Cold weather challenges when heating with mini splits

Hello Brownstoner Forum,

This is our first winter in our newly renovated townhouse where we replaced the existing forced air heating system with a Mitsubishi Hyperheat Mini Split system.

Now that the temperature has been really low, between 10-20 F, consistently for days (but no snow), we are putting the system to its first real test.

We have been experiencing large swings in the performance of the system in the past days, that cannot be explained by the swings in the outdoor temperature. The indoor unit in our living room was blowing 65F air at one point (the room temperature was set to 70F) while just a few hours later the temp of the blown air went up to 75F. In general the system struggles to keep the temperature at 70F on our parlor floor, while it is performing much better but not flawlessly on the top floor. I do see swings in the temperature of the air blown, but in general the temperatures are much higher: between 70F and 85F

It is a 3 story brownstone with a garden rental and our duplex on the top 2 floors. We have one outdoor unit for every floor (MXZ-3 C30NAHZ). The outdoor unit for the rental is in the backyard and our outdoor units are on the roof. I went through the installation specs and we seem to be well within the limits of distance / height difference bw the indoor and the outdoor units. We have individual remote thermostats for each of the indoor units (MRCH1).

My questions:
– what can cause the above described performance swings in the system?
– why is the indoor unit blowing colder air than the room temperature when it is supposed to be heating
– what can explain the performance differences between the systems for our two different floors?
– would wind protection for the outdoor units on the roof help? if so, how should we do it?

thank you.

Bence

beneec

in General Discussion 7 years and 5 months ago

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CarmenR | 7 years and 5 months ago

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Oh, I should mention we have Fujitsu units with 7 total handlers on 3 floors, 2600 sqft, 2 compressors on the roof. I am not sure of the BTUs anymore. They’re two years old now.

monaco | 7 years and 5 months ago

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The technology is available with mini splits now and I use them exclusively to heat our 3,000 sq ft brownstone.. Look into the Fujitsu extra low temperature heating or Mitsubishi hyper heat. I have the Fujitsu installed, 2 exterior condensers (36k btu each) and 8 interior air handlers (2 per floor). The cost during winter months has added approximately $200/month to electric bill. I would estimate during this unusual cold snap the cost this month could double to $400.

CarmenR | 7 years and 5 months ago

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I will update to say that we’ve been using the splits in some rooms to suppliment our radiators and they do seem to be pumping in hot air, however I definitely wouldn’t want to rely on them for 100% of our heat. Our house isn’t quite at “passive” level but it’s very well insulated and it’s still a bit chilly in rooms with a lot of windows during this cold snap even with the radiators running basically 24/7 for the past few days. I don’t even want to think what our electric bill would be if we relied on the splits alone, although the air coming out of them is, in fact, hot.

Master Plvmber | 7 years and 5 months ago

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The technology isn’t there yet. I routinely install boiler-heated radiator heating systems in homes that are also installing mini-splits and VRF systems for heat. They work fine until about 30° outdoor temperatures and then they don’t.

CarmenR | 7 years and 5 months ago

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We have spits but also have baseboard and radiators for this reason. We heard from multiple sources that the splits have a hard time keeping up if the temp is below 30 consistently. I would honestly check with your hvac sub or contractor and ask them-ours very specifically advised us against relying on splits only for heat for this reason. Good luck.

namahs | 7 years and 5 months ago

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I’m no expert but I do have Fujitsu ductless units for a few years. From my experience, when the outdoor temp drops below 30 degrees. The unit struggles to blow consistent hot air in. I heard the same from my friends.
There are newer units that are more effective on lower temps now. As far as if they can really perform, thats a different story.