sizing new attic based AC - 5 ton or 3.5 ton?

Chris_51

My attic based AC has a leak somewhere. In 2006, we replaced the outside compressor. We left the original air handler that was from the late 90s.

I think the contractor just looked a the old one and used a 5 ton. That seems big for 2400 SQ feet of upstairs space that it cools.

One contractor says a 3.5 ton should be enough. Does that sound right?

Hot_water_fan

No, they need to do a proper cooling load calculation. Could be 2 tons all the way up to 5 tons. 

One way to check would be to observe the 5 ton during a hot day. It should run nonstop during the afternoon. 

bburd

You do not want an oversized air conditioner. In addition to costing more and being noisier (assuming existing ductwork is not changed), it will not dehumidify well.

EBEBRATT-Ed

The old general rule used to be 600sq ft/ton which puts you at 4 tons.

But that is likely to be too large in this day and age.

Bigger is not better with AC. To small a unit will not cool the space.

To large a unit will not control the humidity level.... you will feel cold and clammy.

A good heat gain calculation is what you need

I I was going to guess i would say 3 tons

Chris_51

Thanks everyone. One contractor said the air handler in the attic was grossly oversized. He though 3.5.

My cousin says I should get a heat pump based AC so I can add some heat on the shelf days before the boilers need to run for heat. That seems complicated, but maybe.

Hot_water_fan

My cousin says I should get a heat pump based AC so I can add some heat on the shelf days before the boilers need to run for heat. That seems complicated, but maybe.

It’s extremely easy to replace an AC with a heat pump. They’re basically the exact same thing and you get an extra heating method for more or less free. 

Chris_51

Hot_water_fan said:

My cousin says I should get a heat pump based AC so I can add some heat on the shelf days before the boilers need to run for heat. That seems complicated, but maybe.

It’s extremely easy to replace an AC with a heat pump. They’re basically the exact same thing and you get an extra heating method for more or less free. 

Thanks. I'm not sure how well the heat will work with the attic based system. But if it is he same price, or only a bit more, it's worth a shot. I'm not sure why the contractor didn't suggest it. Or the online place.

If anyone in SE Michigan wants to quote putting this in, LMK. I have a good deal on the parts.

ChrisJ

Of course you should know heat pumps do add more failure points like a reversing valve and they do go bad and they aren't cheap to get replaced.

But they do also add some things that are beneficial like a crank case heater.  

Chris_51

ChrisJ said:

Of course you should know heat pumps do add more failure points like a reversing valve and they do go bad and they aren't cheap to get replaced.

But they do also add some things that are beneficial like a crank case heater.  

I see. I'm going to be moving soon. Either this year or in the next few. I like to do things right. I've been told that makes it complicated looking. I did a two stage hot water boiler for the primary heat. To me I have a backup, more efficiency, and have a bunch of valves for potential maintenance. To the ladies it just looks complicated. I might just go with AC. I'll get some quotes.

Hot_water_fan

Heat pumps are not more complicated than AC in any meaningful sense. They are one of the dominant heating methods in the US, very vanilla and commonplace. 

ChrisJ

Hot_water_fan said:

Heat pumps are not more complicated than AC in any meaningful sense. They are one of the dominant heating methods in the US, very vanilla and commonplace. 

The reversing valve alone is enough is a reason to avoid it if you can.   

That's very meaningful.

Hot_water_fan

Ha come on. A valve that needs to reverse a few times per year is worth skipping adding a second, potentially cheaper heating source? What is the failure rate vs all of the other things that can go wrong?  If valves are constantly failing, seems like a great reason to never install a hydronic system ever again! 

ChrisJ

Hot_water_fan said:

Ha come on. A valve that needs to reverse a few times per year is worth skipping adding a second, potentially cheaper heating source? What is the failure rate vs all of the other things that can go wrong?  If valves are constantly failing, seems like a great reason to never install a hydronic system ever again! 

The fact you just put a reversing valve in the same category as valves in a hydronic system suggests you need to do some reading my friend.

You might as well compare it to a kitchen utensil.

I said it needs to be weighed in the decision.  It's not a deal breaker but it's certainly something that should be avoided if it's unnecessary.

Hot_water_fan

It’s not a big deal, it’s especially not a big deal if the OP is moving in the next few years. It can break like anything else, but let’s be real. Millions of US homes use these systems, their lifespans are long. It’s the second most common heating method after furnaces. 

ChrisJ

Hot_water_fan said:

It’s not a big deal, it’s especially not a big deal if the OP is moving in the next few years. It can break like anything else, but let’s be real. Millions of US homes use these systems, their lifespans are long. It’s the second most common heating method after furnaces. 

Regardless I feel it should be mentioned.

For the OP as well as others reading this thread many years from now.


But I think I do see your point on hydronics vs forced air now.

HVACNUT

In over 35 years I've had maybe 4 or 5 reversing valve failures. Divide that by 17.348 billion heat pumps I've serviced in that time, and that's a durable stat. They can be a complete PITA to replace though. 
I'm not being a heat pump advocate, just stating of all the potential failures on a heat pump, the reversing valve isn't high on my personal list.

ChrisJ

HVACNUT said:

In over 35 years I've had maybe 4 or 5 reversing valve failures. Divide that by 17.348 billion heat pumps I've serviced in that time, and that's a durable stat. They can be a complete PITA to replace though. 
I'm not being a heat pump advocate, just stating of all the potential failures on a heat pump, the reversing valve isn't high on my personal list.

Since I'm sure you haven't serviced 17.348 billion heat pumps I'd like this stat stricken from the record.
You're probably also only 35 years old.


That said, I said reversing valve because it is a PITA and does happen, and isn't part of a cool only system.

pecmsg

ChrisJ said:

HVACNUT said:

In over 35 years I've had maybe 4 or 5 reversing valve failures. Divide that by 17.348 billion heat pumps I've serviced in that time, and that's a durable stat. They can be a complete PITA to replace though. 
I'm not being a heat pump advocate, just stating of all the potential failures on a heat pump, the reversing valve isn't high on my personal list.

Since I'm sure you haven't serviced 17.348 billion heat pumps I'd like this stat stricken from the record.
You're probably also only 35 years old.


That said, I said reversing valve because it is a PITA and does happen, and isn't part of a cool only system.

Now that depends on the system.
Some energize cooling some heating. Most if not all minis are heat pumps.

ChrisJ

pecmsg said:

ChrisJ said:

HVACNUT said:

In over 35 years I've had maybe 4 or 5 reversing valve failures. Divide that by 17.348 billion heat pumps I've serviced in that time, and that's a durable stat. They can be a complete PITA to replace though. 
I'm not being a heat pump advocate, just stating of all the potential failures on a heat pump, the reversing valve isn't high on my personal list.

Since I'm sure you haven't serviced 17.348 billion heat pumps I'd like this stat stricken from the record.
You're probably also only 35 years old.


That said, I said reversing valve because it is a PITA and does happen, and isn't part of a cool only system.

Now that depends on the system.
Some energize cooling some heating. Most if not all minis are heat pumps.

What depends on the system?

pecmsg

ChrisJ said:

pecmsg said:

ChrisJ said:

HVACNUT said:

In over 35 years I've had maybe 4 or 5 reversing valve failures. Divide that by 17.348 billion heat pumps I've serviced in that time, and that's a durable stat. They can be a complete PITA to replace though. 
I'm not being a heat pump advocate, just stating of all the potential failures on a heat pump, the reversing valve isn't high on my personal list.

Since I'm sure you haven't serviced 17.348 billion heat pumps I'd like this stat stricken from the record.
You're probably also only 35 years old.


That said, I said reversing valve because it is a PITA and does happen, and isn't part of a cool only system.

Now that depends on the system.
Some energize cooling some heating. Most if not all minis are heat pumps.

What depends on the system?

In what potion the reversing valve fails! Heating or Cooling?

ChrisJ

pecmsg said:

ChrisJ said:

pecmsg said:

ChrisJ said:

HVACNUT said:

In over 35 years I've had maybe 4 or 5 reversing valve failures. Divide that by 17.348 billion heat pumps I've serviced in that time, and that's a durable stat. They can be a complete PITA to replace though. 
I'm not being a heat pump advocate, just stating of all the potential failures on a heat pump, the reversing valve isn't high on my personal list.

Since I'm sure you haven't serviced 17.348 billion heat pumps I'd like this stat stricken from the record.
You're probably also only 35 years old.


That said, I said reversing valve because it is a PITA and does happen, and isn't part of a cool only system.

Now that depends on the system.
Some energize cooling some heating. Most if not all minis are heat pumps.

What depends on the system?

In what potion the reversing valve fails! Heating or Cooling?

I think you misread my comment.
I said cool only systems don't have them. Not that they fail in cool.

ratio

Not all mini splits have reversing valves, but the ones that do, it's a you-know-what to replace. Not at all like inside a big o' heat pump. In a 2 ton Mitsubishi, 95% of the piping is contained in the volume about the same as a 5 ton recip compressor.

Chris_51

We did the install. We did the heat pump option. Besides the nuances discussed, I was sort of taken back by the size. It really is big. In a perfect world, it would be less of an eye sore on the other side of the house. Then not in the afternoon sun either.

On the AC side, my wife said it is far cooler than the old one. The installer didn't on time, so I've been on a trip since the install. I'll check it out on Tuesday. She said the heat is nice. The boilers are tough when it isn't very cold. We have a lot of water, so spring and fall it tough.

SuperTech

So what size heat pump did you go with? All the new condensers are huge compared to the older ones regardless of if they are heat pump condensers or cooling only.

Chris_51

3.5 I had no idea they are so big.

clammy

Not to be tool but Chris j how many straight ac systems have u put in and how many have you serviced . How many heat pumps have u installed or service ? It really easy to arm chair it from a computer but it’s a different story when u have to hand the customer a bill . Just wondering how many bills have you made out for ac/ heat pump repairs and boiler installs and service calls and repairs in the past month ? Just wondering since you’re the expert w professional advice to give to all but how much coin have you pulled in working in this field . I’m just wondering being you seem to be an expert in all fields and on all mechiincal things . Your shoulders should be exhausted from all that weight of your huge brain of yours . And people wonder why the real knuckle dragging mech don’t post or comment to much . Here’s your reason . Thanks Chris for making the guys who have been reading and studying for years and working in the field doing the work, repairs and all that goes w working in the field and have attended dans seminar abiding decades ago and have come here for years just sit on the side lines and never comment why you got all the answers . Hats off buddy . Keep throwing your advice from the mass of experence from years in the field that you have gained from what one boiler in your own home I guess that makes you a expert .This is why the real pro s rarely post why the resident pro got all the answers.
I ll always be lurking and rarely post why should ,there’s a group of better then me already here ,will they be the ones you call when things don’t work ,they don’t do this for a living
But they will tell you what ever body did wrong and how your being charged to much . Hey buddy grab you tools and walk a mile in our shoes and earn your pay through doing it not talking it and at the end of the day let’s see if you turned a profit or will go home hungry ,different story when u thinking about working for food on your table so keep babbling on .Installing one boiler in your own home does not make one an expert nor does reading all of dans books do it either . Try spinning a wrench and working on all this ancient stuff day in and day out for 35 to 40 years then maybe you can walk the talk . Enough said Peace and good luck
Reason why real pro don’t comment anymore

Chris_51

You are a tool for sure.

The question was whether it was worth it to go to a heat pump or just stick with an AC only.

The contractor that originally quoted didn't even offer a heat pump option. That was my research.

I didn't expect a refrigerator sized condenser. It blocks the window it is so big. There are other options. You know it. I know it.

clammy said:

Not to be tool but Chris j how many straight ac systems have u put in and how many have you serviced .

clammy

This has nothing to do w your post . I was commenting on Chris j expert options on all subjects . As for the large condenser well that goes w just about all new condensers and the higher the eff the bigger they are and some are just down right gigantic . Possibly your contractor should have enlighten you of your new units size in comparison to your existing unit . This is really not an issue if installing builder grade low end condenser they usually a bit smaller then say a 16 seer and higher seer condensers. I looks like they at least put the unit on a riser which is a good thing and it s trane equipment so it’s not junk and if properly installed and charged should provide you with many years of trouble free operation , as long as maintenance is preformed ,the use of hepa filters if returns are sized to handle the pressure drop or at least be religious on 30 day throw away filters ,condensate flushing and trap cleaning , and outdoor coil cleaning aside from all electric component checking ,fan indoor and outdoor for proper operation ensuring the longevity of your investment . Sorry if I offended you it was not meant for you . Peace and good luck clammy

Mad Dog_2

To lighten up the mood...What the H--- is a Tool anyway? My kids started using this term like 8 yrs ago...Who comes up with these buzz words & disseminates them??
Is there a Minister of Buzz words?  EG..Gravitas, At the end of the day..I can't wrap my head around this..Going foward...For Sh---s & Giggles..Wicked Pissah, et cetera.  Mad Dog 🐕 

Chris_51

Wrong Chris. I get it. Sorry.

clammy said:

This has nothing to do w your post . I was commenting on Chris j expert options on all subjects . As for the large condenser well that goes w just about all new condensers and the higher the eff the bigger they are and some are just down right gigantic . Possibly your contractor should have enlighten you of your new units size in comparison to your existing unit . This is really not an issue if installing builder grade low end condenser they usually a bit smaller then say a 16 seer and higher seer condensers. I looks like they at least put the unit on a riser which is a good thing and it s trane equipment so it’s not junk and if properly installed and charged should provide you with many years of trouble free operation , as long as maintenance is preformed ,the use of hepa filters if returns are sized to handle the pressure drop or at least be religious on 30 day throw away filters ,condensate flushing and trap cleaning , and outdoor coil cleaning aside from all electric component checking ,fan indoor and outdoor for proper operation ensuring the longevity of your investment . Sorry if I offended you it was not meant for you . Peace and good luck clammy

WMno57

@Chris_51 Thank you for the follow up with photos. Too many people come to this site with questions, and then never share the outcome of the project.

Mad Dog_2

I hate that too!!  You get involved and they go away happy, and forget we're still waiting here.. ha ha 😂 🤣 😆 😄. Mad Dog 🐕 

Brad White

Chris_51 said:

Hot_water_fan said:

My cousin says I should get a heat pump based AC so I can add some heat on the shelf days before the boilers need to run for heat. That seems complicated, but maybe.

It’s extremely easy to replace an AC with a heat pump. They’re basically the exact same thing and you get an extra heating method for more or less free. 

Thanks. I'm not sure how well the heat will work with the attic based system. But if it is he same price, or only a bit more, it's worth a shot. I'm not sure why the contractor didn't suggest it. Or the online place.

If anyone in SE Michigan wants to quote putting this in, LMK. I have a good deal on the parts.

A conventional heat pump can work well especially for shoulder-season heating, where you need a lighter touch and to take the chill off on cool but not frigid mornings. Heating from above, if that is what you will be doing, is less critical when the temperature difference to outside is say 25-30F vs. when it is 50F plus. Of course your level of window area, insulation, all of those factors play a huge role.

A heat pump at least becomes in-effect a back-up system for those times and not need to run your main source of heat. (You still can of course, this is about your unique situation, house dynamics, solar gains, all that.)

Over a season, you can find out what the balance-point is, that point where heat pump heating performance meets the comfort levels you seek before it drops off.

Point being, getting heat pump operation along with cooling does not cost all that much more and gives you and your future buyers options.

My $0.02.

Mad Dog_2

Wow 👏  The Great Bradley T. WHITE in the house 🏠 !!!!  Mad Dog 🐕 

Brad White

ChrisJ said:

Hot_water_fan said:

Heat pumps are not more complicated than AC in any meaningful sense. They are one of the dominant heating methods in the US, very vanilla and commonplace. 

The reversing valve alone is enough is a reason to avoid it if you can.   

That's very meaningful.




I dunno Chris.
I have specified dozens of multi-zone VRF systems which include as many reversing valves as you have indoor zone units. Have yet to see a failure. Likewise over the past 40 years I have specified even more, some in large houses and commercial applications. The equipment failed after a good 15-25 years but rarely the reversing valves. No more often than a TXV or yes, the compressor or condenser fan, those things that operate many more hours than a reversing valve.

There is one AC system associated with you that I know of and I helped you gladly to make decisions. It turned out OK and I was happy to help you sort that out.

But for the love of all that is holy, please respect those such as Clammy, Matt and others who install these for a living, as a habit and a default.

My Dad, also an engineer (designed hydraulic valves for rocket gantries and bridge movers, etc.) would take his drawings to his machine shop where the machinists would critique his work. Then talk to the pattern makers who prepared the masters for the foundry. Every one of them had oil on their hands. Listen to them. Learn from them. "Always listen to those with tools in their hands and less so to people with pencils", he said. That advice has demonstrated its value my entire career. I am grateful for those in the field who took the time and knew why.

Please think about this before you lecture outside of your lane.

Charlie from wmass

Real pros don't comment because they are busy working or tired of arguments over minutiae. Chris isn't why.