My Mitsubishi Split System works great all summer but now I find that it heats wonderfully all day but then ices up outside during the night when the temp falls below 0. At -2 it stops altogether and the fins are nothing but a block of ice….it's too cold and damp from heavy dew for it to defrost. Only in the morning when the sun hits it does the ice begin to melt… How do I stop this from happening? Can I put a protective covering over it of some kind ?
Split System Air Con icing up at night only..
freezingheat-pump
Related Solutions
If both your central air handling fan and outdoor compressor are running and your not seeing any sort of cooling after a 10-20 minutes than I'd venture a guess and say it's time to call an AC repair service.
I'm curious what you are pushing in that makes the compressor turn on? Are you somehow bridging the main capacitor? Capacitors tend to be a common point of failure on AC units and can actually be changed fairly easily. Here is a pretty good article detailing the steps to do so.
To be honest though I'd guess that the bigger problem is the fact that you can get it running and it still isn't putting out cold air. The whole situation sounds like it's low on coolant (have you had it filled in the last few years?). AC's usually have a low coolant pressure switch that will keep the unit from running to prevent damage to the compressor when there isn't enough pressure.
Call an AC service out, many will diagnose your problem free of charge and be able to give you a much better idea of your problem. Be warned though, if your system is old and uses R-22 prices have shot up in the last year.
Maybe. It depends on your climate, comfort range, and behavior.
You may be able to get away without supplemental heat sources if you can tolerate temperatures 10ºF or more above or below the setpoint and if you keep the bedroom doors open most of the time. Are you planning on installing an HRV or ERV? If so, that will help with mixing. Cooling can be more difficult to distribute because of solar gains. I'm writing mainly with heating in mind because that is my experience.
Note that even if your MSHP rated heating capacity is sufficient for your calculated heating load, you'll want to make sure it has enough capacity at the design temperature. Some units maintain capacity pretty well at cold temperatures (Mitsubishi FH series and Fujitsu RLS2-H series), but others do not.
Most people who attempt single-point-source heating with MSHPs install electric resistance heaters in other rooms and/or install a circulation fan to assist with mixing of air.
Supplement with electric resistance heaters
Common options here include electric baseboards, radiant cove heaters, and wall-mounted panel radiators (plug-in or hardwired). Occupant control here is key because you want to maximize usage of the MSHP and only use the resistance heaters when necessary.
For my 1000 sf ranch semi-deep energy retrofit with MSHP, I plan on installing radiant cove heaters in the bedrooms and bathroom. The bedroom heaters will be controlled by line voltage programmable thermostats (e.g. Honeywell P-8001) set lower than the MSHP, and only at night when bedroom doors will be closed. The bathroom heater will probably just get a cheap dial thermostat. Dana Dorsett on GBA talks about using occupancy sensors to control the radiant cove heaters.
Use a circulation fan to help mix air
Some people use low-wattage bath fans (e.g. Panasonic WhisperGreen) to pull air from the living space and supply it to the bedrooms. This still might not be enough to keep the bedrooms close to the heating setpoint because the supply air delta T is so small (only 5–10ºF). If you do room-by-room heat calcs (part of the official Manual J spreadsheet), you can compare the room design heat loads to the Q=1.08*CFM*DT for the circulation fan airflow, to help determine if this is sufficient or you need to supplement with electric resistance heaters.
Some people might not like having a bath fan grille in their living room, even if it is decorative. And running the ducts can be tricky, especially if your R-50 ceiling insulation is already installed. I'm thinking about installing a variable-speed in-line fan in my (unvented, conditioned) crawlspace to distribute air to the bedrooms from the living room. You can't beat the low-watt draw of the WhisperGreen bath fan though.
You probably would not want to use your existing furnace fan and ducts to help distribute the air because it is likely an inefficient motor and duct leakage would be bad if your ducts are in unconditioned space.
Location of single indoor head
In the main living area sounds like a good location. Here's what to think about:
- You want it to be able to "see" into as many rooms as possible.
- Length of the refrigerant lines running between the indoor and outdoor units is also a consideration. There's an efficiency penalty above 25 ft. The Mitsubishi comes pre-charged with refrigerant for up to 25 ft. Lines longer than that need more refrigerant to be added.
- Installation on exterior walls is much easier than interior walls.
Best Answer
I am assuming that this unit is a HEAT-PUMP. If it is, check with the manufacturer as to the lowest an ambient temperature this "heat pump" is supposed to run in. If that temperature is say 20 degrees, then the heat pump should shut down at that temperature and heating will continue on the 2nd. stage heating system. If it is "all the time" then okay we need to check for reasons for the icing of the coil. The outdoor coil of a heat pumps, when running in the heating cycle will ice up, under certain conditions, but should defrost when the ice builds to where it impedes the heating capacity. If your unit does go into the defrost cycle but does not properly defrost the coil, then there is a problem with one of the controls that initiates the defrost, senses the amount of icing, or terminates the defrost cycle. When operating properly the ice will be defrosted off the coil at any outside ambient temperature. Now I have a couple questions: #1- have you ever witnessed the defrost cycle? If so, was all the ice defrosted off the coil when the heat-pump terminated the defrost cycle and the outdoor fan started running again? #2 Do you live in a snow area and if so is the outdoor unit elevated off the ground as it should be? I used to set my installations at 12" above the ground. If the servicing company can't get this unit to defrost properly then you need to find a new servicing company with a tech that knows Heat pumps. They are a different kind of animal and can stump even knowledgeable service techs and factory reps.