Gravity feed advice needed for power outages

[Drewby]

Hello,
I would like to setup my Royall boiler with some cast iron radiators that can be gravity fed in the event of a power outage. This boiler is a Royal 6300 and will be setup with a samson draft regulator to keep the temperature from getting too high.
Currently my plan is to have the boiler sit in a room just below the rads. My question is as to what size of pipe I should use for a decent flow and how far can I go horizontally from the boiler. I plan on using 3 rads that are medium size (not sure on the sqft) but they will put out 12,500 btus each from the calc I did awhile back. Two of the rads will be almost above the boiler and the other I would like about 40’ away and 8’ above the return on the boiler. Can I go this far horizontally and how much slope should the pipe have?

This gravity loop will be setup with a smaller bypass section that will allow it to be pump fed with a zone valve most of the time but can turn into straight gravity if needed by turning a ball valve or two
The purpose of this is to have about 35-40k btus from the rads and another 20k btus or so from the large boiler itself that can keep the house warm in emergencies.
Thanks in advance.
Drew

 

[maple1]

I heat our house with wood & rads when the power is out.

You need lots of straight up, from the boiler outlet, before you go sideways much. Our upstairs zones send lots of heat up there. Those are around 14' up before they go around the zones. Our downstairs zones are only adequate at best. They go around 4' up before going around the zones. All 3/4" standard copper baseboard stuff. Bigger diameter should do better. My horizontal sections have no slope. And I have a 1" normally open zone valve for dumping, on top of the boiler, also on a short bypass loop to the zones, tied to an aquastat - when the power goes out, it opens automatically. It also opens when the stat senses an overheat.

EDIT: Such a dumping setup should really be a standard part of a boiler install anyway, for boiler overheat protection.

 

[Drewby]

So is your main line 1” then? I assume it would flow better if I did it with cast iron rads and 1.5”-2” pipe. I would not have any more than 8’ if vertical distance between the boiler and the radiator.

 

[maple1]

Depends what you call 'main line'. My lines back & forth between boiler & storage are 1-1/4". Same size as boiler tappings. My zone headers are 1-1/4" also. With 3/4" branching off to zones. So all near boiler piping is 1-1/4". Except the 1" short separate dump/gravity loop out the top of the boiler. It branches to 3/4" when it hits the zone piping. I made that 1" to accommodate the 1" NO zone valve. Which when it comes right down to it, I am not sure actually flows much more than a 3/4" - think I read later that both size valves have the same guts. Maybe.