The gas should be set to start coming on at some temp say below 30-40 degrees to supplement the heat pump if needed. If the system is newer you should be able to run the heat pump down to around 20 degrees with little help from the furnace. If gas prices are cheap I would cut the heat pump out at 30 and if they are high I would let the heat pump run on down to at least 20.

There are a lot of variables to figure in what the best temps are to lock things in and out at. I have all electric so it pays for me to go ahead and let my heat pump run a lot longer than if I had gas back up.

The best thing is having the lockout options and having them set up close. There will be times if it is say 25 and no wind that the heat pump might be able to do the heating with no help but on a windy day that it is snowing the supplemental might run some. The trick is to find the best cut off point to shut the heat pump down and run back up only.

I would start with blocking the back up heat source at any temp above 35 degrees. There is no way to freeze pipes if it is allowed to run below 35. I am sure there is little chance at even 30 unless it hovered right around 30 for a very extended period which is very unlikely.

I would shut the heat pump off at anything below like 5-10 degrees or just let it run and adjust it up from there if it seems like it is running forever. This all depends on what you pay for gas compared to electric. You might want to shut the heat pump off at anything below 20.

why would you even have a heat pump in NJ gas is cheaper then electricn NJ

wisehvac - Because at 40 degrees, even cheap gas is more than electricity (unless electricity is really high). He also has an option when gas goes up.

In NC, where everything is cheap, I have mine set on 35 degrees. I figure the $ break even point might be a tad lower but I also feel like it helps relieve stress on the compressor not having to run at freezing temps.

When gas goes back up, I'll set my point lower.

Wisehvac - and another reason - where I live the baseload is nuclear. So my heat is carbon free when I am running the heatpump. Now I don't know about NJ.

Tried to get the best of both worlds here with the high efficiancy dual fuel setup. I have a 10k solar system on my house so I do get a large amount of free electric. I have totally rebuilt the house from the ground up. 3 seperate heatpumps with modulating gas furnace backups on each. 6" exterior walls with R-19, R-55 in ceilings. All new Anderson windows, siding, roof, etc. Now that it is almost finished I'm ready to move out of state if the cost of everything in NJ keeps skyrocketing. Will the last person to leave please turn off the lights!

May be wrong, but I think countryboymo's a little mistaken. I thought that generally dual fuel systems won't run both the gas and compressor at the same time. May have gotten beyond that with modulating gas valves, etc, but I have doubts. Not the same as with backup electric strips.

I might adjust the changeover point above 23F just for comfort's sake. Our Honeywell will only do 5F increments, so not as much flexibility there.

Dual fuel units should be set to changeover at the balance point or below. The balance point is the temperature at which the heat pump can just keep the house at the set point if it essentially runs continuously.

As the outside temperature drops below the balance point, the house will fall below the set point, a degree or two at first, more as the outside temperature drops further.

An economic/comfort decision has to be made regarding the switchover temperature. That requires knowing your operating costs for the heat pump and the backup fuel at the switchover temperatures being considered. A temperature versus COP (Coefficient Of Performance) chart will be required for the heat pump.

Current production heat pumps produce useful heat down into the teens or single digits.

Agreed on the Honeywell 5 degree increments. Not too flexible.

It also depends on the temperature your thermostat is keeping your house. If you're on vacation and lower the thermostat 20 degrees then the cutover to gas can also be lowered 20 degrees.

Look at it like this. The average US residential natural gas rate right now is about $1 per therm (100,000 btu). The average electric power rate in NJ is about 14 cents/kWh. You can check your own rates and run this calculation for yourself. One kilowatt of electric power in a very efficient heat pump is probably going to move about 10,000 btu per hour of heat into your house, and that's for 14 cents at your probable rate. I believe that's calculated at like 45 or 50 degrees to get that "optimal" performance figure for the heat pump. And again that's if the heat pump is among the most efficient you can get.

So now look at natural gas. It you have a 90 percent efficient furnace it is going to give you that same 10,000 btu of heat for about 11 cents. So basically the gas is cheaper no matter what, so I'd set the changeover at like 40 degrees. Save some wear and tear on your gas furnace on those cool spring or fall days when you don't need much heat, but basically at these rates your serious heating needs should all be handled by your furnace. If the rates change you can do it differently.

I hadn't seen this before but the latest state by state residential price for natural gas had NJ at 87 cents/therm as of March 2017. If true for you this makes the gas furnace even cheaper by comparison.

This is an old post, but it is worthwhile commenting. If the break even point is 20-25 degrees, you then have to calculate if the heat pump can provide enough heat to maintain the indoor temperature set point. You would have to do a load calculation at that temperature, and compared it to the heat pump's documentation. It may turn out the heat loss is greater than the output of the heat pump. If it is the switch in fuels has to occur at a higher temperature.

Our heat pump is 15 years old at this point. I've found that I needed to change the temperature where it switches over to gas from 35F to 40F because it no longer could keep up. Not sure whether that's age on the compressor or maybe refrigerant loss (we've had a couple times over the life where it's lost most of the charge over 4-5 years and we've had to reload it; our service company was never able to definitively locate a leak point with the super slow loss of charge).

But one day before too long I'll get to buy a new one, I assume.

Very true Mike! Even if your system CAN keep up with the heating requirements at that temperature it may take significantly longer to bring your home up to a comfortable temperature when returning home if you have the thermostat set to decrease the temperature while you are not home.

The temperature you want to keep the house at, outside temperature, heat pump size, insulation, ducting losses (if not sized or installed properly), number/size/type of windows and many other variables all play into that calculation. Since I think this post is aimed at homeowners who want to "do it themselves" and I would say an accurate load calculation is above the ability of most homeowners, it may be easier to do the guess and check method on that end.

Set the temperature found above as the crossover temperature on your thermostat and see how the system performs on the days just above your crossover temperature. If the system can keep the house at the set temperature and warm up the home in what you personally consider a reasonable amount of time when you get home, then great! If you find that the system runs continuously and cannot heat the house to the set temperature OR takes too long to warm your house up when you get home on those very cold days... bump your crossover temperature up.

If you have to bump the crossover temperature up, realize that while it is not the most efficient $/heat, it is required to provide the comfort you are looking for.

Personally, I have my crossover set at 35 for no other reason than I love the hot air coming out of the vents when it gets that cold out. That is an example of me paying a little extra for the comfort I want out of my system.