Economic Balance Point

shortfuse

Nathan, now I am registered. Please do not be offended by my e-mail address, although it is real, it was created by my administrative assistant, and is used mostly to prevent my regular e-mail from filling with spam.

My user name is representative of my personality.

Nathan

No Problem.... Thanks for Registering!

__________________
Nathan

"He is no fool who gives what he cannot keep to gain that which he cannot lose." - Jim Elliot

GoodmanMan

BamaCracker, P.E.:

Well boys, what are you waiting for? He registered. He apologized. He axed two more questions. Seem's like the ball's in your court now.

Bamacracker

My apologies for the delay. Not being a regular contributor here, I neglected to check for responses in a reasonable period of time.

Shortfuse,
Yes; by plotting the temperature change in the house (sounds like you have the right idea, measuring at several dynamic locations and plotting an average) you could develop a ballpark value for the thermal lag of the house. Remember that thermal lag is affected by delta T, so the thermal lag at 70 ID and 50OD would not be identical to the thermal lag if the temp was 70ID and 20 OD. But, if you had the heating/cooling off, and gathered the thermal lag data over a wide range of delta T, you could develop an equation that would represent the whole, and extrapolate the points you needed.

But; if you don't want to take my 25º SWAG as the gospel truth , you could use the data you have laid out already, and the data you will have collected, and calculate an approximate economic balance point. I still caution you to use that approximate as a starting point, and be sure your outdoor stat is adjustable, so you can experiment with different settings.

So you take your heat pump performance curves, and plot a btu/watt graph for the area of interest. Since we are ballparking this, assume your indoor coil and ductwork to be new and clean. Since the performance curves you have will give you the BTU output at different OD temps, but not the specific current draw at those temps, you will have to measure the current draw yourself, or assume them to be constant. (not valid, but reasonable) Then, you convert the current draw from watts to $$ by using your local power company's billing schedule, and plot this against outdoor temps.

Next, using the data you already posted about your gas furnace, use your gas company's current cost and reported heat value (it's not constant, but without your own calorimeter, what else can you do?) per gallon or pound, and convert the fuel consumption to $/btu. Overlay this graph onto the first graph, so that the junction of these two lines is your economic balance point as regards heat output.

Now, since you offered the assumption that the heatpump could heat your house down to zero (you already reported that it will not, but I see your reasoning), the economic balance point is no longer a function of the house at all, but a function of fuel cost and mechanical efficiency only.

If you wish to wade through the mathematical gymnastics, you could then plot the $/BTU data against your home's thermal lag and heat loss/gain and get a more realistic value, but as we already examined, that becomes more and more a waste of time and energy.

Let me know how it turns out. I'd like to continue this discussion here, for the sake of all, but if you get tired of waiting for me to return, my e-mail is my username plus the following: hvac@yahoo.com (I do not post them together so as to avoid spam bots)

BamaCracker

I was sound asleep when I realized, I have made an assumption that is most likely not valid. I could not go back to sleep until I post it here:

Most people in our industry know that heatpumps are more efficient mechanically than any other form of heat production because only a portion of the heat is created, the rest being gathered from some remote location (usually outdoor air). Because most people know this, I made the assumption you knew it Shortfuse, even though you have no reason to know it, not being in our line of work.

Even at low temp, heat pump coefficient of performance should be more than 2. That means you put in 100 Btus of electricity, and get out 200 Btus of heat to the space.

But a high efficiency furnace (you already noted yours was 90.5%) has a cop of less than 1, meaning if you put in 100 Btus of gas, you get 90 Btus of heat, and not all of that goes to the space being heated.

So, even assuming a COP of 2 (very low) for that heat pump, the price for a BTU of electricity could be 2X as much as a BTU of gas, and the heat pump is still the cheapest way to heat.

Now, maybe I can get back to sleep.

shortfuse

BamaCracker:

Thank you for your conformation of my suspicions and assumptions. I am familiar with COP and it's meaning with respect to HVAC. As you explained, in heat pump mode, we gain heat from both mechanical methods (heat from compressing the freon) as well as transfering heat from the exterior to the interior of the dwelling. The heat from compression helps the efficiency in heat pump mode but works against us in cooling mode. Resistance heat, in theory, has a COP=1. I would guess that un-vented gas logs also have a COP=1. I look at COP as an efficiency multiplier.

The performance curves I have obtained for my heat pump are dependent on the interior coil, but at temperatures in the 47 degree range, the COP is 3 or better with certain interior coils. ("Money for nothing, chicks for free!")

As for the current consumption, I haven't been able to locate a chart or graph for my unit that relates KW to BTUs with respect to outside air temperature. While I have measured the unit's current using a clamp on ampmeter, the power factor of the compressor motor and fan motor must be considered since it has a significant effect on the KWH that the power meter actually registers. A pure resistance load has a PF of 1 thus volts x amps will yield a very accurate value of the watts consumed. An inductive load, such as a motor, will have a PF of less than 1 and will vary to some extent with load.

Given the current price of propane at $1.49 per gallon, the btu's available in a gallon, the efficiency of the propane furnace, and electricity at $0.085/kwh, it is easy to see that the heat pump is a clear winner.

However, as was mentioned several times in this thread, if the heat pump can't keep up with the demands of the dwelling, none of this matters anyway. The auxilary heat must come on regardless of the cost.

The economic balance point is only a factor when it's less expensive to produce a btu of heat from propane, etc. and the installed heat pump can meet the demand at a given outside temperature.

Now all of this leads me to questions about the proper sizing of a unit in order to meet both the heating and cooling requirements of a given dwelling and achieve maximum effiency of operating costs. Maybe I should save these questions for another thread.

Thanks to all for their input and comments.

drpsr69

I have fixed furnaces and heat pumps for engineers who worked at carrier and designed the units they can't fix in their own home.There is a way to figure the economic balance point based on the effiency of the heatpump and furnace. The electric cost and propane cost. It may be true that no equipment operates at the design effiency.But you can come up with a economic balance point .I just looked through my old literature and can't find it but i will call carriers service manager and find out how it is done.I live in syracuse,n.y. which is carriers home at least for now.the economic balance point is probaly more like 40 degreesor higher with a 90% furnace.

gideon4k

I've been following this thread with interest. I have two questions:
1. Is there a duty cycle (i.e. 90%, 70%, etc) that will cause damage to one's heat pump (I have an R410 system)? I've heard of them being used even down to 0 deg F, but I'm not sure if that's okay or not.

2. I also have a propane furnace that burns 1 gallon per hour. Would it be possible to incorporate all of the variables shortfuse was asking about by taking some measurements at a given outdoor temperature using each of his heating methods? Example: He turns on the heat pump and measures the current it draws while running and times the duty cycle when it is 30 degrees outside. Then he turns it off and runs just the propane furnace with the same outdoor air temp and measures the duty cycle and uses the efficiency & output BTU's/hour from the label to calculate the amount of propane burned in the same amount of time. It might look like this:
0.085 Cost Per Kw/h 0.8 Hours 4 KW consumed while running 0.272 $/hr to heat the house with heatpump at 30 degrees F 2.95 Cost Per Gallon 0.25 Hours 1 Gallons consumed per hour while running the propane furnace 0.7375 $/hr to heat the house with propane furnace at 30 degrees F
Could a person do this?

crmont

That's basically how I would do it. A real time test with installed equipment.

There is one element to the "economic balance point" that is not usually factored; the defrost control. There are basically two methods of logic.

Many heat pumps (goodman, carrier, lennox...) use a simple temperature thermostat in direct contact with the outdoor coil. Unnecessary defrost cycles on these units are common and will happen below 40 degrees or so. On a windy day an unnecessary defrost can last 5 min. The indoor expansion device directly affects the performance of a defrost cycle. A fixed meetering device takes longer to flood the evaporator thus a longer defrost cycle.

Some manufacturers (trane, rheem) use what's called an "on demand" defrost control. This control uses two sensors; coil and air. These sensors are thermistors that measure the temperature difference between the ambient air and the coil. As the coil ices up the temperature difference increases. This is a much better way to control the defrost and is consistent.

If I had a goodman heat pump I would disable the defrost cycle and set the balance point at 38 degrees and call it good. If i had a trane I would take the time to record the duty cycles as you suggested.

nangab

I stumbled onto this thread, and find it so very informative. I can see just how little I know about the HVAC industry, and how the systems operate. I have been in facility maintenance for the past 7 years, and at this time I am in the 2nd trimester of a local vocational school for HVAC training. Most of the threads I have been to don't come close to the keen, sharpe, information presented here. Thank you

HeatPro

I appreciate the attention to detail involved in 'drilling down' to definitive answers; participation in the details is the 'fun' of being an engineer. I realize that contractors aren't necessarily that emotionally involved in detailed physics computation, so would be looking for more practical, simpler methods to "get'er done," so the "Yup 25F" would be enough to go off to other things. Your post sequences are a "wake-up call" as to what is really involved in getting to the most accurate point, so are a good education. Appreciate it.