Replies

1. Expert Member
   Dana Dorsett | Sep 25, 2015 01:29pm | #1

   It's seems unlikely that 450' of conditioned space with R19 walls and only 30 square feet of window ( or is that 60 feet total, 30 on each wall?) has a true heat load anywhere near 9000 BTU/hr (20 BTU/hr per square foot, which is higher than a lot of 2x4 construction would be) let alone 11,000 BTU/hr @ -2F as long as it has glass in the windows. It's conceivable the load could be as high 6500 BTU/hr, but it could also be as low as 5500 BTU/hr if it's very air tight.

   If you were paying attention to air sealing details during the rehab it's going to be far tighter than "average". What was the BTU/hr number associated with air infiltration in your calculation?

   With the air tightness presumtion of "tight" what is the calculated heat load at 20F, 30F and 45F?

   The mean temp in Greenfield in January is about 22F, the December mean temp is about 30F, and the March mean temp is about 35F, eyeballing it from Weatherspark datasets, so don't want it to be maxed out at 20F, but modulating rather than cycling when it's in the 30s. The minimum modulated output of the FH09 at +47 F is 1700 BTU/hr, but for the FH12 it's 3700 BTU/hr. Ideally you'd want the thing to be modulating rather than cycling whenever it's actually cold out:

   http://usa.mylinkdrive.com/uploads/documents/4560/document/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf

   http://usa.mylinkdrive.com/uploads/documents/4561/document/MSZ-FH12NA_MUZ-FH12NA_Submittal.pdf

   The HSPF doesn't really tell you much about it's operational efficiency in a zone 5 climate, nor does it tell you the efficiency when the unit is oversized or undersized for the load. From a raw efficiency point of view it'll do somewhat better if oversized by 25-50%, but from a comfort point of view you'll be better off if it's modulating when it's below 40F (or at least by 30F) rather than cycling. If it turns out the FH09 is undersized for the load you'll get better as-used efficiency out of the FH12, since it'll be modulating down to a much lower outdoor temp than the FH09. If running flat-out full speed at +15F it won't have a COP better than about 2, but if modulating it'll be in the mid-2s or higher.

   When cycling on/off they're somewhat less efficient than at minimum-modulation, but the room temperatures also take much bigger swings. The temperature swings are sometimes solvable by retrofitting a wall thermostat, but that's also a significant cost-adder, since they are manufacturer-specific items, and produced in fairly low volumes.

2. 
   bac478 | Sep 25, 2015 04:58pm | #2

   Dana let me try to answer your questions:
   1. 3x5 window, qty 2 per wall; qty 2 walls (N and S), = 30sqft/wall or 60 sqft total window area
   2. 1 ACH air change assumed using guideline here http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm
   3. If I set infiltration to tight I get 5566 Btu/hr @20F, 4453 Btu/hr @ 30F, and 2783 Btu/Hr @ 45F. Used calculator here http://www.loadcalc.net/load.php#

   Let me explain 2 and 3 above. I used two calculators as a sanity check. One takes in ACH as a number and gives guidelines of what to set it at with 1 equated to "leaky". The other calculator just has a subjective drop down ranging from very tight to very poor with 5 options. They also operate slightly differently, one bases results on heating degree days and the other lets you input an indoor and outdoor temp and calculates the heat req for those parameters only. I used outdoor @ -3 so the results are a worst case number. Results as follows:
   Calculator 1 (-3F outdoor, 70F indoor):
   1. At very tight I get 8126
   2. At average I get 9925
   3. At very poor I get 12983

   Calculator 2 (-3F outdoor, 7200 heating degree days)
   1. At .33 ACH I get 6782
   2. At .5 ACH I get 7533
   3. At 1 ACH I get 9740
   4. At 1.7 ACH I get 12831

   Sounds like the goal is to keep the load above the min modulation limit for as much of the time as possible without going so small that the unit can't keep up on an "average" winter day. Clearly in fall spring there will be cycling because outdoor temp is close to desired indoor temp.

   At 20F the FH09 maxes out at about 7500 Btu/hr
   At 20 F the FH12 maxes out at about 9300 Btu/hr

   One of the calculators allows me to change the outdoor temp and adjusts load accordingly. I can use this to figure out around what outdoor temp the unit would start to cycle
   1. The FH09 would hit min modulation at about 55 deg outdoor
   2. The FH12 would hit min modulation at about 36 deg outdoor

   Looks like the FH09 will keep up almost all year and will avoid cycling maybe 10 months out of 12. Are there things I am failing to consider?

   Thanks

3. 
   Reid Baldwin | Sep 25, 2015 05:06pm | #3

   If the ACH on the second floor is from stack effect, the air that is leaving the 2nd floor will be replaced by air coming upstairs from the 1st floor, which is presumably already heated.

4. 
   bac478 | Sep 25, 2015 06:57pm | #4

   That is a good point and I hadn't thought of it. Thinking FH09....

5. 
   bac478 | Sep 26, 2015 11:33am | #5

   Just looked into the fujitsu units. Looks like the 9RLS3H is comparable. I estimate I am going to need about 35' of lineset and the Fujitsu can handle up to 49' with the factory refrigerant charge in the unit. The Mitsu is only good for 25'. I would have to find a tech with a refrigerant scale to meter out the additional charge. I can handle the vacuum/nitro purge myself.

   Looks like the Fujitsu has a min load rating of 3100 Btu/hr so it would cycle more than the Mitsu in the transition seasons and hurt efficiency a little. The "h" version (Fujitsu) looks like it has a built in pan heater, additional drain holes, and some structural enhancements for cold weather. Advertised down to -15F heating.

   That said I think the mitsu is a better fit based on load but the Fuji would be easier to install because it wouldn't require additional refrigerant.

6. Expert Member
   Dana Dorsett | Sep 28, 2015 05:54pm | #6

   The 9RLS3H is more comparable to the FH12 than the FH09, in heating applications, including it's minimum modulation level @ 47F. At -5F the max capacity of the 9RLS3H running at max speed puts out about 14,000 BTU/hr the same as the max capacity of the FH12 at +5F.

   The "nominal" heating output of 12,000 BTU/hr of the 9RLS3H is about what it delivers at -15F(!). So from a heating capacity point of view the 3/4 ton Fujitsu is a somewhat bigger unit than the 1-ton Mitsubishi, and it'll have you covered even if the worst case numbers in your calculation runs prove true (not likely.)

   Note, that as the temperatures drop, so does the minimum modulated output. The min & max output at 47F is just the data set for 47F. I'm sure at 30F the

   If you were building with air-tightness in mind you'll be tighter than the BuildItSolar guidelines, at least for the part that you were rehabbing. This is why mechanical ventilation is almost always necessary in new tight construction. But the heat loss related to that is exaggerated, even under Manual-J methods, since the air isn't all entering or leaving via a single straight & round hole. There is significant "heat exchanger effect" in real-world random leakage- the air never enters the house at the outdoor temperature, nor does it leave the exterior surface of the house at the interior temperature. It's not like a 90% efficient HRV, but it's not a straight pipe- it's something in between. The higher-R you make the house, the bigger the infiltration loss numbers appear compared to the other load factors, but you have to understand that they're large relative to typical in-situ reality, often by a factor of 2 or 3.

   Even with blower door test numbers you don't know where the leaks are or how big they are, just the aggregate size. But the location matters in terms of how much heat exchanger effect you get out of it.

7. 
   bac478 | Sep 29, 2015 06:08am | #7

   Dana - clearly you have a lot of experience with sizing these units. Based on the information provided which seems most appropriate; the 9k Btu Mitsubishi, the 12k Btu Mitsubishi, or the 9k Fujitsu?

8. 
   Dana1 | Sep 29, 2015 08:27am | #8

   For a fairly open & connected 450' space with reasonable convective connection:

   http://usa.mylinkdrive.com/uploads/documents/4560/document/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf

   If it's a couple of doored off rooms with little convective distribution between them:

   http://www.fujitsugeneral.com/PDF_06/Submittals/9RLFCD%20Submittal.pdf

   (Oversize the ducts, keep them short.)

9. 
   bac478 | Sep 29, 2015 11:47pm | #9

   There are two bedrooms and a small 1/2 bath upstairs. I was going to mount the head in the hallway above the stairs to one side if I went with the ductless. See floorplan attached with location highlighted in yellow. This is pretty similar to the Carter Scott arrangement on this site. This is what I was planning on doing so I designed around it.

   If I move a light fixture I could probably install a drop ceiling above part of the stairs with a return grille right underneath the ducted unit. Then I could have a plenum off the supply side with about 10" of retangular duct off of each side (like a giant tee) that went into each room. Terrible sketch attached: blue is the unit, red is the ductwork. This would provide distribution to each bedroom with very minimal ductwork. It is more work than the ductless.

   Currently it is me and my fiancee, the second bedroom won't be used. Open doors during the day shouldnt be an issue. I was thinking heat would distribute pretty well. I was worried about cool air falling down the stairs but there is a door at the bottom. Is the ducted worth the extra work & materials, lower efficiency (HSPF 12 vs 14), and higher min heating temp (-5F vs -15F) for better distribution?

   
10. 
    Dana1 | Sep 30, 2015 08:38am | #10

    If the heat loads of the individual rooms were 1500 BTU/hr a single head would work fine, but they're roughly twice that. A ducted unit guarantees that it all just works.

    The efficiency number tests are for some arbitrary zone 4 set of conditions with an outside design temp of +17F, sized exactly for the load. The test doesn't reflect the true efficiency of modulating units, or the beneficial t effects of modest oversizing. The larger coils of the larger units give them a higher efficiency at some intermediate load when running at 1/3 modulation vs. a smaller unit running at 2/3 of it max speed. Since the ducted Fujistu units all modulate down to the same 3100 BTU/hr @ 47F, there isn't a comfort hit for oversizing. The nominal capacity of the 9RLFCD is 12,000 BTU/hr and will already have some oversizing factor built in for the likely load. But if you wanted to be sure, you could bump that to the 12RLFCD, and have guaranteed margin, and despite the lower HSPF numbers, it'll be more efficient than the 9RLFCD as-used:

    http://www.fujitsugeneral.com/PDF_06/Submittals/12RLFCD%20Submittal.pdf

    The down side other than the modest uptick in upfront cost is the higher air volumes- it might feel pretty breezy even at mid-speed. My gut tells me based on the range of numbers presented is that the 9RLFCD is already approaching the optimal oversizing factor, and the improvement of going to the 12 would be pretty small. If the true load is at the high end of the range presented the 12 might be a better choice.

    For ease of setting up the condensate drain, it might be better to install it in the top of the bathroom ceiling, due to the proximity of the plumbing chases, but that's not the only factor. Routing the refrigerant lines or placement of the compressor unit may trump that.