View Full Version : Mitsubishi Compressor Pre-Heat requirement?

05-01-2011, 01:05 AM
Hello all.
I have been looking for an answer and don't know enough to find one anywhere at this point.:rolleyes:

I was an R12 Automotive AC technician for 20 years, but that was 20 year ago, so I know the theory, but am far behind the current state of the art.

I just installed a Mitsubishi mini-split system and had a
professional vac out lines and start it up. It is working well, but I understand that this unit has a pre-heat on the compressor. I would like to turn it off to save money, but don't dare unless I understand the requirements.

I live In Southern California where it never (that I can remember) gets below 0 Centigrade. Can I turn off the pre-heat option on my compressor? It is a simple dip switch and even though the default is "on", If I can turn it off, I can save $100 per year in electricity.

Oh, this is R410A system with MXZ-2A24NA outdoor unit.

thanks for any responses.
Calvin Thomas

05-01-2011, 06:59 AM
if you are talking about the compressor heater this is used to keep the oil warm and boil off any refrigerant from the oil so do not recommend turning it off

05-01-2011, 05:43 PM
Thanks for the reply.

Yes, it is compressor heater, and it uses 50 watts. Is it "really" necessary for it to run all the time in Southern California weather? I wonder if it is needed in more inhospitable climates, but not that important in mild climates like mine?

One reason I wonder that is because it was a firmware update to the Mitsuibishi controller some time back that made "Preheat on" the default. So.... it must have been "Preheat off" as the default before that update. I know that Mitsubishi doesn't want to replace compressors and can't count on end users or installer to always pay attention to these details, so they must have decided to make the default "on" and leave it up to us to decide if we can safely turn it off.....
I am just trying to find out that exact fact, but don't know where to go. I thought this forum would have the most knowledgeable and scientifically trained people on the subject so I came here to ask for your oppinions and ideas....

thank you for taking the time to answer.
I appreciate it.


05-01-2011, 06:58 PM
i think the pre heat on the controler is for when the unit is in heating mode to stop the indoor fan untill the coil heats up to stop cold draft

05-01-2011, 07:28 PM
No. Sorry, I wasn't that clear in my explanation.
I am indeed talking about the pre-heat on the compressor as you said in your first post. Pre-heat default is controllable via a dip switch on the controller circuit board AND the controller can be updated by firmware to change the default bahavior. In original firmware (when Mitsubishi first built these units), pre-heat was "OFF" by default and the switch had to be turne "ON" to enable pre-heat. On a firmware update (I don't know how far back now, but at least 2 levels back) Mitsubihi changed the default from "Pre-heat Off" to "Pre-Heat On". Now, that behaviour is reversed and you must move dipswitch to turn pre-heat "Off".

What I am truely interested in is an idea of how cold the 410A refrigerant has to become to need the pre-heat to be always on.

For instance......

What outside temperature does the R410A compressor need to stay above to remain safe? What temperature will R410A condense into the Oil?
Obviously, the Pre-Heat is to keep the compressor above this critical temperature, but what is that point?

Failing that, where can I get that information? I am not smart enough to understand all the scientific jargon I find on Mitsubishi's and various other websites.
Sorry for the long postings...

Thanks again for your help

05-01-2011, 08:21 PM
Finally tracked down a copy of the nearest manual for your model. We now believe you :D

10-1. Pre-heat control <MXZ-2A20NA - 1 , MXZ-2A20NA - 2 , MXZ-3A30NA - 1 , MXZ-4A36NA >
If moisture gets into the refrigerant cycle, or when refrigerant is liquefied and collected in the compressor, it may interfere
the start-up of the compressor.
To improve start-up condition, the compressor is energized even while it is not operating.
This is to generate heat at the winding.
The compressor uses about 50 W when pre-heat control is turned ON.
Pre-heat control is ON at initial setting.
It is not such much that R410a will condense into the oil but that all refrigerants will migrate to the coldest section of the pipework system.

This tends to be the compressor in air conditioning systems as it it mounted outside and the indoor unit will naturally be warmer during cold ambient conditions.

When the compressor has a quantity of refrigerant sitting in it's sump during the off period it will be partially mixed with the oil and can cause severe damage to the compressor when it starts up. As the pressure drops in the sump during start up the refrigerant will boil out of the oil causing an oily foam which gets lifted up into the compressor itself.

This liquid mix, oil/refrigerant, gets pumped through putting extreme hydraulic pressure into the moving parts. Depending on the design it can snap pistons, con-rods, snap valves etc.

The reason therefore of having the crankcase heated it to keep the liquid refrigerant out of the compressor during the off periods.

I think perhaps that you could take an educated guess as to when you think the outdoor temperatures are going to be low enough to be a potential problem.

You have to weigh up the cost of running the heater against the price of having a new compressor and electric pack installed.

install monkey
05-01-2011, 09:00 PM
keep the system running at cool 18 and the crankcase heater will hardly be on!
dont worry about a 50watt heater if u want to save on electric turn the room setpoint up a degree and keep the filters clean.

05-01-2011, 09:36 PM
I know about the extreme damage. :off topic:
When I was very young and very foolish, I was told about cleaning carbon from a combustion chamber on a car, by running water through the intake, and combustion chamber and out the exhaust. At that point I owned an old Triumph GT6, and cleaned it by running a water hose to one carb (3 cylinders) while running the motor on the other carb (3 other cylinders). I let the RPM's get too low, and apparently I was running a bit "too much" water. Once one cylinder filled up with more water than the combustion chamber could hold, I heard a very loud "BANG" and the motor stopped.

It started right back up, but then I started noticeing a knocking noise.

3 days later the crankshaft broke.:o

I learned that day that it is not possible to compress water...
I believe you 100%:D

05-01-2011, 09:36 PM
The 50 watt heater is less than the average light bulb.

05-01-2011, 09:53 PM
I was thinking about that, but the 50 watts is the amount it takes to energize the compressor winding. That's what they use for the heater.

So I think anytime the compressor is running it will be over 50 watts, and when it is not running it will be 50 watts. no time will it be under 50 watts even when it isn't doing anything at all.

It seems to me what I need to do is to determine the phase of the refrigerant at any particular temperature. If it is liquid, I have a problem. If it is gas, then I won't. I don't have any gauges, so I can't measure it, but perhaps I can find the standard pressures of R410A in this system, then I should be able to calculate the liquid or gas phase of the refrigerant

Unfortunately, I am not sure how to calculate it. I think it can be done from some of the graphs I find in the manuals, but the nomenclature is not so easy for me to decipher. It has been 20 years, and I was just a replacement tech, not an engineer....

Thanks to everyone that is helping me out. It becomes easier for me to decide things if I have a bunch of people looking over my shoulder....:D


install monkey
05-01-2011, 10:08 PM
the indoor fan will be a minimum of 25watt for a 2.5kw wall mount.
larger mitsubishi systems-city multi the inner bearing can contract if crankcase heater isnt energised for over 24hrs-36hrs if its cold.
an electric kettle can pull 10-12amp on a 240v sopply.a louvre motor operates at 7watt. a 50 inch plasma tv pulls 3amp.oil is thinner when warm and lubricates better.look at the price for a replacement compressor,inverter kit(for a full warranty) plus labour-or inconvenience to yourself

06-01-2011, 03:47 AM
energizing the start winding will consume more than 50watts, induction and resistive loads will ping the power meter more than 50watts

06-01-2011, 05:56 AM
An odd thought. Have a timer on the a/c circuit for the times you will not be operating the a/c. Switch off the electrical feed when the a/c is not required.

Would that help?

06-01-2011, 02:08 PM
An odd thought. Have a timer on the a/c circuit for the times you will not be operating the a/c. Switch off the electrical feed when the a/c is not required.

Would that help?

Yes. That was my first thought. That will work and in worst case, I can do it. I don't think the compressor can be damaged if it is powered off for 3 months twice a year.

But on second thought, couldn't the same thing happen anyway? Since the power is off for 3 months, won't the refrigerant migrate to the oil and the damage can be done?

The thought that any company would build an heat pump that can self destruct during startup unless it is pre-heated makes me scratch my head. Which is true? Which is most likely to happen?

I believe that the pre-heat is something that can save 0.001% of the units sold from self destruction, so.... Mitsubishi decided to turn it on to save warranty costs.

It is no skin off Mitsubishis nose if suddenly their heat pumps are less efficient. Any testing for electrical usage was most likely done with the pre-heat default off. Now they can turn it on in newer models, and reduce warranty costs. It is a win-win solution for them, but not so much for the consumer that pays $500 over 10 years for electricity.

If anyone knows how I can calculate the R410A liquification inside the unit so I can make an educated guess as to the severity of this problem, I would truely appreciate it.

Thank you everyone for your ideas.

06-01-2011, 07:18 PM
won't the refrigerant migrate to the oil and the damage can be done?

The thought that any company would build an heat pump that can self destruct during startup unless it is pre-heated makes me scratch my head. Which is true? Which is most likely to happen?1. Yes the refrigerant may migrate but if you power up the unit for several hours before it is used then the crankcase would have been heated the the refrigerant boiled off.

2. It is not just heat pumps. The majority of compressors have crankcase heaters, depending on their location in the world.

A lot of equipment instructions specify the number of hours that the system must be powered before use. For example a VRV unit may require 6 hours whereas a main chiller may specify 12 hours.

If isn't a case of avoiding warranty claims by the manufacturer; if you kill the compressor by pumping liquid the analysis of the compressor will show it and your warranty will be voided.

06-01-2011, 07:38 PM
Lets look at some figures.

50W x 20 hours = 1Kwh

1 Kwh = 0.18p (current average UK rate)

1 week = 168 Hrs

168/20Kwh/week = 1.51 week

3 months = 13 weeks

13 weeks x 1.51 = 19.63 ($30)

$30/3 months = $10 month

Small price to pay for peace of mind when starting your compressor.

06-01-2011, 11:34 PM
Well said brian. If it was me I would be looking at it not as how much I could save but how much it could cost me if my compressor goes because of trying to save a few pennies. Only today changed a heater in the comp of a chiller. Unit was tripping on low oil dif. Customer is waiting on new compress for circ 2 as one of there maintenance lads riged it to run over xmass with no heater when it burnt out. 5k new comp instead of a few quid for a heater element??

07-01-2011, 04:02 AM
I understand. It is insurance, and I appreciate insurance the way anyone does, but if I lived on top of a mountain and someone wanted me to purchase flood insurance, and quoted me $600 for next 10 years I think it is understandable to ask "how likely is it for me to be flooded sitting on top of this mountain?"

One thing I noticed as a factory certified mechanic for Toyota and Honda was that people that bought extended warranties never needed them. People that did not, always needed them because they abused them.....

OK. Failing any definative answers from anyone else, I will follow the advice to power down and during power up, wait for preheat before I turn on each time.

Thanks to everyone for the advice. I appreciate to know your thoughts.
Thank you