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Discussion Starter #1
Currently I have 2 100 gallon propane tanks. They are powering two Rinnai direct vent heaters. At some point I might want to add a 10-20 kW Generac generator (kW rating depends on whether I switch to non-electric heat for other parts of house).

The two heaters are RHFE-263FA-P / RHFE-1004FA-P.

The ratings on the side are [263 / 1004]:
Manifold pressure: 10.1" WC / 3.7" WC
Minimum gas supply pressure for input adjustment: 11.0" WC / 8.0" WC

Do I need a larger tank (or more tanks) to run the generator?

I won't be doing this myself as I don't touch gas.
 

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Ask your propane supplier, they're the experts. When I was doing my gas fitter training, we spent about 10 minutes on propane tank/cylinder sizing because, as my instructor said, it's only the supplier who really needs to know.
 

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Also concider having them install the tank under ground.
Sure looks better then having a baby hipo out in the yard.
With a larger tank it will need to be located away from the house not next to it.
 

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2-250 gallons tanks. Or 1-500 gallon tank. Generators use a large amount of gas, so the tank needs to have a lot of surface area for the propane to boil off to vapor. 100 gallons tanks don't have enough surface area, so if you were on the generator on a cold night, you won't have enough gas pressure to run the furnaces also, and maybe not enough to keep running the generator.

2-100 gallon tanks, only gives you 160 gallons of propane after they are filled(they only fill them to 80% to have room for expansion).
 

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For reference information only, I have one 250 gal. above ground tank that does a good job supplying a 80,000 btu furnace at the same time as a 36,000 btu water heater when the outdoor ambient temperatures is -21°F.
 

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Generators use a large amount of gas, so the tank needs to have a lot of surface area for the propane to boil off to vapor. 100 gallons tanks don't have enough surface area, so if you were on the generator on a cold night, you won't have enough gas pressure to run the furnaces also, and maybe not enough to keep running the generator.
Ayuh,... Agreed,... The tank valves tend to freeze up, under high usage like that too,...
 

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For reference information only, I have one 250 gal. above ground tank that does a good job supplying a 80,000 btu furnace at the same time as a 36,000 btu water heater when the outdoor ambient temperatures is -21°F.
A single 250 gallon tank should have enough surface area to handle a 5000 watt generator.

With a 5000 watt generator averaging 4000 watts continuous output, it will use roughly 1 gallon an hour. Your 80,000 BTU furnace if its running continuous would be using .87 GPH. So your 250 gallon tank right after it was filled would handle the furnace and a 5000 watt generator for 106 hours by content. It would only suffer when the tank got low. Say 50 gallons left.

The reason I recommend 2-250 gallon tanks. is because you the OP may not be able to get a propane delivery just before a power outage. And 80 gallons of propane may only last him 40 hours. After that, he might as well not have a generator.
 

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Discussion Starter #8
The Rinnai units currently heat the first floor, supplemented by electric in some rooms. The upstairs is all electric.

Possible options:

  • Propane boiler and switch to hot water - leave Rinnai units on "low" setting as a backup in case of boiler failure. Future addition of 10 kW generator so that well pump has plenty of startup overhead. Stick with electric domestic hot water. Will likely need to bury propane tank.
  • Oil boiler and switch to hot water - leave Rinnai units on "low" setting as a backup in case of boiler failure. Future addition of 10 kW generator so that well pump has plenty of startup overhead. Stick with electric domestic hot water. This might also mean more propane.
  • Stick with electric heat (oven/cooktop electric already) with Rinnai units and use 20 kW generator. This might also mean more propane.
I'll check with my propane supplier for their thoughts as suggested by the first reply, but based on the responses I need more propane. I haven't seen diesel gensets in use for home standby (such as Kubota LowBoy) in my area.
 
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