just replace unit with the same ton rating?

[brando1118]

Hello,
I just want to say that this site is fantastic!

OK, on to the question....

I have a 30 year old single Carrier 3 ton split A/C system. I live in a 2000 sqft., 3 level town home of the same age. The AH is located on the middle level. The duct work is all hard line. The first level is just an entrance with stairs and an exit to the garage, but for informative purposes I will count it as a level. It does have a vent going to it.

I had an estimate from an HVAC tech that came highly recommended from numerous people. When I asked him about doing a Manual J, he said I didn't need it. He said to just replace the old unit with a new unit of the same tonnage. (Is that a word?) He want's to replace the AH too, which is fine.

Now I've done some fairly extensive changes to my town home. I've knocked out walls, replaced appliances and done some electrical and plumbing. One thing I've learned in doing this is that the hippie contractor that built my place did it as cheap as possible and all appliances and most of the workmanship were bottom of the line CHEAP! So I'm assuming that his choice in A/C systems was inspired by the same line of thought. I know my A/C unit is inefficient. But I also theorize that the A/C unit is under tonned because it was cheaper. My place is never comfortable in the extremes of the seasons.

I suggested again to the contractor that maybe we should do a Manual J to see if my home is under tonned and he still refused saying that the duct infrastructure is built to handle 3 tons and no more. If I went up in tonnage (there's that word again) then I would have to provide more duct sqft. to accommodate the larger ton capacity. Is this true? Or is it a good thing that I'm asking this question on the forum?

Thanks so much for your help and advice.
-Brandon

[mjk_na]

First of all, since you have not felt comfortable with your existing unit, AND your contractor is refusing to use Manual J - you should find other contractors.

Get a few bids from different contractors based on SAME technical requirement:

1. Heat gain and loss calculations using Manual J.

2. Use of psychrometrics and Manual D to size your air flow requirement into each room, fresh air and recirculated air mixing ratio, and to size your duct.

All the best :)

[dash]

First Man. J to size the system and determine the cfms (air flow)needed per room.

Second Man. S to select the correct equipment,per the capacity required by Man. J,and the capacity of the equipment at the desired indoor and outdoor design conditions.

Third Man. d to check the design of the ducts.Some modifications may be needed or not.This part can be a problem as ducts are not all accessable to change sizes if needed,so you may want to live with ,say a minor temperature difference between rooms.


You can also look at increasing the insulation,etc., to reduce the size system that will be needed.

With a system as old as yours it may not be producing the full rated "tonnage"(that word again),that it should or could.

[kevinmac]

Being that you have a three level town home, I would opt for a split system. This way you can control the floor to your liking independantly, and probably save on cooling and heating costs since you have the flexibility of control and each unit would be smaller then then a 1 unit system.. Just my opinion.

[brando1118]

mjk_na, dash, kevinmac thanks for the replies....

I've got a lot of learning to do. I didn't even know there was a Man D and a Man S.

As far as the split system, I just can't see how the costs could be justified. There is NO room in my attic. It is cross beam construction with 2x4's all over the place! And the third floor is all bedrooms and bathrooms. I don't want a AH in one of the bedroom closets. I know my set-up is not ideal, so I am willing to live with some shortcomings. But not too many!

I've just replaced all windows with Low E glass and there is a foot of insulation blown in the attic.

I think I've done all I can to help out the ole' Carrier.

Thanks for the informative replies!
-Brandon

[danglerb]

I am in almost the same boat you are, 1850 ft2 townhome built 27 years ago with a Carrier furnace, but a Day & Night (CHEAPEST) 3 ton compressor in the back yard.

Get three bids, more information is good.

Ton or tonnage is just shorthand for 12,000 btu of cooling, so 3 ton is 36,000 btu, but btu is what window air conditioners are rated in, and ton sounds a lot bigger. Ironically the tons seem to also convert pretty well to the cost in pounds sterling. ;)

Without getting technical, an old cheap AC unit that is running properly with the right amount of freon and clean coils is going to do the same amount of cooling as a new top of line AC unit, its just going to use more power to do it. Chances are good your old unit isn't hitting on all three (running well, freon, clean), so a new same tonnage may have some more cooling to start with, but without checking over your old unit thats just a guess.

As much as these pros like the manual J etc. I am not sure that some estimates run through a spreadsheet would convince me I don't feel hot. Why you feel hot might be a better approach to solving the problem. Why and how much are you willing to pay to correct it since going up to a 4 ton (1 ton per 500 ft2) may cost some serious cash essentially replacing all or most of the stuff buried in walls and attics.

If you feel hot because of humidity, there may be other ways to approach the problem.

I am sticking with the 3 ton size of my old system, but getting a higher efficiency (not more cooling, just lower operating cost) compressor and a more flexible variable speed furnace (that can help lower humidity).

[bartdude]

By replacing your windows, you've made a tremendous insulation improvement in your house. It could even be possible that with new windows, you could use a 2.5 ton unit.

Old windows vs. new good windows = night and day!

[kevinmac]

You don't have to have the 2nd airhandler on the third floor. I have seen them on the same floor next to each other. It's all in the ductwork, and where the returns are.

[Workhorse]

Without getting a proper man j It's hard to give you an answer. I can say that you CAN'T go down in tonnage (yes, that's a word). Just by what you are saying with the upgrades, doing a 3 ton would work. You origanally had a 3 ton, (old quick rule of thumb-1 ton per 600 sqare ft) which at that time was undersized, from what I get from your post. You could actually probably go up 1/2 ton. It also seems the tech you had out origanally didn't know how to do a Manual J, as a lot of them don't, or he just didn't have time.

Ultimatly I would have either that company come back out with the knowledge that you want a Manual J done and/or call another company to do the same. Good luck.

[seatonheating]

A LOT of mis-information on this thread............


I'll leave it alone.

[Workhorse]

Originally posted by seatonheating
A LOT of mis-information on this thread............


I'll leave it alone.

Don't leave it alone. Post what you think is right, maybe (most likely) I'll learn something.

This isn't just for the home owner but also for the rest of us too.

[pstu]

Here is what I am hearing from the OP, correct me if I am wrong:
1) The old AC system does not keep me comfortable
2) Therefore it is undersized.

Is that too much oversimplification? I am trying to inquire whether there could be *other* reasons for lack of comfort. For example, perhaps the duct system is not delivering air where it is needed in the proper quantity. Too-small ducts are far more common than too-small compressors. I suggest a good place to start would be with someone trained by the National Comfort Institute:
http://www.nationalcomfortinstitute.com/

NCI trains techs on tools to measure air flow, building leakage, and indirectly duct leakage. In theory they can take certain measurements and tell you how many BTUs your system is actually putting out. The world of AC techs is a barbaric one IMO, at least connecting with this training will give you someone more systematic than average.


If you were comfortable then I would be much more comfortable with "present experience" as a substitute for Manual J etc. But since you are not, it may only be prudent to start with Manual J heat load model -- it's the most objective method we have. I predict somewhat of a hassle finding a tech who is comfortable doing it (and it's not free) but when you do, it's probably a guy you should have a long term business relationship with.

Separately, let me say a variable speed air handler with two-stage furnace can really improve winter comfort. I am a homeowner in S.Texas and we use AC much more than heat, if you told your location I forgot. It would be much more helpful if I could tell you whether two-stage AC would do the same for summer comfort, but I don't know from personal experience. A couple of friends in this climate have two-stage AC and praise it highly. But if you put this top of the line AC on a crappy duct system, even one which is too small, you may not get such great results. In dollar and cents terms, you will do better with a single stage AC. In either case I urge you to get the VS air handler.

Hope this helps -- Pstu

[perel]

Cheaply built condo probably means undersized flex everywhere - quite possibly more problems such as flex runs that have fallen off the boots and are pumping conditioned air into the attic, etc. I would definitely have someone who knows ducts (NCI is a good starting place) check the airflow on your existing duct system vs. ManD.

If this is part of the problem, and you're not going to spend to correct it (could be VERY expensive to change ducts, depends on your exact situation) a variable speed air handler could help *some*. They're NOT a substitute for properly sized ducts, but they are better at dealing with undersized duct systems than non-variable speed blowers. The air will get there, it will just make a bit more noise at the registers.

Do have a new heat load done, especially with the changes you've made - balancing the airflow may help even with your existing system, but you should know the correct size for the new one. One other thing to make sure of is that the contractor doing the Man J knows what indoor conditions you want. (If you want 75 inside don't let them assume 78, etc)

[leapfrog]

A significant number of original air conditioning systems are oversized.

http://www.fsec.ucf.edu/bldg/pubs/ACsize/index.htm

You can't size air conditioners by "rules of thumb".

Forget the current size of your A/C - it means nothing.

Find a contractor that will agree to do a heat gain calculation using Manual J and then size the equipment with Manual S and confirm the duct requirements with Manual D. Then you will have found a professional.

[seatonheating]

Originally posted by leapfrog
A significant number of original air conditioning systems are oversized.

http://www.fsec.ucf.edu/bldg/pubs/ACsize/index.htm

You can't size air conditioners by "rules of thumb".

Forget the current size of your A/C - it means nothing.

Find a contractor that will agree to do a heat gain calculation using Manual J and then size the equipment with Manual S and confirm the duct requirements with Manual D. Then you will have found a professional.

I left it alone until someone gave good, sound advice. Listen to leapfrog, he is right!

[brando1118]

Thanks everyone for the informative replies.
Seatonheating, please let us know what information we are lacking.
Thanks,
-Brandon

[seatonheating]

Originally posted by brando1118
Thanks everyone for the informative replies.
Seatonheating, please let us know what information we are lacking.
Thanks,
-Brandon

None, leapfrog said all that needs to be said.

[danglerb]

Maybe this manual J stuff is more regional? I have asked every company I talk to about doing a manual J, and those that know what it is use some more casual method of their own, most boiling down as far as I can tell to 1 ton per 500 ft2, or you need a bigger one.

Worst things about our townhouses is the insulation R rating in the walls, and plenty of air flow with everything closed up. Surprizes me the termites are willing to eat the wood.

[pstu]

I do know there are a wide variety of sizing methods used, and not all of them are junk. First of all, the tons/sqft method involves a lot of stupidity because it is absolutely blind to insulation, shading, orientation of the house, window amount size type and orientation, lots of things that obviously matter.

But I read of a utility which wanted to encourage Manual J type sizing, and it ran into literally dozens of methods other than Manual J which HVAC techs used. Some were simplified versions of Manual J, some home-built spreadsheets, it seemed everybody had his own method. The utility needed to be fair, to not dictate terms to the techs, so they evaluated every method and accepted (for rebate purposes) those which came within 10-15% of proper Manual J. About half passed.

I can give you a link to the published paper if you really want, just don't have it now and cannot afford the time to look it up right now.

That said, there is not much reason to expect the techs you have been talking to, are in the upper half. And the problem of techs wanting to do anything but the right Manual J method for sizing, exists in my S.Texas area too.

As I see it, the elite of the HVAC field actually do good engineering when they size and design systems. And they are outnumbered by the guys who just want to sell something, maybe a few parts and labor, maybe some freon and labor, but who really are not comfortable with a numbers oriented (i.e. ACCA) approach. Really appliance oriented. These guys figure if their company doesn't sell compressor and condensor every few days, they will go out of business. Maybe they are right in a business sense but I hope not.

Hope this helps -- Pstu

[leapfrog]

Originally posted by danglerb
Maybe this manual J stuff is more regional? I have asked every company I talk to about doing a manual J, and those that know what it is use some more casual method of their own, most boiling down as far as I can tell to 1 ton per 500 ft2, or you need a bigger one.Your region must be in the minority.

Cut and paste this link - apparently the auto-URL feature doesn't handle it correctly.
http://www.achrnews.com/CDA/ArticleInformation/coverstory/BNPCoverStoryItem/0,6152,168472,00.html

[danglerb]

Keep in mind that survey had 48 HVAC guys respond, but the whole group was based on people that already use a computer enough to be using that site, and plenty of the HVAC guys I see don't seem too comfortable with a calculator. The survey also, as they admit, those who don't do a manual J were less likely to respond, but still 38% who responded don't do it.

Seems to me the manual J isn't to "tell" you what to use, but to give you an idea of what should work, a range check so gross sizing errors don't happen, and classic CYA. I still don't see how you can argue with what works, if I have a 3 ton and it works fine for me, what value does a manual J that shows I need a 4 ton have to me?

Also whats the bottom line, lets say the manual J says 4 ton, what happens if a 3 ton or 5 ton goes in? First case I believe is the standard for all motels in Texas, either that or the spec comfort zone indoors is 79/79 (temp/RH). Second case is that even on the hottest hot humid days its cool and dry, and most of the time otherwise the compressor runs a shorter time.

No matter how the replacement goes I am not too sure I would want to pay to replace the ductwork unless it was "in general" undersized.

[tlchvac]

POSTED BY danglerb
"Keep in mind that survey had 48 HVAC guys respond, but the whole group was based on people that already use a computer enough to be using that site, and plenty of the HVAC guys I see don't seem too comfortable with a calculator. The survey also, as they admit, those who don't do a manual J were less likely to respond, but still 38% who responded don't do it.

Seems to me the manual J isn't to "tell" you what to use, but to give you an idea of what should work, a range check so gross sizing errors don't happen, and classic CYA. I still don't see how you can argue with what works, if I have a 3 ton and it works fine for me, what value does a manual J that shows I need a 4 ton have to me?

Also whats the bottom line, lets say the manual J says 4 ton, what happens if a 3 ton or 5 ton goes in? First case I believe is the standard for all motels in Texas, either that or the spec comfort zone indoors is 79/79 (temp/RH). Second case is that even on the hottest hot humid days its cool and dry, and most of the time otherwise the compressor runs a shorter time.

No matter how the replacement goes I am not too sure I would want to pay to replace the ductwork unless it was "in general" undersized."

Did you ever consider that a Manual J calculation is not only used to size your ac to keep YOU nice an chilly but also to keep you HOME healthy....????? I.E mold growth,bacteria production yada yada yada ..

Now please think about this when you are giving advice to subjets you likely shouldnt be

Thank you

[danglerb]

I'm not giving advice, or pretending to know anything, just asking questions and giving my opinion as a consumer. I am surprized none of the professionals in all of these sizing related threads going on now haven't mentioned the health issues, so whats bad, too big, too small, wrong ducts?

Maybe growing up with swamp coolers explains my many flaws.

[beenthere]

Originally posted by danglerb


if I have a 3 ton and it works fine for me, what value does a manual J that shows I need a 4 ton have to me?



It shows that some one either accidentially or intentionally screwed up the calc.



Also whats the bottom line, lets say the manual J says 4 ton, what happens if a 3 ton or 5 ton goes in?

An undersized unit would As you said not maintain temp.

The 1 ton over would maintain temp, and provide you with high humidity most of the time, and give you some nice high utility bills. Along wiht possible mold and eventually heath problems.

No matter how the replacement goes I am not too sure I would want to pay to replace the ductwork unless it was "in general" undersized.


If you need to go 1 ton in size, the old duct is undersized.

[dan sw fl]

Originally posted by brando1118
Hello,
I just want to say that this site is fantastic!

I have a 30 year old single Carrier 3 ton split A/C system. I live in a 2000 sqft., 3 level town home of the same age.

The AH is located on the middle level.
The duct work is all hard line.

Thanks so much for your help and advice. -Brandon

Seems like a properly operating 3-ton unit
should be sufficient for 2,000 Sq. ft townhouse.

Townhouse implies adjacent wall(s) are not adding load.

Location? MN or TX

It's all about location, windows & orientation.

[seatonheating]

Originally posted by danglerb
Keep in mind that survey had 48 HVAC guys respond, but the whole group was based on people that already use a computer enough to be using that site, and plenty of the HVAC guys I see don't seem too comfortable with a calculator. The survey also, as they admit, those who don't do a manual J were less likely to respond, but still 38% who responded don't do it.

Seems to me the manual J isn't to "tell" you what to use, but to give you an idea of what should work, a range check so gross sizing errors don't happen, and classic CYA. I still don't see how you can argue with what works, if I have a 3 ton and it works fine for me, what value does a manual J that shows I need a 4 ton have to me?

Also whats the bottom line, lets say the manual J says 4 ton, what happens if a 3 ton or 5 ton goes in? First case I believe is the standard for all motels in Texas, either that or the spec comfort zone indoors is 79/79 (temp/RH). Second case is that even on the hottest hot humid days its cool and dry, and most of the time otherwise the compressor runs a shorter time.

No matter how the replacement goes I am not too sure I would want to pay to replace the ductwork unless it was "in general" undersized.

There you go again, trying to give "other" homeowners advice. Your opinions need to take a break, bud. You are not educated enough on this subject to be giving these opinions. Like I said before, the mods need to put you in your place. The proper way to do the job(no exceptions, not even for you :( ) is to performa heat loss calc. using Manual J, equipment selection per Manual S, duct sizing per Manual D. Anything less and we are doing our customers a disservice.

I think Mark (Beiser) said it best once. He said "I can't believe how many underperforming systems I run across that the customer is happy with". That speaks volumes and you don't know why, we do.

[bartdude]

Originally posted by workhorse
Without getting a proper man j It's hard to give you an answer. I can say that you CAN'T go down in tonnage (yes, that's a word). Just by what you are saying with the upgrades, doing a 3 ton would work. You origanally had a 3 ton, (old quick rule of thumb-1 ton per 600 sqare ft) which at that time was undersized, from what I get from your post. You could actually probably go up 1/2 ton. It also seems the tech you had out origanally didn't know how to do a Manual J, as a lot of them don't, or he just didn't have time.

Ultimatly I would have either that company come back out with the knowledge that you want a Manual J done and/or call another company to do the same. Good luck.

What I meant to say was that if he added the blown in insulation, and new windows (which are, I think could be the biggest improvement in the houses heat loss/ gain...ie, going from single pane metal clad to low-e vinal or such), its possible that the heat gain went down enough that a smaller unit would work. But only by doing the loads could you tell that.

And being that the unit is 30 years old, the chances of it providing its full rated capacity is almost 0. It may not even be providing 2.5-tons of cooling. It may have worked fine in that house 30 years ago but without the Man J calc, taking into account the new windows/ glass, its all a guess now.

[danglerb]

Originally posted by seatonheating

Originally posted by danglerb
Also whats the bottom line, lets say the manual J says 4 ton, what happens if a 3 ton or 5 ton goes in? First case I believe is the standard for all motels in Texas, either that or the spec comfort zone indoors is 79/79 (temp/RH). Second case is that even on the hottest hot humid days its cool and dry, and most of the time otherwise the compressor runs a shorter time.

No matter how the replacement goes I am not too sure I would want to pay to replace the ductwork unless it was "in general" undersized.

There you go again, trying to give "other" homeowners advice. Your opinions need to take a break, bud. You are not educated enough on this subject to be giving these opinions. Like I said before, the mods need to put you in your place. The proper way to do the job(no exceptions, not even for you :( ) is to performa heat loss calc. using Manual J, equipment selection per Manual S, duct sizing per Manual D. Anything less and we are doing our customers a disservice.

I think Mark (Beiser) said it best once. He said "I can't believe how many underperforming systems I run across that the customer is happy with". That speaks volumes and you don't know why, we do.

Why have a forum open to residential consumers if the answer to all questions is to trust the professionals who know, but won't say what why or how. If anybody says something that you don't like, why not shine the light of truth and help everybody out?

I don't want to let out any secrets, but maybe you have noticed that plenty of HVAC pros have opposing opinions on all these issues. For whatever reason a good half don't use a manual J. Why is it that I as a consumer need to know the methods and calculations the contractor uses, can't I trust their professional opinion?

Should I make sure they used Ohms law to figure out the wiring size?

From a consumer standpoint its very hard to tell which is a "engineering" tool and which is a "sales" tool, and if it walks like a duck and quacks like a duck, I tend to think its a duck unless I find out otherwise. Some home contractors are not entirely ethical honest tradesmen, and they don't wear black hats to make them easy to spot. Most consumers have no ability to judge the merit of professional work of any kind, it works or it doesn't, so we are easy prey, know it, and don't really like it.

[xv80satisfiedcust]

danglerb,

I've never seen "Manual J" and my HVAC contractor did not do a "Manual J" study and none of the companies I talked to mentioned it.

And I wasn't going to mention it because they MIGHT see dollar signs in their eyes (I will agree with you on that).

As far as new construction goes, many builders oversize the equipment because they have overweight customers who expect instant cooldown.

In my opinion, the HVAC contractors that DON'T do a load calc don't have to worry. They are not the homeowners paying the utility bills. And, most consumers don't know how much more in utilities they will waste with oversized equipment. There is plenty of SCIENTIFIC (does this scare anyone?) research posted on the subject. (Ask HVAC Talk member Martik - she could quote several statistics).

I used HVAC Calc to calculate my sizing and HVAC Investor to tell me which SEER/HSPF was the best payback.

Anything else is guessing - just ask jeffw_00.

[pstu]

That cover story from ACHRNEWS, I want to give a great big thanks for posting that! But it appears too few people have read it through, they quote that online poll saying about half of pros use Manual J. They neglect to cite the paragraph on page 4: ..."At 20 seminars we asked how many jobs, other than new construction, had a J or D calc done, and the answer came in every time at two to three percent".

IMO many of the points DanglerB makes have at least some validity, but I would dread seeing him act on what he suggests. Sorry DanglerB but some of the things you say lead one to think of a loose cannon... and doing the wrong thing with HVAC occasionally leads to lawsuits for a few hundred thou or million. The pros who argue against you have a whole lot of good points on their side.

Sad to say I have found getting Manual J done by a tech as a stand alone job, is difficult to get. And I have been willing to pay in the triple digits to get that done, I consider it cheap insurance against a much more expensive job. And good heavens, Manual S and D calcs are just as hard to find. My heresy is I DO believe there can be decent alternative methods to compare with Manual J that can help with pre-built structures. Perhaps including (but not limited to) measuring runtimes of an existing system. But only the Manual J is an ACCA approved method, as far as I know.

I think there is a great big gap between customer expectations and what today's techs can install. The customer IMO will only be happy when air conditioning CAN act as a troublefree (mostly) appliance, when he CAN make use of those shutoff levers on so many supply grills, all without pushing the HVAC system into dangerous territory. And without doubt, the customer wants the liberty to choose "Hemi Power" equipment so he can do whatever, and pay no price except energy consumption. He is accustomed to doing that with cars and trucks, and whoever can sell him the same concept for his house, will be giving him what he really wants deep down.

The customer I see (in my imagination, I'm a homeowner) really doesn't want to *know* about sizing a system just barely big enough to get by -- though I understand many of the reasons why. And he doesn't want to have to know anything about ducts, he starts out thinking any monkey can do good enough with ductwork. He wants a butt-simple system to operate, that will always do what he asks (reasonable or not) and won't call any more attention to itself than a kitchen appliance. And today's educated HVAC tech is trying to sell him on several foreign concepts, it's really an engineering consultation when you do things the ACCA way.

And then you have the less educated techs... the less said about them the better.

I think the ideas of the late David Debien have a certain amount of merit here. I don't necessarily agree, but wish there were more ongoing discussion about this. There just could be more than one way to skin this cat.

Best wishes -- Pstu

[danglerb]

Mold is the new treasure chest for lawyers.

I think its potentially a serious problem, but problem or not its a hot button litigation issue. Once a house has mold its close to impossible to get rid of it permanently.

The main thing that confuses me on this size thing is that I have 2 months of 100+ and 4 months of 80-100 every year, what do I really need a 2 ton and a 4 ton each with its own set of ducts etc.?