Mitsubishi HyperHeat Installation
questioning if my PVA‐A36AA7 & PUZ‐HA36NKA was properly installed and operating correctly.
I Recently had a Mitsubishi HyperHeat PVA‐A36AA7 & PUZ‐HA36NKA installed and have been tracking power consumption on it at the breaker. I’m finding that when the temp is below 35F, the power consumption is higher than expected per the submittal sheet. I had the installer do a check on the refrigerant pressure (he checked high pressure only) at 35F Ambient at the outside unit and it was about 325 PSI when it had been running for 10 minutes. I was told this was in spec. I made a temperature measurement today on the inside unit piping with fan on high at middle setting of static pressure and see a delta of 32F. The 5/8″ pipe being around 130 and the 3/8″ pip around 98 after running it for about 10 minutes. Is this a valid place to measure temps, or do I need to measure the discharge and suction pipes at the outside unit near the compressor per this information off the web
The suction temperature should be above 35°F and below 65°F entering the compressor and the discharge line temperature should be below 220°F and above 150°F (typically) on a properly functioning package unit.
| The unit should be properly sized for the home as it does not need to cycle often and run times are about 10-15 minutes. The unit was sized by a professional installer. |
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These systems use variable speed compressors. They're driven with three phase, variable voltage, variable frequency. There are some basic charts in the service/technical manuals that can be referenced, but there is no "spec" to consult regarding system pressure. The system pressure changes based on the load. The TD across the indoor coil is not a fixed value, under higher load conditions the compressor will ramp up and you'll have higher discharge temp and higher TD across the indoor coil. The power consumption on the submittal is a rating from a test under fixed conditions in a lab environment. You're not running under those exact conditions so it's not a good reference point.
Your concern is that if the system is installed correctly. That's impossible to answer. First, is the system properly designed? Did they do an accurate Manual J and use Diamond System Builder to verify output at design temp? Are the air ducts sealed/insulated properly and sized to provide proper airflow in accordance with the applicable loads? Did they use line sets with PE insulation and if so, did they properly seal the ends to prevent pinholes from trapped condensation? Did the use the correct size and type wiring? Did they pressure test the system with 600 psi nitrogen? Did they evacuate the system below 500 micron using a micron gauge and do a blank-off test to verify no pressure rise and ensure the system is completely dry prior to releasing refrigerant? Did they properly elevate the outdoor unit? Did they check the static pressure and adjust the system fan accordingly? These are just a few things that are required to properly install the system.
The first thing to consider is does it satisfy the setpoint and keep you comfortable. If it does, that's a good start and it seems it does as your only complaint seems to anticipated energy consumption based on the submittal. Typically high energy consumption in an ASHP system comes from improper design (equipment selection, sizing, etc), or low charge due to a system leak. This will result in an increasing energy consumption over time as the charge is reduced. The compressor has to work harder and harder to maintain the setpoint, lack of refrigerant reduces head pressure and discharge temp. Eventually it will run at full speed and not be able to satisfy the load. In this type of system, the velocity of the gas in the piping is what returns the oil back to the compressor. As the charge runs out, the compressor will eventually fail if the issue is not identified and repaired. Usually leaks are a result of bad flare connection, and lack of proper leak using pressure and vacuum prior and during evacuation. Proper flare connections are easy to make and extremely reliable but nevertheless bad flares are still common.
Long story short, there's probably nothing wrong here. Continue to monitor the consumption, log it if you're that concerned. Get to know your system. If you see a trend of increasing consumption at a given temp, then contact your installer. If the consumption seems consistent over time as you monitor things and the system is performing properly and keeping you comfortable, then everything is just fine. 325 psi means nothing without understanding what the voltage and frequency of the compressor is at time of measurement. I will say that this is fairly low pressure which would be expected during low load conditions. 35F is fairly warm, sounds like the system is doing a good job of maintaining/modulating as it should, but again that's impossible to tell based on pressure alone.
Dave
Did the installer follow proper installation procedures as you noted in your reply? I bet not as typically they cut corners to get to the next job... The "good enough" attitude so prevalent today.
So it is appearing that the unit is operating as designed or close to it based upon responses from you and others. If I was paying 16 cents per KW (delivered) then I could still save $$$ at lower outside temps. But at 37 cents per KW (delivered) below the mid 30's is not cost effective.
You can do a quick efficiency check with the power meter and a decent temperature gauge.
What you want to do is set the thermostat to max temp and the fan to high speed. Might need to open a window or two to avoid overheating the place.
The unit will take a bit to ramp up to full power, 5 to 10 minutes, once at steady state you can measure the supply and discharge air temperature of the air handler.
To calculate the BTU delivered you can use this formula:
CFM*deltaT*1.08=BTU/h.
So in case of that PUZ running at around 1000CFM, you would expect around a 35F delta between the air return and air supply which would deliver 38000 BTU/h to the house.
From the power meter you have the electrical consumption of the unit to calculate COP.
COP=BTU/3412/kW
So if your unit is using around 4.5kW at the above conditions you are looking at:
COP=38000/3412/4.5=2.48
The one unknow is the actual CFM which depends a lot on ducting so take the results with a slight grain of salt but it will tell you if the system is running in the ballpark.
This is absolutely spot-on, but not something a typical homeowner would be able to do. Accurate temp probes, current clamp? Do you have a manometer also, you can check the static and verify CFM! Well done.
Here is a spreadsheet I put together. As noted, my main issue is that when the outside temps drop below 30's the COP is not met. Perhaps this is expected in real world conditions where a defrost cycle is required. During one cold day around 16F, the unit was pulling 30 Amps. Much higher than the datasheet. Sadly, I had not put together a spreadsheet before getting caught up in the heat pump hype and how it would save me so much over my gas boiler. My error in not doing this as it would have been more cost effective to install a 97% efficient boiler. House is a 2002 high end build. All the ducting was present with the older AC unit of the same capacity. Cooling with the old unit was not an issue. I have a FLUKE dual channel thermocouple for temps, and, I do have a manometer and will take some measurements. Also note my KW/H costs are quite high with National Grid
In our area, there is no real cost advantage to using a heat pump over a natural gas system either. There is no real payback for the conversion from natural gas at current rates. Fuel oil and propane obviously a different story… being an ASHP heat pump design/installation/service company I make sure all of my natural gas customers understand this when they inquire. We still get people who want to convert, but it’s typically for sustainability purposes, or their system is hydronic or steam and they want the AC. The test Akos outlined perfectly will confirm output, but keep in mind the rating on the submittal is not a product of running the system under full load and high fan. The test they do is under a chosen, fixed condition that is intended to reflect the average efficiency in a specific climate setting (Region 4)….and testing also has recently changed. There is no simple way to verify efficiency accurately with a rating from a submittal. You can install a digital kWh meter on the system, or an energy monitor, and this will allow you to easily keep an eye on your consumption. When you said 30A, is this 30A per leg or 15A per leg? Obviously you must be an engineer or technician, and highly competent. I see you’re also apparently a Mopar man! A good friend of mine has an original 71 hemi charger r/t, and also a six pack version of the same car. The GTX is one of my favorites, congrats!
My first car in High school was a 63 Dodge Polara 500 Convertible with a 383 automatic with suregrip. Awesome burnout vehicle. Next was a 70 Road Runner. Air grabber, Pistol Grip, side stripes, 383. Bought it in 71. for $1700. The owner brought the car to the local garage my buddy worked at. It was backfiring and missing under load. The owner mechanic was a Chevy Lover, told him that the engine was going so sell that piece of junk. I took it out for a couple of full throttle runs. Yep fouled spark plugs. I got bad, told him I was worried about the engine and I had the car for $1700. Sold my 69 GTX convertible 3 years ago. No rust or rot. Wish I did not sell it.
Apologies GTX, I should have pulled up your spreadsheet prior to asking about your measurement. Excellent job, I constructed something very similar when I first embarked on my ASHP journey 12+ years ago. What’s incredulous here is your KWH cost!! .37?!!!! That’s extremely high, we’re nowhere near that. We’re .12-.16 typically.
Yeah, in Mass Chews it. NE has high rates unless your town has it's own light dept. 2 towns close to me with light departments are at 16 Cents per KW delivered. National Grid are robbers
Additional information as of this morning.
35F, outside coil is not icing (visual verification)
25.5Amps @236V (5782 Watts (this includes inside unit fan and control board.) (at breaker)
5/8" pipe indoor temp 152F, 3/8" pipe 97F
Return duct before April Aire filter .35" water
Indoor fan set to high, static pressure set to medium
Between top of filter and blower inlet .45" water
Supply duct .1" water
Oh man you’ve had some nice ones and good score….fouled plugs! Sorry to hear you sold that GTX….ragtop to boot yikes. On the bright side someone new gets to appreciate and care for it.
Those rates are insane. Solar companies must be booming there.
So am I understanding correctly your total static is about .55” wc? Assuming no electric aux heater with .1” on supply side? Middle static setting on that is .50” so that would be fine. This is good info, did you have a chance to check the output as Akos detailed?
Was a good manual J completed to ensure this system is large enough to satisfy load at design temp? Around here back in 2002 everyone was still insulating with fiberglass so infiltration rates were much higher than current builds where we have blower door test requirements. Loads on retrofit projects are much higher per square foot than newer, higher efficiency builds. If you’re getting close to balance point at those lower temps it may be running wide open which of course will not give you the rated COP you were expecting.
Any issues satisfying thermostat?
No issues keeping the house warm at say 68F when it's 20 outside.
No AUX heater, If needed my hope WAS, the gas boiler, but when it's cheaper to run the boiler than the heat pump, that will be what I do. I need to find a thermostat that can read the outside temp, and switch between heat pump and boiler based upon outside temp. I heard there is a thermostat out there that can be programmed do do this. So far, have not found one.
I have the KUMO Cloud app on my phone. Currently,. I have a schedule set up. Problem id that despite turning the heat pump off, either by the app or the red link wireless thermostat, when the schedule change time happens, it's turned back on again. Seems like these HVAC companies have software designers they pick off the street.
I think the static pressure is not additive. .45" between the filter and blower would include the .35 from the return duct. Are you adding in the supply duct at .1"
“The unit should be properly sized for the home as it does not need to cycle often and run times are about 10-15 minutes. The unit was sized by a professional installer. “
When properly sized a variable speed heat pump like yours should almost never cycle off. It should run for day or weeks at a time adjusting its speed the keep the house at its set point. The only time it should cycle off is when it gets so warm outside that the min speed over heats the house.
I do not expect you field calculated COP numbers will ever be comparable with the laboratory calculations done under controlled conditions. The big factor you have zero control over is the indoor and outdoor humidity that will wildly change the numbers.
Note if you do replace the thermostat with an aftermarket unit it will almost certainly prevent you unit from its variable speed feature and force it to cycle on and off.
Note I find that changing the indoor temp setting on a heat pump is a bad idea. Pick a temp and leave there 24-7 for months at a time.
Is it my imagination it sure seems like a lot of Mitsubishi and only Mitsubishi owners are complaining about a lack speed modulation?
Walta
345 psi on the high side is low. You should run at 400-425 psi with compressor discharge temp around 140f on high indoor set temp and high fan speed. I would venture to say you are short on gas. Also- at over 150f discharge temp you are short on gas. And lastly a 3 ton at 20f outdoor should run at 3.5 kw … so yes- you are short. And yes - it is that simple. Really.
Compressor speed and resulting pressures vary greatly based on load. They don’t run at a fixed pressure. You can watch the system respond on a laptop using the Mitsubishi service tool. You can see target head pressure change as load changes and you can see discharge temp and pressure change as compressor ramps up or down. Voltage and frequency have a broad range. It’s 3 phase for a reason. Sometimes target pressure is 450psi, sometimes it’s 350. There is no way to tell if the pressure is correct based on outdoor temp alone. The test Akos outlined is exactly what needs to be done. If you’re short gas you’ll be short output. When charge gets short you’ll lose heat capacity (and cooling capacity). Discharge temp has to be measured right off the compressor and it can be a lot hotter than 150. That pipe temp at the air handler is good check the split on the air handler. As it gets colder that temp will go down as will the split. Check the output, Akos knows his stuff. I completely agree with the post that advises not to set the system back. Set it and forget it, let the system modulate under most conditions that’s best. They don’t recover quickly so the energy that’s saved during setback is lost when unit runs wide open under full load trying to catch up. Yes it will cycle some especially under lower load conditions. During heating season and normal winter cycling shouldn’t be that frequent.
Akos is correct ofcourse. But good luck getting realistic CFM. We design ducts with software and we have the choice of air handlers so we have all the variables... and yet the CFM usually is so off in reality than calcs it is not even funny. So good luck getting a good CFM reading. Because of the impracticality of calculating real CFM I have discussed the matter with engineering as some times it is not practical to weight the gas (I design large office and commercial setups and often lineset length and diameter in walls is unknown ) and their bottom has always been - in heating high side 400-425psi in most cases at full tilt and 120-130psi on low side in cooling. Also in heating look for 3-5 degree subcooling. Thats all. There are target pressures and degrees of subcool and superheat no doubt, but when running full out those are the numbers I just mentioned. Quick and dirty and has always been good enough.
Absolutely agree, full load removes variables and is why the output test specified was under full load....but there's no indication from the post that the tech who put gauges on it had the system under full load. Sounded like he just put gauges on it and read them without forcing the system into a fixed condition. Just fyi...for Mitsu, high discharge temp limit is 243 F, full load discharge temp ~ 220F with compressor at full speed. As far as CFM goes with these particular systems, much smaller and simpler than what you're used to dealing with. I've not noticed any issues with actual being much different than rated as long as static is within spec and air handler is set up properly. Both of these should be verified at install of course. I'm not saying they're perfect, but the output testing I've done (same test as described by Akos, with maybe slightly different constant to account for elevation change) has always proven to be pretty accurate on both ductless and ducted units. Actual output very close to design model output. GTX - yes return and supply static add together they're both a measure of restriction. You can be very restricted on the return side (high static) and very open on the supply (low static) but the fan still needs to overcome the restriction. The static ratings for your Mitsubishi air handler are .3, .5, and .8. That would be the static on the return side added to the static on the supply side. Again, as Akos stated this is not exact but it will definitely be close enough to identify if you're having output issues which would cause excess energy consumption. If you have a leak, you will lose output and it will continue to do so until you have performance issues. If installer did job properly, leak test was done (we use nitrogen 600 psi) and micron gauge during evac. We also blank off the system with micron gauge connected at the piping to check pressure rise and ensure system is leak free and dry. I have some 12+ old year single zone hyper heating units out there that do not have basepan heaters. The drain holes do freeze shut during extended cold (from defrost water, snow, etc) and the ice backs up into the coil. This can and has caused holes in the outdoor coil. Charge gets low and you wouldn't believe how much energy a little one ton hyper heating unit will consume when it's screaming wide open all the time trying to satisfy. In those cases we replace the coil, add the base pan heater, evacuate the system and weigh in a new charge and reinstall it.
CFM for that air handler is below. Attached is a design for this system, according to the model air discharge temp off the unit should be about 100F and for this P series unit it will hold that pretty darn close even at 0 deg F outdoor, it was 99F at 0 deg F outdoor. We typically use the SVZ series on the MXZ-SM units. The P series stuff is great but man, it's costly. I'm interested to do some testing on the air discharge temp and I'll have to ask them about it. The split should be somewhere around 35 deg F under rated conditions, I think you measured 32.
Airflow Rate at Cooling, Dry CFM 788–956–1,125
Airflow Rate at Heating, Dry CFM 788–956–1,125
Can I download this tool without being a diamond contractor?
Mitsubishi service tool
As for the questions around it being properly installed as per the points Akos made.... I very much doubt it. I may have to pay to have it properly checked out by a diamond contractor. I can make the temp measurements at the compressor but not much more than that. In theory, as a new install with 25 feet of piping, the install doc status that the unit pre-charge is all that is required. I wish I was there for the install as if something the installer was not doing, I can pick that up. Thanks to all who have responded.