
Originally Posted by gary_g
No catch, but a few things you should know:
 Heat pumps produce 95degree air at 32F outside. As the outdoor temp drops the temp of the air produced by the heat pump also drops. Furnaces put out "hot" air; 120F or so.
 In brutal cold temps, a heat pump may run constantly if there is no dualfuel system (allelectric home).
 Heat pumps have a shorter life expectancy than straight a/c units because they run during all 4 seasons.
Take care.
I am just looking to take a good 'bite' out of what a all propane heating bill would be. I have no qualms with having the setpoint around 35 or so (maybe higher?); I just want to take a large dent out of my propane bill. I don't want to sacrifice any interior comfort, so if that means my propane fires a little bit more then would be optimally efficient, I'm okay with that.
And, how much shorter does shorter really equate to? In the area that I'm in, the heat is used from probably midoctober thru late April. May and June are usually freebie months, as would be Mid September to at least Mid October. So assuming that's three months right there w/ no activity, and 2/3 duty during the other 9, I wonder how long on average it would last before I have to pull it out?
If in the end it wasn't worth it b/c you have to switch the unit out faster (thereby spending more money in equipment costs), that to me would be a big gotcha.

Alrighty...Suppose you start your 20mpg car and take off immediately knowing you have 2 flat tires. The car IS moving forward. Are you wasting Gas? The flat tires represent the temperature below where your OD Stat SHOULD be set. At this point, you should take your other "Car" which only gets 15mpg because it is better suited to the task.
We are talking about 2 different things. You are talking about the efficiency to convert fuel or electricity to heat. I am talking about money leaving your wallet.
If you are paying for something to do a specific job and it can't do it, you are wasting money.
Electric resistance heat is 100% efficient but if it is sized too small to heat your home but you run it anyway, you are wasting money because it's not doing the job you are paying it to do.
Yes, the heat pump is supplying heat and most likely more efficiently than the furnace but the furnace is actually cheaper to run below the Thermal Balance Point. If you let the HP run below the Thermal Balance Point you are below your Break Even Coefficient of Performance (For Natural Gas: BECOP=293 X AFUE as a decimal X ($ per KWH / $ per 1000 cu.ft.)

Originally Posted by jaguar36
This is an excellent question, how do you find the anwser to it? I can't seem to find any real specs for the heat pumps on Trane or Lennox's website, Is it not published, or is it just hidden away somewhere?
You need the model numbers of both the condenser and indoor coil. It must be a matched set.
Once you have the model#s, www.ARIDirectory.org provides heating btus at 47F and 17F (as well as other info).
Goodman (www.GoodmanMFG.com) provides the most complete technical info to homeowners. They have expanded heating and cooling data for all of their heat pump systems under "Product Specs".
Take care.

Originally Posted by shodinjido
We are talking about 2 different things. You are talking about the efficiency to convert fuel or electricity to heat. I am talking about money leaving your wallet.
No, I am talking about efficiency of turning money into heat. Using the numbers Gary provided the Heat Pump is more efficient than my furnace down till 16F. Around here it only gets to that temp for a day or two each year. So basically all of the time the heat pump is more efficient at turning dollars into heat.

Originally Posted by jaguar36
No, I am talking about efficiency of turning money into heat. Using the numbers Gary provided the Heat Pump is more efficient than my furnace down till 16F. Around here it only gets to that temp for a day or two each year. So basically all of the time the heat pump is more efficient at turning dollars into heat.
Yes, more efficient but costs more money out of your pocket to run. The HP has to run longer at lower temperatures and continuously if it isn't supplying enough BTUs. So, you are pumping more money into a piece of equipment that will not perform. Where I'm from...that is wasting money.
You go ahead and set your Outdoor thermostat wherever you like. I'll continue to set my customers OD thermostats where it saves them money (even when the furnace is running).

Heatpump technology was not that prevelant in south jersey because a lot of folks grew up with oil heated, hot water baseboard or scorched air. They liked the comfort generated by those old systems. A heatpump will run longer delivering lower temperature air to the structure. In many cases, the home builders dictate to a contractor that each home will cost x amount of money for HVAC, and duct design is poor because of the budget. This causes the systems to be loud, and uncomfortable in many cases. If designed properly, they work fine. Many people in that area, contractors included are "used to" oil or lp heating. Heatpumps have not been fully embraced. The dual fuel set up works well.

Originally Posted by flange
Many people in that area, contractors included are "used to" oil or lp heating. Heatpumps have not been fully embraced. The dual fuel set up works well.
Ahh That makes sense, Thanks Flange
Originally Posted by shodinjido
So, you are pumping more money into a piece of equipment that will not perform.
It may not perform, however while it is running its is more efficiently converting my dollars into heat (and thus saving me money), regardless of if it can handle the full load or not.
Anyways, since you seem unable to grasp this concept and have therefore decided to turn this thread into a flame war, I'm gonna scamper off.
I do wanna say thanks to the folks like Gary_G and Flange who provided actual useful information though.

Originally Posted by jaguar36
I do wanna say thanks to the folks like Gary_G and Flange who provided actual useful information though.
Best of luck, Jag.
I do want to stress the importance of getting the model numbers of the condenser and indoor coil and looking them up on the ARI website. Some systems are short of their rated capacity (Example: 3 tons is only 32,000 btu instead of 36,000 btu). You need to know this as an educated homeowner.
Take care.

Originally Posted by jaguar36
Ahh That makes sense, Thanks Flange
It may not perform, however while it is running its is more efficiently converting my dollars into heat (and thus saving me money), regardless of if it can handle the full load or not.
Anyways, since you seem unable to grasp this concept and have therefore decided to turn this thread into a flame war, I'm gonna scamper off.
I do wanna say thanks to the folks like Gary_G and Flange who provided actual useful information though.
Information is what you make of it.
I'm not flaming anyone. I am trying to help you understand that there is more to this than the limited information that you have. You have only performed one part of a series of calculations required to know where to set your OD thermostat to save money...ACTUAL DOLLARS.
Trust me. I do understand that the HP is more efficient at converting Dollars to heat at certain temperatures. I've been in the HVAC field for 15 years. I actually know what it takes to calculate where to set od thermostats. Do you know what your Break Even COP is on both pieces of equipment? Has an accurate Load calculation been performed? Do you have the heat output figures for the HP at specific temperatures? Have you graphed the Economic Balance Point?
The information you have will not tell you where to set the stat to keep more dollars in your pocket! PERIOD

Originally Posted by air311
That's what I started thinking, those old farmers simply set the thermostat lower. And I suppose different blends of LP put out different BTUs. Maybe since heat is needed 67 months out of the year here, we get some hotter burning stuff. Not sure if there's a difference in blends. I have oil heat in my home, and seriously considered switching to natural gas when I replaced my furnace in December. Gas company wanted to much to run the line in though unless I would have installed a natural gas water heater. With the heat pump I just put in, I should save a quite a bit next winter.
I wish the gas company here would have put the line in for free for me if I'd have gotten a natural gas water heater... it would have been worth it. They wanted $80 per foot for the 200 foot distance ($16,000) so I decided it wasn't worth it.
Originally Posted by shodinjido
Information is what you make of it.
I'm not flaming anyone. I am trying to help you understand that there is more to this than the limited information that you have. You have only performed one part of a series of calculations required to know where to set your OD thermostat to save money...ACTUAL DOLLARS.
Trust me. I do understand that the HP is more efficient at converting Dollars to heat at certain temperatures. I've been in the HVAC field for 15 years. I actually know what it takes to calculate where to set od thermostats. Do you know what your Break Even COP is on both pieces of equipment? Has an accurate Load calculation been performed? Do you have the heat output figures for the HP at specific temperatures? Have you graphed the Economic Balance Point?
The information you have will not tell you where to set the stat to keep more dollars in your pocket! PERIOD
He was accurately stating his breakeven COP on each piece of equipment and the fact was that the breakeven COP was 16F. You were unhappy because at 16F his heatpump is not producing enough heat to maintain the temperature in his home. His argument being that even though the temperature in his house will drop it is still more economical to heat his home (albeit economical does not mean that its enough to maintain temp) with the heatpump than the gas furnace until it reaches 16F. Just because the temperature drops and his furnace runs longer now to raise the temp again does not mean it uses more energy. If the furnace had come on sooner then it would have used energy by being on sooner instead of being on later and longer. The heatpump still provided heat at a more economical cost in the meantime.
A better argument would have been to point out that if his economical balance point is 16F that at such low temperatures his heatpump would require more defrost cycles (either the length of the defrost or the number of them) and that would therefore negate a certain amount of said balance point since during defrost he would be removing heat from his home and dumping it outside. If the balance point is 16F then you would not want to set the lockout temp to 16F due to the increased defrost cycles... you would want to set it higher... perhaps 20 or 25.

Originally Posted by platchford
I wish the gas company here would have put the line in for free for me if I'd have gotten a natural gas water heater... it would have been worth it. They wanted $80 per foot for the 200 foot distance ($16,000) so I decided it wasn't worth it.
He was accurately stating his breakeven COP on each piece of equipment and the fact was that the breakeven COP was 16F. You were unhappy because at 16F his heatpump is not producing enough heat to maintain the temperature in his home. His argument being that even though the temperature in his house will drop it is still more economical to heat his home (albeit economical does not mean that its enough to maintain temp) with the heatpump than the gas furnace until it reaches 16F. Just because the temperature drops and his furnace runs longer now to raise the temp again does not mean it uses more energy. If the furnace had come on sooner then it would have used energy by being on sooner instead of being on later and longer. The heatpump still provided heat at a more economical cost in the meantime.
A better argument would have been to point out that if his economical balance point is 16F that at such low temperatures his heatpump would require more defrost cycles (either the length of the defrost or the number of them) and that would therefore negate a certain amount of said balance point since during defrost he would be removing heat from his home and dumping it outside. If the balance point is 16F then you would not want to set the lockout temp to 16F due to the increased defrost cycles... you would want to set it higher... perhaps 20 or 25.
He is comparing apples to oranges.
Suppose the furnace has 100,000 btu input which equates to 80,000btu output. At 16 degrees, the heat pump is putting out approx 21,200btus.
A true comparison would be: 80,000/21,200=3.77 and 3.77 X 17.59= $66.31
It costs $66.31 for the heat pump to produce 1,000,000 btus @ 16 degrees
It costs $17.60 for the furnace to produce 1,000,000 btus @ 16 degrees
Which is more economical at 16 degrees? Where do you think break even will occur? Not where he thinks!
Like I have been saying...There isn't enough information in this post to know where to set the od Stat.
Last edited by shodinjido; 04102008 at 01:19 PM.

Originally Posted by shodinjido
He is comparing apples to oranges.
Suppose the furnace has 100,000 btu input which equates to 80,000btu output. At 16 degrees, the heat pump is putting out approx 21,200btus.
A true comparison would be: 80,000/21,200=3.77 and 3.77 X 17.59= $66.31
It costs $66.31 for the heat pump to produce 1,000,000 btus @ 16 degrees
It costs $17.60 for the furnace to produce 1,000,000 btus @ 16 degrees
Which is more economical at 16 degrees? Where do you think break even will occur? Not where he thinks!
Like I have been saying...There isn't enough information in this post to know where to set the od Stat.
Wow... I had to read and reread your post several times to find your error. It was an easy mistake to make and I didn't catch it when I first looked at your proof. The flaw in your mathematical proof is the fact that you multiplied the 3.77 x $17.59... but $17.59 isn't the cost to produce 21,200 BTUs... it was the calculated cost to produce 1,000,000 BTUs at 16 degrees. Also, since you used 80,000BTUs you should really keep it simpler by comparing apples to apples and oranges to oranges. Lets figure the cost to produce 80,000BTUs with each form of heat. Let's also redo the math using the cost to produce 21,200 BTUs... which should be the running amps of the outdoor unit.
Ok, I have to estimate the running amps of his unit... I can't find data for this anywhere. I will use the running amps of my outdoor unit as a guide... not a perfect comparison given my SEER rating is 12 and his is 14 but it should be close... also my heatpump is 2.5ton and his is 3ton. My running amps is 9.5... because his unit is 20% percent larger we will multiply 9.5 x 1.2 = 11.4... now we multiply 11.4(amps) x 220(volts) = 2.508kW... now we multiply 2.508kWhr x $.15/kWhr = $0.3762. Ok, so we now know it costs 37.62 cents for the heatpump to produce 21,200 BTUs. Now we multiply $.3762 x 3.77 (80000/21200) = $1.42. Therefore it costs $1.42 for the heatpump (at 16F putting out 22,500 BTUs) to produce 80,000 BTUs. The natural gas furnace (at 80% efficiency) produces 82,400 BTUs per therm but we want the cost for 80,000 BTUs... 80,000 BTUs / 82400 BTUs per therm = 97.087% (of a) therm... 97.087% (of a) therm x $1.45 per therm = $1.41. Therefore it costs him $1.41 for 80,000 BTUs with his gas furnace.
Ok... to recap... it costs him (roughly) $1.42 for the heatpump (at 16F putting out 22,500 BTUs) to produce 80,000 BTUs and it costs $1.41 for the gas furnace to produce 80,000 BTUs.
If you wish to know the price to produce 1,000,000 BTUs at 16F with these figures you simply take 1,000,000 divide by 80,000 = 12.5... now multiply the above figures by 12.5 and we get 12.5 x $1.42 = $17.75 for the heatpump and 12.5 x 1.41 = $17.63 for the gas furnace. His estimate was $17.59 (heatpump) and $17.60 (gas furnace) so he and I were not far off and I was estimating using my 12 SEER to compute his 14 SEER so, if anything, my numbers should be lower.
Care to try again or concede your math was improper?
PS: Jaguar36, my point about extra defrosts still stands... don't set your lockout temp to 16F because the extra defrost time will negate the savings when operating so close to the balance point. Utilize a balance point 5 to 10 degrees higher and/or adjust it based on how much more frequently your unit requires a defrost.

Originally Posted by platchford
Wow... I had to read and reread your post several times to find your error. It was an easy mistake to make and I didn't catch it when I first looked at your proof. The flaw in your mathematical proof is the fact that you multiplied the 3.77 x $17.59... but $17.59 isn't the cost to produce 21,200 BTUs... it was the calculated cost to produce 1,000,000 BTUs at 16 degrees. Also, since you used 80,000BTUs you should really keep it simpler by comparing apples to apples and oranges to oranges. Lets figure the cost to produce 80,000BTUs with each form of heat. Let's also redo the math using the cost to produce 21,200 BTUs... which should be the running amps of the outdoor unit.
Ok, I have to estimate the running amps of his unit... I can't find data for this anywhere. I will use the running amps of my outdoor unit as a guide... not a perfect comparison given my SEER rating is 12 and his is 14 but it should be close... also my heatpump is 2.5ton and his is 3ton. My running amps is 9.5... because his unit is 20% percent larger we will multiply 9.5 x 1.2 = 11.4... now we multiply 11.4(amps) x 220(volts) = 2.508kW... now we multiply 2.508kWhr x $.15/kWhr = $0.3762. Ok, so we now know it costs 37.62 cents for the heatpump to produce 21,200 BTUs. Now we multiply $.3762 x 3.77 (80000/21200) = $1.42. Therefore it costs $1.42 for the heatpump (at 16F putting out 22,500 BTUs) to produce 80,000 BTUs. The natural gas furnace (at 80% efficiency) produces 82,400 BTUs per therm but we want the cost for 80,000 BTUs... 80,000 BTUs / 82400 BTUs per therm = 97.087% (of a) therm... 97.087% (of a) therm x $1.45 per therm = $1.41. Therefore it costs him $1.41 for 80,000 BTUs with his gas furnace.
Ok... to recap... it costs him (roughly) $1.42 for the heatpump (at 16F putting out 22,500 BTUs) to produce 80,000 BTUs and it costs $1.41 for the gas furnace to produce 80,000 BTUs.
If you wish to know the price to produce 1,000,000 BTUs at 16F with these figures you simply take 1,000,000 divide by 80,000 = 12.5... now multiply the above figures by 12.5 and we get 12.5 x $1.42 = $17.75 for the heatpump and 12.5 x 1.41 = $17.63 for the gas furnace. His estimate was $17.59 (heatpump) and $17.60 (gas furnace) so he and I were not far off and I was estimating using my 12 SEER to compute his 14 SEER so, if anything, my numbers should be lower.
Care to try again or concede your math was improper?
PS: Jaguar36, my point about extra defrosts still stands... don't set your lockout temp to 16F because the extra defrost time will negate the savings when operating so close to the balance point. Utilize a balance point 5 to 10 degrees higher and/or adjust it based on how much more frequently your unit requires a defrost.
LOL
Ya you right. I think my brains have turned to mush on this thread.
I still maintain that there isn't enough info to know where to set the od stat. Just because the heat pump shows equal cost at 16 degrees doesn't mean it is economical to run at that temp. Yes I agree that defrosts add to cost but that's not the only thing that should be considered (which is accounted for by the defrost knee in the Economic Balance point graph). The HP is only putting 21,000 btus at 16 degrees. It's doubtful that the ID thermostat would even let the HP run down to that temp due to rate of heat loss (but again, there is no Man J info here).
This is all conjecture. Having all the information needed makes this so much easier
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