Heat pump efficiencies are rated on three thing Seer for AC, EER max power consumption when running on full power input( without the elec. strip backup). This is a number the utilities like for peak shaving purposes. Heating is based upon HSPF, heating seasonal performance factor. HSPF is what you should be looking at. Bryant has a chart that shows outputs or COP's at various temps. Have your dealer get it for you. You can then run the numbers.

The other thing to consider and I think what will really drive your decision is your comfort. Gas will put a higher temp at the register & into the house than a heat pump. HP's heat, but if you get caught out in a 40* rain you are going to want to stand over a gas heat to warm up more. Than the hp's.

What thermostat do you have? I believe the Bryant Evolution thermostat has two lock out temperatures that can only be set in 5 degree increments.

Are the nat gas and electric rates all inclusive of every cost? You should dbl check.

If so, I wouldn't even use the HP for heating, just use nat gas furnace heat. The furnace you have is a modulating high eff model that operates in small increments between 40%-100% operating range. You have the matching Evolution controller?

Since you prefer nat gas heat over HP heat, the DF system for Atlanta was a waste of money. Sorry but that's the truth. I believe you were oversold.

Here are the fuel comparison results. I had to make certain assumptions. I used 97% eff for furnace, 3.00 COP for electric. Understand this is to be used as a guide only.

Cost per 100,000 btu of useable heat
Heat pump: $0.84
Natural gas: $0.50

IMO

@Tiger Dunes: Well, I went back and compared the avg cost for electric and gas by taking the total bill and dividing by the number of therms or KWH's used. The gas comes out to $1.83/therm and electric is .12/KWH. How does this affect the comparison? Where do you get the COP number? I have not been able to find it.

Auggie

You need to double check your nat gas and electric costs. I don't have confidence in the numbers provided in your last post. I would call your supplier and verify over the phone.

Post back and I will be glad to recalculate.

IMO

Tigerdunes: I double checked the numbers. They are accurate and represent the total amount of money spent on each type of fuel of the last 13 months divided by the total number of therm or kwh used. This take into account all expenses associated with each type of fuel (base charge etc.) and is averaged out over the year so will not deflate or inflate the avg cost per unit becuase it takes into account every month of the year. So, how do these numbers change the comparison?

Auggie

Something not right about the nat gas price you provided. I don't have confidence in that number and it is the highest cost I have seen when nat gas is much cheaper in other close proximity markets.

Anyway, I have plugged in the numbers and here are the results. Same efficiencies are used. As expected the results have now flip flopped in favor of electric HP.

Cost per 100,000 btu of useable heat
Electric baseboard: $3.16
Heat pump: $1.17
Natural gas: $1.80

IMO

Tigerdunes: The reason the natural gas number is high is because the naked cost pe therm is very low (.49 per therm as it appears on the bill), BUT when you factor in the other junk charges from the gas company to get the total cost (i.e, the bill you pay at the end of the month) as you suggested I do in a previous post it is substantially more, per therm, especially if you do not use a lot of gas. BUT, the more gas you use, the lower the cost per therm is because the junk charges (which are fixed) are spread out over more therms. Does that make sense? So, at the end of the day, the cost per therm is a moving target which changes based on the amount of gas you use, that is why I used the total of 13 months of what I actually paid for gas divided by the therms used during that period. To be sure, during this time the majority of the heating was done by the HP (electric), so the cost per therm is much higher. My guess is that the costs are going to be about equal but I just don't know. Is there anyone out there that knows how to do this to get a more accurate comparison. This is confusing.

Auggie

I understand nat gas pricing as my distributor is similar in the manner of cost and billing. All recurring charges have to be factored in and I think one should take an average for the therms you will use each billing cycle. Probably in neighborhood of 100-150 therms/mth over the winter heating season months. However, this really depends on how you use or not use your heat pump. I have a dual fuel system and live in the Carolinas. My changeover is set at 32 degrees. It could be set lower. For the last several heating seasons, I have used less than 100 therms of nat gas over the entire heating season. Of course, weather will play a factor in nat gas use.

This is not confusing at all. You just have to your homework and decide what your temp changeover point will be. I suggest you calculate your cost in increments of 25 therms beginning at 50 therms/month all the way up to 200 therms. Unless you are in a tiered electric rate, your electric rate will be fixed. Keep in mind, you have 2 thermal points, temperature and economic. Just looking at it from an operating cost, I suspect you will be better off running HP down to point that it is not able to maintain your inside thermostat setting. That's called your temperature balance point. Still have doubts about that $1.83 cost/therm.

While you have great and top of line equipment, I think you were oversold based on Atlanta location. Keep in mind, average winter temp for Atlanta is around 40 degrees

IMO

This post was edited by tigerdunes on Tue, Nov 19, 13 at 8:38

Post the details of your gas and electric bills. This will help in determining what costs should be used in making the decision for you balance point.

@MIke and Tigerdunes:

I requested bill summaries from Georgia Natural Gas vs. Georgia Power (Electric). Over the last 13 months, I used 973.186 therms of gas and I paid $1,780.08 which works out to $1.83 per therm. Over the same last 13 months I used 23,408 kwh of electric and paid $2,831.30 which works out to .1209/kwh.

I do know that most of the heat was by heat pump last year, as the temperature balance point (thank you Tigedunes) was set at 42*. And as I said before, the cost per therm goes down as you use more gas because there are a lot of fixed costs that are spead over all therms used. Does that help?

Auggie

Just a correction please.

42 is not your temperature balance point. 42 is the arbitrary temperature you have decided to use when switching from HP heat to your backup fuel source.

In your case, temperature balance point refers to the lowest outside temperature your electric heat pump can go and still maintain your home's inside comfort and thermostat setting. When you go below the temperature balance point, your home is losing more BTUs to the outside than your HP is able to produce. This means you are not able to maintain your home's inside thermostat setting. Therefore the reason for backup heat.

If operating costs are important to you, you should use your HP down to the balance point to leverage electric against more expensive nat gas use.

IMO

Utilities typically include a service charge as part of the utility bill. This service charge tends to distort the price per therm or KWh. This is especially true if you don't use a lot of energy.

Take a look at your bill and figure out how much you are paying for the last therm and KWh. These are the costs you want to use in your calculations for the economic balance point.

For Mike

You can not learn the temperature balance point or economic balance point by the rates alone. There are many factors including the quality of the home's building and insulation envelope and the usual comfort inside thermostat setting.

The fact remains that electric will always be cheaper than nat gas. And nat gas furnace operation should only be used when electric HP can not maintain comfort and inside temperature setting or perhaps in this case homeowner simply does not like electric HP heat.

I have found the easiest way to learn the temperature balance point is through trial and error by making adjustments to the changeover point. I recommend starting with 32 degrees and adjust up or down based on what is comfortable for the home's occupants. Electric though will always be less expensive.

Installing dealer though should have run the calculations for OP on both temperature and economic balance point when system was installed using current rates. Bet he didn't know how to do this.

IMO

My heat pump (for one of them) supplement is propane, no natural gas available here. The dual-fuel setup is set to switch at 28 deg-- decided by the contractor. it has worked well.
I chose propane because the 92% propane furnace can be operated from a portable generator in a power outage if necessary. Propane is expensive here, but so is electrical resistance heating.

@auggie1020, If you have rate tiers then would the calculations be the same?

Here is a link that might be useful: Georgia Power Bill Calculator

For Tigerdunes:

"The fact remains that electric will always be cheaper than nat gas."

I am not so sure this statement is always true. I just got my gas and electric bills from PSE&G. My latest rates not counting the service charge is:

natural gas: $0.823 per therm

electricity: $0.176 per kWh

I have 95% AFUE furnaces. Correct me if I am wrong but I don't think a conventional heat pump which will be cheaper to operate than my furnace with my current rates.

I do agree calculating the comfort balance point should be done by trial and error. The OP was trying to determine at what temperature the switch in fuels should be done for maximum cost savings. The simple calculation using COP and AFUE values is not precise but puts you in the range. The problem I saw was the OP had posting energy rates that did not make sense given today's market. I also took issue with the statement by the HVAC contractor that the thermostat will figure what is the right temperature.

No Mike

You misunderstood.

For this customer, his location, and his electric and nat gas rates.

The maximum cost savings will be using HP down to outside temperature where it can no longer maintain inside temperature. In this homeowner's case, economic balance point does not even matter, just temperature balance point.

TD

This post was edited by tigerdunes on Wed, Nov 20, 13 at 17:17

My bad for misunderstanding.

I find it hard to believe the gas rate is correct. Something is wrong in the way he is reporting it, or he is signed up for a tiered service that does not makes sense given the amount of gas he uses.

And Mike you are correct.

I am on Piedmont Natural Gas in the Carolinas and they are known for being high, but nothing like Auggie's rates. I still find his nat gas rate hard to understand.

BTW, my HP has been purring all day long.

TD

Mike: What do you mean, "Take a look at your bill and figure out how much you are paying for the last therm and KWh"? Are you saying to just compare the price per therm vs. price per KWH without taking into account any other costs from the electric or gas company?

Your bill should state how much you are paying for each therm or kWh. This is the operating rate you want to use for the calculation. Don't include any service charges or other fees.

I respectfully disagree with Mike.

All monthly recurring charges must be factored in to the final cost/therm for natural gas. At the end of the day, you take the billed amount and divide by the therms used.

TD

Here is an analogy. Suppose your boss asks if you would like to work a weekend and get paid extra money. It so happens you were planning to paint your house on the same weekend. You want to know does it make more sense financially to work and make extra money and hire someone to do the painting, or should you turn down the offer to work and paint the house yourself?

In order to make the decision you want to know how much extra money you will take home after taxes in order to pay the painter. The tax rates are progressive, the more you make, the higher the rate. In order to do the calculation would your use an average tax rate of your pay check, or would you use the highest tax rate you would be taxed?

The services charges will be paid no matter how much you use the heat pump or the furnace. In a dual fuel system, the calculation you want to do is to figure out how much is the incremental cost for burning an extra 100 therms versus an extra 100 kWh. (I am using 100 as an example). Including services charges will not give an accurate picture of this incremental cost.

If you were doing an oil to gas conversion, and you wanted to calculate how much it would cost to heat a house with gas versus oil, then I agree all gas service charges should be included.

What Mike says makes sense. Since I have to have gas and electric service anyway, the fixed costs associated with those servie are not relevant to the analysis. So, my costs are .49/therm and .05 KWh. Knowing these costs, does anyone know how to compute whether it is more efficient to run the HP or furnace at different temperatures?

Thanks.