Choosing Between Two HVAC Setups for Best SEER
My wife and I are building a house in North Carolina and the time has come for us to decide on our HVAC setup. We’re waffling between the Trane XV20i with communicating air handler and Mitsubishi MXZ compressor with SVZ air handler. On paper the Trane has the higher SEER2 at around 19.5 while the Mitsubishi MXZ + SVZ only comes in at 15.5 because it is ducted (if it were non-ducted it would come in at 20). Since the Mitsubishi equipment is also more expensive, is this a no brainer decision to go with the Trane equipment or does the on paper specs not really compare with the real world performance? Additional context: we’re using open cell spray foam insulation with a sealed attic, so all the equipment will be in conditioned space. Moreover, we’d like to add a ceiling cassette in the garage. According to the Mitsubishi specs if we did that it would be in a mixed configuration and have 17.75 SEER2. Would adding a cassette really cause the air handler to perform better? Or would you really need to take a weighted average to get the actual SEER2 rating?
Thanks!
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It sounds like you’re comparing two different things: a Trane single zone to a Mitsubishi 2 zone. If you got the Trane, you’d still get a garage unit? What’s that cost?
Apologies for the confusion: both are single zone. The Mitsubishi SVZ is the air handler and the MXZ is the outdoor unit. Either way, we'll probably get an indoor unit for the garage. Cost for the Trane equipment would be ~$20,000 and cost for the Mitsubishi equipment (less the garage unit) would be around ~$23,500. Adding in the garage would cost ~$26,000 for Trane + Mitsubishi garage unit and ~$29,000 for all Mitsubishi. I'm mostly curious if the efficiency numbers on paper will match reality and if so, is there any reason to go with Mitsubishi over Trane?
It depends on capacity - what’s your heat loss and at what temperature? If the Trane runs out sooner, then the Mitsubishi might turn out to be more efficient since there’s an unacknowledged backup heat source excluded from the rating. Also, would a single zone Mitsubishi be cheaper? The MXZ is a multi-zone. The SUZ outdoor unit is the single zone version.
Another point is that SEER is cooling only: the Trane is less efficient at heating, which even in North Carolina is probably the larger annual demand.
Last, does sound matter? Usually the horizontal outdoor units are quieter.
Since we'd like to include the garage, we thought we'd investigate an MXZ instead of an SUZ.
Heat loss is about 18k BTU at 24* F. The equipment has been sized for cooling and may be a bit large.
According to the COP graphs, they should perform pretty similar. The HSPF2 numbers seem about the same as well. On paper their heating abilities seem about the same.
Sound isn't a big issue as with the open cell foam we'll have considerable sound deadening.
Great the Trane seems best!
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Have you look to see how the units tested in the NEEP protocols?
https://ashp.neep.org/#!/
Does one or both bits include back up resistance heat?
How oversized will the system be for your cooling load? Do you understand why that could be a problem?
The quality of the installer is much more important than the equipment brand. What does your gut say about the people bidding?
Walta
I wasn't able to find either setup exactly in NEEP, but approximate setups showed results in agreement with the manufacturer specifications.
The installer has quoted back up resistance heat with the Trane, but according to the load calculations and NEEP even at the low temperatures, they shouldn't be need.
According to the Manual J, with three ton units they should be about 1/2 ton large. My understanding with single and two stage equipment sizing equipment too large causes short cycling can limit the unit's ability to dehumidify and cause excess wear. The contractor has assured us that if it is too large that they'll fix it at no extra charge. That said both should be able to ramp down to ~25% capacity, which can alleviate a little oversizing.
We've gotten quotes from several installers, several of which came off as salesmen. This installer sounds like he genuinely knows his stuff and answers my questions head on rather than avoid them. Our general contractor has also done quite a bit of work with them in the past, so they have a good relationship.
At this point, we're leaning towards the Trane setup as it seems to be better on paper and is less expensive.
Do you have the manual J handy? It’s unusual for a heating load at 24F to be so much smaller than a cooling load.
Here you go! Note there's at least one error in that the duct loss should be zero due to the ducts being in the sealed attic. I'd love to know your thoughts!
What are the windows like? The sensible heat loss and gains are similar, but the delta T for heating is > 2x cooling. So unless you have about 10,000 Btu entering through windows, it seems like cooling is overstated. I’d be considering the 1.5 ton options should the cooling come down some.
I think the windows are decent. They aren’t three pane, but are double pane. I know Mitsubishi offers a 1.5 ton, but I don’t think Trane does. Another idea I’ve had is to put in a zone damper and throw some of the excess to an unfinished wing of the house.
It seems strange that they chouse to zero out the data I painted red. That data answers the questions about is this equipment a good fit for your building.
When the HP goes down the back up heat make it an inconvenience instead of a crisis.
Make sure they show you how to lock out the back up heat when the outdoor temp is above your setting. MY setting is 7°f
Did you say the Trane system included the Trane brand communicating thermostat and air handler?
Make sure water falling off the roof will not land on the Trane unit.
Walta
Walta,
Thanks for the tip on outdoor unit placement as well as the emergency heat lockout! We will definitely have to set that to a similarly low temperature.
Yes, I think they’re thinking the 1050 Thermostat. I’m not interested in turning the equipment from variable speed to one or two stage.
If I had to guess, I’d guess we hadn’t decided on equipment when they put together the Manual J and that’s why all the equipment information is left off.