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Hermit Crab
Gonzaga Fan
Under the Sea
Member since Nov 2008
6069 posts

HVAC - adding second unit
Got a feasibility question for you folks. I have a 2500ish SF house with one 4 ton unit (single level house). The way the house is laid out keeps the bedrooms and hallway pretty cool, but it has a hard time getting the kitchen/living room area on the west side of the house cool on 100 degree days.

Was thinking about adding a second unit for the west side of the house and letting the existing unit just cool the east side. Our existing unit is pretty old, and figure I will have to replace it within the next couple of years so this might be a way to split that cost up while also giving that unit some relief until it does bite the dust.

Would the existing unit be overkill for half of the house and cause problems? Would this plan be worth it at all?


bbvdd
Alabama Fan
Memphis, TN
Member since Jun 2009
17196 posts

re: HVAC - adding second unit
how tall are your ceilings?


Hermit Crab
Gonzaga Fan
Under the Sea
Member since Nov 2008
6069 posts

re: HVAC - adding second unit
8 foot in bedrooms, 10-12 in living room


bbvdd
Alabama Fan
Memphis, TN
Member since Jun 2009
17196 posts

re: HVAC - adding second unit
4 tons is too small for a 2500 sq ft house in LA.

The problem you would run into by splitting it having too much for your square footage.

sounds like you need a 5 ton unit or a 2 and a 3


CrawDude
LSU Fan
Baton Rouge
Member since Apr 2019
698 posts

re: HVAC - adding second unit
I have the same amount of conditioned living area and my HVAC is also 4 ton, and 14 years old. But I don’t have the same problem you are describing. I’d have a professional look at you your ducts - are they sized right for the warm part of the house, perhaps you have some duct leakage, perhaps the air ducts need to be balanced so that you are getting more air to the warm part of the house. Professionals can check all that and make corrections, if needed or warranted, cheaper than a second unit.

Honestly if your HVAC is properly sized it is not designed to keep up with 100 degree days, and the western exposure certainly can exacerbate the issue on that side of the house. For Baton Rouge, I believe the max heat design criteria for the HVAC is 92 F. When temps are higher than that for extended days, the system will run non stop during the day and may not keep up with the thermostat setting but that’s normal, and does not indicate a problem.

That said, you can can certainly consider a second unit, of course there are pros and cons to both. When my house was built I in 93, I asked about this with the builder and his HVAC sub contractor and the HVAC sub told me 2,500 sq ft conditioned space was right on margin where they typically installed a single system, much larger than 2,500 sq ft and they usually designed for a second system. Anyway, I was given the option of installing 2 units, at higher cost, but stayed with the single unit and it has been fine.

If you do go down the path of 2 units, make sure you have Manual J, Manual D, and Manual S conducted on your home. These are used to determine heat loss and gain, HVAC sizing, and duct sizing. You don’t want a HVAC contractor that uses “rules of thumb” for sizing.
This post was edited on 6/11 at 11:00 am


bbvdd
Alabama Fan
Memphis, TN
Member since Jun 2009
17196 posts

re: HVAC - adding second unit
Considering he has 10-12' ceiling in the living area, 4 tons is too small.

OP where is your thermostat?



Clint Torres
New Orleans Pelicans Fan
Member since Oct 2011
1704 posts

re: HVAC - adding second unit
quote:

Considering he has 10-12' ceiling in the living area, 4 tons is too small.


Impossible to know without a Manual J.


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Hermit Crab
Gonzaga Fan
Under the Sea
Member since Nov 2008
6069 posts

re: HVAC - adding second unit
Thermostat is in the hall which stays cool, so that’s obviously keeping the unit from running until it gets living room and kitchen cool, but if it were moved it would freeze those rooms. Previous homeowner added a second return in the kitchen to pull some of the hot air out. But it’s pretty small


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CrawDude
LSU Fan
Baton Rouge
Member since Apr 2019
698 posts

re: HVAC - adding second unit
quote:

Considering he has 10-12' ceiling in the living area, 4 tons is too small. OP where is your thermostat?


Not saying you are wrong but that is the purpose of the manual J to spec out the size accurately. Depends on the building envelop - is his house tight, well insulated, leaky, etc My ceilings are comparable to the OP -9 to 14 ft high, and prob average about 10 to 11. I’ve conducted an online Manual J (loadcalc) and the Manual J Calc indicates a 3 1/2 ton unit for my house - I have 4 ton - 2,500 sq ft of conditioned space. I think I have a fairly tight house but I plan to have a blower door test and and room by room manual J conducted to identify problem leakage areas, size a new HVAC that I’ll likely replace in the next year or 2. Also a manual D to check my rigid duct work for leaks, etc.

By no means am I discouraging the OP in considering a 2 unit system as he plans for replacement of his old system, at times I wish I had 2 units, for those pesky times when a HVAC goes down, but I’ve been lucky and had few down times of my HVAC system over the years.


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baldona
Auburn Fan
Florida
Member since Feb 2016
10057 posts

re: HVAC - adding second unit
You should look into a mini split OP. Would probably be the best.

But a couple things, this isn't going to save you money most likely. One larger unit is usually cheaper then 2 smaller units.

Also, you never mentioned duct work. You can't just add a regular unit. You have to add duct work or tie it in which is not likely possible. So that's where a mini split comes in.

It also sounds like its possible you simply add some smaller ducting or larger ducting to your problem areas and do a couple of small changes, and you may be good to go.


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keakar
Member since Jan 2017
10744 posts

re: HVAC - adding second unit
2500 should be a 5 ton unit, i like to have slightly more then just whats required so i go 500sq ft per ton.

have them replace it with 2 units, one unit for the bedrooms and the other unit serves the living spaces, kitchen, living room, den, office or formal dining room.

then in the future if one fails the other half of the house still has a/c. i have had many people thank me for such advice. its better to sleep on the sofa in a/c when bedroom unit fails, or vise versa, then to have the whole house without a/c
This post was edited on 6/10 at 6:25 pm


poochie
LSU Fan
Houma, la
Member since Apr 2007
4262 posts

re: HVAC - adding second unit
This thread is useless without calculations.


fishfighter
LSU Fan
Baton Rouge
Member since Apr 2008
33752 posts

re: HVAC - adding second unit
quote:

4 tons is too small for a 2500 sq ft house in LA.


No it's not. Got about 2700 and I heat/cool with a 4 ton unit with no problems.

OP, start looking for air leaks in your duct work and have your coil clean.


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uway
LSU Fan
Member since Sep 2004
30662 posts

re: HVAC - adding second unit
I also have ~2,500 sq ft, but have a 5 ton heat pump system that leaves the master suite (remodeled/reconfigured older home) poorly conditioned, cold in the winter and warm in the summer.
I am certain that we have duct issues, in quality, routing, size, etc.

A few questions
1. Are new heat pump units better, as in plenty good enough, at keeping the house warm when temps drop below 35 for an extended period of time? I’ve read they use a supplemental heater during those times, so I guess replacing the old outside unit means also replacing the inside unit. Is that right?
A related question is whether you think it would make sense to switch to a furnace and separate AC unit.

2. Since I really seem to only need additional heating and cooling in 2 rooms plus a closet, this seems like the ideal time for a mini split for that area. Do you agree? If so, what do you think of the pre-charged units they claim can be self-installed?

3. I’m thinking the best way to go about this is to first have a pro come and re-do my dilapidated ducting and then observe how the 5-ton single unit does with good ducts. Then if that doesn’t get it done, get a mini-split and replace the heat pump with a smaller heat pump when the old one dies. Make sense?

Thanks for reading this novella.
This post was edited on 6/11 at 4:07 pm


fishfighter
LSU Fan
Baton Rouge
Member since Apr 2008
33752 posts

re: HVAC - adding second unit
Were you live? I looked at heat pump units for my guest house. Cost was only around $200 more. Still, your air handler unit, if it's electric, one just adds healing elements. Gas furnace is different.

My take on a heat pump. Just doesn't have enough for heating and why would I be wanting to run the compressor year round.


uway
LSU Fan
Member since Sep 2004
30662 posts

re: HVAC - adding second unit
Sout Louisiana

I’ve heard good things about newer heat pumps.

A furnace for me would run off my propane tank which is really appealing, but when I got a quote to replace everything and go with furnace it was a nice chunk of change.


fishfighter
LSU Fan
Baton Rouge
Member since Apr 2008
33752 posts

re: HVAC - adding second unit
quote:

I’ve read they use a supplemental heater during those times, so I guess replacing the old outside unit means also replacing the inside unit. Is that right?


If changing the outside unit, I would up grade the air handler. How old is your system? Is it 410A Freon? New air handlers can boost a condenser outside unit seer out put. In fact the system I bought for my guest house is 16 SEER, but with the air handler I bought, that boosted the SEER up to 17.5 seer. Yep, cost a couple hundred more, but should have a pay back in the first year in service.


uway
LSU Fan
Member since Sep 2004
30662 posts

re: HVAC - adding second unit
quote:

? Is it 410A Freon


Don’t know but I think it’s 25 years old. It’s living on a prayer.

quote:

New air handlers can boost a condenser outside unit seer out put. In fact the system I bought for my guest house is 16 SEER, but with the air handler I bought, that boosted the SEER up to 17.5 seer. Yep, cost a couple hundred more, but should have a pay back in the first year in service



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fishfighter
LSU Fan
Baton Rouge
Member since Apr 2008
33752 posts

re: HVAC - adding second unit
quote:

A furnace for me would run off my propane tank which is really appealing, but when I got a quote to replace everything and go with furnace it was a nice chunk of change.


Nothing to running heat strips. My house is all electric. A 4 ton air handler with 15K-20K heat strips cost around $2200, not installed.


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CrawDude
LSU Fan
Baton Rouge
Member since Apr 2019
698 posts

re: HVAC - adding second unit
quote:

I’m thinking the best way to go about this is to first have a pro come and re-do my dilapidated ducting and then observe how the 5-ton single unit does with good ducts. Then if that doesn’t get it done, get a mini-split and replace the heat pump with a smaller heat pump when the old one dies. Make sense?


I’m far from being a HVAC expert, but I have tried to educate myself on the subject over the past year, from what gather experienced HVAC techs state that the most prevalent design flaw they encounter is oversized HVAC units and undersized ducts. My way of saying yes, have your ducts and air handler checked for leakage, air balance, etc. Last week we had a poster having a problem similar to what you described, and another person on this board board suggested he check for air leaks in his air handler and ducts, he found a major air leak in his air handler that he repaired himself and he stated he had immediate improvement in cooling.
This post was edited on 6/12 at 10:58 am


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