Using Two Units To Replace One

Atricaudatus

Question for you HVAC experts:

My father has a large open space being served by a single 15 ton unit. The air is distributed via two trunk line downs the center of the drop ceiling. The unit is old and will soon need to be replaced. He would like to know if he can use two smaller units, say 6 ton or 7, with one connected to each trunk.. Of course he will also split the returns.

Will the units function properly with two separte units, each with it's own t-stat, serving the same area, or will one unit end up doing all the work.

Also is there a way to interconnect the controls or t-stats, or to make both units function off a single thermostat; so that both units operate together.

Is this common, or are we asking for trouble here?
Any advantage to using one unit vs two that we should take into account?

Thanks for your help.

__________________
-Rhett
Licensed electrician and plumber

Lukester

I have never heard of a 15 ton system in a home, unless it is a chiller with multiple zones. How big is his house? Have someone do a manual J load calculation to determine exactly how much air is needed. If 15 tons is required I would consider three systems. Typically a 5 ton A/C is the largest you find in a home, after that it jumps to a 7.5 ton 3phase unit. Trane used to make a 6 ton unit, I'm not sure if they still do.

I just read your post again. If this in fact a commercial project you should be able to use two seperate systems. You may want to consider a two stage system and just use the existing duct the way it is.

Atricaudatus

Thanks Luke.
This is a "commercial" type installation, not a home. It's for the local Shrine Club, which has a large hall or dance floor area. the old 15 tonner is on it's last leg and the members think they can save a few bucks with two smaller units instead of one big one.
Would the two stage solution be more cost effective or would it mean additional equipment that would end up offsetting the cost of just biting the bullet and shelling out 20K for another 15ton unit?
The goal here is to use two smaller cheaper units in place of the more expensive "commercial" sized units.
Sorry for all the questions, but I am an electrician and not qualified to advise on this issue; and of course you're dealing with a committee. So, there is no shortage of "opinions" on the matter.
I have talked up the virtues of C Talk.comn and all the helpful and smart guys (and gals) on this site, that the president asked me to put the question up for your advise.

__________________
-Rhett
Licensed electrician and plumber

Lukester

The only thing about two smaller systems is that they will still be commercial size equipment. Anything over 5 tons is considered commercial and the price reflects that. It would be cheaper to get a 15 ton two stage system for cheaper purchase price and operating cost. Like most commercial applications you don't need the full capacity all the time. With a two stage system you have half capacity on a mild day or a day when there are not many people in the building, when it is hot or packed with folks the second stage is available, it's kinda like having two smaller systems in one. This is only my second post on this site, I just found it myself. I'm hoping the costruction trades will help me with questions in the near future.

Aceinstaller

lukester summed it up pretty well.

welcome to the forum luke

locofoco

Is the original unit a roof top unit. If it is, it would probably be cheaper just to replace it. Why go to the expense of splitting the ductwork and re-doing the electrical? Why cut in a new roof curb? More money.