We have that, it works, and I have little knowledge of the system. What I’d like to know, but can’t find the info, is what the full load wattage is when we are in super cold weather, like we had in the Big Freeze. If it’s 3 5kw heat units, that’s close to what my Generac can put out running on propane, and we had overload problems during the Big Freeze.

If, for instance, the weather gets progressively colder, will the heat units in the heat pump system come on in sequence, as more heat is required, or do all the heat units come on at once.

I need to figure this out. Bigger Generac, maybe…

Indoor air handlers are manufactured with no heat strips. Heat kits are installed at time of installtions. Heat kits are available in different KWs. Generally, 5KW to 25KW in increments of 5KW. Most likely, one would need to pull the cover and read the nomenclature on the heat kit.

To answer your question on staging. Back in the day, we intalled outdoor thermostats to progressively bring on heat strips, usualy beginning at 45 degrees outdoor. But I have seen that in a 20 years. Now heat pumps produce usable heat well below 45 degrees.

Current practice is to use a 2 stage wall stat. When heat pump can no longer maintain set point, room falls 2 degrees below set point, all heat srips are energized along with heat pump. When indoor temp is with 2 degrees of setpoint, strips are shut off. This will be repeat untill as needed.

When "Emergency Heat" is selected. Heat pump is locked out. Strips become only source of heat.

My 22kw Generac got overloaded in 7° weather. I went to the panel and recognized the west unit, could pull 90 amps. And so could the east unit. So, I killed the east side, since wife and daughter were in the west side. I had the intention of swapping them an hour later, but Co-Op power was restored.

The next day I called the fireman that took care of the HVAC and electrical in my house. He is a pure genius with this stuff. I asked if my generator was not large enough. He said, no it is plenty. Then he taught me what had happened.

Electic heat pumps cannot heat up air that is below 25°F. So below that, the strips kick in. And he said I was right, a 90 amp draw from both units totaling 180 amps. He said my ping pong plan was exactly the right thing to do. Fortunately, we do not have that kind of weather very often.

He said cooling the house in the summer draws less power.

That being said. What can you do when on generator?

I would expect you have two or more breakers suppling air handler. One brecker supplies power to blower and first stage heat. The second breaker powers the second stage heat strips. The simplest solution may to turn off the breaker suppling stage 2 heat strips.

This solution depends on how many strips and how many stage options you have.

Hard to answer without more information.

Or...

Switching to Emergency Heat would kill outdoor unit. That will reduce demand.

Good info! Thanks. I’ll get in touch with our HVAC guy to see how we can allow me to manage the heat unit deployment.

I’m pretty sure that my 20kw Generac won’t support full power demand in extreme low temps, since that was my experience in the Big Freeze. Of course, it may not ever get that cold again, but, as they say, “Hope is not acceptable preparation”.

This is how my old style straight heat strip only heat is wired. I just don’t know if one heat strip on two units would be enough to keep us half way warm with the generator running.

I was thinking the strips were staged, Auxiliary used 1 and Emergency used 2 or all.

Due to different number of heat pump systems and controls used over the last 20 years, that statement could have been true.

My description covers most installations in the field today.

Even down at 10F, modern heat pump systems still produce around 50% of what they do at 37F.

While that may not be enough to heat the home to a warm and toasty temperature, it is still meaningful heat. Every BTU provided by the heat pump requires less KW than strip heat. Using the published data for my own system as an example:
-Waaay down at 7F outdoor air temp, my 3 ton heat pump gives me around 15,000 BTU per hour and consumes 2.4 KW per hour.
-Every 1KW of strip heat produces about 3400 BTUH, so it would take 15,000/3400 = 4.4 KW per hour for the strip heat to make exactly the same amount of heat.

The actual amount of heat you need for your home determines everything, but my heat pump holds my house at my preferred night temperature of 65-66 when it’s 20-25 degrees outside – without using the strip heat.

As Lazyjack already said. for generator use, you may find that you can just disable part of your strip heat. You may find that heat pump plus 5KW is enough to get it done with the generator.

Makes sense, thanks

Without some fossil fuel heat, cooking and water heating sources, I dont know of a single residential gen that will carry an home without budgeting power.

An average 1500 sq ft home will have 10 to 15 KW heat.
A water heater will draw 4.5 to 5.2 Kw
At a min just these 2 applications will have a 14.2 KW load

I didn't include...
Baking a cake 5KW
Clothes dryer 5KW

Or charging a electric vehicle. Let's go Brandon.

You might wanna consider re-configuring your HVAC/Heating setup to a dual-fuel stacked Propane/heatpump setup versus the cost of a bigger generator (that has faster/higher fuel consumption).
That's how I have mine setup, and only need a few hundred watts to run the blower on the propane furnace when power is down or things ice over or super cold.

Generators are only 50% efficient (at best) converting propane fuel to electricity; using tons of propane in a generator to run huge heat-strips is just silly - 43% wasted btu's versus a propane furnace.
(A central propane furnace can be 93% efficient fuel-to-heat - your propane will last MUCH longer).


99% of the time the system heats with heatpump. I don't have any heatstrips at all on my system - don't need them.
Whenever power goes down, things ice over (or temps drop into the teens), or when heatpump calls for defrost, then the propane furnace fires up, sipping propane for just the time it's needed.

I debated the use of Propane over heat strips, but it came down to my not wanting propane in the attic.

he might be a genius but he is wrong on the statement that heat pumps cannot heat up air below 25 degrees F.

What I put in this new place (propane) 1 up and 1 down. ran the down system on a 10 KW miller welding gen set. All needed was the blower.

There are a number of expensive remedies mentioned here for some very rare situations. If you had some sort of emergency a propane space heater and a few bottles of propane that you may use on a grill, etc. anyway, you are fixed.

I have vent-free gas logs (cute space heater in the fireplace) and a gas water heater, so none of these issues mentioned could come up. I guess my winter energy needs are somewhat balanced between electric and propane. I just know the lights can go out and we can take a shower and we aren't getting cold. The portable generator will run frig/deep-freezer and lights.

One thing to remember on Heat Pumps, when they go into defrost mode they kick on the strip heat to keep the inside air warm till the unit comes out of defrost. If you disable the strip heat, the air coming out of your ducts will be around 55° till the defrost cycle is complete.
The reason I know this is I had a board on the outside unit go bad and it would not send the signal to the inside unit to kick on the strip heat. Turned it to emergency heat till the repair guy could come out.
Just remember this when setting up your system.
Good luck.

My system shuts down the evaporator (indoor) blower during defrost. No cold air. The house temperature doesn't drop that much during the few minutes of defrost cycle.

I think DannyB is right. Maybe I really don’t have a critical need to fix anything. I have the portable propane and electric heaters, and a fireplace, for emergency heat. The Generac will carry that load easily. But, I do have a need to find out about the heat strips - how many and what wattage. Maybe some minor modifications are in order for the future. I do not want a repeat of the problems during the Big Freeze. That was brutal, and it got mighty cold in this house.
Me, in my recliner…

Just looking at the numbers on this setup( big generator running big electric heatstrips):

22kw generator at full load (100amps) consumes 3.6 gallons propane/hr (per mfgr specs);
3.6 gallons propane x 91,500 btu/gallon (heat content of propane) = 329,400 BTUs/hr that Generator 'consumes' / INPUT

100amps power @ 220volts = 22,000 watts power generated
(lets assume all 100 amps going to heatstrip, and not just 90 amp)
22,000 watts/hr x 3.412 btu's/watt (heatstrip efficiency) = 67,557 BTUs/hr (heat delivered into house) OUTPUT

setup like this is only giving 20.5 % efficiency for the fuel consumed. (running 329k btu's thru motor to generate 67k btu's home heat)
about $7 an hour, and a 250 gallon tank may last you 3 days running straight out.


by contrast, a 5-ton central propane furnace (95% efficiency) will consume 63,000 btu's/hr propane (about .69 gals/hr) input giving 60,000 btu's output (about the same as the above heatstrip setup)
about $1.40/hr, a 250 gallon tank will last you 15 days running straight out.
(of course, you'll need a few hundred watts to power the blower, but you can get this from a tiny portable generator, inverter on a car's battery, or a couple of solar panels & inverter)


Case and point - generator/heat-strips are a terrible combo; maybe too late when already built-out - but for anybody in a planning stage for a new build or renovating an old setup to new - something to consider.
You can easily stack a Heat-pump coil onto a propane furnace when building a new setup to effect a dual-fuel generator friendly solution; sometimes you can retrofit existing without replacing everything; depends.
Equipment sellers sell dual-fuel combo systems already matched too (for new) - https://iwae.com/shop/heating-air-conditioning/full-systems/central-split-systems/heat-pump-gas/

That's funny.

I will add to that, if you take my advice have the old style of space heaters, Dearborn, etc. Cast -- RIP. I sold a blast furnace Dearborn to Cast.

The "new" space heaters are the radiant types. They have a thermocouple. That is a BS safety device to protect you from yourself. If you don't use them pretty much full time they are useless. If you use one often enough it would be the "Prius" of energy efficiency. I have both just because.............

We made do right e with plug in heaters and small generator…. Oh and hold the wife close

He may have said 20°F. It is a year old memory. I have slept since then.

603, your on the right track.

If one was building a home and planned to have a genny with auto change over, there a ways to segregate the loads the genset sees. It requires two panels and dedicated circuits.