Hello, I have been reading about wood boilers for a couple months now. I think one would work well for me and be worth it in the long run. I have alot of questions, one thing that I have been having a hard time figuring out in my head is related to pressurized storage. I understand putting the hot from the boiler into the top of the tank, and returning the cool water from the bottom of the tank back to the boiler. Next, for heating a zone you take the hot off the top of the tank and have the return enter the bottom of the tank, this is what I'm trying to figure out. Is there not alot of weight from the water plus pressure to over come to push the water back into the bottom of the tank? Is this taking a big pump to accomplish or other forces at work to make this happen?
Understanding Pressurized Storage
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sdrobertson
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There is weight, but its a circle in a way. You do have to "push" the water into the bottom, but at the very same time your "pulling" water from the top, so every thing is evened out. This doesn't take much of a pump to accomplish this.
If it's all sealed and you have ALL the air out of the system it's kind of like a ferris wheel. You have the weight at the top balancing the weight at the bottom and it doesn't take a very large motor to spin a 50 ton wheel.
It seems paradoxical that with all that water in the tank at the bottom and only a little in the pipes above that it is like a balanced wheel.
Water only 'sees' the height, not the volume. So the force needed to push the water up the pipes is balanced by the water going back down the other side.
Imagine you have a bicycle wheel off a bike. If you lift the wheel with one finger it takes a lot of force on that one little finger to lift it up. But if a friend holds the axle and you push the bottom of the wheel sideways it won't take much force on the same finger (now that your finger is calibrated) to make the wheel spin and take the bottom of the wheel up to the top (and the top back down to the bottom).
That's why the pumps used in sealed heating systems are called circulators. They don't 'pump' the water UP, they 'circulate' the water AROUND the system.
If there is air trapped at the top, all bets are off.
It seems paradoxical that with all that water in the tank at the bottom and only a little in the pipes above that it is like a balanced wheel.
Water only 'sees' the height, not the volume. So the force needed to push the water up the pipes is balanced by the water going back down the other side.
Imagine you have a bicycle wheel off a bike. If you lift the wheel with one finger it takes a lot of force on that one little finger to lift it up. But if a friend holds the axle and you push the bottom of the wheel sideways it won't take much force on the same finger (now that your finger is calibrated) to make the wheel spin and take the bottom of the wheel up to the top (and the top back down to the bottom).
That's why the pumps used in sealed heating systems are called circulators. They don't 'pump' the water UP, they 'circulate' the water AROUND the system.
If there is air trapped at the top, all bets are off.
I think I see how that works now. Is it true that the water would not be able to circulate back into the bottom of the tank if the whole system is cold, for example when first setting up the system before any heat is applied? This leads to another question, is there only pressure in the system when heated or is there some pressure always present and then it rises when heated? Thanks for your help.
Generally, the water would be ABLE to circulate but it wouldn't unless there were something driving it. The colder, denser water will settle to the bottom of the system if there are no roller coasters it the plumbing. In other words if the pipe leading to the bottom of the tank were all level or downhill. Once you start heating the water it CAN start circulating if there were no valves or other restrictions. And then it will sit there until heat is applied to it or switch on a circulator.
Most modern systems are going to use a circulator to move things along at a controllable rate and time even if it could thermosiphon on its own.
In a sealed pressurized system with the boiler and storage at the lowest level, the pressure there will be at least the pressure from the height of the water in the system, roughly 1/2 PSI per foot of elevation. There is usually a little added pressure to make up at least 12PSI at the boiler and its circulator. It keeps the water from boiling so easily. If the system is sealed there will be an expansion tank to take the added volume of water as it expands when heated. As it expands, it will raise the pressure. The expansion tank will have to be sized to the total system volume to keep the rising pressure within bounds.
There is a lot of reading you can do on this forum by using the search function at the top of the Boiler Room Forum. Pick a term and type it in the box.
I've just described the most basic idea of it. Lots of different ways to do it. And lots of people asking questions and answering them here in the archives.
And welcome aboard.
Thanks for the welcome and the information! I look forward to learning more here everyday. I'll have to put together a rough idea of my plans sometime and see what the forum thinks about it. I am thinking of getting the storage and main plumbing in place first.
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