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coredad 09-25-2009 11:06 PM

Two AC Systems or One...HELP!!!
After about 3 months of gathering info on HVAC systems we finally decided on a brand (American Standard), a model (Allegience 16) and a vendor. We 'hosted' 6 different vendors before deciding one one.

Current system is 3.5ton Trane downstairs (1900 sqft) and 2ton Trane upstairs (1000 sqft). Older system with SEER around8-9.

We decided on and were recommended a 4ton 16SEER/13EER system for downstairs and a 3ton 16SEER/13EER system for upstairs. Got a reasonable price (~16.5k).

During the final talk with the vendor prior to starting the work after the weekend, he says, 'what about a 5ton 20SEER dual system and zoning instead of the two systems?'

Now why would he do that? After months of agony and heat finally deciding on something we could afford and get decent cooling effeciency and lower electric bills another option that we didn't even know existed.

So, the question(s) is, which option makes more sense? Two AC systems draw more current than one but will zoning be an effective alternative? Will trying to supply cooling/heating to two different zones mean that the one unit will be running all the time where with two units only one might be running while the other is off. It is all so confusing when I thought the answer was at hand.

I appreciate any light that can be shed on this delima.


vsheetz 09-26-2009 12:02 AM

I have owned two story homes with single HVAC units - and I have owned two story homes with separate HVAC units up/down. Due to the problem of either up or down being too hot or too cold with a single unit - I would go with separate HVAC units up/down.

Joe F 09-26-2009 03:47 AM

Has he done a heat load calculation?

beenthere 09-26-2009 05:07 AM

Find a new contractor.

Ok. So, what kind of problems have you had cooling your 1000 sq ft second floor, that he wants to increase its size to 3 tons. Thats a 50% increase in size.
Does his quote include duct and register changes to handle the additional air a 3 ton requires over a 2 ton?

Was a load calc done. or just a guess at what size to use.

Do your own load calc, and see what size you really need, before you fall prey to guess work

HVAC Calc Its a 50 dollar fee. And it is well worth it.

hennyh 09-26-2009 07:37 AM

So what's the problem with your current system? Have you been happy with it's performance up to this point?

coredad 09-27-2009 06:41 PM

Thanks for all the reply's.

Heat load to be done tomorrow just prior to starting install. This is when I need to decide 1 (5ton A-S 20SEER) or 2 systems (4ton down and 3ton up A-S 16SEER).

Upstairs has always been hot with 2 tons, one 4x5 window on East and one on West side of house plus large 8x8 on South. I suggested 3 ton to overcome cooling deficit.

Contractor will adjust air returns to account for additional AC capacity. Upstairs will add three 'in-room' 10"x10" returns and downstairs increase size of existing return and increase size of return duct from 18" to 22" (I think this was the number, could be wrong).

After reading some I'm leaning toward staying with the 2 system option but don't want to do something stupid...(my wife would say 'again' at this point).

I really appreciate all the advice.

beenthere 09-27-2009 07:46 PM

Duct alterations either way.

Make sure they do an ACCURATE load calc, and not just fudge it to come out to what they said.

Either way will work, if done properly.
2 systems gives you the advantage of having one working system. If the other breaks down.

Better to sleep downstairs on a Saturday night, then sweat your butt off. And have your wife say I told you so.

sktn77a 09-27-2009 10:11 PM

Stick with the two systems. And I wouldn't be too quick to increase the capacity of the outdoor units. You will have problems with oversized condensers (short cycling, inefficiency, noise, poor dehumidification, shortened lifespan, etc). Are you sure the problem with the upstairs was sizing and not something else?

coredad 09-30-2009 09:43 AM

I want to thank all of ya'll for your advice. A load calc was done and showed 4.5 tons required (I watched). I decided to go with the two units (thanks again for the input).

A 4ton downstairs and a 3ton upstairs. Both are dual stage compressors and so quiet that you almost have to be in front of them to know they are on. The one thing I failed to take into account was the size of the outside units. My old system was not nearly as big as the new ones. The tallest one now sits partially in front of a den window so will have to engineer a distraction from that.

All the duct work was rehung (off the attic floor) and rerouted from the original install (20yrs ago), two new return air ducts upstairs, increased the size of the return air downstairs and replaced and enlarged all the return air ducts to both systems.

Additional stuff was a new decking in the attic for the coils/furnace, etc. New drain pan (and a float switch which I never would have thought about) and PVC lines for drain. New gas line to furnace (I like the electric ignition and no pilot light!).

Replaced all of the old plastic grills with new metal ones and caulked all the joints between the vent body and drywall.

Well, enought of the details. Point is in my very limited experience with AC installs, I think they did a very good job and as I've read in many articles, time will tell if I am right.

So, guess I'll give the company a plug and say that based on what I saw and the answers to so many questions while this was going on, AirLock Mechanical in Cypress, Texas did a very good job and worked in a very professional manner both inside and outside my home.

Once again, thanks for all the help.

beenthere 09-30-2009 10:15 AM

Hope all works out for you.

If you got the 20 SEER you should be ok.
If you got the 16 SEER, then you could have a problem, since with both units running in first stage. They will have a combined capacity of 5 tons or more.

Could lead to humidity problems.

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