complete answer. That settles all of my concerns. This is my first visit to this site. It's a wonderful resource.
Best regards,

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