View Full Version : HVAC replacement and duct work question
10-18-2006, 04:06 PM
Hi. I've been reading all over this site trying to get some questions answered, so I apologize if you've answered these kinds of questions a zillion times and I missed them.
Basically, I have a 1720 sq. ft, 2 story house that has almost 100% sun exposure on the south side, 0% on the north side. There are six people and two cats who live here. I'm replacing a gas/electric, heating/cooling outdoor unit that exploded the other night.
I've had two guys out. One recommends a 3.5 ton system and one recommends a 3 ton system. I'm looking at American Standard YCY and YCZ models. I currently have a 3 ton system that was installed in 1991. It seemed to do fine with the heating and cooling before exploding. But, the upstairs was always warmer in the summer.
I also had one of the guys say I needed to increase my return duct size. I have two currently. Both are 10". He recommended changing the downstairs one (too much trouble to do the upstairs one) to a 14" return.
I've been reading a lot about Manual J and Manual D and am quite overwhelmed by all of it.
If either one did any type of calculation, they certainly didn't share it with me.
I guess I just want to know if any of this sounds outrageous, right on the mark, or if it's just going to be one of those, "You'll just have to do the best you can" types of things.
Thanks for your time.
10-18-2006, 05:10 PM
Going bigger alone won't help the upstairs, if anything it will make it worse. Shorter run times cools down but not up.
2 10" runs probably good for 2 tons of airflow. Fixing that will make a world of difference! Increasing down to 14" will help overall.
10-18-2006, 06:48 PM
Thanks. I really appreciate your quick response. I certainly feel better about my options now.
The upstairs may be lack of return or supply air.
If the doors are left open upstairs 24/7 ,and there's still a temp. difference ,it's lack of supply air,in the cooling mode.
If doors are normally closed,with the fan running ,crack the door,and hold a tissue there,if the tissue lows out the crack,it needs more return in the room.
10-19-2006, 09:58 AM
The doors upstairs are typically open. If I increase the return duct size downstairs to increase the return air, won't that then increase the supply air? I have two 12" supply lines (one for down, one for up) that run to a box that then separates into the separate 6" supply lines for the individual rooms. Does that sound like large enough supply ducts to push the 3 ton system?
Also, the guy wants to make the grill for the 14" duct a 25x20 filter size. That seems huge to me. Does that seem reasonable?
Thanks again. This site is great. HVACs are a major investment and it's nice to have a place to ask all these questions.
Increasing the return duct and grill size ,with a standard PSC indoor fan motor,will increase the supply side air flow,can't tell how much from here.So it may help the upstairs in coolig and it may not.
Fiolter grilles can't be too large,so other then looks ,it a good idea.
I'd prefer that they looked the entire system,Manual J,S,and D,to get the right size system,and ducts,for all of it.
10-19-2006, 02:15 PM
Did anyone suggested to zone the house? It is clear the "upstairs" needs a separate zone. BTW, do not get overwhelmed by Manual J or Manual D. Start with manual D first and then go to J if you ask me.
Same think happened to me and now I want to certify myself as an HVAC pro.
10-19-2006, 02:53 PM
Thanks again for the replies.
Doing a separate unit for upstairs is more a matter of "I don't have that kind of cash" than the guy not recommending it. The majority of our square footage is downstairs, so keeping it with one unit doesn't feel like a bad decision. Especially since it's done a decent job with the upstairs with a pitiful, old unit and the new increased air flow should help even more.
I think I'll talk to the guy about increasing the grill size on the upstairs return vent even if we can't increase the duct size.
And I think I'll have to leave the Manual calculations to the pros. I'll stick to doing taxes and bookkeeping. :)
10-19-2006, 03:02 PM
The zoned room upstairs will be feeded by the same equipment. Basically, one equipment can have multiple zones. Got it?
10-19-2006, 05:54 PM
Don't let the guy talk you into increasing your grill w/o increasing duct size. Increased grill means nothing. You will not get any more air than the maximum CFM for that duct size (10" round = 400-420 cfm). Right now you are getting a maximum of about 1400 cfm(10" & 14"). Okay for a 2.5 - iffy for 3.0 ton. Not near enuff for 3.5 tons. Just some info to throw at your installers.
10-19-2006, 07:10 PM
Thanks, Skeeter_22. I'll keep the upstairs grill alone.
I have maxed out my expense on this job with the unit and increased downstairs return duct. I think I'll give the new system a chance to work before I look into increasing the upstairs return vent. I guess switching to a zone system at that point would make sense. I did read about them and realize I was mistaken on how they work. If it's not working the way it should in a year or two, I may have the cash to put in to a new duct and a zone system, but that will have to wait.
I will definitely throw out those numbers for the installers, if for no other reason than to let them know I'm interested in the information. I always appreciate communication with people who are working on anything I own.
if you increase the filter grille size upstairs ,it will increase air flow ,period.The larger it is the PD(pressure drop),this reduction also the fan to use the difference in energy it was using to overcome the PD ,to move more air.
Now it may not be enough to solve you problems,and the additional air may not go to the rooms you'd like it too,but there will be more air moig thru the equipment.
What size grille do you have now??
10-20-2006, 10:31 AM
The upstairs grill is 10x20 with the 10" duct. I had thought, "Well, I'll be taking out the 14x20 grill from downstairs... I could just put it upstairs if it helps the airflow." I don't have much more room than that since the upstairs return duct is in the ceiling in the hallway.
BTW, I change my filters monthly and they are completely clogged. Does that seem like a unit/duct problem, or is my house just that pathetic? It is only 15 yrs old and I keep it relatively clean. I have been grossed out by the amount of crap in my filters.
Filters are in the grilles corrects?
They are currently undersized,IMO.
Larger filters more surface area,less PD,more air flow,and they won't get clogged near as fast.
10-20-2006, 01:45 PM
Yes. The filters are in the grills. I'll have to look at how tough it will be to increase the return size upstairs. I'll save the parts that come out from downstairs and see if I can use them for an increase.
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