Questions on new zoned Trane XV95
A zoned, 2-stage, Trane XV95 was installed recently in my house. One zone is for the majority of the house, the other is for a much smaller in-law suite. I have some general questions. I'm an engineer and understand enough basic HVAC principles to wonder about some of the things I see.
Do return ducts normally have dampers? Mine do not. I ask because when the house is heating and the suite is not there is considerable "suction" on the suite side. The door to the suite is pulled open if not latched.
The furnace supply and return plenum have a 9" round duct directly connecting the two with a damper inside that is attached to a weighted arm on the outside. The damper opens more when the furnace goes to high heat. What is the damper's purpose?
Is there a limit to the number of dampers that can be wired to a zone? I ask this because two 6 inch supplies are on the wrong trunks. Example: A basement supply heats when the suite does. It should be on the house zone instead. It would be simpler to install a zone damper in the 6" duct than re-route it due to space constraints.
What causes the furnace to switch from low to high heat? Is it a timer on the furnace, or is the rate of rise at the thermostat factored in?
I have attched two cartoons/schematics. One shows the addition heating and the rest of the house not. The other shows the opposite.
Thank you in advance your help.
Returns don't normally have dampers.
It can be done though.
But, it sounds like your short of return when the main zone only is heating.
The bypass damper is too ensure enough air flow across the heat exchanger.
Staging can be controlled by the stat, the zone panel, or the furnace.
Ask your contractor which way yours is set up.
There is a limit, but your drawings indicate, your far from any limit.
Your zone system should have a DATS to limit air temps.
Brand and model # of zone panel?
Thanks for the reply.
It is a Honeywell HZ311 TrueZONE.
Originally Posted by BaldLoonie
I found the installation guide and this may partially answer my own question if I understand it correctly. It says the maximum dampers per panel are limited by transformer size. I assume this is the furnace transformer and I do not know what size the XV95 uses. Maximum damper VA per zone (I assume that is volts x current.. i.e. watts) is 28.8. The dampers are Honeywell and say 24VAC 6 Watts on the plate.
The path of the main return is much less direct than that of the in-law suite. The main has three 90 bends. I'm guessing that is why the in-law suite pulls more air.. path of least resistance.
Are bypass dampers standard in zoned systems?
From the install manual, I learned that a DATS is an accessory called a digital air temperature sensor. I looked in the panel and nothing is connected to the DATS terminals. Just guessing here... bypass dampers will increase the supply temperature and a DATS is there to makes sure things don't get out of hand?
I'll ask my contractor about the staging.
That panel is for single stage equipment.
A bypass is to keep air flow up.
Not to control discharge temp.
If the panel is for single stage operation, that must mean my staging is controlled by the furnace timer or the thermostat, correct? Is there any advantage to having the panel handle staging?
Panels that can control staging, can be set to only use second stage if 2 or more zones are calling.
Tends to prevent short burst of second stage operation. And is easier on the VS blower.
Exactly. Should be on a 432 panel so it doesn't go to high unless the preset % of zones is calling. Or goes to high right away when that many zones are calling.
Are you certain I would want high heat automatically when both zones are on. We have plenty of moderate days when low heat may be adequate for the entire house.
So far, low-heat appears adequate for the in-law suite on cold days.
I think I better understand my system now. These are my primary questions.
1. Two 6 inch supplies are on the wrong zones. The least costly fix for the contractor may be to reroute the lines, but it would screw up my limited shop space where the furnace is located. There was a better way of laying this out initially, but it is what it is at this point. That is why I asked about additional dampers.
2. I don't understand why dampers aren't normally placed on returns. I must be missing something. If only the in-law suite is heating, where does the air for the main house return come from? I don't understand how it would not create a negative pressure in the rest of the house and pull in cold air through leaks (or worst case, through the water heater vent stack)?
The panel would still be set up, that a thermostat has to also be calling for high heat.
So both zones and one stat would need to be calling for high heat.
Your system can have additional dampers installed to have those 2 6" runs controlled by the right stat.
Basically, the only reason it will draw return air from the zone that isn't calling, is because the zone that is calling, doesn't have enough return.(undersized)
Meaning if you had a damper in the returns that shut when then opposite zones return. The main zone wouldn't have enough return.
There are no perfect answers for staging & zoning. Our office has 3 zones, similar furnace. The panel times to high but senses supply temp and if too warm, such as 1 zone calling, drops right back to low fire. So far the system has worked great. Usually if we can change the panel when switching to a 2 stage furnace, we do % of zones calling so when we are at 2 or 3 or 3 of 4 we go to high. In mild weather that might not be needed so I suppose an outdoor stat could be wired in to high fire if someone is that picky. OR a 2 zone panel be used with 2 stage stats only in large zones that can support all the airflow without overheating and single stage stats in the small zones.
What brand panel are you using.
It is a Honeywell HZ311 TrueZONE.
Originally Posted by beenthere
Thanks for the explanations guys. I'm good on the zoning. It sounds like there are lots of ways to set this system up. The mismatched ducts are the only real problem at the moment.
Beenthere: I think we're talking past each other in regards to the returns. Probably a terminology thing. Anyway, I'll probably run my own numbers on the ducts if I can get the time because something doesn't seem right. I don't do residential ducts or HVAC for a living, but know my way around the engineering calculations well enough to get to the bottom of what I'm seeing.
Is there a number I can call at Trane to obtain their fan curves (CFM vs RPM vs pressure)?