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Get Powered Up! Certified Energy Manager Jeff Yago answers your alternative energy questions

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Archive for the ‘Thermosiphon’ Category


Cast iron/copper pipe conversion

Sunday, November 16th, 2008

Hi Jeff,

I’m remodeling my basement and the over head cast iron heat pipes that supply heat to the radiators are positioned low and a bit from the walls. A plumber suggested replacing them with copper pipe and lifting them closer to the ceiling and walls where they can be enclosed in a soffit. My question is will a smaller copper pipe reduce the efficiency of heat supplied by the radiators throughout the house? The current cast iron pipes are about 3.5 inches wide.

Thank you,



Many of these older heating systems used very large size cast iron pipe because they did not have any hot water pumps to circulate the hot water and they operated at very low or no pressure. The large piping provided a kind of thermo-siphon effect which allowed hot water to rise and cold water to return without a pump. In this case, the large pipe size was not due to a high water flow rate. If your new system now has a circulating pump, you will not need large piping since the higher flow rates and system pressure to remove air can transfer much more heat per hour.

A 1-1/2″ copper pipe has a flow rate of over 20 gallons of hot water per minute with a nominal pressure drop, which is probably more than your pump can pump. A 1″ copper pipe can flow 9 gallons per minute. Except for the main headers, I would bet most of your piping run-outs to radiators are only 3/4″, but this size pipe can still flow 5 GPM in each leg which will add up to the main header flow rate. Remember, a smaller pipe costs less to insulate.

Hope this helps,

Jeff Yago



Tuesday, September 16th, 2008


I want to install a simple gravity fed hot water tap in my remote cabin. I am not electrified at this location. My idea is to have a water containment vessel (probably not sealed) above the heat source and copper tubing running out of the bottom of the vessel dropping to about 15 coils around a wood stove pipe. When I open the tap (which is at the lowest point) the water flowing around the coils will heat to a temp relative to the amount of heat the stovepipe is generating. Will this system work, or will the water thermosiphon back into the vessel and provide me no water at the tap? Do I need a sealed system with an inlet and outlet in the water vessel to circulate water as it heats? Thanks for the help.

Richard P.


What you described is not a thermo-siphon system, you are just passing the water around a hot pipe when you open the lower faucet and this is not safe. Without water flow, the water in the copper pipe wrapped around the stove pipe will quickly heat up and turn to steam, then “shoot” out of the tubing and back into the elevated tank.

For a thermo-siphon system to work and not overheat, you need a constant water “flow:” around the copper piping and up into the side of the tank near the top (but still below water level). The bottom end of the copper coil is connected into the bottom of the elevated tank which has to colder water (heat rises).

Your hot water supply piping to faucets should be a separate pipe connected near the top of the tank (also below the water level). As the water is heated in the coil, it will rise and enter the top of the tank, while colder water enters the coil from the bottom of the tank.

If you decide to make this a closed system and under pressure, you will need a separate temperature relief valve and a pressure relief valve, but I recommend that you keep it simple and not pressurize the tank.

Make-up cold water can be a separate cold water line connected near the bottom of the tank using either a manual valve or a “commode” type float valve to let more water in as the water level drops.

Be safe,

Jeff Yago



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