HVAC Question for the Pros

[NotByWorks]

Need to replace a 31 year old AC that finally died. House is 100 years old, not too tight, and very close to Galveston Bay so humidity control is essential.

The existing unit is an oddball design which has a 3.5 ton condenser paired with a 2.5 ton evaporator. There was a guy here in Houston 30 years ago touting his design as superior in removing the suffocating Houston humidity. To his credit, this system did keep the upstairs comfortable. I suspect this system today would not meet government energy standards even if it was a good design.

So what is the cooling rating of the current system? Is it really 3.5 ton based on the condenser, or is it closer to 2.5 ton limited by the evaporator?

I have a bid for new unit with a 3.5 ton condenser and 4 ton evaporator because his vendor doesn’t make a 3.5 ton. My concern is this system may end up oversized and not remove enough humidity. Would a 3.5 ton condenser with a 3 ton evaporator work?

Thoughts from the Hide?

 

[Milf Dots]

If you can afford it, maximize the insulation and seal everything around openings. Optimize your ductwork and vents, too.

 

[Short-bus]

Wonder if mini split systems might be more economical and efficient for you. Worth a looksie, but it depends on the size and layout of your house. If it's a sizeable home, with a lot of rooms, mini splits might not be for you. I bought a mini split for my 700sq ft shop that was suppose to be for a 1500sq ft home. It's an inverter design, so it throttles up and down as necessary to be as efficient as possible. Works for me.

Branden

 

[ironman56]

My 1500 sq house had a 2.5 ton unit that finally quit last year , the contractor that quoted me 8k to install a 3ton unit said the 3 ton would fit the inclosure ,it would not , I ended up installing a 2.5 ton Goodman complete system myself for 3000. And couldn't be happier with it , but I'm perrty handy , have all my own tools and a can braze copper as well as most HVAC guys , I'm in Central Texas so the humidity not as bad as the coast but it ain't Arazona either, like Milf dots said invest in insulation and seal all opening that will help alot and match the equip to the sq ft of the house , I have no experience with mini splits , but on existing construction they kinda look ugly ,at least the ones I've seen , on new construction where u can hide the plumbing not so much.

 

[longshooter]

Evaporator.

And, with a larger system, you might have to add ductwork to gain air handler CFMs.

 

[Kwtaylor]

The 3.5 ton condenser will over work itself trying to push through a 2.5 ton evaporator and then freeze up the A coil. There would need to be massive air flow to prevent it.

The 2.5 ton condenser/3.5 ton evaporator makes for more A coil surface area to dehumidify. Is there any re heat in the system?

 

[NotByWorks]

The 3.5 ton condenser with 2.5 evaporator were in service for 31 years. Kept the house comfortable and never froze that I was aware. Central City AC was behind this design and pushed against efficiency measures that ignored humidity.

From all I have read, an oversized evaporator reduces compressor head and efficiently chills the air, BUT it does not cool the air enough to condense the water. In the extreme you get the cold wet towel effect.

What I am trying to figure out is the effective cooling tons of the original 3.5 ton condenser/ 2.5 ton evaporator.

To me the 2.5 ton evaporator would be the limiting factor.

I am trying to decide whether to get a new 3 ton system or a 3.5 ton. My gut tells me the 3 ton would work just fine but the AC contractors say to go with the 3.5 ton system. The risk of going too small is it might not cool on the very hottest days. Risk of going too big is less humidity control.

I know just enough about this to get into trouble.

As for mini splits, they are great in the right situation, but not so much on this application. I have one in a cabin and it works great.

 

[Stevo86]

The 3.5 ton condenser with 2.5 evaporator were in service for 31 years. Kept the house comfortable and never froze that I was aware. Central City AC was behind this design and pushed against efficiency measures that ignored humidity.

From all I have read, an oversized evaporator reduces compressor head and efficiently chills the air, BUT it does not cool the air enough to condense the water. In the extreme you get the cold wet towel effect.

What I am trying to figure out is the effective cooling tons of the original 3.5 ton condenser/ 2.5 ton evaporator.

To me the 2.5 ton evaporator would be the limiting factor.

I am trying to decide whether to get a new 3 ton system or a 3.5 ton. My gut tells me the 3 ton would work just fine but the AC contractors say to go with the 3.5 ton system. The risk of going too small is it might not cool on the very hottest days. Risk of going too big is less humidity control.

I know just enough about this to get into trouble.

As for mini splits, they are great in the right situation, but not so much on this application. I have one in a cabin and it works great.

He did that to drop your saturated suction temp, lower coil temp = colder coil= more moisture removal….not a terrible idea… but not a great one…honestly surprised it didn’t die years ago from the oil being washed out of the compressor. I’ve seen it done on larger tonnage equipment and it worked fine… but that was with digital unloading scrolls compressors that monitor saturated suction temps and would unload when it got to that set point.

I know on residential carrier infinity offers humidity control and as someone mentioned above about reheat, you can use the electric heat strips as reheat to remove moisture. Also Lennox offers their humitrol system as an add on but you would have to re pipe the system for hot gas bypass. Never seen it done on residential personally but maybe someone else can offer up some experience there.

A whole home dehumidifier along with new system might not be a bad option. Just my 2 cents worth, there’s always more than one way to skin a cat.
edit: I don’t see the guys post who mentioned reheat… I laughed at it, but meant to like. Come back to the conversation. Your on the right track

 

[Milf Dots]

If it were me I'd do a 3.5 ton system with a dehumidifier option.

 

[DarnYankeeUSMC]

Need to replace a 31 year old AC that finally died. House is 100 years old, not too tight, and very close to Galveston Bay so humidity control is essential.

The existing unit is an oddball design which has a 3.5 ton condenser paired with a 2.5 ton evaporator. There was a guy here in Houston 30 years ago touting his design as superior in removing the suffocating Houston humidity. To his credit, this system did keep the upstairs comfortable. I suspect this system today would not meet government energy standards even if it was a good design.

So what is the cooling rating of the current system? Is it really 3.5 ton based on the condenser, or is it closer to 2.5 ton limited by the evaporator?

I have a bid for new unit with a 3.5 ton condenser and 4 ton evaporator because his vendor doesn’t make a 3.5 ton. My concern is this system may end up oversized and not remove enough humidity. Would a 3.5 ton condenser with a 3 ton evaporator work?

Thoughts from the Hide?

Read this following post again.

He did that to drop your saturated suction temp, lower coil temp = colder coil= more moisture removal….not a terrible idea… but not a great one…honestly surprised it didn’t die years ago from the oil being washed out of the compressor. I’ve seen it done on larger tonnage equipment and it worked fine… but that was with digital unloading scrolls compressors that monitor saturated suction temps and would unload when it got to that set point.

I know on residential carrier infinity offers humidity control and as someone mentioned above about reheat, you can use the electric heat strips as reheat to remove moisture. Also Lennox offers their humitrol system as an add on but you would have to re pipe the system for hot gas bypass. Never seen it done on residential personally but maybe someone else can offer up some experience there.

A whole home dehumidifier along with new system might not be a bad option. Just my 2 cents worth, there’s always more than one way to skin a cat.
edit: I don’t see the guys post who mentioned reheat… I laughed at it, but meant to like. Come back to the conversation. Your on the right track

This ^^^^^^^ has your answer. It won't come cheap but it's better to cry once than sweat your balls off and end up paying to have it done right.
Also: you are not starting from an ideal situation with the lack of insulation and whoever you choose to do the system needs to be informed. You need to get that taken care of before over sizing the replacement system. Not doing so will bring your worst nightmare into your home. Black Mold!!
Sizing the system as a bandaid for the insulation issue is a very bad idea

 

[kthomas]

Wonder if mini split systems might be more economical and efficient for you. Worth a looksie, but it depends on the size and layout of your house. If it's a sizeable home, with a lot of rooms, mini splits might not be for you. I bought a mini split for my 700sq ft shop that was suppose to be for a 1500sq ft home. It's an inverter design, so it throttles up and down as necessary to be as efficient as possible. Works for me.

Branden

And if you go the mini split route, buy the equipment and install yourself, or have a handiman do it.

Commercial mini-splits are ~$8k+ in my area. The DIY route will save you about 70-80%, so a huge chunk of $.

 

[NotByWorks]

Stevo, DamnYankee,

Thanks for the thorough analysis. I agree with everything you guys said.

As for the old house, I have added just about all the insulation I can without dismantling the house. Just another one of the joys of owning an old house.

I am curious what you would guess the effective cooling tonnage was for the 3.5 condenser and 2.5 evaporator? Seems to me it would be closer to 2.5 than 3.5.

 

[NotByWorks]

And if you go the mini split route, buy the equipment and install yourself, or have a handiman do it.

Commercial mini-splits are ~$8k+ in my area. The DIY route will save you about 70-80%, so a huge chunk of $.

This isn't a good fit for the mini split. I did installed one in a cabin and it works well.

 

[Stevo86]

Stevo, DamnYankee,

Thanks for the thorough analysis. I agree with everything you guys said.

As for the old house, I have added just about all the insulation I can without dismantling the house. Just another one of the joys of owning an old house.

I am curious what you would guess the effective cooling tonnage was for the 3.5 condenser and 2.5 evaporator? Seems to me it would be closer to 2.5 than 3.5.

I’ll plug some numbers tomorrow and see what it comes up with, it’s really not that uncommon for a coil to be slightly smaller than the advertised tonnage, I know I’ve seen it several times with lennox stuff on schools, first time I saw it on their submittals i called my rep and questioned it, they sent me the AHRI matchup sheets, they’ve been running 15 years now with no problems Or if u wanna send me a drawing of the house I’ll run a heat load on it for u. Won’t take much time. I will say I don’t have much experience with coastal stuff as I’m in north Tx. Humidity is still a bitch here tho.

 

[NotByWorks]

I’ll plug some numbers tomorrow and see what it comes up with, it’s really not that uncommon for a coil to be slightly smaller than the advertised tonnage, I know I’ve seen it several times with lennox stuff on schools, first time I saw it on their submittals i called my rep and questioned it, they sent me the AHRI matchup sheets, they’ve been running 15 years now with no problems Or if u wanna send me a drawing of the house I’ll run a heat load on it for u. Won’t take much time. I will say I don’t have much experience with coastal stuff as I’m in north Tx. Humidity is still a bitch here tho.

Thanks for the offer. I'll PM you tomorrow.