Replies

1. 
   Patrick_OSullivan | Oct 20, 2023 10:08am | #1

   CFM is dependent on the unit. Think of it this way (for cooling)... if unit A outputs 60 F air and unit B outputs 50 F air, unit A is going to need to deliver a higher CFM to achieve the same cooling, though both units might be able to satisfy the overall load.

   Think of the order of design as:

   1. You size the load (Manual J).
   2. You choose the system for that load (Manual S).
   3. You design the ducts for that system (Manual D).

   Each step is an input into the next. The CFM spec from the unit is an input into duct design to make sure the air delivered to each room is proportional to the heating/cooling load for that room.

1. 
   pnw_guy | Oct 20, 2023 10:26am | #2

   Do the rooms furthest away from the air handler have a bit of trouble at lower air flows, though? When the unit is running at its lowest CFMs, and the air is travelling through the ducts relatively slowly, it seems that by the time the heated or cool air gets to the furthest away vents, it will have lost so much of its heating/cooling capacity that it won't really heat or cool those far away rooms?

1. 
   Patrick_OSullivan | Oct 20, 2023 10:31am | #3

   This is why duct design is a real process and shouldn't be left to rules of thumb.

   Energy Vanguard has a lot of info on their site. You can start here: https://www.energyvanguard.com/blog/basic-principles-duct-design/

1. 
   pnw_guy | Oct 22, 2023 09:32am | #4

   In other words, if your house is a typical house with poorly designed ductwork, variable speed air handlers might indeed present problems due to the bad ductwork?

   (My house was built many years before I moved into it, and I'm not about to rip open all the floors, walls, and ceilings to fix the ductwork....)

2. 
   norm_farwell | Oct 22, 2023 11:37am | #5

   In terms of coming up with an optimal system for your house, there are a number of considerations.

   1) how much total heating and cooling (you’ve got this with your manual j and fuel use history)
   2) how to deliver that on the indoor side (ducted/ ductless)
   3) what outdoor unit(s) to serve those indoor units

   A couple of rules of thumb that should figure into decision making (and I am assuming you are in a heating dominated climate here)

   1) ductless is almost always simpler to install and more efficient than ducted
   2) multiple smaller outdoor unit compressors are always more efficient than large multihead ones because the turn down ratios are better and there will be less short cycling
   3) the advantage of ducted units is the ability to downsize heat pump capacity (and therefore compressor size) because an optional resistance heater can be installed to handle peak loads. Also the ability to zone distribution in a compartmentalized floor plan— bedrooms for example.
   4) the disadvantage of ducted systems is the potential expense of insulating ducts and the need to rework inadequate existing ducting (for example undersized branch ducts, constricting registers, inadequate return air capacity etc)

   So applying these and other considerations to your situation…

   I’d explore to possibility of adding a ductless head that might allow you to lower the ducted capacity. For example maybe you have some area of the house that has an open floor plan—great room, dining roomand living room etc.

   If you end up with two ducted air handlers sized to heat load, I’d look at down sizing them and adding optional resistance heat to cover peak loads. And do not locate these air handlers in unconditioned space under any circumstances. Ducts should be in conditioned space if all possible; if not they should be scrupulously air sealed and highly insulated.

   The mxz sm48 is a big unit and takes a branch box. So if possible I’d recommend putting indoor units on their own separate smaller outdoor units (suz ka**nahz*) to minimize compressor size and short cycling. Two small units can handle the low load parts of the year much better than one big compressor. Also it might be a bit cheaper since you eliminate the branch box.

   The svz air handlers have some flexibility to meet existing duct capacity because they have three static pressure settings: .3 .5 and .7. So your installer can “tune” the blower on the air handler to deliver the necessary cfm—within limits. It’s not going to be able to overcome multiple serious flaws in existing ducts.

   Be sure to specify the optional mhk2 wireless wall mount thermostat with any mitsubishi indoor unit. And pay close attention to where these are located, again to avoid short cycling.

   Hope that’s helpful. Good luck.

3. 
   walta100 | Oct 22, 2023 12:05pm | #6

   Duct design for the Asian systems needs to be done carefully because they tend to use lower horse power motors that work at lower pressures. Your duct work will need to be larger with wider and fewer bends.

   Using rules of thumb and wild guess all to common in the HVAC industry may well leave you frustrated and uncomfortable.

   Getting used to a variable speed heat pump is not easy if you have lived your whole life with an oversized gas monster. When we hear the furnace kick in, we know the meter is spinning like crazy and it is costing us money and when it off it is not costing us money. The variable speed unit will run nonstop for weeks sometimes months at a time if it is not oversized and you have a cold spell. The speed will go up and down as needed to provide just enough heat to keep the temp from falling. Only when the lowest possible speed makes more BTUs than is required will it turn off. You have to get used to it running it will be your new normal.

   My Rheem system seems the keep the fan speed constant so I see my delt T change with the compressor speed often I will see 11°F when on low and on the rare occasions when it runs on highest speed, I will see 24°F deltas. Other brands may change the fan speed to keep the delta the same.

   Yes, 400 CMF per ton is standard but 24000 BTUs = 2 tons and 800 CFMs so 735 sound close enough assuming the two units have been tests as a system by the manufacture and are listed as a matched set. I would not except any combination of units not listed as compatible by the manufacture.

   Walta

4. 
   bmul23 | Mar 13, 2024 05:24pm | #7

   First off, thank you all for the wealth of knowledge. I thought I would close the loop on this thread to hopefully help someone in the future. I ended up hiring an independent person to perform my manual J, S and then D although I did specify Mitsubishi equipment. It's worth noting that my wife strongly vetoed the wall mounted air handlers due to cosmetics. Additionally, because of significant incentives (I'm in the north-east) the 1:1 hyper heat units didn't qualify for many of the incentives and would have been significantly more expensive as a result. The work was done by an HVAC contractor this winter.

   I'm very happy with the system although the install had some hick-ups as it was a ductwork retrofit. It kept the whole house warm in some temperatures actually below our design temp. I chalk this up to the manual J building in some wiggle room as I've read it has an extra 10-20%. Most notable is that you can't really hear the system running at all even on the high fan speed. The outdoor unit is near a window but is extremely quiet, so much so that the next door neighbor even commented on it.