Heat Pump Not Running Efficiently
I have a 48k unit outside, 5 heads in the house
2 – 12k
1 – 9k
2 – 6k
I leave only two 12k units on in heating mode on auto fan while the other units are off when we go to bed I put another unit on at night so 3 units running the 2- 12k and 1 – 9k. Its not that cold out yet (New England weather is crazy) some nights range from 20f – 30f and the days 30f – 45f
I noticed today having the 2 – 12ks on at 70 temp on auto fan to maintain we woke up to 66 degrees and it stayed like that for awhile until i had to adjust.
we are using between 40 and 50 kwh over night.
am i doing something wrong, should i be running all my units at once on the same settings to keep a load on the unit to help with a min load.
Also during the cooling season if i would just run 1 unit say 1 6k i would hear this noise constantly from the unit outside.
https://www.youtube.com/shorts/z1p2HGSrm9k?feature=share
please let me know to help as i bought this unit to help save me money going from Electric Baseboard to Heat Pump to fully heat my home.
Model: MXZ-8C48NAHz2
House sqft about 1700
the unit is about 3 – 4 months old.
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Replies
That noise sounds like metal rubbing on metal to me.
Call your contractor and have them start a warranty claim for replacement of the compressor.
Regarding the energy consumption, assuming 50kWh is being used overnight just for the heat pump, and assuming an overnight average COP of 3 (based on the table at https://www.greenbuildingadvisor.com/question/need-to-know-mitsubishi-heat-pump-cops-at-temps-below-17-degrees-f), it should have given you:
50kWh*3 = 150kWh = 512kBTU of heat
If overnight is about 10 hours, that is 51200BTU/hr. Maybe some of that was lost to the friction evidenced by the noise. But it still implies that it was running constantly at full throttle overnight, which implies that there is a problem with the device or it just can't keep up with the load (i.e if the house is very leaky). Heat loss calculations for your house should have been performed by the contractor who installed it, so they would know if it was performing as expected.
In summary, there are lots of variables here and you need to tackle them starting from the most obvious (the compressor noise) to the least (leakiness).
Hi Danan,
There was no heat loss calculations done. the noise does go away only if i run either at least 1 12k the units are on auto and i do see them kick on once and awhile so none are on full bast at any time. if i put the unit on full blast it can reach temp then i put it back on auto and from what i can tell it will maintain but over night we wake up because auto cant keep up with the drop in temp? just not sure about all of it.
I'm sorry to hear this Bobby. That system is gigantic for a house that size, which usually does lead to poor performance and Mitsubishi has advised against it. For some context, 54,000 Btu (that unit's rating at 5 degrees) is 66 Amps at 240V. Many older houses have only 100 Amps total for all electricity, so I'd find it unlikely that yours uses 2/3rds for them just for heating. As a comparison, if I heated my similarly sized home with electric baseboard, it'd use only about 20 amps at 5 degrees.
I'd leave all heads on 24/7 as a start.
How much electricity were you using with the baseboards per year? Multisplits are more efficient than baseboard, but their COP is usually in the ~2 range.
Hi Paul,
Only lived at my house for about 2 years now, and my electric bill was always high during the heating months close to 900$ with the electric baseboards. we used over 3k kwh during the month of January.
so we then decided to jump on the hype train for the heat pumps and i was told i would save money even given the contractor my yearly bill for heating.
What are my options and best route of action when it comes to getting the efficiency
So all units on auto at 70 to 70+?
That's about the best you can do without major changes, yes. I think you will still save money vs. baseboard and maybe the multi-split was the best your house could fit since retrofits involve compromises but I'd prefer to see ductwork, fewer zones, and a smaller system. If you have them, can you post the previous bills? Ideally, kwh, start and end date for some winter months.
Here is an excel file with my usage for the year. i didn't get the unit until 3 months ago which were were not using much in the beginning because the weather was actually pretty nice out.
I bought one of those Emporia Vue Electricity monitors but have not hooked it up yet so i can monitor the usage a little more closely.
As you can see, you use about 2 more kwh for every additional heating degree day (base 65, using Boston weather). Assuming a day with an average temp of 0, that's 65 HDD. So your heat loss is 2kWh/HDD65 x 65HDD65 x 3412 btu/kWH / 24hour/day = 18,000 btu/hr. So you're about 3x oversized and ideally you'd have something around 24kbtu or even lower, considering you could always keep the baseboard for the coldest day of the year.
> we used over 3k kwh during the month of January.
Assuming that was mostly heating energy, it works out to an average of 33kbtu/hr if running constantly. That's quite high for a house of that size, implying that your house by itself loses a ton of heat, irrespective of the heating technology used. That's a symptom of leakiness and poor insulation.
> so we then decided to jump on the hype train for the heat pumps
Heat pumps aren't hype, but they aren't magic either.
To use a car analogy, you can't get Prius efficiency out of an F150 by dropping two Prius engines into it. Similarly, an oversized heat pump is not going to operate optimally efficiently on a leaky house.
I'd start by calling the contractor about that noise though, and show them the same energy usage information you are sharing here.
Thanks for the Reply, when it comes down the oversized heat pump i trusted the contractor knew how to gauge the size of the unit for my size house. i just wish things worked out differently, maybe if i put all the units on and up the temp to adjust i can make it efficient . trying to be hopeful
Seems to me you have a major failure on your hands if you have the thermostat set for 70° and the unit runs nonstop for hours at 30 outside and it can only maintain 66° inside.
If the above statement is true call for warranty service today.
It is hard for me to tell how abnormal the sound is from the video but some motors do make strange sounds when the electronic are commanding slower speed operations. The unit should be in high-speed mode if it is 4 degrees off its set point.
With heat pumps I like to record the indoor air temps entering and exiting the unit along with the outdoor temp and keep a log so I understand what normal operation looks like. At some point there will be a leak and difference between incoming and outgoing air will shrink. Only because of the log will be able to establish a normal number. My guess is if the unit is running at full speed for some time and difference between incoming and outgoing air is under 25°F there is a problem when it is above 20°F outside.
I think it is a mistake to change the thermostat setting of a heat pump pick a number and let it run 24-7. I say set it and forget it turn it on in the fall and turn it off in the spring.
Walta
Thanks for the reply Walta seems the set it and forget it is the way to go with these.
Bobby,
It is quite possible that the unit is not oversized unless your house is a very well built house with current energy code minimum insulation and good air sealing. If it is oversized it is likely only oversized by maybe 15-25% which should not affect the operations of the unit significantly. It sounds like there may be a mechanical issue with the unit.
I would follow Paul's recommendation to see how the unit operates with all units 5 zones operating 24/7. This should indicate if it is a sizing issue or a mechanical issue with the unit. If it operates properly with all the units on then it may be a sizing issue but if there are still issues it may be a mechanical issue with the unit.
How is your house constructed (what are the r-values for the walls, floor, and roof/attic, are your windows newer high efficiency, etc)?
Regards,
Josh
It looks 3x oversized unfortunately for Bobby.
I cant go back to the contractor on this? i mean how should i handle this i feel like i got screwed.
Deleted
Did they do a terrible sizing job? Yes. But unfortunately it's impossible to get 5 zones and only 18-24kbtu out of a Mitsubishi multi-split. So they would have to install a lot of ductwork to make it happen. Getting a COP of 2 is probably the best case scenario here. Still half the electricity as before! Just not 33% of the previous electricity.
Appreciate all your help!
Thanks for the responses and advise.
> I cant go back to the contractor on this? i mean how should i handle this i feel like i got screwed.
If you can't go back to the contractor after 4 months, then it sounds like you had an unorthodox business arrangement. Contractors normally factor the cost of addressing post-install issues into their prices, especially for something as complex as an HVAC system.
If you really can't go back to the contractor, you might consider informing Mitsubishi since your contractor seems like they would be a brand risk to them, and presumably the contractor is certified by them if they installed Mitsubishi equipment.
Mitsubishi could refer you to another contractor to try to help mitigate/repair any issues, but beyond warranty covered items, you might need to pay. Hopefully you didn't pay much for the installation, given the inability to go back to the contractor.
When i said I cant i didn't mean it that way, i am in contact with the contractor as we speak but just getting some advise on what i can use to help my case with them, they been some what responsive but not as responsive as i would of liked.
what makes it a bit worse is they removed my electric baseboard heat in order for me to stay "efficient" in order for me to get my rebates through the state.
Hi Josh,
The house was built in in 72 mass save did come in prior to me buying the home. they insulated the attic and the joist in the basement as for the walls not sure but the windows are on the newer side. for the windows that i thought were leaky i try to seal the best i can during the winter time.
I would have your contractor confirm the communication wires are landed properly to match the refrigerant piping to make sure that when a head calls for heat (or cooling) the refrigerant is flowing to the right head.
I believe there was article on GBA a while ago that addressed this particular mistake on an install.
Hi Kyle,
I had the electricians come out and double check this and they said it is correctly connected.
The system may well be oversized but when the complaint is that the system is not able to keep the house at the set point something is wrong with one if not both unit and it has nothing to do with being oversized.
The symptom from oversized would be short run times at low speed with long period of the unit being off. Yes, the power usage would be somewhat higher than a right or undersized unit but not 33% more power usage as some have suggested.
Walta
First step is to make sure your unit is running properly. To do this, open a couple of windows and set all the units to max fan and max heat. Wait for a bit for the unit to ramp up to full and measure intake and supply temperature with decent gauge such as a digital meat thermometer.
The 8C48 should deliver 46500BTU to the house, this means that you should see 34F to 40F temperature rise across the 12k units, a bit less on the smaller units as the max fan speed is about the same. If you have a power meter or clamp on current meter, it would be good to check outdoor unit power consumption. You should be seeing 4.7kW (21A) draw.
If you don't see this, most likely the refrigerant charge is off. This could be a leak or the unit was not filled properly during install. Should be a simple fix. Calculating the refringent volume on these is not simple, I would ask the tech to show exactly how they come up with it. You can do the math yourself as well to check:
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10007\M_MXZ-NA2%20NAHZ2_INSTALL_RG79D595H11_01-20.pdf
Page 11
These units are very sensitive to refrigerant volume and efficiency can really fall off if they are running low.
I know the contractors did a pressure test but they did not do it for a full day more like an hour. and the vacuumed the lines. They have an excel sheet they enter the line set and apparently calculates the amount to add to the Unit after its Vacuumed.
as for how much he put not sure, and if he got the line set length completely accurate.
I think you (and maybe your contractor) will find the emporia monitor useful as that's what I have on my 2014 Mitsubishi 36k 2 head (24k & 6K) unit in a 70s ranch of similar square footage in Ohio- and it's been enlightening. beyond possible mechanical issues, three things might be helpful for the long term.
-First, from the suggestion of a post on GBA I tried putting my fan on the lowest setting and it seems to help the unit run at a lower level vs. running at a higher wattage and cycling more when on auto. To be fair, I haven't crunched the numbers, but today it ran at ~600w for the whole night, which I find ideal. How this works when the weather dips into the 20s and below remains to be seen.
-Second, my MSZ heads used to give false temp readings. My heat pump used to run flat out for a short time then shut off and had trouble keeping up when temps dipped into the 20s or lower. I put on the wireless adapter and in addition to phone app control, It allowed me to add an external temp/hum sensor (PAC-USWHS003-TH-1) which was a game changer. Now it's a whole different heat pump after those sensors -much more consistent. This was before the emporia monitor, but when I first moved in the outdoor unit outside of the bedroom would loudly ramp up and down to the point I switched back to the electric baseboards to sleep. I haven't heard it in that way since the external sensors. Your mileage may vary, however.
-Third I would agree Walta and leave the thermostat alone -just let it run the same 24/7 . It definitely is more comfortable, and the heat pump doesn't struggle/have to run flat out to warm the house up.
Ill have to setup the Emporia this weekend get some more detailed data.
I ran all 5 units last night left them at 70 except the 2 12k units i had at 75 just to keep a load on the unit outside it never hit 75 but it was warm in the house.
i went outside every hour until about midnight and i was using about 3kwh on the whole house while people were sleeping so i left it over night same temp same everything and after 7 hours it was at 21kwh used i am not sure if this is good or not? but i think it might be a bit better.
I left all the units the same and went to work while my wife works from home i will have to check how much is used when i get back from work to see what the difference is.
I might try all 5 units same temp and check every hour to see if it can satisfy the room and just coast at that temp and check the usage.
I seen a lot of water underneath my unit probably from the defrost mode this morning i assume this is normal but i hear like a whooshing sound coming from the head unit when the defrost kicks on and the coils on the head unit get a little cold.
Ill check out the external temp/sensor portion they may be expensive though?
Couple things:
1. Your compressor is prolly fine. For now. That is how they sound when they might be starved for gas. Or you just have a noisy one- they do exist and can run forever like that.
2. I think I hear excessive liquid line noise- I would measure (you should be running 90-130 psi on the low side on those at full tilt) with gages, but off the cuff I say you are low on refrigerant. Now without being there l can’t tell you exactly what is going on, but you have either gas or valve issues. Run a delta t on the indoor unit and see if you get 20-25f raise across the coil. I am suspicious you will not….
3. You don’t know if you are oversized as being low on refrigerant will severely reduce heating capacity as it is In Your case - 50 kWh over night means your unit ran flat out over night. There is a problem and oversizing ain’t it for now.
4. Get wall thermostats for the bigger heads. IDU temp sensing is inaccurate and will waste you money as the unit system will short cycle.
Good luck and let us know
When it was summer time and we were running it in Cooling i had the guy come check the coolant but it was cold out day so he said the reading may be off but he check the temps on the pipes and said they look good and we ran each unit one by one full throttle then all of them at once.
I did bring up about the PSI should be closer to 130 and he said they don't do refrigerant on this unit like that has to be measured because it doesn't do superheat or something like this i could be wrong.
Basically saying the unit comes with refrigerant in it then measure the line sets and add what mitsubishi recommend per ft for the line set i don't remember how much they added.
Is it easier or harder for them to check my refrigerant in the summer or winter?
Winter or summer it does not matter much the correct way to check the refrigerant level is to remove the refrigerant into a container on a scale and measure the change in weight.
This is very different than the old school way system was checked in the past. We use to connect a gage look at the pressure and add what we thought was enough. The old way will not work on a mini split he may get it close enough to produce heat but it will not operate a peak performance.
The only reason to add refrigerant is because some has leaked out!
The correct fix is not to replace the missing refrigerant and collect some money that is poor workmanship.
The leak must be found and repaired if you want more than a temporary solution.
Finding the leaks is difficult time consuming and there is no guarantee that it can be located. Failure to locate the leak is frustrating as is coming to the conclusion that an expensive or unattainable part is the source of the leak.
My guess is 80% of Heat pumps are replace because it is more cost effective to replace the entire system than locate the leaks.
Walta
Walta, mini splits are not black magic. Activate test mode to make sure compressor runs at 100%, wait 20 minutes for pressures to stabilize and measure or even have the main computer in the ODU display the high pressure and compressor discharge temp if capable. High pressure should be 435 - ish and discharge temp should be 110 plus outdoor temp for Fujitsu and Mitsu. Service manuals for both list acceptable values if tech wants to double check, but those are the numbers. Yes- pull charge and weight is best, but often especially in larger buildings lineset length is unknown so charging by compressor discharge temp and high pressure (in heating mode) will get you close enough.
Once the compressor is locked at 100% speed (usually 100 rps) one can troubleshoot those modern wonders fairly easily
Try leaving it in heat mode, high fan. Auto will allow too much of a swing to recover from on cold nights.
I did notice this leaving it on heat Auto its hard for the unit to keep up to temp thats for sure during the night.
But i have been leaving all the units on 68 and some of the rooms end up getting to 74 - 77 i thought there were suppose to stop throwing heat once the temp is reached either that or heat is rising.
i will try the Fan high and have it set temp so it can maintain ill let you know.
Also if any of you are from the boston area and have anyone they can recommend me to reach out to that is possibly really knowledgeable or certified with mitsu heats pumps please let me know as well.