The project: a 3 car garage (36 x 30) over a full conditioned basement with conditioned 2nd floor above the unconditioned garage. About 2000 sf of conditioned space, half above and half below the 1000 sf unconditioned garage. Southwest Colorado, 7500 feet, low humidity. HVAC-Calc outdoor design temps of 87/-1, indoor design temps of 76 cooling, 70 heating.
ICF basement, R30 SIP walls above grade, R40 SIP gambrel roof. An ERV or HRV system is required for the SIP’s
All three levels will have additional 2x4 R13 stud walls on the inside. As a result, total wall insulation both above and below grade will be massive, effectively around R40, comparable to the roof.
Heavy insulation (at least R19) will separate the basement ceiling from the garage floor and the garage ceiling from the 2nd story floor.
We will use a Trane XL14i R22 heat pump with TWE037 variable speed air handler and Trane’s residential zoning system.
For you folks who might remember -- like JRbenny, MarkBeiser and Dash -- we’re some of the unlucky people whose original contractor on our main house sold us the wrong air handler (TWE037) for our 3 ton XL19i heat pump. We kept the TWE037 when we swapped in the correct TWE040 air handler for the XL19i, because we knew we could use it for this new project.
Unlike our house where no contractor would bid on the retrofit ductwork, we have several of them falling over each other to bid on this new construction system. Fortunately one of our favorite local companies has recently become a Trane dealer, so we will use them for this project. They have no real experience with Trane’s residential zoning, however, so I’m asking some questions here to hopefully tap into the collective experience of HVAC-Talk regulars on system sizing issues.
The middle garage level will be unconditioned, but we plan to use it as a bypass dump zone. This works extremely well on our XL19i system where we use our normally unconditioned greenhouse/sunroom and basement hallways as dump zones to avoid bypassing back into the return plenum when minimal air flow is called for.
Obviously there will be no returns in the garage, and a powered damper will keep the bypass dump duct closed except when the bypass is active. We know we will need some pressure-sensitive ventilation in the garage since it will not have returns. We don’t have gas or propane, so pressure issues related to gas water heaters, etc. don’t apply.
Unlike the first system for our passive solar house where we needed a lot of zones to separate sun-warmed areas from other areas, the new building needs only two zones and a bypass, one for the basement, one for the second floor and the bypass dump into the garage.
But unlike our first system, we’re not using a 2-stage heat pump. So whatever size heat pump we choose will run whenever the system is on, even if just one of the two zones is calling.
After our prior experience with an indoor coil that couldn’t handle the heat output from the outdoor coil (420psi head pressure, repeated compressor limit shutdowns), we want to make absolutely certain that the components we choose for the new building will work properly together
HVAC-Calc doesn’t offer an option for SIP walls with additional R-13 stud walls inside them, so our load calculation results are a bit higher than they should be. Calculated loads without taking those stud walls into account total about 10k btuh cooling and about 26k btuh heating. The cooling load for the basement is only 3.5k btuh, and if our other basement is any indication, the new basement might actually never need AC at all. Of course, it’s highly unlikely that the basement zone would ever call for cooling on its own anyway, except in the case of testing or human input error.
According to its service facts document, our TWE037 air handler can be connected to anywhere from a 2 ton to a 3.5 ton heat pump. But we’re no longer dumb enough to take the service fact tables at face value. We know there are going to be risks/costs/benefits for each size heat pump.
The choices: we’ll use either a 2 ton, 2.5 ton or 3 ton XL14i R22 heat pump with the TWE037 air handler. The dealer recommends at least the 2.5 ton unit, but our experience with our other system makes us worry that 2 tons might be the better choice. The price differences are so small that they don’t factor into the choice at all, which is as it should be.
In our part of the country we don’t have to worry about excess humidity, but short cycling is still an issue for lots of other reasons that you guys know better than we do. Since we can always dump a fair amount of air flow into the garage we feel that a 2 ton unit would not be too big despite the small cooling load – but we could be wrong about that and we know it.
Can anyone provide us with input on the risks/costs/benefits of the 2 ton vs. 2.5 ton heat pumps? Or whether we should just buy a smaller air handler and give away the TWE037? Can Trane’s residential zoning even be used with a single-stage heat pump?
Thanks for any help. If you’ve actually read this far, you qualify as a serious HVAC techno-geek.