The obvious question is how did your existing 4 ton AC perform when it was running properly? Have those dealers who recommend a 5 ton AC condenser said anything about modifications to your ductwork system to handle 25% additional CFMs? Keep in mind you size to the average weather conditions, not extreme!

IMO

The current 4 ton unit has been at this home for 18 years. I think it's pretty safe to say you need a new 4 ton unit.

After 18 years what has changed other than the system being old?

It easier to fix a unit that is slightly undersized than a unit that is oversized.

If you want a manual J, I would do it... but I would charge you for it. The cost varies with the size / complexity of the home.

I service the Katy, Texas area.

If your house has 2X4 wall construction then you can assume the insulation is R11 or R13 for your manual J calculation. You can try both to see if it makes a difference in equipment selection. Go into the attic and see what insulation you have. It there are batts measure the height or lift it up to see what is printed on the paper backing. If it is blown in then estimate the average height. The calculation of height to R value is simple.

The contractor who did the load calculation should give you a copy as part of his quote. Ask him what he used for insulation values. If he doesn't answer then I am skeptical he did a real calculation.

Is the duct work and located in the attic? If it is I would spend money improving and insulating the duct work before investing in a bigger equipment. If you are in a humid area, then a small under size of equipment is better than over sizing in my opinion.

I really don't understand the problem here. If lovdyg wants a properly sized AC unit and the industry standard to properly size an AC unit is by using a Manual J calculation, then why not get a contractor that performs a Manual J as part of their bid process. This doesn't seem that complicated.

In the area of the US where I live, the utility company will give some new AC buyers a rebate for buying a new energy efficient, AC unit. One of the utility company's criteria for the rebate is that the contractor has to provide the Manual J calculation they performed to correctly size the AC unit for the house.

Here's a link that shows that the trend towards energy efficiency in the US is for states to require a Manual J as part of their codes for new build homes.

I'll tell you why Bob,

Salesmen that perform a manual J can skew the numbers. You could even call this a 'miscalculation'. Intentional or otherwise is hard to prove.

Salesmen have a motivation to sell 'more'. Because as it turns out their pay is determined by how much they sell.

An air conditioner (properly sized according to manual J calculation) is OVER SIZED 90% of the time.

Heat of day 3-6pm Under heat wave 3-7pm. 24 hours in a day. 3rd grade math from there.

The air conditioner is only properly sized for 3 hours a day. Then throw in seasonal changes from spring to summer and summer to fall.

A manual J will not solve all your problems. It's a tool in the tool box for those who use it properly. Do as you wish, don't say I didn't warn you.

This is why you pay to have one performed if that is what you want.

Based on his reasoned comments and commitment to those on this site, I would have ‘Austin’ do the Manual J and gladly pay him for his time (on-site & traveling) and expertise!

IMPO

SR

Most HVAC contractors will not do a proper load calculation even if you offer to pay for it. My definition of a proper calculation is entering the dimensions of all rooms, the sizes of windows, the direction the house faces, with the appropriate indoor and outdoor temperatures in to software which follows the Manual J calculation. Estimating by area and the number of registers is not a proper calculation.

There are many excuses as to why they don't do it. Some are insulation values are not known, the original builder puts in the right size, the numbers can be skewed, it is too hard, it is not accurate, and I have many years of experience so it is unnecessary.

I can tell you stories about some of the licensed contractors who did "calculations" for my estimates several years ago. In one case I told the contractor I need a copy of the Manual J calculation in order to qualify for the NJ state rebate. He said no problem, he would reduce the price by the rebate amount. Another state the rebate application was too much paper work. The application is one page which the Manual J calculation is attached.

Can you think of any other item that you would hire a licensed contractor to install in your house without doing a measurement? Then why are HVAC installations different?

I live in CA, a load calculation must be presented to the city inspector for permit sign off.

Load calculations are very easy to fidget with to get to a desired answer. When it's done in that manner, the unsurprising and intended outcome is to spec larger equipment than is necessary.

Bottom line...... if the old 4 ton unit cooled adequately, then he needs a 4 ton, not a 5 ton.

Austin, where do you get your "oversized 90% of the time" statistic from?

mike_home, I'm not the one setting the HVAC industry standard here. The HVAC industry says Manual J is the standard that is supposed to be used. The buyer goes online and many HVAC contractors say the only correct way to accurately size a new AC unit is by using a Manual J calculation.

Elmer Fudd, that is why you get several bids from contractors that complete a Manual J calculation. There should be a consensus for the correct size AC unit.

I still don't think this issue is that big of a deal. If lovdyg wants a contractor to perform a Manual J then just make that part of the bidding process. Why should lovdgy do this? Because the HVAC industry says it's the standard and that's why the trend is for states and utility companies to require it. IMO the US should just make it mandatory across the whole US so there isn't this back and forth between contractors.

There have many posts on this forum of people trying to hire someone to do a Manual J calculation and have been unsuccessful. It is something most contractors don't do because it is not required. It is a conflict of interest. Why spend time and money doing a calculation in install smaller equipment? Smaller equipment means less profit.

mike_home, you make the perfect argument for why Manual J should be mandatory for all HVAC contractors. It would protect the customer from hacks and unscrupulous contractors. Smaller equipment means less cost for electricity over the life of the AC unit and less cost up front which saves the buyer money. The electricity savings is why the utility companies and many states are making it part of their mandatory requirements to have their HVAC contractors perform Manual J.

Here's a good explanation from Dean Heating and Cooling on HVAC Talk Forum that explains the benefits of using a Manual J and also talking about proper duct design (Manual D).

"A load calculation known as Manual J is the only correct way to determine heat loss and heat gain. Once equipment size has been determined, the duct system is designed by Manual D. These are design protocols formulated by ACCA. Duct design takes into account the location and potential gain (or loss) of having ducts in an attic, crawlspace, or even exposed on a roof.
You are correct the installing a larger air conditioner on an undersized duct system is pointless. We have all seen it many times.

Frequently, putting in smaller equipment delivers greater comfort when the existing equipment is too big. For example, a 4 ton unit on a 2.5 ton duct system can only deliver 2.5 tons worth of air flow. The oversized equipment still uses the electricity for 4 tons of cooling but the delivery system still limits what arrives in the space. The equipment actually uses more energy because of inadequate air flow. Plus it cycles more often than smaller equipment so its life is shortened."

Here's a good discussion about Manual J calculations also from the HVAC Talk forum. There's an HVAC designer and an HVAC contractor that post over there that seem to have the most input on Manual J. Their user names are "dan sw fl" (AKA Designer Dan) and lkapigian.

Austin, where do you get your "oversized 90% of the time" statistic from?

Suns cycle of rising and falling. There is no bigger HVAC load, and this load varies which is why your system is Oversized 90%. In other words, common sense.

24 hours in a day. Heat of the day builds and builds until 3pm to 6pm. (3 hours / 24 = 8) 1/8th of a day under normal conditions is your AC properly sized.

Under extreme heat 3-7pm (4 hours / 24 = 6) 1/6th of a day. Extreme heat last under most cases a few weeks to a month. This is NOT a reason to 'FURTHER' over size your air conditioner.

Seasonal changes: it doesn't get quite as hot and the heat lasts less. Sun sets earlier. Without suns heat there is very small loads. Loads are the primary ingredient to manual J calculation. Again, common sense.

It would protect the customer from hacks and unscrupulous contractors.

So who is going to pay for the manual j police to ensure the manual j was done correctly?

The single biggest advantage you have is time. If the current system 4 ton, 3 ton, 5 ton whatever size it is was installed in your home for 10, 15, 18 years and the only reason you are replacing it is that it is broken. Do yourself a favor and replace it with the size that it is.

Heat loads vary... all the time. Sometimes it's hot, sometimes it's not. There is no such thing as a 'concise' heat load calculation.

With this said, I have come across recent flagrant improper sizing. But these situations are extremely rare. I'm lucky to come across one of these over the course of every couple of years or so.

I service the Katy, Texas area.

Ray,

Your post validates two things we are discussing on this post. There are excuses why a heat load calculation should not be done, and there is flagrant improperly sized equipment installed in the field.

Right on, Mike. The "that's what was there, it must be the right size" attitude is especially unthoughtful knowing the kind of hacks and shortcuts that are taken with HVAC systems in new homes. To have the normal practice of continuing the original mistake with another iteration of the same doesn't demonstrate professional skill and knowledge. Sorry, I'd hire someone else.

In my recent complete equipment replacement, my contractor's attitude was "What's there must be oversized (rather than right-sized), let's do the calcs and see what we find". Indeed, each piece of equipment was determined to be oversized and was replaced with one with less output. Performance is great, comfort is improved from longer cycles (but with on/off cycles all the same), and operating costs have declined. For me, skilled expert service in HVAC must include finding the best solution alternatives for the circumstances, not simply doing a competent installation job.

The other point that the contractor from Dean Heating and Cooling made is that if the AC is not sized correctly for the house that will cause it to cycle on and off more frequently so the lifetime of the unit is shorter than normal. That's another expense that can be saved by using a Manual J to get a correctly sized unit.

The other thing that Elmer Fudd mentioned is that he has a more comfortable environment after having properly sized AC units installed. That's another reason to have a Manual J calculation performed.

One furnace 135 down to 110 or 115 (I forget), the other 80 to like 45. 80% efficiency old and new (mild weather area).

Mike, I did say it's rare to find it.

How rare is rare. Less than 1% of the homes I go to.

If the home has been there for 18 years like in the case of this thread / I would go back with the 4 ton.

There have been threads posted to this board in which a heat load calculation was performed. The equipment was later determined to be sized wrong and removed from the property.

Anyone can come in with a guess and be wrong (if you are not building a home / a manual j heat load is an educated guess due to factors used to get the numbers you get). You must know insulating factors to perform a heat load. This is beyond the math to put it all together. The life of the house (age) and the monthly cost of unit operation can all be clues toward that goal.

If a contractor was to go into every home and proclaim the AC was oversized, that contractor wouldn't last long in this business.

How do I know this? because larger units sell. People often complain the unit isn't large enough. This complaint is lodged when we are experiencing a heat wave. You think a HVAC contractor going into a home in this heat has a fighting chance of getting someone to reduce the size of their AC during a heat wave. In many cases it's hard enough to get someone to agree with you that they should go back with their current size of equipment.

Currently in my area of Katy, Texas we are seeing temperatures normally not seen until late July and early August.

Just like every heat wave before, this too shall pass. Proper size is still oversized 90% of the time. Those numbers do not lie.

1/8 of a day. 100% of a day / 8 (100/8 = 12.5 --- 100 - 12.5 = 87.5% throw in seasonal changes puts you easily at 90% oversized rate.)

Always do what you can to reduce heat-gain so U can get below 4-Ton units; it is very difficult to design for enough airflow with larger tonnages...!

Indoor coil pressure drops will usually be .43" & filter drops due to lack of enough filter area - will be another .3" PD for a total of .73"; above even a .7" Rated blower, leaving virtually no Available Static Pressure (ASP) to move air through the duct system. You need .08" for proper friction Rate Design...!

You need .2" of ASP *100 is 20/250-TEL is a required .08" Friction Rate Design... you usually can't even get .1" ASP with a 4-Ton unit...!

Austin, do you normally factor in and measure what udarrell is talking about?

There's a limit to the amount of science I put forth. If this works of Udarrell, I'm pretty sure he's cornered this market niche.

I am in the process of attempting to drop the cooling tonnage of my current home in Katy, Texas from 4 ton to 3 ton, but there is much more to this and very costly upfront. It may wind up (due to air flow concerns) having to use a 4 ton air handler in conjunction with a 3 ton condenser. It will probably be an inverter system or minimally a 2 speed as it is now.

This method of reduction will not work for every property. For the primary reason most homes have large banks of windows and windows are huge heat loads.

How I am going about this: Reinsulated attic to roughly R30-R38, repaired major thermal break at fire place flue.

Here are a couple of pictures before I fixed this problem:

I will drop the size of the windows from current size to 3'x3' size (4 of them)

I should easily shave 1 ton of cooling load from this structure (aka my current home in Katy, Texas).