Lots of info on generators but I have not seen my question asked anywhere so thought I would try here.
Wife has health problems and must have AC in hot humid weather. So I got a whole house 22k Generac installed, works great. But it does drink propane quite freely, about 3.5 gal an hour at full load.
In the winter time if we have a long term power outage I was wondering if a small portable generator could be back fed into the system. I would shut of manual breaker to power company and the breaker at the 22k generator.
Question is, would that cause any problems to the Generac switch.
Small generator would supply enough power for well and furnace.

You understand the generator will use less fuel the less load it's under?

Have you contacted your installer or Generac to verify that this amount of fuel use is not excessive?
My only generator experience is a 6k gasoline so I'm no expert but wow, that seems high.
I know L.P. is less efficient than natural gas or gasoline but?


Because of age I was thinking of a whole house generator. We are on natural gas but gasoline is looking better all the time.

quote:

Originally posted by mjlennon:
You understand the generator will use less fuel the less load it's under?

^^^This.

3.5 gal an hr per spec sheet from Generac at full load, about 2.5 gal at 1/2 load.
Had it for about 3.5 yrs, by sound it only runs one speed but no tack on it to tell.

It does a nice job, in less then 10 seconds after the power fails it is running and powering the house.

Yes, a standard generator runs at a set speed (tuned to generate 60hz). The more load you have, the more fuel it takes to maintain that speed.

22k is a heavy load. Whole house A/C, electric heat types of loads. My electric hot water heater is only 4500 watts.

If you are running excessive loads like that, look into a diesel powered generator. Would be more efficient than lp by far.

Most all generators run a constant RPM depending on load, the amount of hertz they put out (and volts) is determined by RPM. They just use more fuel to maintain the same RPM's with a heavier load. You don't want to run a really light load on a generator as they meter fuel poorly at light loads and can glaze cylinder walls, etc.. A 22kw will use considerably less fuel for a 3.5kw load, not as much less though as a 4 kw generator.

To answer your question, Yes You could get an outlet and transfer switch etc to run a portable 3.5 kw if you're going to run a load that light, but chances are you won't run a load that light as there's the hot water heater, appliances, etc. etc.. Then you have to keep adding gas to it. Diesel would be most efficient, then gas, then LP. But you already own the LP generator and have the tank. The beauty of LP is there's no issues with old fuel, algae in the fuel, water in the fuel etc. Like you'd get with gas or diesel.

60hz generators usually run at 1800 rpms for the larger ones (most all diesel and some gas) and 3600 rpms for some of the smaller ones. 1800 rpm ones are much quieter.

The first part of your question is easy, yes you can wire your house so that you can switch between multiple generators in the way you outlined. The second part of that is more complicated in what is the best way to do it.
I'm assuming that you have a generac automatic whole house transfer switch. So my first choice would be to just wire in a small 2 circuit manual transfer switch wired to your panel with a 30 amp portable connector. In this market you really get a choice of 1 circuit 15a, 2-6 30a so you will need to be at 30a for most well pumps.
But real design work would take a bit more information on your end.
And no it won't bother the generac transfer switch per se as that switch wouldn't have clue you were doing it (the way the generac circuit switchs work).
Hope that helps. Me I'd get a bigger tank system and have the whole house online, but I get it can be expensive.

The cheapest way to do this would be to get the appropriate generator interlock kit for your breaker panel.

https://www.amazon.com/slp/generator-interlock/

Wire the new generator breaker to an appropriately sized generator inlet on the outside of the house.

https://www.lowes.com/pd/Relia...er-Inlet-Box/3133095

When the power goes out, you would need to manually switch off enough breakers that your portable generator could run the remaining circuits, then switch over to that using the interlock and shut off the Generac. This may not be appropriate for DIY, but shouldn't cost too much to have an electrician install.

You probably don't want to do that. That's a good solution when you want to flip back and forth to manually manage your power requirement versus your generator output. A little light and then a little refrigerator and then a little freezer and then a little furnace?
But in this case he has a whole house generator already. If you want to run stuff just fire it up and run stuff. He wants low fuel and long run times on two specific circuits (as I understand it). So why not just give that...

quote:

Originally posted by gjgalligan:
3.5 gal an hr per spec sheet from Generac at full load, about 2.5 gal at 1/2 load.
Had it for about 3.5 yrs, by sound it only runs one speed but no tack on it to tell.

It does a nice job, in less then 10 seconds after the power fails it is running and powering the house.

Unless its woefully mis-sized for your home, there's no way it should be running at 100% of capacity the whole time. I have a Generac 20kw gen set and auto switch running a 5 ton heat pump along with the rest of the house, and it runs at approximately 50-60% of capacity. Just to cut some load on the generator, I run the hot water tank manually, and ask my wife not to dry clothes unless absolutely necessary (i.e. 220v loads).

I'd speculate that if your gen set is running at 100% of capacity, you have an issue to address.