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What determinest t he pressure in a hot water boiler?
Brad White
Member Posts: 2,399
determines the pressure.
You want enough pressure to get the water to the top of your system and add about 4 or 5 PSI on top of that.
Say you have a 3-story house with say, 10 feet per floor and 30 feet from the fill point to the top of the system. Water height exerts 0.433 lbs. per square inch (PSI) per foot or looking at it another way, every 2.31 feet of height exerts one PSI.
Either way you look at it, that 30 feet will require about 13 PSI just to fill it to the top of the highest radiator. Add 5 PSI on top of that and 18 lbs. should be your cold fill pressure.
When heated, this could well climb to 25 PSI but your rise from 15 lbs. to 25 and the assumption that this return to 15 PSI when cold is consistent, tells me that you need more expansion tank. Not more cowbell, more expansion tank. You may have a large-volume system which also contributes to larger proportional expansion. Typically, heated from about 50 degrees to the 180-200 range will increase water volume by about 4 percent.
The larger your expansion tank the lower this "hot" pressure will be. (Similarly, the colder your water the lower will this pressure be, but you still have to size the tank for the highest operating temperature.)
The valve setting should be set to the cold fill pressure and no more. Go get a tape measure, determine your height and start there.
If all things pan out and you need more expansion tank, no harm in buying a second one piped in parallel with the first. Just make sure both are pre-charged to the same pressure which should match the cold-fill pressure.
You want enough pressure to get the water to the top of your system and add about 4 or 5 PSI on top of that.
Say you have a 3-story house with say, 10 feet per floor and 30 feet from the fill point to the top of the system. Water height exerts 0.433 lbs. per square inch (PSI) per foot or looking at it another way, every 2.31 feet of height exerts one PSI.
Either way you look at it, that 30 feet will require about 13 PSI just to fill it to the top of the highest radiator. Add 5 PSI on top of that and 18 lbs. should be your cold fill pressure.
When heated, this could well climb to 25 PSI but your rise from 15 lbs. to 25 and the assumption that this return to 15 PSI when cold is consistent, tells me that you need more expansion tank. Not more cowbell, more expansion tank. You may have a large-volume system which also contributes to larger proportional expansion. Typically, heated from about 50 degrees to the 180-200 range will increase water volume by about 4 percent.
The larger your expansion tank the lower this "hot" pressure will be. (Similarly, the colder your water the lower will this pressure be, but you still have to size the tank for the highest operating temperature.)
The valve setting should be set to the cold fill pressure and no more. Go get a tape measure, determine your height and start there.
If all things pan out and you need more expansion tank, no harm in buying a second one piped in parallel with the first. Just make sure both are pre-charged to the same pressure which should match the cold-fill pressure.
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"
-Ernie White, my Dad
-Ernie White, my Dad
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Comments
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Does the water pressure set at the feed valve determine the boiler pressure? My boiler is a little on the high side under fire,about 25lbs and water temp 180.Not very far to go befor the saftey valve lets go.At rest the gage reads 15 lbs. Can i adjust the feed valve to a lower setting?The feed valve has a lever on top to manualy bring in water I think that feed valve has an adjustment under that lever dome.Should i just get a new valve?0 -
are you sure
Are you reading the triadicator on the boiler and if so, how sure are you it's reading correctly? I would not suggest to you to start changing anything. If it's a concern have an annual inspection done and request they check it then...0 -
More cow bell
Brings back the memories. I think that was one of the best skits SNL ever did.0 -
Please call a professional Larry. I saw the other thread that you started. Playing around with relief valves and PRV's can be dangerous.0 -
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