Heat pump efficiency?

Discussion in 'HVAC Heating & Cooling' started by Giles, Jan 5, 2010.

  1. Giles

    Giles Retired tool & Die and Mechanic

    Messages:
    110
    Location:
    N.W. Alabama--Florence--
    I have a Carrier high efficiency heat pump that is about eight years old. I also have a much older, natural gas central unit, that is tied into that duct-work of my home.
    This gas unit was part of the original 18 year old system. I know this gas unit is not very efficient because of age, but it is nice to have a working "backup" heat source.
    I live in N.W. Alabama so heating is not a major concern but money saved is money earned.
    Can someone correct me if I am wrong? ---It is my understanding that generally, a heat pump is the cheapest source of heat to a certain temperature. A heat pump starts to become more expensive when the temperature falls below 40 degrees.
    When the outside temperature is 35 to 40 degrees, I use my older gas furnace.
    I know there are many circumstances, other then fuel cost, that need to be considered and this is a hard question to answer.
    I ask this question because I have only lived in this home for a few months.
    All opinions would be appreciated.
  2. Thatguy

    Thatguy Homeowner

    Messages:
    1,460
    Location:
    MD
    Assuming 100% efficiency, 1 Therm of energy can be had from approx. 0.71 gal of fuel oil, 100 cubic feet of natural gas, 1.1 gal of propane, 29 kwh of elec heat, 8.4 kwh of heat pump heat (COP = 3.5), 4.2 kwh of heat pump heat (COP = 6.9) & 14 pounds of wood.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,809
    Location:
    New England
    ANy time the temp drops too low, or you call for a lot of heat quick (say after a set-back for the night), the heat pump controls may activate the resistance heaters - so, during that time, you're not getting any 'multiplcation' effect from the heat pump as you're using strictly electric through a strip heater to warm the place up.
  4. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    And since the thermal efficiency of a electricity from your average coal fired generation facility is around 33% resistance heating during cold snaps is a loser compared to gas in many cold climates. (Geothermal excepted...but there is that very high installation cost to consider.) I sure am glad I haven't had a heat pump with strip heaters for the past two weeks...

    A heat pump with gas backup probably makes the most sense efficiency wise, letting the heat pump do what it is good at, while the furnace does what it is best at.
  5. Dana

    Dana In the trades

    Messages:
    2,785
    Location:
    01609
    There's no estimating the cost until we know what your utility rates are.

    What is your total cost per kwh?

    What is your total cost per therm/decatherm/CCF or however it's billed in your neighborhood?

    Natural gas pricing has crashed in most of the US in the past year, and as I recall AL is a net NG production state, which may mean your well below the national average. But you're also probably part of the cheap electricity hydro/nuke/coal electricity TVA zone. But this is all conjecture- we need the numbers.

    The gas unit is likely to be rated ~78% AFUE if it has a standing pilot, 80-81% if it's electronic ignition. With aging & typical oversizing it's likely to be in the 70-75% range for as-used AFUE. Assume you get ~73000BTUs of heat into the ducts for every therm you burn.

    The heat pump will probably still have COP of 2.0 at 25F, and above 40F it'll be 3.0 or more. So, assuming you have $0.75/therm gas and $0.07/kwh electricity lets convert your gas efficiency to $/kwh

    for $0.75 you get 0.73therms of heat output . At 29.3kwh/therm that's equivalent to 21.4kwh, so you're paying $0.75 for 21.4kwh of heat, or 3.5cent/kwh of heat.

    If you're running with a COP of 2.0 and paying 7cents/input kwh for 2kwh of heat, that's also 3.5cents/kwh.

    But at a COP 3.0 you're paying 2.3cents/kwh.

    If the heat pump can keep up at lower temps without using the resistance backup (or you disable the resistance), if that's the range you're paying you'll be better off just running the heat pump always. If by simply avoiding deep night time setbacks the backup resistance never kicks on it'll cost the same or less as having to recover from the setback with resistance heaters or the gas furnace it may be less expensive overall.


    But the numbers are everything- what do you pay?
  6. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    Agreed.

    Unfortunately, its output would be trailing off at that point, meaning the strip heaters will be running during the times of highest demand (although in his case gas would be used.) With average daily temps here hovering in sub 10 F range and likely to be dipping to sub zero for the next two days it would all be resistance heating for an all electric.

    His climate is likely mild enough to gain advantage from higher COP through the vast majority of the heating season. However, his 40 F breakpoint is probably about right cost wise.

    For where I live 3/4 of my heating load in a typical year comes from 3 months of winter where the average daily temp is 30 F. Electric runs 12 cents/kwh here and is supposed to bump up a few more times in the coming years to pay for a new plant. I use $1 ccf for gas as a basis, but this year's winter tariff is $0.70/ccf.

    There is not much affection for heat pumps locally. My HVAC guy has one and likes his, but I suspect it is massively oversized allowing it to keep up during colder weather. Others I know that have them regret the decision. I imagine with the low temp rise uninsulated ductwork in the corners of the home would make for a very uncomfortable temp profile in winter.

    I would like to have a geothermal heat pump system...if somebody else paid for the well...:D
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,809
    Location:
    New England
    The biggest complaint I have with a heat pump is the comfort level on a cold day that doesn't trigger the emergency heat - the moving air out of the thing is much cooler than a 'typical' furnace, and is dry. This warm, dry air blowing against you can create a chill factor. And, it is often not setup to lower the fan speed for the heating season, and is optimized for cooling, where the higher fan speed can help.
  8. Giles

    Giles Retired tool & Die and Mechanic

    Messages:
    110
    Location:
    N.W. Alabama--Florence--
    Dana---Thanks for trying to help. I don't understand a lot of what you say, but, for now, all I can say is that I just got my second bill and I used 100 units of natural gas and the charge was $120.93. Also on the same bill, I used 1739 units of elect. cost $154.83.
    I don't think this helps, but I will call tomarrow and find out what they call a unit and the price.
  9. zootjeff

    zootjeff New Member

    Messages:
    4
    Location:
    Portland OR
    A picture is worth a thousand words on this. Here is a post I put on a different forum..

    I was curious how much my heat pump costs compared to Gas or resistive electric heat (space heaters, etc). So I decided to do something with the heating tables that came with my heatpump and coil.

    This exercise really helped me to understand the principals and trade-offs of what the factors are in using the various methods to heat my house. Hopefully you find it usefull too.

    I got my electric and natural gas utilities. I calculated the price per kW based on my bill total and usage.. I used a small and a large bill for comparison.

    The next step was to put the gas and heat into the same units for energy. Electricity comes in price per KilloWatt Hour, and Gas comes in Therms. A therm is 29.3 Killowatt Hours.

    I ended up with a price of 10.2 cents per kWh for Electricity and 3.9 cents per kWh for Gas.

    This shows that if I was heating with purely resistive electric heat, I'd be spending over 2x what I pay for gas heat.

    The next step was to figure out how this compares to a heat pump. The Tables that came with my heatpump show the heating parameters so I can calculate how much heat it will move for how much energy goes in (work) at a given temperature. All the numbers are based off an internal house temperature of 70 degrees. By using conversions from kW to BTU I found that at 65 degrees F outside, the heat pump uses about 2.3 cents per kWh, and at 40 degrees F it uses about 2.9 cents per kWh.

    It actually is cheaper than gas until about 13 degrees F based on my calucations. To do the comparison, I calculated the Output heat of the furnace at 92% of the input power. The Furnace allways costs 4.2 cents per kWh no matter what the outdoor temperature.

    See the spreadsheet below.. The calcuations are based on my 1900 sq/ft house in Portland OR with a 92% 80,000 BTU furnace and 2.5 ton 14SEER HeatPump. My heat load is a pessamistic approximation, but shows the concept.

    [​IMG]

    Then the other thing this spreadsheet did was help understand the Heat Load of the house. This shows how much power is lost from the house from the difference in indoor vs outdoor temperature.

    The heat pump is able to transfer less and less heat as the temperature outside goes down. But the house leaks more and more heat. These two factors are why you allways see backup heat with a heat pump system.

    This graph shows this relationship of heat transfer output of the heat pump compared to that of my 2 stage gas furnace vs temperature.

    [​IMG]


    This makes it very clear that for my system, even though it is cheaper to use the Heat Pump all the way down to 10deg F outside, it won't be able to keep up. If this is my real heating load from my house then my Heat Pump will only keep up until about 37 degrees. This means that when it is 40 degrees it will be running 92% of the time. So I might want to bump up my backup heat switch over to more like 45 degrees to give the heat pump the option of staying in it's defrost cycle a bit longer if it wanted, etc..

    This also shows about how often my furnace runs. If it is 50 degrees outside, then my furnace could stay in the first stage and run 33% of the time to keep up.

    Bottom Line Summary:
    Electric space heater heat: ~$0.102/kWh
    Gas heat: ~$0.042/kWh
    Heat Pump Electric heat: ~$0.026/kWh


    My next step is to put a data logger on my furnace and actually measure how long it runs in it's various stages to validate my load assumptions..
  10. Giles

    Giles Retired tool & Die and Mechanic

    Messages:
    110
    Location:
    N.W. Alabama--Florence--
    zootjeff---THANKS--you responded to my question with very good commints. It appears that your concerns are quite similar to mine and you have done the research. Very informative post.
  11. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    zootjeff,

    That's a pretty good way of doing it. There was a small error in your gas pricing: the customer charge should be subtracted out (this adjustment didn't appear to be needed for the electric.) To do this take the three points do a linear plot and subtract the zero intercept (it works out to $13.36/month.) Resultant gas price is about $1.01/therm. At your 92% efficiency the cost of kwh heating output is $0.0375/kwh. This will move your crossover point for gas to ~23 F from 13 F.

    I'm looking at a 95-96% furnace. Local gas prices run about $1/therm (much less than that this year) and $0.12 kwh for electric (and scheduled to rise.) That would put my gas heating rate at $0.036/kwh output and heat pump electric at about $0.04/kwh (3.0 COP at 30 F...our average daily temp for three months of Winter.) Adjusting your curve for my utility costs the crossover point would be at about 35 F.

    Your furnace looks to be around 2x oversized based on your climate and home size. That's good news for you as the heat pump will likely keep up down to your economic break point in the mid 20's.
  12. zootjeff

    zootjeff New Member

    Messages:
    4
    Location:
    Portland OR

    Agreed. I didn't want to pull that number out because I wanted to account for it. Instead I used a large bill where the usage is much much greater then the affects of flat fee customer charge.. Ideally my chart gets smarter and takes into account duty cycle and usage at a specific temperature and accounts for how big my bill is actually going to be. If you dropped gas completely, you wouldn't be paying the fixed cost, so I think it should roll into the usage.


    My electric company actually accounts for this by charging a lower rate for the first 250kWh and a higher rate for the amount over that. The result is that it looks like the 10 bucks a month customer fee goes into the noise. I like having the total price per kWh include the flat rate because most of my gas and most of my electricity go to heat. ( I currently use electric space heaters in some rooms.) You can call the price what you want but at the end of the day, I pay a utility bill for a certain amount of energy..
    Last edited: Jan 18, 2010
  13. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    When one is doing this sort of cost analysis it is the incremental cost that matters for the econ. This would differ of course if one could eliminate a utility, which is a possibility for gas if you don't use gas for water heating, clothes drying, or cooking. What one is really looking for is the delta in the bill from option A to B to C. Like you, I account for the total number when I'm tracking my overall utility use. From the standpoint of paying the customer fee the incremental cost impact on my utilities is becoming quite large, because I've done a pretty good job of minimizing the incremental use. This is most noticeable for me with gas in the summer months when it is just the water heater...and water all of the time since the base fee effectively doubles my cost there each month.

    The incremental effect comes up a lot in plant project economics. One can get charged an arm and a leg for new usage...but get almost no credit for reduced usage. It was noticeable with instrumentation and process simplifying projects because even though manpower needs would be reduced, one could only take credit for it if a defined personnel reduction could be implemented. Cutting out operators, technicians, supervisors or maintenance personnel in an existing facility is no fun!
Similar Threads: Heat pump
Forum Title Date
HVAC Heating & Cooling Help! Exchanger on Pool Heat Pump Leaking! Mar 8, 2014
HVAC Heating & Cooling Electrical question fir 5 ton pool heater heat pump Feb 28, 2014
HVAC Heating & Cooling heat pump problem Feb 8, 2014
HVAC Heating & Cooling Heat Pump Fan Not Spinning Feb 3, 2014
HVAC Heating & Cooling Residential heatpump control board question Jan 20, 2014

Share This Page