Trane XL900 Thermostat Question

orchid1961October 27, 2012

We just installed the Trane XL16c gas pack. The contractor said that the XL900 tstat does not work with the XL16c because the XL16c does not have the communication module. It's hard to believe since the XL16c has dual stage heating and cooling, variable speed blower etc. Please advise. Thanks.

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Your contractor is correct. I see no need to use the XL900 since the XL16c can't make use of many of its features. The Honeywell VisionPRO IAQ is similar in how it controls humidity with the variable speed blower and 2-stage compressor. You could ask them to install this and make sure they wire it so that it drops the blower speed to 80% on a call to dehumidify. Or alternatively the XL803 is a good thermostat for this unit. It can only overcool by 1-3 deg to control humidity (set by installer), but this in combination with the Comfort-R feature will work quite well. Comfort-R is an enhanced blower ramping algorithm that helps improve humidity removal on a call for A/C. This needs to be enabled on the unit by the installers.

Did you end up getting any ductwork modifications that we discussed in your previous post? Just curious.

Hope the new system performs to your expectations. The XL16c is a great unit.

    Bookmark   October 27, 2012 at 5:25PM
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ryanhughes, as always, thank you very much for the info.I'll stop bugging my contractor for getting technical support on the XL900 then. I believe he installed the XL803 this morning (while looking for a better thermostat). But for some reasons, the tstat does not have the option to turn on/off the dehumidification setting, will need to check with him.

About the ductwork modifications, I love to discuss with you about it to see what do you think. To recapture our previous conversation, in the original duck design, there are two supply boxes, the primary box (16" duck delivers air to 1300 sqft) is fed from the gas pack and the second box is fed from the primary box (12" duck delivers air to 600 sqft). Currently, I have 18" flex coming off the plenum which branches off to two supply boxes with 12" and 14" flex ducts. In the morning of the installation, I went down the crawl space and noticed that the second supply box is about 30-32ft from the plenum. Best practices call for duct run within 25ft, otherwise, the air velocity would drop quite a bit and energy would be wasted. If I know about the long stretch flex duct, I probably would stay with the original design. Am I being too critical?
The contractor did do the Manual J load calculation, it showed the cooling load size of 2.7 T for 1964 sqft. I have 4 rooms with vaulted ceiling so it's probably OK to round up to 3T. However, he did not do Manual D calculation; I wonder whether the 14" duct could cover 1300 sqft. Should I move one of the 8 flex duct that covers 150 sqft to the smaller supply box?

Being totally ignorant about the HVAC, I've tried to learn something about it (with your help, of course) in order to understand contractors' proposals. I hope that everything will work out.

    Bookmark   October 27, 2012 at 11:26PM
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If a full room-by-room Manual J was done, the contractor will know how much airflow each room needs and duct sizes can be determined accordingly. Does the 18" run the full 30 feet with two branches, 12" and 14", or does it run a shorter length before branching off to a 12" and 14" running to the boxes? Do you know if any balancing dampers were installed in the branches? It's hard to make definite recommendations without seeing it in person. How does the airflow seem throughout the house on both high and low stage, both in the rooms served by the first box and the box 30 ft. away from the unit?

    Bookmark   October 28, 2012 at 10:44AM
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Hello ryanhughes, I'm sorry that I misspoke, the 12" and 14" ducts branch out from the supply plenum, each with a damper. The main 14" flex duct is branched horizontally straight out, the 12" duct is branched from the top of the plenum, they mold the flex duct into an elbow shape. The 12" duct run is about 32 ft, covers about 650 sqft. This morning I turned on the unit to high stage, I went to all the rooms and checked for air flow velocity; I noticed that the air flow from all the rooms covered by the 12" duct is weaker. At low stage, the airflow in all rooms are weaker but the difference is not very noticeable. I hope that you could understand what I'm trying to describe. I'm sorry for not being very descriptive. Thanks.

    Bookmark   October 28, 2012 at 12:19PM
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The Manual J load calculation was done for the entire first floor not room by room (I guess that my house has a pretty open floor plan, the rooms specified in the Manual J printout do not map to all the rooms in the first floor). Anyway, I'm glad that I did not change the current load of the second supply box.

    Bookmark   October 28, 2012 at 12:26PM
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Well, it's not surprising that the airflow out of the rooms served by the 12" is weaker -- less airflow is carried by the 12" duct (compared to the 14"), and it's a longer run. However, it still may be enough to satisfy the load of the rooms served (~650 sqft). A room-by-room load calculation would tell you the CFM needs of each room, from which you could then balance the system to obtain the required airflow throughout the house. Not all contractors have the equipment (i.e., flow hood) required to properly balance a system. So, you'll have to see how the system performs -- if it's keeping the temperatures even, great. If not, some balancing is in order. I'm guessing both balancing dampers are wide open right now? Your contractor can adjust the dampers to provide more airflow to the further rooms (by slightly closing the 14" damper), if needed. How long is the 14" flex run?

    Bookmark   October 28, 2012 at 9:38PM
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Yes, both dampers are wide open. The 14" flex run is about 15". I guess I need to wait for colder weather to evaluate the performance of the unit and to determine the changes needed. I had high hope that the new duct layout and the new unit with higher energy inefficiency and size would show obvious improvement. But it was not the case, the master bed room shows weaker air flow, may be its ducts are at the first two slots of the main supply box. Well, on the positive note, I'm having heat, just in time for the cold temperature this coming week.

    Bookmark   October 28, 2012 at 10:06PM
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If the 14" flex run is only 15" off the main plenum, then that's where a good portion of your air is going. Air follows the path of least resistance. Surely that damper can be closed somewhat to force more air to the further rooms. What's the point of having the balancing dampers if they're both left fully open? You have chosen great equipment -- now you just need to work with your contractor to get the system properly balanced. Best of luck.

    Bookmark   October 28, 2012 at 10:31PM
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