So the last hard freeze in 21, a family member was not home but their generator thru a code and did not stay on.

I'm wondering if the 4 ton and 5 ton heat pumps could be too much for the 28 kw natural gas generator. The generator has worked fine a few times since then, but never during the winter.
In 2021, nothing else was on in the home except the 2 heat units and a couple light bulbs.

Perhaps you can look at the data plates on the units and determine what wattage or amps they draw at max. Or at startup etc. seems that data would be listed or easily found through Trane.
Or back into by looking at what size breakers control them and voltage. 220-240v?
If remember I think voltage x amp = Watts.
I'm sure a lot EE guys on here can confirm.

It's close. That 4 ton is probably about 14kw and the 5 ton is likely about 16kw.

But that is a total shot in the dark scientific wild arsed guesstimate based on my years as an energy engineer....

Get the ratings off the units and can get more accurate.

Also is the gen rated at 28kw for natural gas? Or rated at 28kw on propane.... for example rated at 28kw on propane will make less kw on natural gas....

Its called cold load pickup. Even though the home has only been without power for however long the throw over takes, both heating units are going to try and start at once and whatever else is on in the home. The in-rush amps will trip the gen off due to the electrical load it is trying to pick up.

We see it on a large scale when trying to bring load up on electrical distribution circuits when they trip during cold. Close the circuit in on all the heating load and the circuit trips. That is when I was on the utility side of the business.

It's not the "4 and 5 ton" rating, for the heat pumps, it's also if the supplemental heat strips are involved.
A 28k generator could/should readily handle the amperage needed, for the compressor side of the equation.

Yes, startup amperage will be high, so a staggered start would be best, or essential...but when, or if, the heat pump(s) cannot meet the desired (thermostat setting), the resistance heat strip amps needed will be very high (most come with THREE 60 amp breakers!).

Thanks for the replies. It sounds like the best plan is to only run one of the heaters at a time if power goes out and the generator is on. It's a little complicated with a 91 year old and different home health care ladies there.
Not being near to re start the generac if it fails is what concerns me most.
In the last hard freeze of 2021, 2 copper pipes froze and leaked in the attic because the generator didn't stay on and there was no heat. Luckily I was there when the leak started and shut off the water.
ETA I wish the custom home had natural gas furnaces and not heat pumps so this would not be an issue.

A poster above addressed this, but the issue is likely the the "emergency heat" that is tripping the generator. In a deep freeze like that, the heat pumps won't be able to keep up, so the heater kicks on the emergency heat strips which draw a LARGE amount of amps. Sometimes you can lockout the emergency heat on the thermostat, depending on how fancy/programmable the stat is.

Best thing to do is just run one unit.

Another option would be to turn off the breakers feeding the heat strips (or maybe just 1 or 2 of the typical 3).

Mas89 said:

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So the last hard freeze in 21, a family member was not home but their generator thru a code and did not stay on.

I'm wondering if the 4 ton and 5 ton heat pumps could be too much for the 28 kw natural gas generator. The generator has worked fine a few times since then, but never during the winter.
In 2021, nothing else was on in the home except the 2 heat units and a couple light bulbs.

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Are your units single speed or variable speed?? What is the model numbers? If you have a Trane XV18/20 series condenser then they are communicating systems and they are very finicky on the low voltage side - especially with generators. I have sold many Trane XV20 systems over the years. If they are 2 speed or variable speed then you don't need the soft start kit as they automatically start on the lowest speed and ramp up as necessary. With either of these 2 options I would tell you to get voltage regulator and the supply houses that we buy our equipment from it's a combination surge protector and voltage regulator.

If you have a single speed condenser it starts up at full 100% load on start up because that is all it can do. On single speed equipment with a home generator you will want a soft start kit.

I own Wise Guys Heating & Cooling here in Cypress/Houston area. 346-482-2691

This is the one most customers purchase for single speed condenser (directly off Amazon ) and then we will install it for you.

Guys…..I don't know the technical terms. I have two Trane heat pumps with power strip backups. I have a Generac 24.5KW generator that kicks on automatically. I forget the name of them, but there are two switches (one for each unit) that keeps them from running at the same time when on generator power and I can easily set the priority of which gets to run first…then the second one will not come on until the first goes off. 4,400SF under HVAC. As long as it's neither oven or clothes dryer is on, it keeps the house toasty without tripping. Sorry I don't know the technical terms…I'm a bean counter.

"Newest" version is the Flex. I think they're the same as the 368 just renamed. There was a bad firmware batch but they'll replace if they fail. (B34)

28kw could power the whole block if you installed soft starts on HVACs. That's a big gen.