zoned hvac problems - compressor cycling on and off due to DATS
I recently finished a 400 sq ft third floor attic and had an existing 2.5 ton which controlled 2nd floor replaced with a 3.5 ton unit and honeywell zoning, with single stage furnace and blower. I have now had the chance to test it during the summer. the 2nd floor (around 1500 square feet) cools great, the 3rd floor is the problem, when cooling 3rd floor alone, after a couple minutes, the DATS flashes green and the blower stays on, but the compressor cycles on and off and repeats for less than every two minutes continuously until temperature is reached. I am sure I will have the same problem only reversed in the winter with heat with the DATS overheating.
After researching, I think I understand the issue, but I do not know what my options are. My unit is trying to push 3.5 ton of AC through 1 ton of duct work when only the 400 sq ft third floor is cooling, so the temperature drops low and quick causing the DATS to shut off the compressor until its temperature rises to safe range, then repeats. This all happens in less then 2 minutes to shutoff and restart and repeats, which has to be terrible for the unit and my electric bills.
What options do I have for this now? I thought zoning was meant for different rooms, but this cant be normal. The 3.5 is great when both floors are cooling, but it is obviously oversized when just 3rd floor is going. How is this delt with in zoning applications? I realize the DATS is keeping my coil from icing up, but how can I cool my 3rd floor without this problem? I spent $ on this whole setup, and have contacted the contractor and waiting for a response. Would retrofitting a variable speed blower help? Or possibly adjusting the dampers so some of the cool air goes to 2nd floor when 3rd floor is cooling? Please help with some suggestions, i want to have some ideas for when the contractor calls me back monday. Thanks for any help.
Last edited by beenthere; 07-27-2013 at 08:36 AM.
Unless the DATS is set too high, it's doing it job. That's the problem with throwing 3.5 tons into 400 sq ft. One solution might be to have the other dampers set so they don't close completely so their is more airflow through the system. You are right that this is a good idea. Variable speed blower might not do much. It can push more air into the little zone but those ducts and registers may start howling.
Though it's hindsight, that little area should have been a unit of its own somehow. Too bad they don't make conventional 1 ton A/Cs anymore. I used to put a lot of them in for rooms like that. But now minisplits have become more common & affordable.
400 sqft shouldn't have added 1 ton of additonal load. 0.5 tons at most and that's assuming the existing system wasn't already oversized.
With zoning you should in most cases have 2 stage equipment when the smallest zone cannot handle at least about 70% of the airflow. They should have put in a 3 ton 2 stage unit. It would likely resolve your issue.
Only cheap solution now is as mentioned above, you'll have to adjust the min position on hte other dampers to bleed off some airflow, or add a dump zone somewhere that you don't mind being overconditioned like a hallway.
That 400 sq ft didn't add a ton of load. It even reduced the load of the second floor.
May need to slave the addition to the second floor.
your installer does not have a clue about installing a zoned system and i would have hime come out and make it right ,a 3 ton 2 stage system may have been a better choice
Hi everyone, thanks for the advice. I am going to fight to have this taken care of properly. Worst case scenario, i will have to live with it and adjust the dampers to release some cool air into second floor upon third floor demand.
Originally Posted by catmanacman
It sounds like my best case scenario would be a 2 stage replacement? I can not have a separate unit up there, a minisplit will not even work, so zoning is my only option. My questions on this are - Are 2 stages just for the heat, ive heard of a 2 stage furnace, but would I need a 2 stage AC Compressor as well as a 2 stage furnace?
Obviously the contractor is going to try to take the cheapest way out, but I am going to fight for what I can. Can HVAC units be returned, or once installed is it a done deal? This is a fairly expensive American Standard setup, I cant imagine the contractor taking that kind of a hit without going to court or something.
Not much of a market for used units. So doubt he'll even consider swapping out the furnace and A/C. Without you paying a lot.
He might be able to add a dump zone to the second floor.
Model numbers of equipment and zone panel?
Sent from my HTC VLE_U using Tapatalk 2
American Standard Allegiance 13 R410A Condensing Unit 4TTB3042D1000B
Originally Posted by BaldLoonie
American Standard Coil 4TXCB048BC3
American Standard Freedom 80 1 Stage Gas Furnace AUD1D000A9401A
Honeywell TrueZone HZ311 zoning
Rarely does anything good come from zoning single stage equipment, especially when one of the zones is relatively small.
Is there a bypass damper? Zoning system will control DAT and compressor cycling is a result. A bypass damper will at least provide proper air flow across the dx coil and static pressure control. Increasing static a little may get more 55° air to the zone and satisfy it quicker. VVT is common commercially but there always seems to be trouble zones, areas that can never be satisfied like foyers and small IT rooms. You may want to get more air to that zone.
I didn't write the book I just read it!
Yes, there is a bypass damper, perhaps i could cheat a little with the adjustment. It seems to me that pushing a little more air to the room might help, its currently not coming out that strong at the registers. Because ,at this point, during 3rd floor only cooling, the majority of the cold air is bypassed and going right back through the system and then getting supercooled and dropping the temp at the DATS quickly. perhaps that, along with bleeding a portion of air through the 2nd floor would help enough to improve the situation. I realize the setup is far from ideal at this point, but max budget was spent on this so i have to look at cheap options. I wouldnt even mind as much if the DATS tripped after 5 minutes or so because then it wouldnt happen as often because temp would be reached before it happened, but every 2 minutes on and off is ridiculous.
Also, just additional info, for the 400 square feet, I have 4 ducts run, although i do not believe it is oversized duct work. I guess also i dont want to push too much air too quickly because then it is not dehumidified and i will have moisture problems, is that correct?
Bypass damper throwing 40 degree air in the return won't help keep the DATS happy.
You may have more problems in the winter. The 311 has a fixed high limit way above what today's furnaces can handle. So when the 3rd floor is only zone calling, the furnace will cycle on its own limit which isn't meant to frequently open. If you wake up cold one morning, it's because the limit stuck open from opening too often. We had a customer that bought a house where a 311 was put in. I told her to get on that contractor for a 321 or 432 where the limit could be adjusted so the DATS shut off the furnace before it overheated.