15kw furnace and generator

[Dsg319]

Has anyone ever done this?

Typical 15kw furnace has a 60amp and 30amp OCPD.

The way I understand is they cycle to handle the load?

If one was wanting to try and still have some heat with a smaller say 13kw genset could one only utilize the 30amp OCPD and make sure that it also is powering the logic board and fan?

Or will it heat until it goes to change sequences and never heat the other elements for not having the additional circuit therefore never changing back to oringal state on sequencer?

Not and HVAC guy. So even if it’s impossible would still like to know how it works.

 

[Shaneyj]

Coincidentally I troubleshot a bad main on Sunday. It tuned out the 15kw furnace is what exposed the bad main. GE residential is crap BTW.
On that unit the 60 was powering the control transformer.
Based on amp clamp readings alone, there was no sequencer.
With house temp at 72 I turned up t stat to 74.
With the 30 off, 45 amp load.
30 on it was 65 amps.
I would’ve expected one or 2 of the heat strips the drop out but saw full load.
I’m not an hvac guy either but hope this helps.


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[Shaneyj]

I should add; I got him limping until Poco shut down his service the next day for MB replacement by shutting down the 30A circuit on the furnace. (Leaving a 45A load - which should work with a 13kw generator)


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[James L]

I had a 15kw furnace with heat pump.
Controls and fan were on the 30 amp circuit.

I used a load shedding device on the 60 amp circuit, which was my plan all along

One thing I didn't know beforehand is that the load shedding device was 120/240 so it needed a neutral. I was so glad that I ran 2 empty 3/4" Smurf tubes from the panel to the furnace room on the rough-in, so when I pulled the circuit I put the neutral in

 

[Birken Vogt]

Has anyone ever done this?

Typical 15kw furnace has a 60amp and 30amp OCPD.

The way I understand is they cycle to handle the load?

If one was wanting to try and still have some heat with a smaller say 13kw genset could one only utilize the 30amp OCPD and make sure that it also is powering the logic board and fan?

Or will it heat until it goes to change sequences and never heat the other elements for not having the additional circuit therefore never changing back to oringal state on sequencer?

Not and HVAC guy. So even if it’s impossible would still like to know how it works.

Is the furnace just resistive elements or does a heat pump do the heating most of the time?

Here in California, we might have a heat pump with some many kw heat strips that run half an hour per day for defrost cycle.

Lock them out when on generator power but also be thinking of what will happen when they are not active.

If it gets really cold outside, will the heat pump still be able to function? Or will it try to switch to backup heat only?

Will the furnace controls complain if you start interrupting picking and choosing what runs and what doesn't?

I am not an HVAC guy but I do try to understand as much as I can how they work so I can make generators interface intelligently.

 

[James L]

Is the furnace just resistive elements or does a heat pump do the heating most of the time?

Here in California, we might have a heat pump with some many kw heat strips that run half an hour per day for defrost cycle.

Lock them out when on generator power but also be thinking of what will happen when they are not active.

If it gets really cold outside, will the heat pump still be able to function? Or will it try to switch to backup heat only?

Will the furnace controls complain if you start interrupting picking and choosing what runs and what doesn't?

I am not an HVAC guy but I do try to understand as much as I can how they work so I can make generators interface intelligently.

Depends on the system. "Backup heat" doesn't really mean anything

There's a difference between supplemental and emergency.

Many heat pumps only function efficiently (effectively) down to about 25⁰ - 28⁰

Below that temperature, the the heat pump might still run, but call for the strips as supplemental heat. If that happens, it may call for all the strips at once, or in stages.

Some heat pumps stop altogether below the effective threshold. And again, may call for strips all at once, or in stages.

 

[Birken Vogt]

Depends on the system. "Backup heat" doesn't really mean anything

There's a difference between supplemental and emergency.

Many heat pumps only function efficiently (effectively) down to about 25⁰ - 28⁰

Below that temperature, the the heat pump might still run, but call for the strips as supplemental heat. If that happens, it may call for all the strips at once, or in stages.

Some heat pumps stop altogether below the effective threshold. And again, may call for strips all at once, or in stages.

It all comes down to knowing what you are doing.

You have to design it to work with what you have.

If I had a system that would not function below 25° on HP only, and the generator could not handle all the heat strips, then maybe figure out a way to only engage some of them, just enough to keep the house warm in the worst condition.

Some manufacturers (Kohler) have load management that can operate in stages for something like this.

Most of the time, I find that a 20 kw generator can handle the entire HVAC system, if only just barely. If the rest of the house were to also load up at the same time, it might overload. So out of expediency, I will use the main thermostat power wire to load shed the entire HVAC as long as the generator says it is overloaded. Then when the rest of the house settles down (water heater satisfies, oven and stove top satisfy, dryer finishes) the HVAC will reengage and no one will be the wiser.

 

[Dsg319]

This is an all electric resistive heat strip furnace only no heat pump.

 

[Birken Vogt]

You will have to study it and understand how it works. Or just turn off the big breaker and see how well it runs on the small breaker only for a heat cycle or two.

Making electric resistive heat off a fuel burning genset, extremely expensive way to go.

 

[LarryFine]

I think every electric furnace I have seen had the control and blower fed from the primary (10kw) circuit.

 

[James L]

I think every electric furnace I have seen had the control and blower fed from the primary (10kw) circuit.

And that's the key. I've seen them on the small circuit, but it was a 15kw thru-the-wall furnace with heat pump but into the same unit also on the small circuit