HVAC Advice

[3inSlugger]

The setup:
1. Have a 2001 house which had a poorly sized unit (2 ton for 1300sf)
2. Contractor A installs new HVAC upsized to 3 ton. Money was spent for a new condenser, heater, and compressor.
3. Front room does not get enough warm air to keep one-year old son warm during winter.
4. Wife calls Contractor B for an invoice and review of the system. ConB states returns were not upsized for the larger 3 ton unit and the unit is being choked out and therefore not generating enough pressure in supply plenum to pressure son's room (the longest run). ConB also states that he thinks a return plenum would help out a bunch too. ConA and original setup did not have return plenum.
5. I get ConA back out and he does not think that the returns are the problem. However, he does install a damper on the Master Bedroom supply ( it was often too hot) and an air scoop on the son's supply line at his cost. This seems to have helped a bit but not completely.

Now I'm a trained civil engineer. I understand fluid dynamics and friction losses. I understand capacity and air flow. What I don't understand/have time to look up myself is what your average 3-ton unit needs on the return side and the efficacy of a return plenum.

The facts:
3-ton unit
current 8" return from master bedroom
current 14" return from hall (where thermostat is located)

Con B wants to 8>12 and 14>16

HVAC Contractors and/or MechEs, what say you? Who is blowing hot air or can't tell?

 

[SPDguns]

I HATE word problems.......!

 

[Tanis143]

One other thing to look at is how your son's room is fed. Is it a small line directly from a main branch or is it on a secondary branch? Also does your son's room door stay open (a closed door with no clearance at the floor level will create a negative air pressure and prevent air from flowing in from the register)? Are there any thermal leaks around the windows/walls?

I'm no HVAC guy, but I've been dealing with the same issues with my 45 year old house that has floor vents that are more than likely blown out and I've asked those same questions. I was told my unit (2.5 ton) was getting plenty of inflow, it was my vent lines that were causing the problem (front half of the house, which is closest to the blower unit, stays warm/cool while the bedrooms and bathrooms get much less air and the MBR gets hardly any airflow at all).

 

[Glock 40]

Are the other issues your not considering on that cold room? North wall? 3 exposed walls including to garage or outside? Large windows? I had similar issue in a new construction house. Unit was large enough one room had a 7 degree difference from rest of house. It had 3 exposed walls, very large picture window and was on NW corner furthest room from unit. Ended up having to get a strait run to that room with a damper at system so I could adjust for winter and summer. Also had a damper added for the master that had 2 vents that was closest to the unit in attic.

 

[Buddhaman]

Return should be ~200sqin per ton. So 3 ton unit needs 600sqin of return. We upgraded to a 5 ton unit and had to increase the return sizes.

 

[TwoForFlinching]

Have you tried adjusting your dampeners to equalize flow through each run?

 

[3inSlugger]

One other thing to look at is how your son's room is fed. Is it a small line directly from a main branch or is it on a secondary branch? Also does your son's room door stay open (a closed door with no clearance at the floor level will create a negative air pressure and prevent air from flowing in from the register)? Are there any thermal leaks around the windows/walls?

I'm no HVAC guy, but I've been dealing with the same issues with my 45 year old house that has floor vents that are more than likely blown out and I've asked those same questions. I was told my unit (2.5 ton) was getting plenty of inflow, it was my vent lines that were causing the problem (front half of the house, which is closest to the blower unit, stays warm/cool while the bedrooms and bathrooms get much less air and the MBR gets hardly any airflow at all).

Closed door makes it worse but still happens with open door. We are working on getting a new front window as well.

EDIT: Two separate straight runs from the far end of the supply plenum

Are the other issues your not considering on that cold room? North wall? 3 exposed walls including to garage or outside? Large windows? I had similar issue in a new construction house. Unit was large enough one room had a 7 degree difference from rest of house. It had 3 exposed walls, very large picture window and was on NW corner furthest room from unit. Ended up having to get a strait run to that room with a damper at system so I could adjust for winter and summer. Also had a damper added for the master that had 2 vents that was closest to the unit in attic.

Check check check. Large N window, 3 exterior walls, but already has two straight runs (6 & 8) going to it. It is also the largest run.

Return should be ~200sqin per ton. So 3 ton unit needs 600sqin of return. We upgraded to a 5 ton unit and had to increase the return sizes.

Currently: 14x14 and 14x30 = 616 sqin so right there..

Have you tried adjusting your dampeners to equalize flow through each run?

Only dampener is on MBR supply. That HAS been helping.

 

[Glock 40]

Well sounds like you got the trifecta. Does the room have solid floors or carpet? Carpet would probably add some insulation if it doesn't have that currently. Also is house on slab or crawl space? If its crawl space you could be loosing a lot of heat from floor also. Make sure vents are closed currently. Do you have adequate insulation over the room in the attic? Don't normally get much sun if any on NW corners so it makes keeping temps regular a pain. It sucks my current house my office is on NW corner and its always cold in here. 2 exterior walls and 3rd is Garage. I finally said screw it and I got a space heater I run most of the time I am in here. I wouldn't suggest that for a room with a child obviously. Also on the window you can add some heavier curtains to try and keep from loosing heat that way.

 

[TwoForFlinching]

Only dampener is on MBR supply. That HAS been helping.

I'm no HVAC dude, but I'm in a similar situation. 1330sqft, 3ton r22 unit. When I bought the place, it would be hot in farthest runs, downright cold in the shortest. Built a new plenum, installed balance dampeners for each duct, balanced them, now it's 68 in every room. Added a second 24x16 return hole in the wall for my 80% closet unit.

 

[Dirty Dave]

Typically its 400 cfm per ton
3ton=1200 cfm
8" duct 200 cfm
14" duct will do 1000 cfm
You can oversize to reduce noise or overcome length or restrictions. Your 8"&14" should work considering your filters are clean... I like to have a RA in every bedroom for comfort with the door closed.. if you were short on return it would show up in the summer running the AC.. it would freeze up.. sounds like infiltration from windows and lack of insulation in the walls... hard for anyone to actually diagnose over the internet tho LOL