Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

HP Performance Experiences

Options
Interesting piece on the experience those with heat pumps had during last week's New England's deep freeze. Anyone here have their own experience with it?

https://www.bostonglobe.com/2023/02/08/science/heat-pumps-had-their-first-major-test-last-weekend-heres-how-it-went/
«1

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,650
    Options
    I'll chime in -- as you may know, we put a Carrier minisplit into an apartment in Cedric's home, as the tenant likes it warmer than the rest of the building is kept. Cedric still did all the heavy lifting, effortlessly holding a 75 degree inside to outside temperature difference, with considerable (20 to 40 mph) wind.

    The Carrier was able to boost that another 5 degrees in the apartment, which is what it was set to do -- but it worked very hard indeed to do it, and had to run continuously. It couldn't raise it more than that. I was happy with that -- but I'd not have wanted to depend on it for the main heat source, as I doubt that it could have done it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    JakeCK
  • JakeCK
    JakeCK Member Posts: 1,417
    Options
    I'll chime in -- as you may know, we put a Carrier minisplit into an apartment in Cedric's home, as the tenant likes it warmer than the rest of the building is kept. Cedric still did all the heavy lifting, effortlessly holding a 75 degree inside to outside temperature difference, with considerable (20 to 40 mph) wind. The Carrier was able to boost that another 5 degrees in the apartment, which is what it was set to do -- but it worked very hard indeed to do it, and had to run continuously. It couldn't raise it more than that. I was happy with that -- but I'd not have wanted to depend on it for the main heat source, as I doubt that it could have done it.
    But it was sized to do what you wanted it to do right? Which means it worked flawlessly for it's intended purpose at what could only be seen as the far extreme of what would be expected of it or any heating system?

    How far below design temperature were you?
  • Jamie Hall
    Jamie Hall Member Posts: 23,650
    Options
    20 degrees below design for Cedric, and to be honest I didn't really expect the heat pump to produce much at those temperatures. It did use a LOT of electrticity, though. Unfortunately, I don't have a good way to determine exactly what it is drawing in terms of Kwh. Wish I did. Bottom line, though,as you say -- it did what I had hoped it would do, but it certainly didn't come close to persuading me it could substitute for the steam.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • gyrfalcon
    gyrfalcon Member Posts: 167
    Options
    I have a 12k btu Mitsubishi h2i for my poorly insulated sunroom . During the pre Christmas sub zero cold snap in Indiana, it did just fine. It did use more electricity as COP drops in colder temps, more defrost cycling too. I’m happy so far.  
    Now, my old Slant Fin Galaxy did just fine too,  it’s oversized and not dependent on outside air .  It still “short cycled” . 
    Slant Fin Galaxy GG100(1986) , 2 zone hot water baseboard, T87 Honeywell thermostats. 
    Hot_water_fan
  • ChrisJ
    ChrisJ Member Posts: 15,919
    edited February 2023
    Options

    20 degrees below design for Cedric, and to be honest I didn't really expect the heat pump to produce much at those temperatures. It did use a LOT of electrticity, though. Unfortunately, I don't have a good way to determine exactly what it is drawing in terms of Kwh. Wish I did. Bottom line, though,as you say -- it did what I had hoped it would do, but it certainly didn't come close to persuading me it could substitute for the steam.

    If you have no ay of knowing what it drew in terms of kwh, how do you know it used a LOT of electricity?

    As the evaporators on my monitor tops get colder, their power consumption drops due to lower load on the system. That would suggest a heat pump at 0F consumes less power than at +30F. Colder evaporator, especially on the edge of the system's capability means less gas being supplied to the compressor. Less gas means less load.

    You also get less output too of course, but either way both power consumption is down as is the load on the motor and pump.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
    pecmsg
  • WMno57
    WMno57 Member Posts: 1,408
    Options
    ChrisJ said:

    As the evaporators on my monitor tops get colder, their power consumption drops due to lower load on the system. That would suggest a heat pump at 0F consumes less power than at +30F.

    That's interesting, but I can think of two additional electrical loads on home heat pumps that would be greater the colder it gets.
    The colder it gets outside, the more electricity used to defrost the outdoor coils.
    Inverter heat pumps run the pump at variable speeds. Do the controls run the compressor at the fastest speeds (more electric) on the coldest and warmest days?
    I know next to nothing about heat pumps.
  • ChrisJ
    ChrisJ Member Posts: 15,919
    edited February 2023
    Options
    WMno57 said:

    ChrisJ said:

    As the evaporators on my monitor tops get colder, their power consumption drops due to lower load on the system. That would suggest a heat pump at 0F consumes less power than at +30F.

    That's interesting, but I can think of two additional electrical loads on home heat pumps that would be greater the colder it gets.
    The colder it gets outside, the more electricity used to defrost the outdoor coils.
    Inverter heat pumps run the pump at variable speeds. Do the controls run the compressor at the fastest speeds (more electric) on the coldest and warmest days?
    I know next to nothing about heat pumps.
    I had thought about the defrost heaters and I'll agree with that one.
    As far as speeds, they would max out at one point and then it just is what it is.


    Even though I personally do not want a heat pump, I wouldn't assume they use more power and work harder when it's cold out. Even if the compressor runs faster, is that really "harder"? Yes. it would suggest more power consumption, but we need tests.

    I feel we need some actual real world measurements taken before we can make a fair decision. Otherwise I'm going to stick with they work easier and use less power when it's extremely cold out.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • hot_rod
    hot_rod Member Posts: 22,476
    Options
    You could get one of those KilOWatt meters, plug it into that to record power consumption. Really no need to guess or speculate🤔
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    mattmia2
  • ChrisJ
    ChrisJ Member Posts: 15,919
    Options
    hot_rod said:

    You could get one of those KilOWatt meters, plug it into that to record power consumption. Really no need to guess or speculate🤔

    All of the ones I've used, that are affordable are only 120V though.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,917
    Options
    I feel we need some actual real world measurements taken before we can make a fair decision. Otherwise I'm going to stick with they work easier and use less power when it's extremely cold out.


    You can look at the manufacturer's specs:


  • Jamie Hall
    Jamie Hall Member Posts: 23,650
    edited February 2023
    Options
    We do need more tests, for sure. On the thermodynamics -- the key factor (other than the defrost strips -- there[s a reason for that high value fuse! -- more power is required from the compressor as the temperature difference increases, for a given load. The principle is this: the load is powered by condensing the refrigerant on the high side, and is directly related to the mass of refrigerant condensed. The heat energy released on the high side is provided by evaporating the refrigerant at the low side temperature. The power required from the compressor is proportional to the mass flow of refrigerant and the pressure differential; the latter is proportional (more or less linearly) to the temperature difference (for a given highside temperature, the low side pressure must be lowered as the temperature on the low side drops, but the high side pressure remains more or less constant).

    oh and how do I judge power demand? By looking at the electricity bill, between tears...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    gyrfalconunclejohn
  • hot_rod
    hot_rod Member Posts: 22,476
    Options
    Define affordable? This is the brand that a geo contractor I know included on all his HP installs.
    The CoOp power company that we were on in Missouri gave me an old style analog meter and base to install on my geo HP
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Hot_water_fanGGross
  • WMno57
    WMno57 Member Posts: 1,408
    Options
    "There’s no official “cold climate” standard for heat pumps just yet. But the next Energy Star standard for air-source heat pumps, launching in January 2023, will include a certification mark for cold-climate heat pumps, signifying a suitable level of low-temperature performance and efficiency."
    https://www.consumerreports.org/heat-pumps/can-heat-pumps-actually-work-in-cold-climates-a4929629430/
  • ChrisJ
    ChrisJ Member Posts: 15,919
    edited February 2023
    Options

    I feel we need some actual real world measurements taken before we can make a fair decision. Otherwise I'm going to stick with they work easier and use less power when it's extremely cold out.


    You can look at the manufacturer's specs:


    I mean,
    That chart basically says exactly what I said, no? Going by the "rated" column as I doubt maximum would ever matter in real world performance.

    @Jamie Hall Thank you for that detailed explanation.
    As you remember, I have no experience with refrigeration systems. :|

    Unfortunately your reasoning assumes the same output capabilities at all temperatures.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,917
    Options
    I mean,
    That chart basically says exactly what I said, no? Going by the "rated" column as I doubt maximum would ever matter in real world performance.


    I think yes, rated does what you say. But that's not super relevant - cold climate heat pumps use higher frequencies to get their low ambient performance, so yes kw increase as temps drop.
  • ChrisJ
    ChrisJ Member Posts: 15,919
    Options

    I mean,
    That chart basically says exactly what I said, no? Going by the "rated" column as I doubt maximum would ever matter in real world performance.


    I think yes, rated does what you say. But that's not super relevant - cold climate heat pumps use higher frequencies to get their low ambient performance, so yes kw increase as temps drop.
    Then why doesn't the chart you posted show that?
    Maximum is maximum, not real world expected performance.

    I'm not interested in winning an argument, that gains me absolutely nothing.
    I'm interested in learning what really goes on with these units.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,917
    Options
    Then why doesn't the chart you posted show that?
    Maximum is maximum, not real world expected performance.


    I would expect the units to run at maximum when called upon. I don't think needing max output at 0 degrees out is that uncommon, seems like expected behavior. Now, I agree that most of the time you won't need max output at 45 degrees.
  • ChrisJ
    ChrisJ Member Posts: 15,919
    Options
    We have a cheap LG heatpump at the office...
    I should see if I can get a reasonbly priced KWH meter to watch it's behavior.

    It certain works.......... the only issue I've had is the indoor filter gets clogged a lot faster than I expected and it simply cannot cope with it. So I keep that clean and everyone's happy.

    It was a much better alternative than the fiberglass lined ductwork that was in use by heavy smokers for years.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Jamie Hall
    Jamie Hall Member Posts: 23,650
    Options
    There are actually at least three different problems happening here. The relatively minor one is the power demand required to deforst the cold side coils, if they need that. Hard to get around that -- whether one is dealing with a heat pump for space heating, a heat pump for space cooling, or a heat pump cooling a refrigerator. Condensing ambient water on the outside of cold side coils is actually beneficial -- provided the condensed water doesn't block the air flow. However, the other two problems -- which are related -- are quite intractable. As I noted, the heat output of a heat pump is related to mass flow of the refrigerant and the hot side temperature. For a given hot side temperature, it is directly proportional to the mass flow. So for space heating, we can sort of slide over that -- space heating usually targets a remarkably small temperature range. Cold side is another matter. The mass flow is still determined by the desired heat output on the hot side -- but the pressure isn't. The pressure has to be much lowe (minor eminder -- we are looking at absolute pressures and temperatures here, not relative ones of our familiar gauge pressure and Fahrenheit or Celsius temperature scales). To get down to the bottom line, for a given heat output we are going to need a lot more compressor power with an outside ambient of, say, -15 F than we are for an outside ambient of 45 F.

    Perversely, the required power output on the hot side goes up almost linearly with decreasing outside ambient temperature, but the available output of a given heat pump drops dramatically with decreasing outside ambient (that table shows that quite clearly -- thank you @Hot_water_fan !).

    This is, I think, a "gotcha" which will not be a problem for the conscientious and thorough installer, but could be a real booby trap for the unwary -- but it's worth noting that if the application needs the power output of the unit in the table, but wants it at -13 rather than 45 (rated temperature) the application will need two of them, rather than just one -- with, I might note, a correspondingly higher electricity demand.

    This may be part of why the installation worked for me. Cedric's output (time averaged) was much higher at -15 than it would have been at 45 (good thing too!), This, if one thinks through it, negated the need for greater output from the heat pump and so the significantly reduced output from the heat pump wasn't a problem (although it did work noticeably harder). This is in contrast to the case where the heat pump would have been asked to provide the entire power requirement; while it would appear that a heat pump could have done it, it would have to have been a much higher power unit (a problem not unique to heat pumps -- think about it; a boiler capable of heating a structure at -15 is going to be much bigger than needed at 45!).

    Geothermal source heat pumps, of course, are not subject to the performance drop with ambient.

    As a highly application specific addendum -- in my specific case, the heat pump is not a money saver, except only in that it allows running much of the building at a significantly lower temperature than that specific apartment. Even at high ambients, its COP is such that, on a BTUh basis and in my specific area at my specific fuel and electricity costs, it still costs more to run than Cedric. Your mileage, as they say, may vary...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • pecmsg
    pecmsg Member Posts: 4,961
    Options
    Thermo Dynamics. Pressure Temperature.

    In Heat mode the indoor unit aka condenser is fairly constant 65 - 70°F supply air.

    The outdoor unit AKA Evaporator temperature will continue going down with the ambient temperature. Lower temperature, lower operating pressure, lower energy usage!

    This does not include defrost that will consume more as the defrost cycles increase.
    ChrisJ
  • Jamie Hall
    Jamie Hall Member Posts: 23,650
    Options
    "The outdoor unit AKA Evaporator temperature will continue going down with the ambient temperature. Lower temperature, lower operating pressure, lower energy usage"

    Um...no. Sorry. The hot deck pressure is approximately constant. The cold deck pressure, as you note, is less. The power requirement is related to the difference in pressure, not the cold deck pressure alone, and as a result goes UP as the cold deck temperature drops.

    Thermodynamics and physics are pretty relentless...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Hot_water_fan
  • ChrisJ
    ChrisJ Member Posts: 15,919
    Options
    pecmsg said:

    Thermo Dynamics. Pressure Temperature.

    In Heat mode the indoor unit aka condenser is fairly constant 65 - 70°F supply air.

    The outdoor unit AKA Evaporator temperature will continue going down with the ambient temperature. Lower temperature, lower operating pressure, lower energy usage!

    This does not include defrost that will consume more as the defrost cycles increase.


    That's exactly what I thought, but Jamie is saying otherwise.

    How do inverters behave in these conditions? I've only worked with fixed speed compressors.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • pecmsg
    pecmsg Member Posts: 4,961
    Options
    @Jamie Hall

    Less pressure, lower suction pressure, lower refrigerant volume, less energy usage!

    @ChrisJ

    The computer takes into several data points, pressures, temperatures fan speeds. Well above my pay grade!
  • Steamhead
    Steamhead Member Posts: 17,008
    Options
    During a cold snap a few years ago, similar to the most recent ones, we had to replace a steam boiler in a house that had these new "cold-weather" heat pumps. They worked well enough to keep the house from freezing, but could not get the inside temp warmer than 55° F. The owners arranged to be out of town during the replacement, since that was pretty much intolerable.

    Good thing this family was not seduced by the cold-weather heat pump hype, and kept their steam system.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • ChrisJ
    ChrisJ Member Posts: 15,919
    edited February 2023
    Options
    Steamhead said:

    During a cold snap a few years ago, similar to the most recent ones, we had to replace a steam boiler in a house that had these new "cold-weather" heat pumps. They worked well enough to keep the house from freezing, but could not get the inside temp warmer than 55° F. The owners arranged to be out of town during the replacement, since that was pretty much intolerable.

    Good thing this family was not seduced by the cold-weather heat pump hype, and kept their steam system.


    I'm going to assume those heat pumps were sized primarily for cooling, and the heating was an added bonus?

    If that's the case, it's not really fair to use that. In fact, it sounds like a huge benefit because +55 is fantastic in regards to loosing a lot of plumbing.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Steamhead
    Steamhead Member Posts: 17,008
    Options
    ChrisJ said:

    Steamhead said:

    During a cold snap a few years ago, similar to the most recent ones, we had to replace a steam boiler in a house that had these new "cold-weather" heat pumps. They worked well enough to keep the house from freezing, but could not get the inside temp warmer than 55° F. The owners arranged to be out of town during the replacement, since that was pretty much intolerable.

    Good thing this family was not seduced by the cold-weather heat pump hype, and kept their steam system.


    I'm going to assume those heat pumps were sized primarily for cooling, and the heating was an added bonus?

    If that's the case, it's not really fair to use that. In fact, it sounds like a huge benefit because +55 is fantastic in regards to loosing a lot of plumbing.
    No idea. We didn't install them. Just going by what they were told.

    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • Jamie Hall
    Jamie Hall Member Posts: 23,650
    Options
    Oh dear. OK. Step one. Power output (BTIh) from the hot deck is dependent only on the mass flow to the deck, and on condensing temperature (the latent heat released per unit volume is related to the phase change temperature) and on hot deck pressure (the relationship of the condensing temperature and the hot deck pressure is characteristic of the refrigerant). NOT on volume. Step two. The volume of refrigerant in the gas phase is directly related to the absolute temperature of the refrigerant and directly related to the mass, but inversely related to the absolute pressure. Thus, for a constant mass flow (power output on the hot deck) you need a greater volume at lower pressure. Step three, the compressor power demand is directly related to the absolute pressure difference for a given mass flow. The required pressure at the cold deck is, again, a function of the cold deck temperature and is a characteristic of the refrigerant. It is not linear with absolute pressure or temperature, but is more or less linear if those are related to the triple point of the refrigerant. More simply, the colder the cold deck is, the lower the pressure must be to evaporate the refrigerant.

    So -- bottom line. For a given power output from the hot deck, we have a fixed mass flow, fixed temperature, and fixed pressure. For that power input, from the cold deck, we have the pressure lower at lower temperatures. Since compressor power is related only to mass flow (constant) and pressure difference (greater with lower cold deck temperature), the power required for the compressor increases as the cold deck temperature decreases.

    It would be nice if it were the other way -- but that yields an immediate absurdity, or rather two. First, if it were the other way, as the cold deck temperature approached the hot deck temperature, more and more power would be required, to the point where if they were equal the power required would be extremely high. Second, as the cold deck temperature decreased, the power requirement would be reduced, to the point where at some low cold deck temperature no power would be required at all.

    Heat pumps are wonderful devices, in all their guises. They are not, however, magic, nor can they violate the laws of thermodynamics.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    bburd
  • ChrisJ
    ChrisJ Member Posts: 15,919
    edited February 2023
    Options

    Oh dear. OK. Step one. Power output (BTIh) from the hot deck is dependent only on the mass flow to the deck, and on condensing temperature (the latent heat released per unit volume is related to the phase change temperature) and on hot deck pressure (the relationship of the condensing temperature and the hot deck pressure is characteristic of the refrigerant). NOT on volume. Step two. The volume of refrigerant in the gas phase is directly related to the absolute temperature of the refrigerant and directly related to the mass, but inversely related to the absolute pressure. Thus, for a constant mass flow (power output on the hot deck) you need a greater volume at lower pressure. Step three, the compressor power demand is directly related to the absolute pressure difference for a given mass flow. The required pressure at the cold deck is, again, a function of the cold deck temperature and is a characteristic of the refrigerant. It is not linear with absolute pressure or temperature, but is more or less linear if those are related to the triple point of the refrigerant. More simply, the colder the cold deck is, the lower the pressure must be to evaporate the refrigerant.

    So -- bottom line. For a given power output from the hot deck, we have a fixed mass flow, fixed temperature, and fixed pressure. For that power input, from the cold deck, we have the pressure lower at lower temperatures. Since compressor power is related only to mass flow (constant) and pressure difference (greater with lower cold deck temperature), the power required for the compressor increases as the cold deck temperature decreases.

    It would be nice if it were the other way -- but that yields an immediate absurdity, or rather two. First, if it were the other way, as the cold deck temperature approached the hot deck temperature, more and more power would be required, to the point where if they were equal the power required would be extremely high. Second, as the cold deck temperature decreased, the power requirement would be reduced, to the point where at some low cold deck temperature no power would be required at all.

    Heat pumps are wonderful devices, in all their guises. They are not, however, magic, nor can they violate the laws of thermodynamics.


    Ok Jamie I have a question.
    When dealing with gasses and pumps, if we have a pump with an inlet pressure of 50 psi and an outlet pressure of 200 psi.

    Does it pump the same amount of gas as it would if the inlet pressure was 5 psi and the outlet pressure was 200 psi?

    What about an inlet pressure of a slight vacuum and an outlet pressure of 200 psi?

    is the volume of gas pumped the same? Is the load on the motor the same?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • hot_rod
    hot_rod Member Posts: 22,476
    Options
    Doesn't it always come down to proper design, application and installation :)

    Air to air HP in one form or another have been around for 30 years or more. It's fairly well know what they can and can't do. Technology is getting better, inverters, vapor injection, newer refrigerants, etc.

    Same goes for GEO based HPs, well documented track record on them.

    A2W are a bit newer on the hydronics scene, but the limitations are well known and documented by the manufacturers.

    So it comes down to what the buyers expectations are, if and when a supplemental heat source is wanted or needed. The numbers are out there to determine how the systems will perform at most any condition.

    It important that the buyer become educated on HP pros and cons.

    No doubt with any "new" and "in Vogue" components, the snake oil salesmen amongst us will cloud the waters and leave a trail of unhappy customers.

    Don't blame the technology for mis-application or over hyped sales.

    It's a lot like water softener sales people. One device solves all you water problems or concerns :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Larry WeingartenGGrossHot_water_fanyellowdog
  • Jamie Hall
    Jamie Hall Member Posts: 23,650
    Options
    To answer @ChrisJ , if we are talking about a compressor operating on a gas, the answer is no -- the work required per unit mass of gas varies with the pressure difference. It is much easier to see this conceptually if, instead of varying the input absolute pressure we hold that constant and vary the output absolute pressure. It doesn't make much difference to the mathematics which. It is also MUCH easier to conceptualize if we look at a positive displacement compressor, such as a piston type -- or a bicycle tire pump. Consider the bicycle pump. As one draws up the piston (or lengthens the pump, for the fancy type which clips on the frame!) a certain volume of air is pulled in -- and that volume and mass is the same every time one pulls up on the piston. Now one pushes the piston down. The air is pushed out into whatever one is pumping up. If it's one's air mattress, at perhaps 2 psig, that isn't that hard to do and one can do it fairly fast. Now take the same pump and hook it up instead to the tire on one's truck, at 80 w at one is working a lot harder -- because the resistance on the pump is that much greater. If one changes one's thinking to varying the input pressure rather than the output, it is a little harder to visualize, but suppose one is receiving air from a 20 psig source -- the piston will actually be pushed up if one lets go of the handle, but will take the same effort to push down (for the same output pressure) as if it were a 0 psig source. Now drop the pressure to -10 psig. One will have to pull up on the handle considerably harder to get the air in.

    Now work is force times distance (actually the integral of force with distance, but let's not get fancy). In the plus 20 psig case, the source gas does work on the pump on the intake stroke, and this work must be subtracted from the work one does on the pump for the compression stroke, so less total work input (from you!). In the -10 psig case, not only does one have to do work to compress the gas on the compression stroke -- one has to do more work on the intake stroke to get the gas in there in the first place.

    With a piston pump, if one wants to compress a given mass of gas, one must make fewer strokes if the intake pressure is high (more mass comes in per stroke) than if it is low. If one wants to compress a certain volume of gas, however, then one must also decide whether that volume is going to be measured at the inlet or at the outlet. If we are looking at output volume at a certain output pressure, we have to have more strokes if the input pressure is low than if it is high or, if we make the same number of strokes, the output volume will be less with low input pressure than it is with high.

    It sometimes helps to look at extreme cases. In this case, consider what happens if the input pressure is the same as the output pressure -- there is no effort to move the piston at all.

    The above also applies to other types of fixed displacement compressors, such as scroll types or Roots blowers, but not to centrifugal or axial flow types, such as turbochargers or the axial flow compressors for gas turbines, which have a more complex relationship of flow to delta P -- but the end result is the same. Greater delta P for a given mass of gas is going to take greater power.

    For heat pumps -- or for that matter drag racers or aircraft gas turbines! -- we don't care what the volume is. We care about the mass flow.

    If you want to get a headache, consider that the relationships for gasses are very different from liquids. In gasses, we are usually talking about adiabatic compression (look it up) where temperature has to be taken into account as well as pressure. The end result, on a qualitative level, is the same, but the relationship between pressure and volume isn't linear (as the pressure increases, not only does the volume decrase, but the temperature increases) while in liquids the volume doesn't change although the pressure does.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ethicalpaul
    ethicalpaul Member Posts: 5,795
    Options
    I have to butt in just to poke at my friend Jamie a bit...

    As a highly application specific addendum -- in my specific case, the heat pump is not a money saver, except only in that it allows running much of the building at a significantly lower temperature than that specific apartment.


    So you're saying it's not a money saver, except that it is?


    Also regarding heat pump defrosting, don't they just run backward for a while to use some of the interior heat to thaw themselves? Modern ones aren't using resistive heating for that, right?

    NJ Steam Homeowner.
    Free NJ and remote steam advice: https://heatinghelp.com/find-a-contractor/detail/new-jersey-steam-help/
    See my sight glass boiler videos: https://bit.ly/3sZW1el

  • Jamie Hall
    Jamie Hall Member Posts: 23,650
    Options
    Something like that, @ethicalpaul !
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    ethicalpaul
  • pecmsg
    pecmsg Member Posts: 4,961
    Options
    Oh dear. OK. Step one. Power output (BTIh) from the hot deck is dependent only on the mass flow to the deck, and on condensing temperature (the latent heat released per unit volume is related to the phase change temperature) and on hot deck pressure (the relationship of the condensing temperature and the hot deck pressure is characteristic of the refrigerant). NOT on volume. Step two. The volume of refrigerant in the gas phase is directly related to the absolute temperature of the refrigerant and directly related to the mass, but inversely related to the absolute pressure. Thus, for a constant mass flow (power output on the hot deck) you need a greater volume at lower pressure. Step three, the compressor power demand is directly related to the absolute pressure difference for a given mass flow. The required pressure at the cold deck is, again, a function of the cold deck temperature and is a characteristic of the refrigerant. It is not linear with absolute pressure or temperature, but is more or less linear if those are related to the triple point of the refrigerant. More simply, the colder the cold deck is, the lower the pressure must be to evaporate the refrigerant. So -- bottom line. For a given power output from the hot deck, we have a fixed mass flow, fixed temperature, and fixed pressure. For that power input, from the cold deck, we have the pressure lower at lower temperatures. Since compressor power is related only to mass flow (constant) and pressure difference (greater with lower cold deck temperature), the power required for the compressor increases as the cold deck temperature decreases. It would be nice if it were the other way -- but that yields an immediate absurdity, or rather two. First, if it were the other way, as the cold deck temperature approached the hot deck temperature, more and more power would be required, to the point where if they were equal the power required would be extremely high. Second, as the cold deck temperature decreased, the power requirement would be reduced, to the point where at some low cold deck temperature no power would be required at all. Heat pumps are wonderful devices, in all their guises. They are not, however, magic, nor can they violate the laws of thermodynamics.
    Copeland performance report on a 3/4-HP 
    Look at the rated amps as evaporator temp drops

    ChrisJ
  • Matt_67
    Matt_67 Member Posts: 295
    Options
    For a good comparison the heat loss needs to be charted against the heat pump output. Hypothetically (using numbers from the chart above) if a room with an interior temperature of 68 degrees had a heat loss of 6,730 BTU's when its 30 degrees outside it would require 436 watts of electricity per hour to keep it heated. If that same room required 10,281 BTU's at 0 degrees outside that is 949 watts. More than double the electric usage for 53% more heat. That's what makes the heat pumps more painful than fuel based heat which is linear.
  • ethicalpaul
    ethicalpaul Member Posts: 5,795
    Options
    I finally found a home heat pump system you would actually buy @ChrisJ !!


    NJ Steam Homeowner.
    Free NJ and remote steam advice: https://heatinghelp.com/find-a-contractor/detail/new-jersey-steam-help/
    See my sight glass boiler videos: https://bit.ly/3sZW1el

    ChrisJGGross
  • ChrisJ
    ChrisJ Member Posts: 15,919
    edited February 2023
    Options
    I think, maybe the easiest comparison I can make is, take a vacuum cleaner and measure it's power consumption. Block the hose, and notice the motor speed increases substantially, while power consumption drops.

    This happens because the lower the pressure, the less "stuff" there is to pump, so load drops.

    It's just the way it is.........The same happens on forced air systems as you restrict the return side.

    I never said the efficiency increases. The system output drops as the outdoor temperature drops, and as it does so does power consumption other than obviously longer run times. So overall power consumption may go up, but the actual power being drawn does not, it drops.

    @pecmsg I kinda thought this was just common knowledge? I guess not?

    The other point I had was, they certainly aren't working harder, they work easier when it's cold out because less load on the compressor. Until pressure drops too much and causes weird things like oil starvation etc.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • ChrisJ
    ChrisJ Member Posts: 15,919
    Options

    I finally found a home heat pump system you would actually buy @ChrisJ !!


    Should weigh around 14,000 pounds...... if it's built like the original and just bigger.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
    ethicalpaul
  • Jamie Hall
    Jamie Hall Member Posts: 23,650
    Options
    Ah, @ChrisJ -- you've just run into the booby trap built into centrifugal blowers! They do indeed use less power when the air flow is restricted (so do centrifugal pumps). What you say is quite true -- less stuff, less load, uses less power (although efficiency may drop). For any such device (liquid pump or air blower) there will be a specific speed and pressure difference at which the power demand is the greatest. We see that all the time on pump curves, but we don't see it much on blowers or fans (the same curves can be drawn). And you won't see it at all on positive displacement devices.

    But... as I've said, in refrigeration work we aren't interest in volume pumped. We are interested in the mass flow, and we don't need less stuff as delta T increases, we need more...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ChrisJ
    ChrisJ Member Posts: 15,919
    edited February 2023
    Options
    Ah, @ChrisJ -- you've just run into the booby trap built into centrifugal blowers! They do indeed use less power when the air flow is restricted (so do centrifugal pumps). What you say is quite true -- less stuff, less load, uses less power (although efficiency may drop). For any such device (liquid pump or air blower) there will be a specific speed and pressure difference at which the power demand is the greatest. We see that all the time on pump curves, but we don't see it much on blowers or fans (the same curves can be drawn). And you won't see it at all on positive displacement devices. But... as I've said, in refrigeration work we aren't interest in volume pumped. We are interested in the mass flow, and we don't need less stuff as delta T increases, we need more...
    Refrigeration compressors are pumping gas and behave exactly as I said my friend.

    Regarding centrifugal blowers and pumps...I seem to recall them actually doing the same if you restrict the output as well.  Obviously not the same with a  compressor.




    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • unclejohn
    unclejohn Member Posts: 1,833
    Options
    When the suction pressure drops you are using less power but you're capacity drops with it. You no longer have say 24k btu output from your 2ton HP. Less cap less power less heat. There is no magic 
    ChrisJ