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Discussion Starter · #1 ·
I have a Goodman model CPKJ36 heat pump that works in conjunction with a Janitrol propane furnace as the seconday heating system. A Goodman AFE 18-60 dual fuel board is used along with a Honeywell T8411R thermostat to control the heat pump and propane furnace. There are no resistance heating elements. The rating on the furnace indicates that it's a 100,000 btu unit with an output of 90,500 btu. I assume that this means a 90.5% efficiency.

The system does not have an outside thermostat at this time. The Honeywell T8411 thermostat is an earlier model and has a 5 degree differential between setpoint and room temperterature before calling for auxillary heat. New production units have a 2 degree differential. Setting the thermostat to emergency heat will always engage the propane furnace when heat is called.

My question is how to determine the optimum point for the system to switch from heat pump to propane furnace. My electric cost is .085 per KW and my propane costs are 1.49 per gallon.

Currently, the system will call for heat pump first, and if the heat pump is unable to meet the heating demand and the room temperature drops more than 5 degrees below setpoint, the propane furnace will begin operating. As soon as it brings the room temperature to within 5 degrees of setpoint, the propane furnace will shut down and the system will attempt to heat with the heat pump. This cycle will continue with the temperature of the room remaining 5-6 degrees below setpoint until I manually switch to emergency heat. If the outside temperature is above 25 degrees, the heat pump alone can usually meet the demand.

It would seem logical to install an outside thermostat to disable the compressor and redirect the stage one heating signal to the propane furnace, if the outside temperature fell below a certain level. I am certain that the optimum point to set this thermostat is related to the outside temperature, the efficience of the heat pump at lower temperatures, the cost of electricity, the cost of propane, and the efficiency of the propane furnace.

I would appreciate any comments that you may have about the above.

Thanks in advance.

Discussion Starter · #4 ·
Still Puzzled

Although the heat pump can still meet the demand at 25 degrees, it must run almost constantly. At 25 degrees, the propane furnace can run at a much lower duty cycle and easily meet the demand.

Therefore, it would seem that the performance curves for the heat pump would be part of the equation to calculate the point where it cost the same to use electric or propane given a certain outside temperature and a fixed demand to heat the house.

In other words, the house requires a certain number of btu's per hour to keep it at the desired temperature. The heat pump can produce a certain number of btu's per hour at a specific outside temperature using a certain number of KWs. The propane can produce a certain number of btu's per hour using a certain number of gallons of propane.

It would seem that if the above is correct, an equation can be written where the cost of electric, propane, and the outside temperature are inputs and it would calculate the balance point.

Any ideas????

I forget to mention that the propane furnace is a model GMPN 100-4 Input 100,000 and output 90,500 btu.

52 Posts
You are considering operating cost only. The price of propane doesn't have anything to do with a heat pump's balance point. What if propane prices go up? Would you go outdoors and readjust the balance point?

I am very sorry to hear about your GMPN. That is a very very inexpensive model and will not tolerate even the slightest over heating. Keep your air flow up or the innards of the furnace will bust.

Discussion Starter · #6 ·
GMPN-100 Blues :(

Sorry, the title of my original post is "Economic Balance Point" not "heat pump balance point". The word economic implies $$$. And yes, it would seem logical that if the price of propane had a significant change, it would be prudent to re-adjust the outdoor thermostat accordingly.

The higher the price of propane, given a stable electricity price, the more inefficiently (lower outside temperature) one can operate the heat pump and still break even (economic balance point).

Now it beginning to sound like the factors are as follows:

1. The price of propane.
2. The number of btus in a gallon of propane.
3. The efficiency of the propane furnace.
4. The cost of electricity.
5. The cost of operating the blower in the furnace (electric blower motor).
6. The efficiency of the heat pump translated to the number of KW hours necessary to produce a certain number of btus at a specified outside temperature.
7. A good algebra teacher.

Have I omitted anything???

Oh, and there's always the point where the wife says "it's too cold in here, turn on the *%@$ gas!" Otherwise known as the heat pump bitching point.

52 Posts
Yeah I understand you are talking about economics but when saving money damages the equipment the cost of repair offsets any monies saved.

When your heat pump goes into defrost it will blow cold air unless there are heat strips energized.


I have a PROPANE furnace. And like any 2 stage heating system that uses a heat pump for stage 1, the PROPANE furnace is on during defrost just like the heat strips are on during defrost.

Defrost = put the system in AC mode, bring some heat from the house to melt the frost or ice from the outside coils.

Discussion Starter · #10 ·
In Spec.

It may be hell on the compressor, but you can't tell it from the high and/or low side pressure readings nor from the amperage readings on the compressor. Pressures and amperage are about the same as normal AC operation on a summer day.

So can we please return to my original question and try to remain focused. (Is this better Nathan?)

Hint: 1 gallon of propane ($1.49) contains 91,600 BTUs of heat, and 1 KWH of electricity ($0.085) will produce 3,413 BTUs of heat using resistance strips.

Now, if anyone has access to the performance data for the Goodman heat pump I specified in my first post and can determine the number of BTUs of heat it will produce at a given outside temperature and the number of KWH necessary to power the unit, my question could be answered.

Hey, what's the point in having a forum if I have to answer my own questions?

Discussion Starter · #13 ·
Most of the answers, Yes.


You are correct. I do have most of the answers except for the performance data for my heat pump per post #10.

I am an electrical engineer and design computer systems for industrial process control, so I have a reasonable understanding of the various systems associated with a heat pump. I had courses in thermodynamics in college, so I also have some background knowledge of the theory of heat pumps.

I do, however, lack the practical day to day knowledge that a seasoned professional HVAC contractor would have aquired while working in the trade.

My motivation to post on this forum was in hope that the post would be read by a seasoned HVAC professional who is familiar with the concept of my question regarding the economic balance point and would be able to post an equation/formula where I could "plug in" the variables and derive the desired answer.

Had a similar dialog occured in my company as in this thread, my message to my employees would be: it's ok to admit that you don't know the answer to a question and it simplifies the communication process to just admit it right off so I can move on to another source of information. No one is expected to know everything.

It's interesting and amusing to note all of the "non answers" that were posted in this thread. Irrelevant material is often worse than no material and is a waste of one's time to sift through it.

So, the remaing question to Nathan is: Would the answers be of a higher quality and more relevant if I registered?

Discussion Starter · #16 ·
My Apologies

First of all, as an engineer and an HVAC professional, allow me to apologize for the way my fellow engineer has approached this subject. Many engineers have a problem with people they suspect are not on the same "level" with them, and since engineers as a whole do not communicate effectively with non-engineers, they tend to come across a little demanding and unfriendly.

That doesn't make it OK, but I am sorry for you guys to have to deal with it.

Now, on to the matter at hand:

Dear Unregistered:
If you are indeed an engineer, then you are familiar with unknowns. The question you have asked has many unknowns. Some of these unknowns can be calculated or measured. For instance; paramount among the information necessary for your equation is the heat loss and gain of your house at different outdoor temperatures, and the thermal lag associated with your home's construction. Also measurable or able to be looked up is the average temperature change per 12 hours in the area where you live.

But, unable to be looked up or calculated is your specific system's heat out put at a given indoor/outdoor temperature set. Even though the manufacturer publishes data at 17º and 47º outdoor temp, those data points are independent of the indoor unit, ductwork efficiency losses, the coil and filter cleanliness or lack thereof, and many other variables associated with a mechanical system's operation.

So, let me run down a verbal description of this equation you are asking for:

Heat leaks out of your house at a set amount per hour based on variables such as the indoor/outdoor delta T, construction style, indoor air velocity, outdoor air velocity and air exchange rate.

This heat leakage is offset by many factors, including the heat output of the heating system, heat output from appliances, lights, electronics, people and animals and solar gain, to name a few. Thermal mass of the construction style also plays a role in your balance point, as heat is stored in the mass of the house and then given back to the space as a function of temp change and specific heat indices for all the masses.

Now that we have an equation for your "NEED" for heat, we can begin to look at those variables associated with the "economic balance point" of your system.

Your heatpump balance point seems to be, based on your reported observations, about 25º outdoor temp. Assuming for a moment that your indoor temp is not variable, let's examine the thermal lag of your home. How long does it have to be 25º outdoors before your system begins to fall behind and call for furnace heat? This would give you an idea of the thermal lag factor. Let's assume for this discussion it is 4 hours. (a reasonable number). So we need to examine how often the temp falls below 25 for more than four hours where you live. Your heatpump balance point may not be 25. It may be 35. I hope you are following me here, because I am getting tired of this analysis.

Let's move on. The ONLY way to determine accurately your heatpump's KW/KW output is to put your entire house inside a psychrometric facility and measure it. Even if you know the COP, EER and other rating points (you can look them up at of the outdoor unit, or even if you have that data for the indoor/outdoor unit combination, you still do not know the data for your installed system. (refer back to ductwork)

You also need to know the exact efficiency of your furnace as it is installed and maintained. The rating you gave us is the manufacturer's rating, and is for reference only. The conditions at which those reference data are collected do not exist in most occupied dwellings.

Now, based on your prior expressed knowledge, and the information I have given you here, you are the ONLY person who is in a position to apprehend the information you are seeking. Unless you would like to pay me to come to your house and figure it out. :D

The original response you got from Steve was given to him by me. This response was based on 20 years experience designing, selling and maintaining HVAC systems, a Master's Degree in mechanical engineering, and 4 years of experience as a design and specification engineer for HVAC manufacturers. I stand by it as the only reasonable response to a question such as yours in a venue such as this. A P.E. stamp gives me the ability to approval of disapprove calculations such as you are asking us to give you, and I summarily disapproved this one, based on lack of knowledge and information I know will remain missing.

Now, to your behavior. You came to a website to ask assistance from professionals. They gave you their best response based on the information you gave them. You then berated, ridiculed and corrected them. If you (assuming you are what you claim to be; an electrical engineer) were asked to help design my home's electrical distribution network, would you be happy to hear me say you were wrong and had no clue what I was asking for? It is my humble opinion you owe these gentlemen an apology, but since this is the Internet and you think you are anonymous, I doubt one will be forthcoming. In fact, I'll wager you are all poised right now to give us all a piece of your anonymous mind, right?

Save your typing for someone who cares. :(

Discussion Starter · #18 ·
My Aplology


Thank you for your insightful answers to my original post. You are correct; some of my earlier remarks were uncalled for and rude so I offer to all whom I offended and in particular Steve, my apology.

I understand all of your analysis and appreciate your enumeration of many of the variables that influence a heating or cooling system in the real world.

However, I would appreaciate your continued analysis from a more static or theoretical perspective using the assumption that the heatpump can meet the heating demands of the dwelling down to 0 degrees F or lower.

I have obtained the performance curves for the Goodman CPKJ 36-1A and can see that the theoretical heat output is dependent on the indoor coil among other things.

I also realize that producing an equation that would yield an exact answer and work in all cases would probable be impossible for the reasons you have stated, therefore, I am seeking more of a "ball park" type of equation where the most important variables you have descirbed can be parametic in their use.

Thanks in advance.

Discussion Starter · #20 ·
Thermal Lag


I have re-read your post several more times and would appreciate your additional comments/analysis regarding the following:

Disregarding, for the moment, solar gain, outdoor air velocity, heat gain from people, appliances, etc. and focusing mostly on thermal mass and air exchange, and assuming the duct system is properly sized, using a temperature data collection system that recorded the indoor air temp at the return(s) (with blower motor only operating, heat is off) along with the outdoor temp, could a relationship be derived that gave a reasonable indication of thermal lag for a specific dwelling given certain delta t's?

I haven't used thermodynamic equations in many years, but I seem to recall that the simplified relationships involving heat gain/loss are non-linear with respect to delta t. Is this correct?
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