AC Replacement Options

akaDigger

I currently have installed in my home an AC system that features a 3 ton, 12 SEER condensing unit, a 3 ton fan/coil unit w/ piston metering device. No heat (radiant floors). The duct system is engineered and balanced. AHU and ductwork are in an unconditioned attic in southern Indiana. We can experience dew points of 75°F plus for weeks at a time. Normal would be low 70s. I installed the system in 1999. It has run and cooled my home faithfully with no problems other than one rodent gnawing incident and one replaced contactor in these 19 years.

I am set to retire this equipment and replace with new. The current system has always performed as an oversized system would. We went with rule of thumb selecting it. Two things I never liked about this system were the fan speed, I'm guessing around 1300 CFM, and lack of dehumidification. I'm downsizing and going with multiple fan speeds and a TXV.

Will simply applying these measures get me to where I want to be?

EBEBRATT-Ed

Yes. Size the unit correctly and don't oversize. If the ductwork is now 3 ton and you downsize you will get quieter operation and better dehumidification.

Standards for ductwork have changed for the better in 19 years. It's probably a good time to check to see how well the ductwork is sealed and the quality and amount of insulation on the ductwork

GW

Carrier and Bryant recommend 350 CFM per ton. You could do a two stage 3 ton, but I’m just guessing too.

Small recommendation, don’t say condensing, say condenser. I normally just say ac unit.

GW

akaDigger

I have a lot of confidence in the ductwork integrity. The equipment I selected is a 2.5 ton, 14 SEER single stage outdoor unit paired with a 3 ton variable speed ECM unit w/ TXV indoors, for 16 SEER and a utility rebate. I plan to use a thermostat with humidity control to slow the fan in dehumidification mode. Not sure yet what the base fan setting will be but hope it's no higher than 1050 CFM. It will be what ever it has to be though.

GW

Yes a 2 1/2 ton will be at 1000 or so.

Big Ed_4

I would say your on the right track .. I would do an heat load calculation and see how close you are , then take it from there... Rule of thumb 3ton = 1200 sf home ... I would assume no skylights , normal windows with trees ? How close did I come ?

akaDigger

I don't really want to open a can of worms here but... I did a cooling load analysis way back before we selected the current unit. A cooling contractor friend of mine said my home needed 4 tons. He based that on 1600 sq ft. My calculations, following a formula I found in The Journal of Light Construction, came out closer to two tons. I compromised, I thought I was correct but deferred to experience, a bit. I was assured that I was making a big mistake.

My 1952 built home was taken back to bare bones, expanded and redone in 1999. I did every bit of the work myself except finishing drywall and laying the new brick. It's heavily insulated with fiberglass (trimmed to fit perfectly). R40 attic, R 15 exterior walls, even R30 floors (due to radiant). Only 16 sq ft of glass facing west. I did another free analysis the other day here. Just over 24,000 BTU. I was tempted to go with 2 ton but chickened out.

The current system, when installed, had a cheap Honeywell thermostat and would cycle the equipment constantly. With the input from another friend, an HVAC tech, we changed t-stats and lengthened the on times for better dehumidification. That helped a bunch but the longer run times had you wishing the damn thing would shut off toward the end of a cycle as you were freezing. Uncomfortable temperature swings. The system would cycle even on the very hottest days. We get temps in the 100s here.

So, I'm compromising again. Upsizing from my cooling load analysis but downsizing from an oversized system.

I should mention that I have 40 years working in the pipe trades. Steamfitter, plumber mostly. I have installed many industrial and commercial sized cooling jobs. Five ton up to 16,000 ton. I have many friends in the HVAC service sector as we are all in the same union and collaborate on the same projects. My experience with DX cooling is almost exclusively with installing. We always had HVAC service techs to start and service equipment. I'm recently retired but still get all my discounts at the local supply houses.

SuperJ

akaDigger said:

I have a lot of confidence in the ductwork integrity. The equipment I selected is a 2.5 ton, 14 SEER single stage outdoor unit paired with a 3 ton variable speed ECM unit w/ TXV insidoors, for 16 SEER and a utility rebate. I plan to use a thermostat with humidity control to slow the fan in dehumidification mode. Not sure yet what the base fan setting will be but hope it's no higher than 1050 CFM. It will be what ever it has to be though.

I started to type that you should use a dehum capable stat and ahu to slow the fan, but you already figured that out. A bunch of stats have dehum droop control to run the cooling a little longer if it's humid.
The good ECM air handlers are pretty configurable for fan speeds, I don't think you'll have a problem with the base fan speed. If you do, you could always use the recirc or heating settings to go lower instead.

Might have to do a bit rebalancing since your taking a bunch of CFM out of the system.

If it were me I'd go with the 2ton system. It's not like they stop cooling the second your load goes over 2tons. You can manage your peak heat load with blinds/curtains, and it only lasts for a couple hours then you'll be glad for the longer run times.

I'm most uncomfortable laying bed on nights were there just isn't enough load to run the AC long enough. On a hot afternoon I don't really care if my house is a degree or two above the cooling setpoint, as long as the humidity is handled.

ratio

If you have an air handler, you should be able to get heat strips for it, which will let you do true dehumidification with it, i.e. reheat the air so the temperature doesn't crater. The Honeywell 8321U stat (my favorite) will do it, as well the Carrier Edge Pro (hate 'em), I'm sure many others as well. Too bad no one that I'm aware of makes a residential hot gas reheat setup.

If you do go with the electric heat strips, consider carefully the sizing so that you don't inadvertantly end up heating/cooling when in dehum mode.

akaDigger

SuperJ said:

akaDigger said:

I have a lot of confidence in the ductwork integrity. The equipment I selected is a 2.5 ton, 14 SEER single stage outdoor unit paired with a 3 ton variable speed ECM unit w/ TXV insidoors, for 16 SEER and a utility rebate. I plan to use a thermostat with humidity control to slow the fan in dehumidification mode. Not sure yet what the base fan setting will be but hope it's no higher than 1050 CFM. It will be what ever it has to be though.

If it were me I'd go with the 2ton system. It's not like they stop cooling the second your load goes over 2tons. You can manage your peak heat load with blinds/curtains, and it only lasts for a couple hours then you'll be glad for the longer run times.

I'm most uncomfortable laying bed on nights were there just isn't enough load to run the AC long enough. On a hot afternoon I don't really care if my house is a degree or two above the cooling setpoint, as long as the humidity is handled.

I was actually fishing for for someone to advocate for less tonnage. And I agree with you on night time cooling. Outdoor humidity rises to near 100% in the wee hours of the morning here. The AC doesn't run much. The Honeywell Vision Pro 8000 will run the unit up to 3° below set temp to control that humidity gain but I'm not sure I really want to feel cooler temps.

The variable EMC units I am looking at don't come in half ton sizes. It's 2 ton or three.

SuperJ

To me the biggest downside to marginally sized AC is the slow recovery if you shut it off, or let your house fill with hot humid air. If you like to leave your windows open until your house is uncomfortable then your going to need some tonnage to recover in short order, but if you keep things closed up and dry, with reasonable unoccupied setbacks, recovery is not a problem.

akaDigger

ratio said:

If you do go with the electric heat strips, consider carefully the sizing so that you don't inadvertantly end up heating/cooling when in dehum mode.

I considered reheating. We did it all the time in large, chilled water applications. It just seems like wasted energy though. There is probably data to rebut this line of thought.

I am installing a 5kw heat strip in the unit. The current unit has one too. I heated the house with it during the remodeling. The purpose in my case is for duct conditioning in winter. One or two short cycles a day in certain weather conditions prevents any condensation inside the otherwise idle ducts. I am happy that in the new system I will be able to turn the fan way down from where it is now. Depending on the requirements of the heat kit I end up getting, I may get fan speed down in the 500CFM range. More heat inside the ducts, less in the conditioned space. Afternoons on cold, sunny days are best conditions for condensation. These are also the best conditions for radiant over heating. I have other condensation prevention measures in place but there is no substitute for warm, dry ducts.

akaDigger

SuperJ said:

To me the biggest downside to marginally sized AC is the slow recovery if you shut it off, or let your house fill with hot humid air. If you like to leave your windows open until your house is uncomfortable then your going to need some tonnage to recover in short order, but if you keep things closed up and dry, with reasonable unoccupied setbacks, recovery is not a problem.

Good point. Thanks for bringing this up. Since the current system was so bad at dehum, we eventually started keeping the windows shut all year round. The screens are all stored away.

ratio

So this is a "project", great news. That opens up all kinds of fun things to do!

I know that reheat seems energy-intensive, but you're aiming for comfort, right? I'd, personally, use an SCR to control the heat strip to a neutral discharge air temp when running in dehum mode (however 5kW might not be enough. IDK). That'd be about the cheapest you can get operation-wise, unless you want to build a frankenevaporator with a reheat coil & tap the hot gas line in your condenser. Doable, but complicated. Should be about the most efficient though.

Once you get reheat of any flavor, that opens up the sizing somewhat. You can go slightly larger for more horsepower when you need to recover but still allows for good dehum.

akaDigger

ratio said:

I know that reheat seems energy-intensive, but you're aiming for comfort, right? I'd, personally, use an SCR to control the heat strip to a neutral discharge air temp when running in dehum mode (however 5kW might not be enough. IDK). That'd be about the cheapest you can get operation-wise.

Could you explain SCR? To a pipefitter, SCR is "selective catalytic reduction".

I absolutely want to keep it simple.

akaDigger

Wow! I did a Google search. This SCR would benefit the duct warming application as well. Longer, cooler cycles equal less wasted energy. And this could be an add on down the road.

ratio

Silicon Controlled Rectifier. The active element in a solid state relay. an appropriate one can be used to PWM the heat strip, to reduce the output from 100% on. Unfortunantly, while I wouldn't call it the opposite of simple, it does add some complexity, in terms of parts and wiring. (You can stop reading here, but I'm going to ruminate on this now.) It will give you a few benefits, however: namely that it will dehumidify correctly over a larger operating range of indoor temps, will simplify the sizing of the heat strips (& possibly ease the match if the 'correct' size is oddball, like 3.2 kW), and if you do go with reheat it will give you the least operating cost vis-à-vis electric reheat dehumidification.

TL;DR: My idea of simple, isn't.

John Mills_5

I'd run 400 CFM/ton but since most variable speed air handlers have dehumidify on demand, get a thermostat that works with that feature. That slows the blower down usually 20% to increase the latent capacity and dry out the air better. That way you have more airflow for cooling on a hot afternoon and in the evenings and overnight, it likely will be slowed down when it runs less & gets more humid.

akaDigger

Another option question: aluminum tube or copper tube? I'm old school and seems that copper is the only way to go. However, the aluminum tube equipment may be just fine. I didn't even know this stuff came with aluminum until I bought a York 1.5 ton set for my garage last summer. Somewhat surprised but didn't give it much thought at the time.

hvacfreak2

Lennox has the only active residential dehumidification option that I am aware of. We have a few of these in a private school but I have never had to go back on after start up to really watch one work ( that's a good thing and a bad thing I guess ).

https://lennox.com/products/indoor-air-quality/humidity-control/hd

As usual always set the condensing unit on a level pad and size the duct to 400 cfm per ton.

akaDigger

akaDigger said:

Another option question: aluminum tube or copper tube? I'm old school and seems that copper is the only way to go. However, the aluminum tube equipment may be just fine. I didn't even know this stuff came with aluminum until I bought a York 1.5 ton set for my garage last summer. Somewhat surprised but didn't give it much thought at the time.

Well, with no feedback, I guess I'll go with the York aluminum equipment.
I got all my old equipment removed. Need to tweak the levelness of the condenser unit pad. Thinking about adding a drip pan under the attic unit. Going to add some real flooring up there too. Pulling fiberglass batts out to find ceiling joists to step on might have been OK for the forty year old that installed it that way. But that forty year old is now fifty-nine and has less that perfect knees.

SuperTech

Don't do it! It sounds like the York aluminum tube stuff you are describing is what's known as micro channel. It's total junk. Leaks all the time and is a pain in the butt to clean.

Go with all copper. Avoid micro channel at all costs!

akaDigger

The outdoor equipment is Microchannel. The technology of the indoor unit is called MaxAlloy™ and is standard tube in fin but is all aluminum.

John Mills_5

York is bring back copper tube outdoor units. You don't have to get a microchannel coil if you want a York

ChrisJ

I live in an area that gets fairly hot during the day but drops into the 60s or even low 50s and very humid at night. I ran window units for a few years and they hated it.

Last year I installed a 2 stage 16 seer 3 ton Goodman system. After fighting with txv issues and finally installing a smaller txv I decided to also install a crack case heater and a ICM fan controller along with a single speed ball bearing fan with high temp winnings for the condenser.

I've never been happier. I run 1200cfm and if the indoor humidity goes above 50% the blower slows down. My typical indoor rh is 43-48% at 72f. The system fires up when it's 50f outside and behaves just like when it's 80f out.


Part of me wishes I never went with Goodman and part of me loves it because of how cheap I got a 2 stage system for.


I'm responding more for the humidity issues than brand. For me,I'd have to do some modifications to pretty much any brand.

akaDigger

For better or worse, my fate is sealed. I have the York AVC in the attic which will be remarkably easier to clean than my old, now removed basic Tempstar unit. I am happy with this choice. The outside unit is a York YCE with Microchannel. I'm a little uneasy given the debate. There are haters but no lovers. It is a roll of the dice for me. I now have two of these units. The one I bought last summer for my garage was York Microchannel. I never noticed before that it was different from what I would consider "normal".

When I looked over my cost and the procedure to clean my existing refrigerant lines I decided to just install a new line set. I get to switch my 7/8" suction line for a 3/4" line with this change. Not a straight forward job as the lines run through a crawl and up through an interior wall. But looks to be fairly easy anyway.
My existing suction line is 7/8" hard drawn and has seven fittings. The new set line have no fittings.

akaDigger

I have now added a Honeywell 20x20x5 filter cabinet. MERV 11, 0.1 inch pressure drop @ 1000 cfm.

ChrisJ

akaDigger said:

I have now added a Honeywell 20x20x5 filter cabinet. MERV 11, 0.1 inch pressure drop @ 1000 cfm.

I'm glad you posted this.
I wasn't even considering a MERV 11 this year for my AirBear because I swore it was a much higher pressure drop than my MERV 8. When you posted 0.1" @ 1000 CFM it made me look it up again.

Looks like I'll be putting in a MERV 11 this year. I think my system can stand the extra 0.02" in high stage. Low stage is 800 CFM and that appears to not even change.

unclejohn

Don't fret the aluminum.

mikeg2015

If its not running continuously for 6+ hours at design conditions on a sunny, humid day, it’s oversized. few 1200sqft homes need more than 2 tons. THere is no rule of thumb.

A good option on a newer VS AHU is to add a thermostat like a Visionpro IAQ with the humidity control output, then wire it to reduce fan speed on demand. The newer coils are already oversized for higher SEER ratings, so latent ratio suffers. That’s why old 10SEER systems actually worked so well to dehumidify. Their probably was more they smaller outdoor coil, and the old recip compressor efficiency, not the small indoor coil.

IF you don’t believe smaller works, I have a 3500sqft 112 year old home with lots of large original windows and poor quality storm windows. R12 attic floor insulation, but otherwise just thick brick walls. I have 3 ton 2 stage condenser upstairs on a Unico (so 2.5 tons actual capacity) and 2 ton 2 stage split system downstairs. Both units would keep up on design day on 1st stage only. So about 3.5-4 tons would cool the whole house on a humid, sunny, 95F+ day. The system would run for 16-20 hours straight and fall behind 2-3F but RH would be under 40%.

If in doubt, go smaller. You can always increase nominal airflow a little for sensible capacity, then use dehumidify on demand to bring it back down to 375-400. Balance the ductwork and use good commercial registers it’s good throw. The cheap stamped steel $5 registers are crap. You can use 1/2 as many $30 registers that will turn air over in the whole room then save a ton on ductwork and labor.

ChrisJ

Everyone insisted I only needed 2 tons in my 150 year old 1600 sqft house even after I did several heat gains on the house.


My 2 stage system runs balls out in high stage on a 95f day. It starts going into high stage when it gets into the high 80s out which means a 2 ton system would already be falling behind. I keep my house cold. I like it cold regardless of what others claim is comfortable.


A manual J is a must unless you have a previous good working system to go by.

ratio

@ChrisJ, don't you keep it like 40° or something?

My 2½T unit (1400 sq ft) runs all day during the summer, but it's due to the ductwork being sized for heating only in the late 20's.

ChrisJ

> @ratio said:
> @ChrisJ, don't you keep it like 40° or something? My 2½T unit (1400 sq ft) runs all day during the summer, but it's due to the ductwork being sized for heating only in the late 20's.

Bedrooms on second floor 68-70 first floor 72ish. Rh in the mid to high 40s


I was very pleased when my design was able to keep the second floor cooler than the first,exactly what I wanted.

ratio

Pretty impressive.

I think it shows that a) you need to know what you want, and b) you need to spend the time to do it right, to get good results.

You can't take shortcuts until you know the long way like the back of your hand!

akaDigger

Wrapping up this project. I have a question though regarding the Honeywell VisionPro 8000 t-stat. My AHU will slow fan for dehumidification. It operates the dehum mode on open humidity sensor. I set the t-stat for "normally closed humidistat". Thus, it will open on rise in humidity? Is that correct?

ChrisJ

> @akaDigger said:
> Wrapping up this project. I have a question though regarding the Honeywell VisionPro 8000 t-stat. My AHU will slow fan for dehumidification. It operates the dehum mode on open humidity sensor. I set the t-stat for "normally closed humidistat". Thus, it will open on rise in humidity? Is that correct?

Correct.

akaDigger

The installation is substantially complete. I am waiting for a warm day to start the AC portion. Temps here in the 40s and 50s.