System Sizing: Trane Heat Pump Residential Zoning & New 3-Story 3-Car Garage
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.
If you can't run return air in the space you are going to supply air to, then how will you maintain any kind of temperature in that space.
<air conditioning> The removal of latent heat and humidity from condition space with air changes per hour.
'Life begins with the journey each day'
Thanks for your reply, aircooled. Here's the (admittedly complicated) story: 3 Floors, basement, garage, 2nd floor. Basement and 2nd floor are conditioned with both supply and return ducts. Garage is unconditioned but used as a bypass dump zone.
When air flow is dumped into the garage, the garage will be positive-pressured without a return, but it will have a pressure-activated one-way vent from the garage to the outside. As unconditioned space, we don't plan on maintaining any temperature in the garage. But any conditioned air we dump into it can only add to the conditioning of the spaces above and below it.
If your garage becomes positively pressurized and you have a one way vent to the outside to vent the positive pressure then you are pulling a vacuum in the house and every cubic foot of air that goes out the vent to the outside will want to be replaced through intrusion into the living space. This means hot, moisture laden air in the summer, and cold air in the winter.
I'd re-think that idea.
You can't use the garage for a dump zone.
The garage may not be connected to the duct system that does a living space.
While the system is off, a car could be ran in teh garage, and fill teh duct system with CO2.
It is a code violation.
At 7500 feet. You need more then 400 CFM per ton, so your larger air handler should not be a problem with the 2 ton condenser.
I would seriously consider a separate means of air conditioning for your garage.
The Trane Residential Zone Control system is very complicated and not to be installed by someone who has not been trained on it. Using a variable speed blower with the TRZCS negates the need for either a bypass or a dump zone. So if you're going to introduce air to the garage, you'll need a zone card and a sensor for that zone. In fact, the whole purpose behind the TRZCS is to eliminate the need for bypass and dump zone. Frankly the Carrier/Bryant zone system is eons ahead of the Trane system. Having said all of that, if you're committed to using a dump zone, I'd abandon the Trane sytem and get a Zone First or Arzel system. Each of those allows for a by-pass or dump zone without the complications of the Trane system.
If YOU want change, YOU have to first change.
If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!
Thanks for all the replies. Beenthere, I also thought it was a code violation to connect a garage to any forced air system. Strangely I have been assured that it is not here. Now, I'm more willing to believe HVAC-talk than our local inspectors and architects, so I will attempt to re-check and get the details and code sections people are quoting us.
If it's a code violation to dump into the garage, then no one is going to install that duct and no inspector is going to OK it, so it simply won't happen.
If we can't bypass dump into the garage, then Trane's relief option will probably be the appropriate strategy. As we understand it, this simply opens dampers to other non-calling zones just enough to move the appropriate amount of air through the air handler.
We appreciate your input that the larger 3 ton air handler will work with the smaller 2 ton heat pump, at least in part due to our high elevation. That's what we need to know.
Diverdan, thanks for bringing the forced infiltration issue to our attention. If it is in fact illegal to bypass dump into the garage then we won’t have to worry about sucking in external infiltration because we won’t be pressurizing the garage after all.
Skippedover, we know that systems such as Carrier Infinity and possibly the other ones you mention are superior to Trane's in some respects. But our only direct experience is with Trane's and it has worked exceptionally well for both heating and cooling in our passive solar house. We're going to stick with it for the new garage building.
TRZCS is new to us, if that's a Trane part number. We've been quoted on part ZSASSMAL010 which appears to be the current version of the system installed in our house 5 years ago (ZSASSMAL0100). The only differences that we've seen between the old and new versions are settings to accommodate the XL16i dual speed heat pump which did not exist 5 years ago.
Our 5 year old system also uses a zone card for what we call the "bypass dump" zone. Our installer preached the virtues of Trane's relief-based bypass avoidance strategy, but relief functioned very poorly in our house. It had absolutely no intelligence and simply attempted to more or less evenly distribute the appropriate excess air flow among all other zones instead of to the zones that made the most sense.
So rather than using Trane's relief option, based on info from HVAC-talk which was confirmed with Trane's zoning people, we use a standard Trane zone damper and a zone card with switches set for bypass, and route the bypass air into zones that can use the excess conditioned air instead of zones that would not benefit from it.
Thanks to JRBenny on Hvac-talk the capability to do this was designed into our ductwork from day 1. No one knew ahead of time whether relief or bypass into the return plenum or bypass dump into appropriate zones would work best. We’ve experimented with all three options and settled on the bypass dump option, based on what has worked well and what hasn't.
lets not forget that if the house is in a negative and garage is positive.....
may as well run a piece of alumaflex from your car exhaust right to your bedroom...enjoy
Its possible your area doesn't enforce the code that prohibits the garage being on the same duct system.
But, that doesn't make it safe to do.