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Heat pump efficiency

Post in 'The Green Room' started by maverick06, Jan 21, 2011.

  1. maverick06

    maverick06 Minister of Fire

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    All,
    I have oil/heat pump/wood insert and have stated that I use wood when its below 45F and let the heat pump run above that temperature, and was asked how I derived that number. But that seemed like it should be an efficiency thread, not just a PM. So, let me try to explain my logic here.

    I have a heat pump, with oil fired backup and a wood stove.

    By default the thermostat runs the heat pump exclusively down to 50F, from 50F to 20F it looks at the oil burner, but hardly ever uses it, only on a defrost cycle, below 20F it is exclusively burning oil.

    I have wondered at what point I should be burning my wood stove.

    I have attempted to figure out what the actual beak even point is. I have also determined that its not possible to figure out. HAHA

    The heat pump is rated by its HSPF, but that is a national average, over the actual winter. I have a trane XR-15. The HSPF (per spec) is, 8, maybe 7.7... I think, with my, non variable speed blower motor. But thats still a national average... so with some fudging numbers, lying, and speculation with the pretty solid spreadsheet here: www.eia.doe.gov/neic/experts/heatcalc.xls I figure that my HSPF is really 6.4 because of where I live. But since I will not be using it when its cold (I will use my wood stove) that changes everything.



    Then guessing the costs,
    $0.16/kwh (generating, transmission, tax, etc).
    $175/cord
    $3.50/gal oil (irrelevant since it doesn't get used)

    And my efficiencies:
    Oil 81% (per the tech who tunes it up)
    Wood 71% per spec

    I am guessing that the break even point between the cost of the heat pump and the cost of wood is somewhere around 45F. Above 46F i run the heat pump, 44-46F it is based on my mood. Below 44F I definitely am running the woodstove. That keeps the oil burner from running much (I use 30 gal/year). And the colder it is, the worse the heat pump's efficiency is. So, If i dont run the heat pump below 45F, the heat pump doesn't have the efficiency as if it was in Philadelphia. based on a national average low temperature map: http://2.bp.blogspot.com/_TRXN9Xw5s...AAT4/Mq9DJw_ZJXs/s1600/temp christmas eve.gif

    it probably behaves more like it was in Atlanta, Ga... which makes it about 7.6, or there about.

    So, for me (with local prices and efficiencies), the cost per MBTU is
    wood: $11.20
    Oil: $31.16
    Heat pump (when operating above 45F): $21
    Heat pump (if run all year): $25

    So, why did i choose 45F, because the math shows that it probably isnt a good break even point?

    I chose that because at 45F+ the fireplace puts out too much heat (A insert full of wood puts out more BTU than needed, even though it is cheaper $/btu, this case would result in wasted BTU) so the house gets too warm, also above 50F I have a hard time getting the draft moving in the right direction in the chimney (found that out the wrong way... twice). So that is how I determined that operating point.

    I hope that cleared up the thought process. I was a bit rushed writing this, but the information should be valid.

    Rick

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  2. seeyal8r

    seeyal8r Feeling the Heat

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    Pretty neat to see the numbers you put to it. When the average daily temperature is below 40 for myself is when we burn wood. This year has caused us to waste a lot since we've burned only at night several times. My firewood costs more like $20/cord for gas, chains, etc and probably 5-6 hours of time. But I figure my time as free since its exercise and I enjoy it. Its hard to calculate that cost or pay.
  3. fireview2788

    fireview2788 Minister of Fire

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    WAAAAAYY too many numbers for me. I do it this way: I heat with the stove if it's going to get into the 30's anytime in the next 24hrs, that way I'm ahead of the heating game. Today will be in the 50's but then drop off so I am keeping a small fire going this morning but will let it die when I go to work and then my wife will have an easy start when she gets home. Heat pump will never come on.

    So far this has saved me an estimated $480 in electric (I have a heat pump with electric back-up).

    fv
  4. woodgeek

    woodgeek Minister of Fire

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    This is my kinda thread.... you can see geek in my handle.

    I live within 10 mi of the OP, and heat my house with a heat pump, and burn wood on the WE and evenings, but in a stove that is so old and inefficient it never 'pays' for me to do it on a BTU basis. It is a nice backup however.

    More to the point, I am doing a DIY energy retrofit on my house, a bit at a time. As an engineer, I want to measure my progress and so I model my house heat loss in detail.

    My preferred measurement method is to run my aux heater for a few hours, and time the duty cycle of the tstat. Since the aux is 50,000 BTU/h, I can compute my heat loss **as a function of temp**. Turns out that this function is non-linear....conduction is linear, but air infiltration causes a loss that depends on the temp diff squared. I can fit my data to a quadratic, and see the square term go down when I airseal, and the linear term go down when I insulate!

    As a side benefit, I can log how many kWh my heatpump compressor and defrost circuit uses during a given time period, and compute my coefficient of performance (COP) as a function of temp!

    The answer: With no defrost, i.e. above 40°F for my controller, my COP is ~3.0, on a Goodman 14 SEER single speed unit, comparable to the OPs.

    At lower temps, my simple defrost controller cycles on a timer, whether it needs it or not. Under dry conditions, the 'uneeded cycle' takes about 4 minutes, during which the aux is energized (15 kW), and the pump is pumping the BTU outside!

    My installer set the timer to defrost every 30 run minutes. My calculation suggests that cuts my COP in half at 30°F (my typical January mean temp), to COP = 1.6. I immediately realized it was running too much and set the jumper to 60 minutes, and got an estimated COP boost to 2.1 at 30°F. Since then I have hacked my defrost circuit with a delay timer to hold off the aux coil for the first two minutes of each cycle, and I get a COP of ~2.3 at 30°F, under dry conditions, lower during heavy frosting conditions. Verified by direct measurement.

    My findings are in line with the HP models that are out there, such as the OP showed.

    So, to the OP: my guess is your COP is 3+ when you are not running defrost, probably 2-2.5 otherwise since you are running oil in the second stage (if you don't count the oil). I think my guess for your cost per MMBTU is a bit lower than yours..maybe $15-16 above 40°F, ~$18-20 on a seasonal average. Worse if you are in defrost too much or have leaky ductwork in unconditioned space.
  5. begreen

    begreen Mooderator Staff Member

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    45°F is our threshold too. But that's based on a combination of factors including draft. We burn wood for comfort and efficiency. Usually we are burning whenever the temps are 40F or below. But we aren't slaves to the thermostat. Our heat pump may run while the stove is going, especially toward the end of a burn cycle. Another factor is draft. Below 45F the stove drafts nicely. Above that, you need to be a bit more careful when opening the door to avoid smoke spillage. Also, in the above 40-45F range we are burning one or two short hot fires a day and letting the stove go out in between. Below 40F we can burn continuously. I'm lazy, so when it's 45F+ out I often will let the h/p handle it unless we are feeling chilly.

    FWIW, we have a Am Std Heritage 16 with an HSPF of 9.0. It's a 2-stage compressor with a dc motor, variable speed air handler. The unit is set to switch to backup (resistance electric) at 22 °F.
  6. jharkin

    jharkin Minister of Fire

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    Ive also arrived at a threshold of between 40 (sunny day) and 45 (overcast) for the same simple reason - above that temp the stove will overheat the house. When we hit the mid 50s solar gain alone keeps the place warm on a sunny day.
  7. sesmith

    sesmith Member

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    I think the only way you could really figure this out would be to monitor the electrical usage of the heat pump at various outside temps. An amp clamp on the wiring in your box, or if you're not comfortable doing this, maybe using one of the inexpensive black and decker whole house electrical monitors could be used. You would then know what the heat pump uses in kw per hour at various temps and could compare it with the other fuels. With an oil furnace, if you know the nozzle size and approx run time each hour, you could have a pretty good idea of usage also at different temps. The wood use could be weighed and calculated based on average lbs per cord.

    One problem with the eia calculator is that it is using combustion efficiency, but is ignoring heat lost up the chimney, if I understand it right. So in real practice, your 80% combustion efficient furnace might be more like 60% efficient in heating efficiency. The 71% wood stove might be more like the 55% they use for the default in the calculator. If you actually measure the fuel used in each instance, you would account for this. If I read your data right wood's cheapest for you at any temp, isn't it?

    FWIW, if I had to buy my wood, my heat pump's break even point over wood would be somewhere below zero. Mine is a GSHP and quite efficient even below zero :) Needless to say, I haven't burned any wood this year since we put the heat pump in, so it doesn't break my heart that this year's firewood is still out in my woods, cut and unsplit. Maybe next year.
  8. maple1

    maple1 Minister of Fire

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    How much do you think your GSHP contributes to your electric bill?
  9. sesmith

    sesmith Member

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    We put it in in Oct. For the month of Nov., my electric bill was about $35 over our average monthly bill over the previous year, the month of Dec about $65. Jan. isn't over yet, but looks like it will be about $90ish. Our electric rate here is about $.11/ kwh (including delivery and taxes). Oil would have run close to 3k for the year. Wood 5.5-6.5 cords average when I heated with wood exclusively.
  10. sesmith

    sesmith Member

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    Easier yet. Use your electric meter to check your usage. You could compare like times of heating with heat pump only vs heating with wood stove only at 2 or 3 average temps to see when it's most cost effective to use the stove.
  11. woodgeek

    woodgeek Minister of Fire

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    I use a B&D monitor to track my electrical usage, and I know my BTU demand. I get a COP ~2.25±0.1 at 30°F outside, and low dew point. I get 3+ w/o defrost, above 40°F.
  12. karl

    karl Minister of Fire

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    One of the things I don't like about Trane is that they don't put their manuals online. Goodman and others do. Anyway. Look in your manual and it will have a chart that shows your heat pump output, electricity input, and COP for whatever temperature you want. You can use that and a calculator and find your cost per BTU for any outdoor temperature.
  13. woodgeek

    woodgeek Minister of Fire

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    While Goodman does publish all the performance specs for my unit, in copious detail, it is not as useful as you would think. The numbers are for steady state running, and do NOT include the effects of defrost, which can shave a full point off the COP if your system is poorly configured by your installer. I think my performance numbers in cold weather are consistent with the goodman tables and my defrost cycling. I do appear to underperform slightly at high outdoor temps, which I can only assume is due to the short cycling of the system. The system uses so little juice in warm weather however, I don't care if the COP there is 3 or 3.5.

    And if you had leaky ducting, it would 'look' like reduced COP. Measuring is good.
  14. Jerry_NJ

    Jerry_NJ Minister of Fire

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    I have a geothermal (aka GSHP) heat pump, about 20 years old. It has a COP of about 4 in low speed, and about 3.2 in high speed. These efficiencies drop a little as my ground loop cools, but the COP is always over 3 regardless of the outside air temperature.

    I like a wood fire, but think it is never saving me money over the HP. That given, I never have a fire unless the outside temperatures are below 40 degrees, mainly for improved draft, and usually don't have a fire until the temperatures go below freezing. I do believe that running the wood fire to keep the heat pump off for 5 to 8 hours allows the ground loop to regain heat from the ground and thus give me higher HP efficiency for the next couple of hours of run. I do not monitor the ground loop temperature so what I say about the ground loop is speculation based on reasoning.

    With an air-to-air heat pump I think the first post is a good plan. I haven't checked lately, but think the these units have greatly improved their efficiency and ability to extract heat from cold air.. down in the low 30s even upper 20s. Defrost was mentioned, and that seems to be one factor that cuts into the efficiency. A geothermal doesn't have to run any defrost cycles. I think if one lives north of Washington DC (east coast usa) a geothermal HP can be cost justified. Twenty years ago I was able to get on for $12,000 and the electric company gave me about $3,000 in rebates. I guess a similar unit today would cost over twice that amount, but still worth it. Mine has been mostly trouble-free, I have had to put about $4,000 into it over the last three years, nothing before that and never a service or inspection fee. I change the air filters myself, that's been the only regular maintenance cost.
  15. oldspark

    oldspark Guest

    How big is the service for the unit including the resistance heaters and do you know how many amps the resistive heaters draw?
  16. begreen

    begreen Mooderator Staff Member

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    Since 2006 our system has been serviced twice. The first service was a system tweak which helped lower our switch-over temp by about 5 degrees. The second was just a checkup and filter change. I've helped this dealer sell several mini-split systems so he did both services for free. I don't remember the total wattage of the resistance coils, have to look that one up. It's substantial, they are on their own 60 amp breaker.
  17. Jerry_NJ

    Jerry_NJ Minister of Fire

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    Just for reference, my electric emergency/supplemental resistive is also two stage. The first is 5,000 watts (about 17,000 BTU), the second is an additional 5,000 watts, giving a total of 34,000 BTU, and don't stand near the power meter when both are running I'll make you cry to see the $$$$ flying by. The program control on mine brings in the supplemental in two stages after some period of time were its second stage (about 34,000 BTU) HP output isn't raising the temperature, or if the demand (difference) is greater than a couple of degrees. The argument can go, don't turn the HP down at night as you'll just pay for the electricity in the morning when you bring the temperature back up. As I use some wood heat, the HP is off typically for about 10 to 12 hours anyway so I go ahead and turn it back to 60 degrees. So, by morning the house is down to 60 degrees. I disabled the second 5K Watt resistive unit so when I kick the temperature demand up in the morning I get only 5K Watts resistive boost. Doing the numbers under that condition I getting about 34 KBTU form the HP and about 17k BTU (and about 10 degrees higher plenum air temperature) and with the HP at COP over 3 I'm still getting a total/average COP of about 2.0. Even for these brief periods (maybe one hour) of resistive boost my cost is competitive with oil heat at $3.50 a gallon, and the way things are going we may see $4 a gallon soon...pray for spring first. Of course, the resistive automatically drops out once the difference between the demand and measured temperature is 2 degrees or less. I usually play the game of bring up the demand slowly if I have the time, a game of see how long I can keep the resistive off. I would like to modify so that I can manually override the resistive heat operation.

    I guess one "Take away" form my diatribe may be that resistive backup units less than 5 KW may not be of much value unless one is heating under 500 square feet of space.

    Program controlled systems are nice, but they do limit operations to their built-in program.
  18. woodgeek

    woodgeek Minister of Fire

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    My resistance backup is 15 kW, or ~50,000BTU, which was my design max load when the system was installed. Works out to ~65A at 240V, so I did need to upgrade my 100A service to a 200A service. The wiring to my attic was pricey.

    While it would cost ~$2/hr at that consumption rate, in >90% of my winter weather conditions I run it about 2 minutes per run hour on the HP compressor (during defrost).
  19. buddylee

    buddylee Member

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    I have not turned my heat pump on all winter. As stated above the unit doesn't cost a lot to operate if you house is properly insulated and you leave the thermostat alone. I have a huge stack of wood so why bother turning the heat pump on ? I think those punching numbers should be splitting wood instead of numbers :)
  20. oldspark

    oldspark Guest

    Some day I am going to put a back up to my wood burner and I just might be better off with a efficient propane furnace, there will come a time as we are getting older we might be gone for a little while when it is cold and running resistive heaters could be pricey.
  21. begreen

    begreen Mooderator Staff Member

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    OS, I'm not sure about your home layout, but I would look at it the opposite way and consider a super-efficient mini-split system. They can have an HSPF of 12.0. That could cover say 80% of your heating needs. Then use the wood stove as backup instead of resistance heat.
  22. oldspark

    oldspark Guest

    There is supposed to be a new one that is good to 15 degrees but the info is hard to come by and can not find it for sale any where.
  23. Jerry_NJ

    Jerry_NJ Minister of Fire

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    A HSPF of 12.0, is that for an air-to-air? I am not familiar with the technical details of how the HSPF is computed, but I have see the Coefficient Of Performance (COP) used on geothermal related as COP = 0.3 HSPF , approximately. Here I don't know what outside air temperature plays. For geothermal, the outside air temperature has (almost) no effect good or bad of the heating efficiency. From the relationship a HSPF of 12 equates to a COP of 3.6, Wow!. My geothermal delivers a COP of 4 only in low speed. It is closer to 3.5 in high speed. If new air-to-air heat pumps can deliver the equivalent of 3.6 at, say 30 degrees and above, it is competitive efficiency wise in North Central NJ. Our winter temperatures, including over night, are over 30 degrees more than half of the time (I estimate). Even with an existing ground loop for my geothermal that can be reused, I think it will cost me about $20K to replace the HP. My current HP is almost 20 years old. This is based on the (not up-to-date) guess that a new ground loop would add another $10K to the cost of a new installation. I am thinking it may be time to just completely replace my heat pump so I bounce my "data" off as the HSPF figure makes me question the cost benefits of geothermal.

    What would one pay for a high efficiency air-to-air with, say, 40K BTU heating output? Here I assume my existing duct system is simply reused.

    I use a couple of cords of seasoned hard wood a year. I enjoy a wood fire, and I think that even with the high efficiency of the geo HP, shutting it down for hours when the temperatures are below 30 degrees not only saves the electricity it would use during those periods, it also gives the ground loop time to recover some of the heat pulled out of the adjacent areas (including water table, as my loops are vertical). Said another way, the geothermal is not really independent of the outside air temperatures...as running drives the loop temperatures down, in fact my loop has antifreeze in it so it can still pump below 32 degrees. I think it never gets that low - still the factory specs include heating COP of 3.4 with the ground loop at 32 degrees F EWT (entering water temperature). East coast electricity runs at about 16 cents per KWH, so high efficiency is especially important. I also have the wood for surviving power outage, which we had in October 2011, when we saw an early wet snow. We were out for 4 days (many saw much longer periods). I used deep cycle batteries to power my blower on the air tight wood insert. So, wood got a test as back-up heat. It worked but the house was "cool", outside in the 30s as I recall. I now have a gasoline 1000 watt generator so I can keep the refrigerator running.
  24. begreen

    begreen Mooderator Staff Member

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    There are whole threads on this topic in the green room. Search for mini split in here and select All as the date range. Here's a couple to get you started.
    http://www.hearth.com/econtent/index.php/forums/viewthread/60402/
    http://www.hearth.com/econtent/index.php/forums/viewthread/66991/

    And there is a lot of info on the web. Just google 'high efficiency mini split heat pump' and you should get lots of links. Some excellent units will continue to extract heat down to zero degrees. It won't be the same amount of btus as at 15 degrees, but still, that is impressive.

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