3 ton or 3.5 ton?

[Teryt]

I have a HVAC background (but haven't worked in it for a few years), and a HVAC contractor friend of mine is buying and helping me install a new package unit. Our townhouse is in Phoenix and is 1400 sq. ft., with two adjoining walls. (The old unit is a 3 ton split system about a zillion years old.)

The overall question is whether to go with a 3 or 3.5 ton, which is only about $70 difference in initial cost. The old 3 ton got us by okay, but admittedly struggled on really hot days and/or when we entertained several people. So in that respect, I think a 3.5 ton would fit the ticket better.

The more specific question is this: Will the 3.5 ton run less enough to offset the higher wattage it will use, and if not, by how much?

Thanks & hope that makes sense!

[coolwhip]

What does the manual J load calc. say? Is the ductwork adequate for the system?...manual D?

[Teryt]

What does the manual J load calc. say? Is the ductwork adequate for the system?...manual D?

We would drop another register in to handle the extra 200 cfm of the larger unit. But all things being equal, will a 3.5 ton unit running a less amount of time offset it's higher wattage?

[REP]

So what you are saying is that you want a unit to hanle a party (group of people) every time it comes on.
Did you or did your equipment "struggle" when it got hot.What does struggle mean? did it maintain temp or not?What have you done to improve the envolope of your townhouse.
Things like insulation and sun screens and shades not only pay you back with energy savings but also comfort too.
So when you look at it that "only $70.differance"could end up costing you hundreds of dollars every year.

[Teryt]

So what you are saying is that you want a unit to hanle a party (group of people) every time it comes on.
Did you or did your equipment "struggle" when it got hot.What does struggle mean? did it maintain temp or not?What have you done to improve the envolope of your townhouse.
Things like insulation and sun screens and shades not only pay you back with energy savings but also comfort too.
So when you look at it that "only $70.differance"could end up costing you hundreds of dollars every year.

I'm not sure, but I think design temp in Phoenix was probably around 105 degrees when they installed this old unit almost 30 years ago . . . It frequently gets hotter than that here, and the 3 ton unit "struggled" to maintain temp. On hotter (than 105) days, if you add some humidity and a few people, then it would not do a good job of cooling off and would run constantly.

And yes, we added a bunch of foam insulation to the roof and garage door, changed windows to thermal pane Low-e, and most lights to fluorescent . . .

So what you are saying is that the decreased running time of the 3.5 ton will not offset the higher wattage - correct?

[WayCoolJr]

With the changes you've made and the efficiency of a new unit you may not have to up size your unit. If in doubt have a contractor come and do a proper heat load calc on your home and then you eliminate the guesswork. Have them check duct work for leakage or restrictions in flex ducts. A/C units are often blamed for poor performance when the failure may be elsewhere in your system.

[VMSCO]

I agree with WayCooJr. Proper sizing is more critical in your area than other areas of the country. There are so many factors that need to be considered, position of the house, how much glass, shade from trees, awnings, window coverings, shades, total insulation. Especially in your area a Heat Load Study would be a very good idea.

Having once lived in Florida for 5 years I know that when high temps hit the A/C can struggle to keep the house at a comfortable level if not sized according to the region. Of course we had to contend with high humidity, where that should not be a problem in your situation. My house which was brand new was built with a 2.5 ton heat pump system and should have been a 3 ton. The builder must have used formulas to calculate the sizing for a home in the midwest. Their home office was in Indiana. The undersizing caused the unit to run endlessly, costing hundreds of dollars more to operate than it would have with a proper sized unit. Plus the house was never comfortable when temps approached 100 degrees.

Getting a Heat Load Study would be worth the investment.

[Teryt]

With the changes you've made and the efficiency of a new unit you may not have to up size your unit. If in doubt have a contractor come and do a proper heat load calc on your home and then you eliminate the guesswork. Have them check duct work for leakage or restrictions in flex ducts. A/C units are often blamed for poor performance when the failure may be elsewhere in your system.

We made these changes in windows & insulation about three years ago and the existing 3 ton still didn't always keep up. Contrary to conventional wisdom, we do get some periods of high humidity in Arizona during our monsoon season from mid-July into early September. It can easily be 110+ degrees with 50%+ humidity, and during those times the old AC runs near constant withthe therrmostat set to 82 (this is also when waking up to 90 degrees at 6 AM). Plus our downstairs has never had enough air flow, so dropping an additional register down there and another 200 cfm, should help that area.

If we look at the 3 ton as approx. 2.57 KW and the 3 ton at about 3 KW, it will take about 1/6th more electric to run, so I guess the question is, will it also run 1/6th less?

[VMSCO]

Teryt,

What you are asking cannot really be determined on a forum. Any input regarding the current draw and cost of operation would require much more information than you could ever provide here. Someone that can physically see, measure and gather facts could give you the answer. At the very best all any of us here could give you would be a wild guesstimate. You really need to call in a pro. If you do call someone in, make sure they are qualified to do a proper analysis. Some dealers will automatically refer your study to their salesman who may or may not be a pro. I would suggest you turn to referrals for which company in your area is truly qualified .

Best of luck

[genduct]

You do have times where you actually have RH humidity to deal with so Right Sizing still remains a challenge, does it not?
Old unit really putting 3T of output? then get the larger unit. I would be inclined to stay with 3T that has worked except during times when you have exceeded design.

[Teryt]

Wouldn't the slightly larger unit running less also be less wear on the unit over time? Could I expect the 3.5 ton to also last 15% longer?

[sammy37]

I cant see why 3 tons would have an issue with 1400 sq feet.
I am curious as to how your going to go from a split system, to a pack?

I bet you have some undersized ductwork. If you could replace, upsize it and run a load calc, I bet you could get by with less then 3 tons. Especially since you've done improvements to tighten up the house already.

[Teryt]

I cant see why 3 tons would have an issue with 1400 sq feet.
I am curious as to how your going to go from a split system, to a pack?

I bet you have some undersized ductwork. If you could replace, upsize it and run a load calc, I bet you could get by with less then 3 tons. Especially since you've done improvements to tighten up the house already.

We have to go to a pack because the space the indoor blower is in won't allow for the newer/larger air handlers. Fortunately the air handler is directly adjacent - no more than 3' away through the ouside wall from the condenser.

You may be right with the ductwork. For sure there is not enough downstairs. That's why we're thinking to run a flex duct directly off the supply side for another register downstairs. So maybe it is more of a airflow thing than a unit capacity issue . . .

[BaldLoonie]

Wouldn't the slightly larger unit running less also be less wear on the unit over time? Could I expect the 3.5 ton to also last 15% longer?

Starting & stopping more frequently is probably worse than a lot of steady running.

[sammy37]

We have to go to a pack because the space the indoor blower is in won't allow for the newer/larger air handlers. Fortunately the air handler is directly adjacent - no more than 3' away through the ouside wall from the condenser.

You may be right with the ductwork. For sure there is not enough downstairs. That's why we're thinking to run a flex duct directly off the supply side for another register downstairs. So maybe it is more of a airflow thing than a unit capacity issue . . .

So, it sounds like your going to set the new unit outside and duct it into the old air handler closet on the other side?
That would work but there are ok ways and really bad ways to do that.
Is the return grill underneath the old air handler? As you know, package units have a side by side airflow configuration, which means that you will have to have a transition made that will convert the airflow to an over/under setup.
The return would then come straight through the wall into the return cavity under the closet and the supply would come into the closet and have a 90 degree elbow up to the ductwork.
If you do indeed have a return grill under the old air handler, then I think right off the bat it will probably be undersized. Also like I mentioned earlier, your ductwork is probably undersized also.Whichever way you decide to do the job, make sure you do it right or you will end up with less cooling than you have now.

[Teryt]

So, it sounds like your going to set the new unit outside and duct it into the old air handler closet on the other side?
That would work but there are ok ways and really bad ways to do that.
Is the return grill underneath the old air handler? As you know, package units have a side by side airflow configuration, which means that you will have to have a transition made that will convert the airflow to an over/under setup.
The return would then come straight through the wall into the return cavity under the closet and the supply would come into the closet and have a 90 degree elbow up to the ductwork.
If you do indeed have a return grill under the old air handler, then I think right off the bat it will probably be undersized. Also like I mentioned earlier, your ductwork is probably undersized also.Whichever way you decide to do the job, make sure you do it right or you will end up with less cooling than you have now.

The old air handler hangs in a space where the whole space is the return. So there is a straight shot for the return with no air restriction. The supply side is also pretty straight-forward, and we are taking additional steps to maximize the airflow (besides just the additional register I mentioned B4.

Also, what someone ellse said about lots of starting and stopping being worse than more constant running makes sense. I know that starting and stopping anything is one of the more violent things you can do to it . . .

[2old2rock]

All these guys have been telling you to get a load calculation. If you don't, it's all just a shot in the dark.

[Teryt]

All these guys have been telling you to get a load calculation. If you don't, it's all just a shot in the dark.

Yup, and I'm comfortable with that. Using a semi-SWAG approach, I feel confident we're in the ball park - and especially since the old unit was a 3 ton and did OK, I don't think we can go too far wrong. (Plus the guy that's helping me had his own residential installation & service HVAC company, with about 15 employees, for about a dozen years before he sold it, and then worked for Trane for about 10 years before going on his own. Between us we have more than enough knowledge to be at least fairly dangerous in our own minds!)

So thanks everyone. I think I'll go with the 3 ton. And to increase air to the 1st floor, run a by-pass supply line to the downstairs trunk and also add another register down there. The 3 ton will run a little more, but won't cycle as much, which I think is a good thing.

[sammy37]

Like I posted earlier, your going to need some custom sheet metal fabricated for your new setup, make sure that it is done correctly! I have seen simillar conversions like this done here and believe me, it wasnt a pretty site.

Maybe when you get the job completed, you could post some pics of it.

[VMSCO]

Teyrt,

Best of luck on your installation. Please keep us posted and let us know how it all works out.