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[Homerglide]

After searching for hours about this and coming up empty, I will ask this:

Does the vent to atmosphere for the water have to be the highest vertical point of the system? If the vent is lower than portions of piping, will the water siphon away?

If the site only allows for the OWF to be downhill from the house and loss to siphon is an issue, how can it be remedied?

[Scott7m]

It's not an issue....  An open system simply means it's not pressurized and the water can expand without ever building pressure. 

It doesn't have to be at the highest point, most of the time there simply at the top of the furnace.  No water will siphon away just make sure to use a good quality boiler treatment to prevent corrosion

[Homerglide]

About the level of the OWB in relation to level of the piping system.

While researching valves, I stumbled upon this publishing   http://www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_10_us.pdf   from Caleffi that covers hydronic heating systems. To see the description of my concern, scroll down to the outdoor wood furnace section 5 on page 18. Figure 5-3 and formula 5-1 is directly related to my original question and should be a concern to any OWB installer.

What it states is; when an open system has piping above the level of the vent at the furnace, there can be flash to steam at low temperatures when the circulator switches off. The more height differential, the lower the boiling point temperature.

[Scott7m]

Why would the pump be switching off?

I've seen pumps on many open systems after years look like new, of course if you were running no boiler treatment things would be different. 

It seems most installs I do are where the house or second floor is well above the level of the boiler, never have had any issues.  However, I sure don't have my pump shutting on and off.  Are you worried that when your pump is off the water will drain back to the boiler or what

[Homerglide]

Why would the pump be switching off?

Depending on the system engineering, the pump may switch off when home thermostat is satisfied. If a HX is placed in the plenum of a forced air furnace and the hot water circulate 24/7, is that desirable?

Do all OWB systems have the circulator running 24/7?

[Scott7m]

Most modern day systems, never have the pump shut off, years ago it was common to have the pump come on with the air plenum fan.  Most of the reason for this was folks skimping by on cheap line and not wanting to lose there heat to the ground.  Now with quality underground line, we just let em roll.  Also, when it makes the domestic water heating much simpler, consistent, safer and so on.

Some folks have fans that never shut off, air circulates in the home all the time and a zone valve hooked to a thermostat determines when heat is allowed into the furnace coil.  Also in regards to hot water, it's much simpler to have a plate exchanger mounted in your hot water heater supply vs running water out through line, through the stove, and back into the house. 

[Homerglide]

Thank you for your added input Scott, I truly appreciate it.

It appears that a circulator always on would make it a non-issue except during a power failure.

[bajonesy77]

My owb is below my house and even during power failures it is not a problem.