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  • Monitoring HVAC Heat Pump Performance

    Background:
    In 2001 I built a fairly large home. Heating and cooling are provided by five heat pumps. In 18 years I have experienced heat pump failure from time to time. The most common problem – about a half dozen times – has been a gradual loss of efficiency caused by a slow leak of refrigerant. Another creeping performance decline has been caused by my occasional failure to clean filters when needed.
    Problem:
    Gradual heat pump performance degradation often goes unnoticed until long after the problem begins. Meanwhile, the additional cost of relatively inefficient heating/cooling rises until the issue is addressed. In my case, the extra cost of pumping more well water for heat pump thermal assist adds to the financial burden.
    Need:
    I need an inexpensive and simple way for my smart home to keep an eye on my heat pumps, informing me when their performance is subpar. While it would be nice to determine absolute performance such as BTU per hour, that’s not essential. I just need to know whether something needs to be fixed.
    Constraints:
    I’m no HVAC expert, but the service people have told me the easiest way to confirm efficient heat pump operation is to compare ambient room temperature to the temperature of air coming out of the registers when the heat pump is running. They say that the differential should be around 15 to 20 degrees F. under normal conditions.
    Therefore, one cannot infer heat pump performance by simple measurement of plenum temperature; a properly functioning heat pump might provide 80 degree air at the register if ambient temperature is 100 degrees, but 60 degree air if ambient is 80 degrees.
    Likewise, one cannot make inferences based on heat pump percent run time. The air conditioning will always run a greater portion of the time on a hot day.
    Hardware:
    My approach is to measure plenum air temperature by placing a sensor at a register. Not wanting to get bogged down with Arduino/ESPxxx/MQTT/1-wire temperature sensing solutions (especially since this is still in the experimental stage), I bought an HS-FS100+ Z-Wave Flex Sensor (without the cable options). I run it on USB power rather than batteries so I can get register temperature measurements at 60 second intervals.
    HS3 Interface:
    I have created events to perform the following actions: Once a minute I compare the current plenum (register) temperature with the one I read a minute ago. I then compare this temperature increment with the maximum difference I have read today and update the day’s maximum difference if necessary.
    At the end of each day, I compare my day’s maximum 60-second temperature increment against a standard constant I have previously determined by experiment. If the comparison is unfavorable, I send myself an email addressing the problem. It’s also easy to store each day’s maximum in a virtual device, which the Device History plug-in can use to create performance charts.
    Call for Comments:
    There may be a better way to monitor HVAC heat pump performance. If anyone can offer an improved solution, please advise.

  • #2
    Rather than build this elaborate system why not just have the system checked every year or two and be more diligent t about changing the filters?

    Comment


    • #3
      So, this is something that I am trying to do as well. I did go the Arduino route.

      I measure the following.

      Inlet air temperature
      Inlet air absolute pressure
      Inlet humidty
      (Inlet is defined as between the filter and the blower intake)

      Outlet air temperature
      Outlet air absolute pressure
      Outlet humidty
      (Outlet is defined as in the supply duct just above the air handler)

      Reference air temperature
      Reference air absolute pressure
      Reference humidty
      (Reference is defined as outside the air handler, but next to it, in the conditioned space)

      I detect the following.

      Reversing valve on or off (reverse on = cool)
      compressor stage 1 on or off
      compressor stage 2 on or off
      aux heating on or off
      de-humidification on of off
      fan on or off

      I determine the actual fan speed (0 to 100%) based on time on, stage 1, stage 2, aux heat, and de-humidify. The unit ramps the fan up and down based on these same things.

      I calculate the duct static pressure based on the outlet pressure minus the reference pressure. There is some auto calibration to zero the pressures when the system is off. If the duct pressure goes too high, there is a problem (too many vents closed?). If it is too low there is a problem (blower not working right?).

      I do the same thing for the inlet static pressure. If it passes a threshold of -0.5 "H2O, then it's time to change the filter.

      I calculate the temperature rise/drop across the coil. Mine is usually about -24F in the summer time. I use a formula that I am still refining to determine what it should be based on the inlet humidity when it is cooling. There is another formula for when it is heating that is based on the outside temperature. (Outdoor temperature is supplied by another device.) After much research, I found that when cooling the temperature drop is determined by the inside humidity more than anything else. When heating, the temperature rise is determined by the outdoor temperature more than anything else (except when the aux heat is on). So far, my readings seem to bare this out.

      I am logging the averages of these readings/calculations daily to watch for trends. I also log daily run times, adjusted for stage 1 and stage 2.



      Comment


      • #4
        I think If you're interested in performance data then you'll have to trend more parameters. Performance is based on the manufactures specs, which can include Subcooling, Superheat, Aproach temp. outdoor dry bulb / Indoor webulb temperatures, static pressure, and wattage within conditions. It's no trivial endeavor to get it accurate and precise.

        Most of your more advanced heat pumps / AC system are doing this internally now. Every VRF/VFV system I worked on has these system build in. They are quite sophisticated.

        Now if you're just looking for basic diagnostic information, I think the following would suffice:

        1) You need to determine what the manufactures uses to check refrigerant charge. It's typically Superheat for units with Fixed Bore meter, and Subcooling for systems that utilize a TEV or EEV. Lennox system use the Aproach Method for older equipment.

        All the above methods require at least 2 temperatures. (Either Suction line or Liquid Line) and (Outdoor dry bulb and or Indoor wet bulb temperature)

        With the exception of Lennox' Approach Method, you will need to also capture either Liquid Line or Suction Line Pressure (depending on Metering device).

        Like drhtmal mentioned, it may be a lot easier to have a qualified company perform this analysis on an annual or bi-annual basis.

        RJ_Make On YouTube

        Comment


        • #5
          Originally posted by drhtmal View Post
          Rather than build this elaborate system why not just have the system checked every year or two and be more diligent t about changing the filters?
          Good question! I hope I can offer what you will accept as a good answer. Let’s do the math.
          Some of my numbers here are a little fuzzier than others, but we’re looking for ballpark figures.
          1. My electricity costs about $0.13 / KWH.
          2. When my well pump runs, it draws 2.4 KW.
          3. My well pump always runs when any of my heat pumps are running. (I have discussed this with the HVAC folks, but presently it is a fact.)
          4. My heat pumps are different sizes, but when running, probably average 2.8 KW each, including power for blowers.
          5. I have a contract for annual maintenance of my HVAC systems, every December. They clean and change filters.
          6. The HVAC technician will be here tomorrow to repair the guest wing heat pump, recently discovered to have lost its cool.
          Some educated guesstimates:
          1. Depending on weather and time of day, heat pumps will average about 20% utilization factor.
          2. Since scheduled maintenance is done only annually, it is reasonable to expect that a degraded heat pump will run for 3 months before ordinary detection.
          3. A degraded heat pump might average 50% efficiency, compared to normal.
          Given all of the above, let’s see where that leaves us:
          1. If a single heat pump ran at 20% utilization 24/7/365, its monthly cost would be:
            0.20*$0.13*(2.4+2.8)*24*30 = $97.34
          2. Suppose it takes 3 months to notice the loss of efficiency, and that efficiency degrades 20% per month for each of those 3 months. The first month costs me $97.34 *1.2 = $116.81. The second month’s bill is $140.16, and the third is $168.20. Thus, I must pay $425.17 to run that poor heat pump for three months, when I ought to have gotten the same heating or cooling for 3 * $97.34 = $292.02. So, the money I have wasted by not fixing the problem when the leak began amounts to $133.15.
          You might ask whether I have purposely inflated some of these figures just to make a point, but I assure you, I have not. The numbers are consistent with my overall power usage, also. In winter, my monthly electric bill occasionally exceeds $1200, most of it attributable to heating costs.
          So, if heat pump monitoring sensors can save me, say, $120 every 3 years, they will have paid for themselves in the first year or two.
          Does that make sense to you?


          Comment


          • #6
            I stopped reading when you said "math :-)

            Sent from my SM-T813 using Tapatalk

            Comment


            • #7
              I don't have a heat pump, however, my approach for my HVAC is overly simplified. I will admit, it may not identify similar issues as designed. I wanted to know when / if the outdoor unit was not running as that is where I have had previous / recent failures. Mine is inspected twice per year by a HVAC professional - well worth the cost IMO.

              While I have events which extend the fan runtime to push the remaining hotter / cooler air out of the duct work to reduce the likelihood of mold. The fan runs full time when the home is occupied too. Surprisingly, after 20 years there isn't much dust in there either.

              Regardless, I have an event triggered when the operating state has been "cooling" for 10 minutes, if the plenum temperature is greater than the cooling set-point, I get an email. I think I received two this cooling season. A longer term solution is to figure out a way mount a small pivoting arm over the fan triggering a door/window sensor indicating the fan is spinning as that fan seems to be the prime point of failure for me.

              Additionally, I send all of my homeseer sensor info to influx / grafana and have a decent amount of data to start identifying trends to future alerts.


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              Comment


              • #8
                Originally posted by ServiceXp View Post
                I think If you're interested in performance data then you'll have to trend more parameters. Performance is based on the manufactures specs, which can include Subcooling, Superheat, Aproach temp. outdoor dry bulb / Indoor webulb temperatures, static pressure, and wattage within conditions. It's no trivial endeavor to get it accurate and precise.

                Most of your more advanced heat pumps / AC system are doing this internally now. Every VRF/VFV system I worked on has these system build in. They are quite sophisticated.

                Now if you're just looking for basic diagnostic information, I think the following would suffice:

                1) You need to determine what the manufactures uses to check refrigerant charge. It's typically Superheat for units with Fixed Bore meter, and Subcooling for systems that utilize a TEV or EEV. Lennox system use the Aproach Method for older equipment.

                All the above methods require at least 2 temperatures. (Either Suction line or Liquid Line) and (Outdoor dry bulb and or Indoor wet bulb temperature)

                With the exception of Lennox' Approach Method, you will need to also capture either Liquid Line or Suction Line Pressure (depending on Metering device).

                Like drhtmal mentioned, it may be a lot easier to have a qualified company perform this analysis on an annual or bi-annual basis.
                As I attempted to explain in the original post, I'm simply looking for a timely warning that I should call in an HVAC technician for service. I really don't care whether my heat pumps are running at 100% or 95% of factory specs. If the technician says my heat pumps are all OK when he does his annual maintenance, that's good enough for me. What I care about is getting guidance that I need to call the service tech. BTW, after the last visit, I had to call him back because I gradually lost all the refrigerant in a heat pump he had just serviced. Why? Because he forgot to replace a valve cap in the refrigerant system.

                Like drhtmal mentioned, it may be a lot easier to have a qualified company perform this analysis on an annual or bi-annual basis.
                Earlier today, I responded to drhtmal with some some significant estimated cost savings I might realize through timely repair of a slowly degrading heat pump system. It seems to me that, aside from the cost of professional testing and analysis, your suggestion of "annual or biannual" testing pretty much throws away the cost advantage of timeliness that I am seeking.

                Comment


                • #9
                  Originally posted by joegr View Post
                  So, this is something that I am trying to do as well. I did go the Arduino route.

                  I measure the following....
                  Wow, I certainly am impressed! I'm not sure how I could implement your ideas and still keep it simple, however.

                  You did make a point that I hadn't considered. This is summer, so I'm not thinking about aux heat. But I do recall from last winter that sometimes the systems were using aux heat, and sometimes not. Obviously, that's a factor to be considered.

                  Your mention of the various thermostat signals brings to mind a different project that I revisit only occasionally. I live in southern Oregon. We have lots of winter storms. The trees blow over, snap the power lines, and cause frequent power outages. I have a backup generator, but it's nowhere near powerful enough to run all the heat pumps. But, I might be able to run one or two of them sometimes. So, the project is to implement a fancy load shedding algorithm (probably Arduino) by dynamically overriding the thermostat signals to keep the net backup power draw manageable. I'm sure you can appreciate that it's not quite trivial.

                  Comment


                  • #10
                    Originally posted by ericg View Post
                    ...I'm sure you can appreciate that it's not quite trivial.
                    Yes, I very much do. I have schematics and even a couple of extra blank PCBs that I made (you would have to solder in the components), should you become interested. Of course, to cover all your systems you'd have to get more PCBs manufactured.

                    Comment


                    • #11
                      I'm not an expert but in my experience monitoring delta-T is a reasonable, if minimal, way to keep an eye on system performance. But rather than in the room it should be measured immediately before and after the coil--or as close as is practical--to remove variables like heat gain/loss in attic ductwork, etc. It's pretty simple and relatively inexpensive to implement with 1-wire DS18B20 temp sensors ($2-3/ea), EDS OW-SERVER-ENET-2 1-wire server (~$100), Ultra1Wire3 plugin ($40) and cabling. Depending on configuration you might also need one or more EDS JB6 junction boxes. My temp sensors are inserted through the fiberglass ductboard return and supply plenums immediately before and after each air handler. 1-wire humidity sensors are available, too, although I've not used them and don't know whether they're suited for use in a ducted air stream.

                      I've not gotten into arduinos yet or I might've done it that way to save a few dollars. And static loss across the air filter would be nice to monitor.

                      edit: just ran across this
                      https://forwardthinking.honeywellhom...oblem-solving/
                      -Wade

                      Comment


                      • #12
                        Yes, I think that does give you the biggest bang for the buck.

                        Comment


                        • #13
                          Originally posted by cc4005 View Post
                          I'm not an expert but in my experience monitoring delta-T is a reasonable, if minimal, way to keep an eye on system performance. But rather than in the room it should be measured immediately before and after the coil--or as close as is practical--to remove variables like heat gain/loss in attic ductwork, etc. It's pretty simple and relatively inexpensive to implement with 1-wire DS18B20 temp sensors ($2-3/ea), EDS OW-SERVER-ENET-2 1-wire server (~$100), Ultra1Wire3 plugin ($40) and cabling. Depending on configuration you might also need one or more EDS JB6 junction boxes. My temp sensors are inserted through the fiberglass ductboard return and supply plenums immediately before and after each air handler. 1-wire humidity sensors are available, too, although I've not used them and don't know whether they're suited for use in a ducted air stream.

                          I've not gotten into arduinos yet or I might've done it that way to save a few dollars. And static loss across the air filter would be nice to monitor.

                          edit: just ran across this
                          https://forwardthinking.honeywellhom...oblem-solving/
                          Thank you so much! I really appreciate your validation of my approach -- and the "Delta T" article you referenced is great.

                          I certainly agree with your suggestion that it's better to measure the temperature just after the coil. I have not done so because I'm still in the experimental stage - proof of principle. I thought about dropping the HS-FS100+ down inside the register, but did not because the metal ducting looks like a Z-Wave inhibitor. Another unfortunate aspect of the HS-FS100+ is the fact that its temperature sensor is buried inside a plastic case which has got to have a high thermal time constant. I have read that typical thermistor sensing elements have thermal time constants of the order of a second or less.

                          If my approach proves useful, I'll take a look at the hardware you recommended and research what others are doing with Arduino and 1-wire cheap temperature sensors. Then I'll implement it in the equipment room where all the heat pumps are clustered. I think I can just punch a small hole in a duct and insert a tiny thermistor. Some of the expansion coils are remotely located, so I'll just have to run crawl space wires to them.

                          Comment


                          • #14
                            Originally posted by ericg View Post

                            .... its temperature sensor is buried inside a plastic case which has got to have a high thermal time constant. I have read that typical thermistor sensing elements have thermal time constants of the order of a second or less.
                            About a year ago for prototyping, I taped an extra sensor in it's plastic casing from https://wirelesstag.net/ to the outside of the metal plenum and achieved surprising results considering thermal conduction. I left it there as it worked well enough - granted, it only detects a 10 degree drop during the cooling cycle. Takes 2-5 mintues to register a temperature drop. Reporting frequency may influence that as well.

                            Comment


                            • #15
                              Originally posted by ericg View Post

                              As I attempted to explain in the original post, I'm simply looking for a timely warning that I should call in an HVAC technician for service. I really don't care whether my heat pumps are running at 100% or 95% of factory specs. If the technician says my heat pumps are all OK when he does his annual maintenance, that's good enough for me. What I care about is getting guidance that I need to call the service tech. BTW, after the last visit, I had to call him back because I gradually lost all the refrigerant in a heat pump he had just serviced. Why? Because he forgot to replace a valve cap in the refrigerant system.



                              Earlier today, I responded to drhtmal with some some significant estimated cost savings I might realize through timely repair of a slowly degrading heat pump system. It seems to me that, aside from the cost of professional testing and analysis, your suggestion of "annual or biannual" testing pretty much throws away the cost advantage of timeliness that I am seeking.

                              Hopefully your HVAC contractor replaced/repaired the leaking schrader core.
                              RJ_Make On YouTube

                              Comment

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