What temperature setting should I use for a heat pump with alternate heat with a Nest?

[bbhaag]

So we have a 2 stage cooling and 3 stage heating HVAC system that utilizes a Nest thermostat. I'm wondering at what temperature should I set the Nest at to kick on alternate heating. It is a 2 stage heat pump and the alternate heating is a natural gas furnace.
In the Nest controls there is a slider bar that lets you decide when the alternate heat should be used in conjunction with the heat pump when the outside temperature drops below a certain point. I'm wondering what temperature should I set the slider at? 32, 40, 50?? I'm just not sure. We live in the Peoria IL area if that helps.

Here are a couple of screen caps I took to hopefully help. One is my wiring and the other is the slider bar I mentioned. I'm having a hard time putting into words what I want to know so if anyone has any questions please ask.

This is how our Nest is wired.

This is the slider bar that I was talking about. What temperature should I set it at so alternate heat kick in when the heat pump is unable to keep up? As you can see right now I have it set at 34*F but I'm not sure if that is ok.

 

[PowerEngineer]

I have an older (30 years older) heat pump that is also backed up by a gas furnace. Ours is set up so that the gas furmace takes over for the heat pump whenever the outside temperature drops below 40 F. My understanding is that this is based on the dropping efficiency of the heat pump as it has to work harder to draw heat from the colder ambient air. I am pretty sure, however, that heat pump technology has improved and so 40 F may not be right for you. I would talk to whoever put in your HVAC system to see what they recommend.

 

[bbhaag]

Thanks for the advice. We purchased the home last fall and when the unit was installed back in 2011 the company that did the install is no longer around. I had an HVAC tech come out a few weeks back to look everything over and he said it is an older unit from 2006.
Our guess was that the previous owner purchased a used unit and then had it installed in 2011. Anyway, thanks again and if anyone else has some advice please let me know.

 

[PowerEngineer]

For guys like me with nerdish tendencies:

https://www.e-education.psu.edu/egee102/node/2090

 

[WilliamM2]

How much is natural gas in your area? Here it's $0.27 a ccf + $0.32 a ccf distribution charge (more to distribute/deliver than the actual gas), and it's been slowly falling for 5 years now. With the price of electricity here, it doesn't make sense for us to use a heat pump at all with our 97% efficiency furnace.

There are a few factors to consider. And dump the nest before you wake up one day with no heat, or no air conditioning.

 

[bbhaag]

How much is natural gas in your area? Here it's $0.27 a ccf + $0.32 a ccf distribution charge (more to distribute/deliver than the actual gas), and it's been slowly falling for 5 years now. With the price of electricity here, it doesn't make sense for us to use a heat pump at all with our 97% efficiency furnace.

There are a few factors to consider. And dump the nest before you wake up one day with no heat, or no air conditioning.

NG in our area is $.21 a therm plus $.36 a therm for the delivery charge then of course all the miscellaneous charges.
Electricity is $.051/kwh and $.056/kwh for supply then of course all the miscellaneous charges.

These are summer rates to so electricity is more expensive and NG is a little cheaper but is appears that for our area electricity would be the better way to heat. Then when the heat pump can't keep up have the NG furnace kick in correct?

Also why say ditch the Nest? We've had nothing but a great experience with it since I installed it last fall when we moved in. Especially now that this summer we ran all the extra wiring that was needed to hook it up properly for our HVAC system.

 

[PowerEngineer]

I think you will find that the heat pump and gas furnace will never both be running at the same time. They are like tag team partners; only one of them can be in the ring at any one time. You want your heat pump to "tag out" before the ambient temperature gets too low for it to keep up. Assuming it is sized properly, your gas furnace can provide enough heat for your entire house at those lower temperatures without assistance from the heat pump.

What William is pointing out is that if natural gas is cheap enough then it may save you money to never use your heat pump (to heat). This is a little tricky to determine because the heat pump's virtue is that it releases more heat into the house than the kilowatt-hours it consumes, about three times as much - but this drops off as ambient temperatures drop. Your cost of gas is $0.57/therm and a therm is the heat equivalent of about 29 kWh, so roughly $0.02/kWh. If your gas furnace is 90% efficient, then the 10% loss bumps the cost for a delivered kWh to $0.022. An electricity cost of $0.107/kWh (if I have that right) is towards the cheapest in the US. Even so, it is about five times more than your delivered gas cost. So even if you get a 3-to-1 benefit from the heat pump from a consumed energy standpoint, running the heat pump for heating may be costing you more (i.e. 5/3 of what heating with gas would cost).

This is all highly dependent on the rate structures for both electricity and gas in your area, and so I would look for guidance from your local utilities and/or energy consultants.

 

[Fanatical Meat]

I can answer this (mostly)
I had one installed last winter, I started at the suggested 30 degrees but that barely hit my electric bill now it’s set to 25 degrees. Not sure what the electric difference was but I know it was fairly trivial.
Tracking electric usage is complicated for us because we have solar

 

[Scarpozzi]

Mine is set to switch to gas at 40 degrees. Heat pumps really start to lose efficiency below 34 degrees if I remember correctly...but the process itself is pretty inefficient compared to extracting heat from a flame... What is cheaper is going to be determined by the NG rates at the time....as but wear and tear on a heat pump probably has a cost too that should be considered.

 

[bbhaag]

Thanks for the advice everyone I really appreciate it. For now I will set the temp at 40 and wait to see what is right for our situation. I will probably come back to this thread a little later in the fall as the outside temps start to drop and we switch from AC to heating.