[bront23]

I got multiple quotes for systems and am trying to decide between two Mitsubishi systems. The house is a colonial with a manual J load that matches the outdoor unit.

System 1) Mitsubishi MXZ-SM48NAMHZ (hyper heat 4 ton) with two ducted air handlers SVZ-KP24NA (one up and one downstairs).
- This seems to be what would be considered "standard"

System 2) Mitsubishi MXZ-SM48NAMHZ (same as above) with two ducted air handlers SVZ-KP30NA (same as above)
- Contractor says the air flow with the two tons air handler is too low for the open floor plan 1st floor (SVZ-KP24NA can only do 735CFM @ 2 tons.

From reading online it seems like all the Mitsubishi air handlers have lower CFM/ton then the standard 400CFM/ton. Why is this the case? Is the 400CFM/ton rule I see all over antiquated or not for variable speed heat pumps?

 

[pecmsg]

2 stories 2 systems.
Especially with minis. Any issues with the outdoor unit everything goes down.

Same size outdoor unit why the difference in indoor?

Your manual J came in at 4-Tons?

 

[bront23]

A bit lower then 4 tons but the units can modulate down and the utility incentives make it worth it for the 4 ton unit. Two separate systems is cost prohibitive in this application.

Is the standard Delta T of 18-22 degrees still appropriate for calculating the air flow?

 

[pecmsg]

I’m well aware they’re variable. Still need to be sized properly!
What did the manual J report say?

 

[bront23]

Contractor didn't do one so I did on Cool-calc. I went room by room measured everything and gave it the actual inputs. 45.5kBTU for heating and 35kbtu for cooling. Cool calc then calls out 1600CFM for the total system.

 

[pecmsg]

Contractor didn't do one so I did on Cool-calc. I went room by room measured everything and gave it the actual inputs. 45.5kBTU for heating and 35kbtu for cooling. Cool calc then calls out 1600CFM for the total system.

What was the load for the 1st Floor?
What was the load for the 2nd floor?
Basements get a separate system and dehumidifier

 

[bront23]

I very much appreciate the help, info below:
1st Floor:
Heating: 25989 BTU/h
Cooling: 21312 BTU/h
Air Flow: 975 CFM

2nd Floor:
Heating: 19454 BTU/h
Cooling: 12615 BTU/h
Air Flow: 611 CFM

I don't have any system for the basement, just a dehumidifier.

 

[pecmsg]

Those #’s look reversed. Same Sq Ft heating should be more for the second floor as well as cooling.

 

[bront23]

I just double checked and according to cool-calc that's correct. I would think it would make sense that the first floor would take more heat because heat would rise and there's no insulation between floors? However, the cooling seems off. The rooms upstairs that face south-west get killed in the summer and fall. However, the south-west facing bedroom is 1868 BTUh cooling while the back bedroom facing NE is 1252 BTUh. The bonus room over the garage has very few windows.

 

[pecmsg]

The 1st floor has Conditioned Space above.
The 2nd floor has unconditioned space above.
With the same Sq Ft the 2nd floor requires more BTU’s /h of heat

 

[bront23]

Perhaps I entered something incorrect in cool-calc but even after some searching I still have a higher heating load on the 1st floor and it's actually less square footage. Probably worth noting that the total heating load seems correct as I've calculated it via nat gas consumption and HDD.

 

[pecmsg]

I doubt that.
Think about it. The 1st floor has conditioned space above. The heat will move slowly to the 2 nd floor.
The 2 nd floor has an attic and roof that’s unconditioned. The heat will move much faster to the cold!

 

[vstech]

Variable load does not mean it can be oversized for the loads.
It means when properly sized, the equipment can go lower in less demanding conditions.
Installing equipment that is too large for the demands will yield bad results.

 

[dan sw fl]

I got multiple quotes for systems and am trying to decide between two Mitsubishi systems. The house is a colonial with a manual J load that matches the outdoor unit.

System 1) Mitsubishi MXZ-SM48NAMHZ (hyper heat 4 ton) with two ducted air handlers SVZ-KP24NA (one up and one downstairs).
- This seems to be what would be considered "standard"

System 2) Mitsubishi MXZ-SM48NAMHZ (same as above) with two ducted air handlers SVZ-KP30NA (same as above)
- Contractor says the air flow with the two tons air handler is too low for the open floor plan 1st floor (SVZ-KP24NA can only do 735CFM @ 2 tons.

From reading online it seems like all the Mitsubishi air handlers have lower CFM/ton then the standard 400CFM/ton. Why is this the case? Is the 400 CFM/ton rule I see all over antiquated or not for variable speed heat pumps?

No comments. ... No location. No detailed Building THERMAL ENVELOPE.
____________ ZIP _ _ _ _ _ ? NO floor areas. No date house was built.

______ _____CFM / Ton
DRY region __ 380 - 440

Humid region 330 - 390

https://www.mitsubishitechinfo.ca/sites/default/files/SB_SVZ-KP30NA_For_MXZ_SYS_201912.pdf
SVZ-KP30NA ___ 613, 744 & 875 CFM