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I've got a Mitsubishi Mr. Slim split ductless system in the "mother-in-law" suite of my house. There are 3 wall units connected to it, one in the front room, one in the bedroom, and one in the living room/office area. The bedroom one is set really low because that room is small, so the unit almost never has to come on.

Currently, the other two units are blowing air that's only semi-warm, and are not keeping their rooms up to temperature. Just a day ago they were able to do so easily. The outside temp has fallen somewhat, but the difference in internal temp seems excessive. I've noticed that there is ice on the bottom third of the cooling grid on the back of the external unit, and I'm wondering if this could be part of the problem. If I get that ice off of there am I likely to get any warm air back?

If so, since the grid is delicate, I'm thinking my best option to remove the ice is a heat gun on low, but I'm not sure what that will do to the cooling grid since it's that really light-weight, easily-damaged aluminum. Would I be okay using the heat gun?

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DAO1 is right, if this happens often, then your defrost capability are suspect, and you should call a technician.

For the time being, if no one is currently occupying the units, the best thing to do would be to run the system in AC mode. This is optimal for 2 reasons: 1) It will cause a more gradual change to any of your system parts, meaning there's less of a chance of parts cracking or splitting due rapid expansion. 2) It will heat the ice from the inside, meaning you shouldn't have to melt the ice all the way though to remove it.

That said, the radiator fins are built for heat transfer and heat alone shouldn't damage them, so a heat gun on setting you could point at your skin for a minute with out pain would likely do the trick without damage.

You may also want to consider using a space heater in the future, especially when the temperature is below 5 degrees F because your ductless heat pump may can have a negative coefficient of performance at that temperature as indicated by Figure 8 in this NEEA study.

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