Undersized Mitsubishi mini-split condenser (5C42NA2)?

[Boston_John]

Long time reader but first time poster. I'm hoping to get some of your excellent advice to talk with our a/c installer about what I think is an undersized condenser unit and a related question about a wall unit.

Part 1:
I have an MXZ-5C42NA2-U1 (42k BTU condenser) to support the following wall units: 2x MSZ-GL06NA-U1, MFZ-KJ09NA-U1, MSZ-GL09NA-U1, and MSZ-GL18NA-U1 (five total wall units).

Just adding the max internal consumption of each of the wall units, 6k + 6k + 9k + 9k + 18k = 48k total, which is over the outdoor capacity of 42k by 6k. Does this mean the outdoor unit is undersized?

I'm expecting that there are some rules of thumb and I don't want to oversize our system, but during the recent heatwave here in the Boston area our system could not keep up. All 5 units were turned on AUTO. The two smaller units ran fine. The three larger units were running with high fan and were unable to produce much cool air (it was cooler than room temperature, but not by much). Both lights were on for all three units, indicating that they were working to reduce temperature.

When I turned off the KJ09NA unit, the GL09NA and GL18NA both responded with cooler air, and within 30 minutes or so, the fans dropped down a notch automatically. By morning, we were back running with low fan/single light on.

This implies to me that the three larger units were probably calling for more cooling than they could receive (or rather, that the condenser couldn't serve up what was necessary). I haven't heard back from the installer but my suspicion is that the condenser is undersized. I totally understand that planning my system for the occasional Boston heatwave is a nice-to-have, my goal here is to understand if the basic math is off. What do you think?

Part 2:
The KJ09NA unit is located in a 750 sqft finished attic space at the top of an open stairwell from the second floor. That number includes closets and a bathroom, so the used area is much less, closer to 500 sqft. There is ceiling insulation and only two windows, but the house was built in the early 1900s, and more importantly, that space will absorb all of the heat rising through the house. Even if the volume should be de-rated because of the sloped ceilings, I think the open stairwell and older construction means it will be more likely to heat up. The installer suggested that if the KJ09NA unit is not keeping up, perhaps I need to add another unit. To me, that seems like the wrong step. If the condenser is undersized, adding another unit would just make that worse. Plus, why would I not just go up in size to either one of KJ12NA or KJ15NA units? For this part, I think I'm trying to understand if the unit is sized properly (and being underserved) or if it's too small for the application.

Thanks in advance for any insight you can give!

 

[Boston_John]

Some updates:

The installer said that it is acceptable to size the indoor units up to 130% of the outdoor capacity, so in my case, this would be imply a ~55k of capacity. This is based on the expectation that not all units will require full capacity simultaneously.

However, now that I look at the spec sheets more, it seems like none of the wall units are modulating... meaning that they request full BTUs when they are on. That can't be right. Is it?

 

[Sponsor]

 

[Fitter30]

Like to know the sq footage of the house and how old are the doors and windows . What shape are their weather stripping? Was a manual j heat load calculated? Has the installer came back a check refrigerant levels. Some
Electric company's offer a energy audit that includes a blower door test which tells how tight the house is.

 

[Boston_John]

All doors new in 2018; 75% of the windows are new in 2018 as well (Marvin casement) with the remainder Andersen 400s installed about 15 years ago. All are in excellent shape with maintained weather stripping. The square footage is approximately 2800, although about 350-400 of that is part of the finished attic space I mention, and two of the rooms have vaulted ceilings (kitchen and bedroom), so from a volume perspective it's a little bit more.
I've had it serviced each year, most recent service was in June, although the installer hasn't been back since I mentioned the concern.

 

[Boston_John]

Bumping my thread as I'm still not sure what to do with my setup. Any help greatly appreciated!

 

[Fitter30]

It is common to have btu capacity in inside unit size verses condenser. Loads are figgered that all units will never be at full capacity under normal operation because units run 24/7. With all units running and calling for full cooling by turning thermostats down with a load what is the suction pressure should be around 140# 49* . Under normal operation do you see frost on the refrigerant lines at the condenser? There shouldn't be. Did this house have air before this mini system and how well did it operate.

 

[Jeff H Young]

Just saw your thread and Im not in any way a hvac expert but its mid august is it working? was you old window units the exact and perfect size? Lastly its built now so whether its over or under or spot on whats it matter? once again not an expert but houses vary , climates vary, customers taste vary.
If I remember right you got 42k btu equals 3 1/2 ton ? my house is about the same size well insulated 2002 vintage came with 5 ton just replaced w 5 ton. no real choice for me as 5 ton is max size Id love to have 2 units but didnt want to reinvent this place too costly but works decent. Im in so cal 70 miles from coast some call the desert ha ha. but for my area 3.5 ton would be undersize on a traditional a/c system.

 

[Boston_John]

Hey guys, thanks for the replies, and thanks for the side note fitter30, I appreciate it. I've been offline enjoying the summer a bit and missed the replies.

Now that it has cooled down I'm going to talk to the installer again and see what we can come up with. I'm still of the impression that we are undersized in that finished attic area. I also think that if we're in 95+ degree heat that the system can't keep up with the total demand (despite what the sizing book says).

 

[Fitter30]

Some utility companies offer a blower door test included in a energy audit at a very resonable price. Might be worth it to see how tight your house is.

 

[Dana]

Hey guys, thanks for the replies, and thanks for the side note fitter30, I appreciate it. I've been offline enjoying the summer a bit and missed the replies.

Now that it has cooled down I'm going to talk to the installer again and see what we can come up with. I'm still of the impression that we are undersized in that finished attic area. I also think that if we're in 95+ degree heat that the system can't keep up with the total demand (despite what the sizing book says).

I too have been off-grid for awhile but...

If the air coming out of the ductless head isn't cool installing a bigger head isn't the solution. Even at max speed there should be a big temperature difference. If the refrigerant lines to those heads are longer than spec there will be a hit in capacity.

Most ductless heads on a multi-split still modulate, but are limited in modulation range by the minimum output of the condenser, and the rate at which other heads are running.

The fact that the half-ton heads were keeping up with their zone loads under the extreme heat wave means they are probably oversized (a common error committed by the "ductless head in every room" approach.)

It's possible that the total refrigerant charge is off. Did they do the math on the refrigerant line lengths/volumes and weigh-in the charge? (Too many installers just guess, and don't fully test the cooling/heating ranges rather than properly commissioning it.) A 3.5 ton condenser should be able to handle about 3000-4000' of reasonably tight insulated conditioned space in Boston.

The only way there would be "... heat rising through the house..." is if the house is extremely air-leaky, and would primarily be an issue in winter. (In winter is the first floor cold and the attic space warm?) In a kneewalled attic room with sloped ceiling the source of cooling season overheating is from the sun-baked 140F+ roof (think of it as an un-glazed solar collector) not the 95F outdoor air. It's a much bigger temperature difference than the walls, and probably doesn't have sufficient insulation to be really great, but it can be improved.

How much (and what type) of insulation is in that cathedralized ceiling? How deep are the rafters? Spot checking the ceiling temperatures with a pistol grip infra-red thermometer (or the l0-res $200 FLIR One IR camera) could be useful for spotting any gaps in the insulation. It doesn't take many square feet of uninsulated roof to make difference in peak cooling load.

 

[Rerod]

Yes don't immediately throw more tons at the issue.. A properly sized AC runs continuously on the hottest days of the year.