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Posted by HeatPro on January 30, 2007 at 08:41:09:
In Reply to: Re: Size expansion tank posted by Dave on January 29, 2007 at 08:21:56:
The size of the tank required is indirectly related to the boiler input and cast iron, only in that the input relates to the water content. A larger boiler with a cast iron or steel boiler MIGHT mean more water. A copper-tube boiler less.
So the REALITY to deal with is that you need to know the AMOUNT OF WATER in the system. As you are going to use antifreeze, you can drain the system water to know how much (approximately) there will be to expand into an expansion tank. You will want a PARTICULAR percentage mixture of antifreeze of the water, so you will know HOW MUCH WATER will be in the system to expand. You will need an expansion tank that accepts the FOUR PERCENT of that total water. Expansion tank ratings supply the ACCEPTANCE volume in their written specifications.
Hopefully, the tank size could relate to the system water volume to be useful, like a #15 for 15 gallons in the system, #30 - 30 gallons in the system. Otherwise, the manufacturer really doesn't care what he makes and is no help.
By knowing that the tank has an ACCEPTANCE volume equal to the TOTAL water content, which can expand to make 4% more water, you can arrive at a tank requirement. It need not relate to a boiler input for cast-iron boilers on 1-inch pipe systems in a copper-tube-wall-hung on 1/2-inch copper-tube world.
It doesn't hurt to have an oversized tank as long as the system cold fill pressure is sufficient to keep the water up to the top of the system (plus 4 lbs to force air out a top vent). You can have more than one tank at the proper location as here:
http://www.heatpro.us/designtree/