Been looking at the Fujitsu RLS3.

There are no Fuji dealers near hear, everyone wants to sell me a Mitsubish Hyper-Heat.

They all say "yeah, 100% output at 5F!"

But when I go read the tech manual:

http://www.ecomfort.com/manuals/muzfhtechservice.pdf

Page 14 "Heating Capacity" and look at the MUZ-FH15NAH it seems what I am reading is around 8-9k BTU at a 5F outside temp.

What am I misunderstanding? Looking at Tech Design manual for the Fujitsu RLS3 it shows a much higher output at 5F like on the order of 2x.

And what I see in Mitsu manual doesn't jive with "100% output at 5F !!"

I've been installing the Mitsubishi for several years with no issues at all; have also installed them in my own home and love them. Great units & very satisfied. Have no idea about the finer details of the efficiency - it's usually either -0- or +20, so we get few extended periods of 5-10o. In the larger picture, you can't go wrong with the Mitsubishi, and you may regret buying a unit which no one near you can service.

If the capacity tables in that document were true I know several people who would have frozen to death this winter when it hit negative double-digits.

Believe the submittal sheets (which are often third party verified test data)

http://www.mehvac.com/media/953333/msz-fh15na_muz-fh15na_submittal.pdf

------------------------------------------------------------------------
Heating at 5° F
Maximum Capacity ...................................................18,000 Btu/h
Maximum Total Input .......................................................3,000 W
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.
.
.
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*4 Heating at 5º F | Indoor: 70º F (21º C) DB / 60º F (16º C) WB
*4 Heating at 5º F | Outdoor: 5º F (-15º C) DB / 5º F (-15º C) WB

---------------------------------------------------------------------

So, at 70F indoors/60F wet bulb (= 56% relative humidity, which is on the high side for wintertime indoor air) and +5F outdoors/5F wet bulb (= rime-icing fog conditions) the thing can deliver a peak of 18,000 BTU/hr, some of which is derived from the heat of fusion of ice forming on the coils.

The capacity tables are a worst case net output including the defrost hit, (" Above data is for heating operation without any frost") which would be huge in a +5F DB/ +5F WB condition. But don't worry- those conditions are pretty rare.

In reality most indoor & outdoor air will be drier than that, reducing the peak capacity figure from 18,000 BTU/hr slightly, but not enough to really matter. The capacity hit from not reaping the heat of fusion at peak rate is more than made up by fewer defrost cycles with drier outdoor air. The only time you're screwed is if your heat load at +5F is EXACTLY 18,000 BTU/hr with no margin, or when you have very saturated humid but cold air such as 5F fog. Under most indoor & outdoor conditions it will be able to deliver a very large fraction of that 18K number, even after accounting for defrost cycles.

Note that at 17F, the rated capacity matches the table value (11Kbtu) in the other document.

>2. Above data is for heating operation without any frost.

So 9K @5F "rated" with no frost (ie, no defrost cycles) is correct, but the claim is that there may be some frost and that this will increase (or decrease?) sustained output by some amount? I'd want to know more about how much (real data, not hand waving).

In general, heating capacity decreases fairly linearly with outdoor temperature - I'd be very suspicious of any data that claimed otherwise. "Maximum" may be a weasel word with "rated capacity" being the useful number.

phishfood: where do you see data that the Fujitsu does better (I see 7K btu @5F for the ASU15RLS3 - and that's in high humidity conditions)?

phishfood: where do you see data that the Fujitsu does better (I see 7K btu @5F for the ASU15RLS3 - and that's in high humidity conditions)?

http://bangorwinsupply.com/wp-content/uploads/2015/02/RLS3-DT.pdf

somewhere around pages 12/13 maybe I read it wrong but I see 22,000 BTU at 5F

You are right - I read the Kw table.

The "rated" value is a special term to be using- it has a legal meaning in the context of an HSPF/SEER submittal. The "rated" heating output is the modulation rate at which the unit is tested for efficiency at both +47F and +17F by which the HSPF is determined. The capacity at +17F has to be at least the "rated" value at a nominal set of dry bulb & wet bulb conditions to be able use that term, but that does not include fully saturated rime-icing conditions.

As a practical matter the maximum capacity is usually pretty close to what the thing can actually deliver over the vast majority of anticipated conditions. Some people feel the need to have some sort of backup heat source to cover the difference, for the fully saturated outdoor air case, but that's a bit silly, akin to oversizing. Rime icing conditions at the 99% outside design temperature aren't impossible, but extremely rare in most locations.