Hot water boiling

johnjohn89

What’s hot water boiler operating pressure? Is that 12 to 15 psi? If it does but why? So 1 psi pressure = 2.31 feet of water head And two story home the average height may vary from 20 to 27 feet! Is that the reason why boiler operating 12 to 15 psi? If boiler operating 15 psi what should be the expansion tank? 12 or 15 ? And why?

EdTheHeaterMan

Static pressure is the pressure needed to get the water to the highest point in the home. That is why we start at 12 PSI as a industry standard. Most 2 story homes with a basement will have the highest radiator about 24 feet above the boiler. That will leave about 2 PSI extra pressure to bleed the radiators upstairs. This is an illustration I used to teach this in my classes on this subject.

STEVEusaPA

Cold fill pressure, expansion tank and boiler feed should all be the same pressure.
Typically distance from boiler gauge to highest radiation, divide by 2.31, plus 5 psi

EdTheHeaterMan

To answer the expansion tank question, we need to understand where the diaphragm tanks came from. The original tanks were in the attic and were on open systems.
When the water expanded, the extra volume went into the tank in the attic. as the water cooled, the water reduced in volume and the water in the tank went back into the system.

EdTheHeaterMan

When it was discovered the this open system idea caused corrosion, we started using closed systems. This is around the same time we started using circulators. So the closed system had an expansion tank in the basement near the boiler in many cases. This tank was large enough to create an air cushion in the top 1/3 or 1/4 of the tank as the tank filled with water. these tanks did not have a bladder or membrane to separate the air from the water. The air pressure in the tank was equal to the water pressure inside the boiler system.

EdTheHeaterMan

Then someone at the Amtrol Company came up with an idea to save space. "What if there was a tank where we could put the air pressure in the tank at the factory?" The tank could be much smaller. That is why an Extrol 30 is just under 5 gallons in size but does the job of a 30 gallon expansion tank. The air cushion is already there. So if you match the air cushion charge in the smaller tank with the system static pressure, you can use this smaller tank and have less water in the system.The movable diaphragm or bladder will move to allow the system water to push against the air cushion. The air will push the water back into the system as it cools off.

If the tank pressure does not match the system pressure, then the tank will not work to the maximum capacity. Low air pressure and the tank may be too small for the system. Too high and the water will not be able to enter the tank when it expands.

I hope this helps

Mr. Ed

EdTheHeaterMan

There is only 1 metering device by the way.

johnjohn89

EdTheHeaterMan said:

To answer the expansion tank questionEdTheHeaterMan said:

To answer the expansion tank question, we need to understand where the diaphragm tanks came from. The original tanks were in the attic and were on open systems. When the water expanded, the extra volume went into the tank in the attic. as the water cooled, the water reduced in volume and the water in the tank went back into the system.

EdTheHeaterMan said:

To answer the expansion tank question, we need to understand where the diaphragm tanks came from. The original tanks were in the attic and were on open systems. When the water expanded, the extra volume went into the tank in the attic. as the water cooled, the water reduced in volume and the water in the tank went back into the system.

, we need to understand where the diaphragm tanks came from. The original tanks were in the attic and were on open systems. When the water expanded, the extra volume went into the tank in the attic. as the water cooled, the water reduced in volume and the water in the tank went back into the system.

Why expansion tank in attic? Sometimes I saw that higher, so curious about why place expansion tank about that high? Most time I see tank just above boiler in residential?

Jamie Hall

The tank was placed in the attic because it was absurdly simple to do that. There was no need for any fussy fittings, no need for any control on air charge, no need for any air removal devices, and the system pressure was maintained at just what was needed.

The problem was, of course, that the tank could freeze up there. So the next step was to take the tank and put it down near the boiler, but now it needed an air cushion in it to stabilize the pressure -- so the airtrol fitting was invented (really big hydropneumatic tanks, used to control well pumps for instance, had additional controls to control the air volume). So long as any air in the system was directed to the compression tank, there was enough air present in the tank to form a nice cushion, and they were -- and are -- completely trouble free. Once in a long time, on certain systems, the air charge isn't adequate -- for various reasons -- and they did waterlog. The solution to that was simple -- close the valve on the pipe from the system to the tank, open the drain on the tank, drain all the water out, close the tank drain, open the valve to the system, and you were good to go.

Unless there really isn't enough air in the system. For instance, if someone gets modern and places another air removal device on the system to get rid of the air. This can deplete the air charge in the tank in a matter of days, if it's a good one. Then the technician, who doesn't know how it works, blames the stupid old compression tank instead of his shiny new air removal fitting and takes the tank out and puts in a nice modern bladder tank...

JUGHNE

The tank was high in the building because it was open top, with an overflow tube as in a WC tank.
The pressure was created by the height of the water from boiler to top of upper tank. So it had to be at the highest point.

hot_rod

Open tanks are still being installed in UK systems. Now a days with substantial insulation jackets to lessen that large heat loss of a hot tank in a cold space. Seems kind of odd, doesn’t it? Especially in a country hurting for energy.

Jamie Hall

hot_rod said:

Open tanks are still being installed in UK systems. Now a days with substantial insulation jackets to lessen that large heat loss of a hot tank in a cold space. Seems kind of odd, doesn’t it? Especially in a country hurting for energy.

Many UK houses -- at least in Scotland where my daughter lived -- also have a gravity cistern for the domestic water which is located in the attic or at least up high. If's usually controlled by a float valve, and fills slowly over several hours. The pressures and maximum flow rates of the public water supplies are very different from what the US and Canada are used to.