Static pressure and ductwork is where im going to be lost to some of your responses, so I'll open with that.
The previous unit was rated for 1200 cfm so I went apples to apples with that effectively. Going on this, the entire time I was under the impression that the ductwork was rated for >= 1200 cfm, i did a ballpark figure at the time.
Old school air handler systems the target design velocity was often ~600 fpm to go with the smallest/cheapest/easiest duct sizing possible, and often missed that target by a bit, yielding static pressures > 1" w.c. along with lower then specified flow. By the system being ridiculously oversized adequate cooling/heating was still maintained despite the actual air flow being smaller than specified. It usually wasn't considered a problem unless the flow was so low that the cooling coil iced over or the overtemp limit safety switch on the furnace heat exchanger was tripping. The low efficiency split capacitor AC motor drives were capable of handling the abuse without overheating or burning out. Though sub-optimal, such systems continue to work, albeit with excessive duct noise and excessive cycling due to oversizing.
An ECM blower motor while highly efficient and quiet, will burn out when chronically subjected to static pressures north of 0.8" w.c., and for optimal performance & efficiency the target max should really be 0.5" w.c. even for those air handlers capable of handling 0.8" w.c.. As a rule of thumb (as lousy as they are), designing ducts for 400fpm maximum velocity (on both supply & return ducts), and using large deep pleated filters (4-6" media) sized as large or larger than the return plenum will usually deliver the goods. So if your original duct design was done well and running 600 fpm velocity when delivering 1200 cfm @ 1" w.c. static pressures, dropping back to a system & air handler that delivers 900 cfm would deliver duct velocity of 600 fpm x (900cfm/1200 cfm)= 450 fpm. The newer-lower static pressure would run about 1"w.c. x (450^3/600^3) =0.42" w.c., which is perfectly fine.
( counting registers and returns and sizing) most are 4x10 )
this is from what i saw ductwise-
2nd fl
(3) 4x10
(1) 2x10
(3) 6x12 return
1st fl
(5) 4x10 duct
(4) 2x10 duct
(3) 6x12 return
Are you mixing duct sizing with register sizing here? The register size is almost irrelevant, since the register boot is at the end of the duct run, imparting a fairly low additional static pressure (unless very undersized or the register is closed.)
What size are the supply & return plenums at the air handler?
A totally crude first rough cut analysis: If those are duct sizes, on the supply duct side it looks like you have 3 x (4 x 10) + (2 x 10) = 140 square inches of duct cross section to the 2nd floor, and 5 x (4 x 10) + 4 x ( 2 x 10)= 280 square inches to the first floor, which together adds up to 420 square inches, or (/144=) 2.9 square feet. At 1200 cfm that translates to an average velocity of 1200/2.9= 414 fpm, which is totally fine, assuming the runs aren't insanely long with lots of twists, turns, and sharp throated ells, etc.. At 900 cfm that's an average velocity of 310 fpm, which is also just fine.
On the return side you have 3 x (6 x 12) = 216 square inches on the second floor (fine for 140" of supply to that floor) and another 216" of return for the first floor, which is UNDERSIZED for the 280 square inches of supply to that floor. Ideally the returns would run 1.25x - 1.5x the supply, but there is some wiggle room there, as long as the return runs are short. The second floor returns are 1.54x supply, the first floor returns are 0.77x the supply- probably never a disaster, but an imbalance between first & second floors big enough to drive outdoor air infiltration when the air handler is running by pressurizing the first floor, depressurizing the second floor. But if the second floor returns are substantially longer (they might be) it may not be worth changing.
the air handler is rated for that cfm within the documentation RH2V3621MTANJA.
Per
the manual the RH2V3621MTANJA has an ECM drive motor (almost always necessary to hit SEER 16 or higher), with 21" x 21" plenum connections, rated 800 cfm - 1600 cfm in that plenum size. At the bottom of the chart on p.8 you'll see that the _3621MT_ will run it's nominal 1200 cfm @ 0.5" w.c., dropping to 1162cfm @ 0.5" w.c. with a strip heater installed (which you
will need, efficiency be damned!) While the table still shows numbers north of 1000 cfm even at 1" w.c. notice that the nominal
power consumption of the blower is roughly TWICE at 1" w.c. than at even higher flow when the static pressure is only 0.3" w.c. . Even though it's "rated" for that, it's never a good idea to run it anywhere near 1" w.c.- they can and DO burn out and a replacement motor can run over a grand(!), installed.
As to why they chose that unit/s?
They were quoted between 8 - 11k just for an amana or york replacement- and those were 14 seer.
the 16 seer was 11k for amana ASZ160361L 3 ton heat with ASPT37C14 air handler.
They are clearly stuck in a situation, and this is what is available for them right now in comparison. They were told by a friend in hvac getting control boards for Bosch later on if something fails could be an issue. Is there a parts shortage or something right now for brands like bosch ?(pandemic related?). This is all that I know.
I liked the bosch but in terms of the max COP 5 degrees rating off ashp.neep returned 1.86. There was a situation of availability and finding a reputable place that will deliver. Also they were told getting replacement parts if anything goes wrong (pandemic or otherwise) isn't easy and this was another important factor in making the choice. --
General consensus is they wont get a mini split unless its central(unitary) for the existing ductwork. However in terms of actual efficiency of the unit I'm aware of what the compact ducted are capable of doing, The units availability is also in question. They can't or won't do cassettes due to the rooms configuration ( asymmetrical ) imagine a corkscrew but vertically. I'd like to try to do as minimal surgery to the house as possible (again not mine) considering the budget and time constraints moving forward.
Availability is a problem for lots of these systems due to pandemic supply chain issues, but unless you're desperate and need it NOW it's better to wait until something more appropriately sized is available. The Bosch units are manufactured by (and I suspect largely designed by) Midea, which is currently the largest supplier of AC equipment in the world. Whether or not the controls for the Bosch units are unique to Bosch is something worth investigating, but I suspect (like many Midea manufactured units) most sub-assemblies are cross referenced to other designs and nameplates.
All of Carrier's mini-splits are now pretty much standard Midea units, many/most even at the cabinet box level (simply a swapped label to make it a Carrier), and going forward it looks like some of their modulating full sized air handler heat pump units are mostly or fully-Midea inside as well. eg: A 2 ton
Carrier /
Payne /
Bryant /
Weathermaker /
Midea 38MARBQ24AA3 / 40MBAAQ24XA3 pairing would be a pretty good fit f0r your loads. They are all exactly the same Midea unit underneath, and there may be even other brand labels slapped onto that design through other distribution chains. Per
the manual the nominal cfm for that 2 tonner at high speed is 882 cfm @ 0.8" w.c. (pretty close to the numbers I've been throwing around), and depending on the duct & filter particulars it can probably come in under 0.3" without major re-hacking of the ducts. It would need a toaster for the cold-snap extremes, but would run decent efficiency with lots of modulation comfort most of the year.
Something like this would better than ANY 3 tonner, and head & shoulders above that low heating capacity Rheem. The Rheem would be OK if your 99% outside design temps were north of 25F, but that's not where you live.