Is this feasible?

[Tony0945]

An old story: I bought HVAC-CALC. It determined that I needed a 2 ton unit, even with a generous safety factor. Bought a 3.5 ton unit because salesman scared the wife by claiming we needed to at least buy the same size as the old unit, in fact he recommended 4 tons based on our square footage. I managed to hold the line at 3.5 and consoled myself that the two-stage unit was OK on low cool. Salesman pulled a switch and wrote down the model number of a single stage unit on the contract.

Last year wasn't too bad, I lowered the fan speed by 80% from default and got halfway decent dehumidification. Last year was dry. This year is not. I have to drop to meat locker temps to get decent dehumidification. I was ready to throw out the money I paid and replace the unit with a new two stage when I got this idea.
Namely to add a two ton complete unit and two stage thermostat to the system, connecting the thermostat's low cool wires to the two ton and the high cool wires to the 3.5 ton. THIS IS NOT A DIY PROJECT.

Is this idea all wet? Would putting two indoor coils in the air stream produce too much back-pressure? I wanted to get opinions from this board before talking to a contractor, because none of them around here will even do a load calc. They probably won't like the unorthodox idea either, so I need ammunition if it's a feasible idea.

[johncavh]

all wet, get a load calc done have it done the right way.

[Kizer]

It seems somewhere I saw an ad for a motor/ module combo that was basically an after market variable speed assembly that could be retrofitted to most air handlers. That along with a TXV, assuming you don't already have one, might help you improve your situation. I think White Rodgers makes the motor assembly.

[contactor]

So now you want 5.5 tons with insuficient air and ducts?
One can of worms to another.
I would start from scratch and find a contractor to size and design the system, Too many people try to figure their size, buy something else because of price, and then try to save face with what they got,
Oops, you just said that.
Good luck, Maybe an out of town contractor, If you find a good one, don't bring up your last idea You'll scare him off too.:)

[ga-hvac-tech]

The correct thing to do is get a professional out there that will do a load calc and set you up with a proper system. They may well suggest some duct modification... if they do, do it!

One alternative that comes to mind is a whole house de-humidifier attached to the HVAC system. AprilAir and Thermastor make them. (Google the names).

If some other system keeps the air dry, then the cooling will work better.

But if it were me, I would get it done right: see first paragraph above.

[ckone180]

An old story: I bought HVAC-CALC. It determined that I needed a 2 ton unit, even with a generous safety factor. Bought a 3.5 ton unit because salesman scared the wife by claiming we needed to at least buy the same size as the old unit, in fact he recommended 4 tons based on our square footage. I managed to hold the line at 3.5 and consoled myself that the two-stage unit was OK on low cool. Salesman pulled a switch and wrote down the model number of a single stage unit on the contract.

Last year wasn't too bad, I lowered the fan speed by 80% from default and got halfway decent dehumidification. Last year was dry. This year is not. I have to drop to meat locker temps to get decent dehumidification. I was ready to throw out the money I paid and replace the unit with a new two stage when I got this idea.
Namely to add a two ton complete unit and two stage thermostat to the system, connecting the thermostat's low cool wires to the two ton and the high cool wires to the 3.5 ton. THIS IS NOT A DIY PROJECT.

Is this idea all wet? Would putting two indoor coils in the air stream produce too much back-pressure? I wanted to get opinions from this board before talking to a contractor, because none of them around here will even do a load calc. They probably won't like the unorthodox idea either, so I need ammunition if it's a feasible idea.

See what happens if you go against technology? Thought you were smarter than a LOAD_CALC, huh?

[mech_head]

or you could put in a hot gas bypass:cool:

[ga-hvac-tech]

Before I write this post, I want to specifically say that I am NOT saying anyone does or does not know their profession. OK, now that I have defused a potential arguement:

There is something we need to remember about load calcs: Variables!
*How does one accurately measure infiltration? (Do you do a blower door test on every load calc?)
*How does someone accurately measure the insulation in the walls? (Do you own a thermal imaging infra-red camera and use it on each wall of each load calc?)
*How does one accurately measure duct loss? (Does anyone do an accurate duct leakage test on every load calc?)

OK. My point was NOT to chastize anyone nor to look down on our industry. My point is to say that load calc's have variables that have to be 'guessed' at, and the results of the load calc are only as accurate as the 'guessing'.

This is just one more reason that a seasoned professional should be doing the load calc, as well as the sizing. While they may not own and use all the expensive gadgets necessary to answer the questions posed above, they do have the experience to understand and make a lot better 'educated guess' than someone that is doing a load calc for the first time.

OK, I am zipping up my flame retardent suit, you can start in on me... :)

[contactor]

I somewhat agree ga-hvac, most info is based soleley on paper and not based on construction technique, anamosities, settling, mistakes nor on actual measurements.
And not to scare you off Tony, your unit may be perfectly sized.
And if not, a dehumidifier may help

[allstar08]

I will say that it would be a supreme waste of money and time to try the two coil thing, which IMO will not work. Explain your situation to the company you are working with, if they do not take the appropriate measures file a complaint with the BBB and your local state office.

Then when you're done doing that install the proper system for your house or deal with what you have. A variable speed retro may work, but can be pricey. Add more ducts or put in a bypass and cool your attic or something, not really a good solution. The only thing I can think of that would do the trick is a two stage cooling system. Just my 2

[Tony0945]

ckone180, it was my wife and the salesman who thought they were smarter than the load-calc.

The existing system is variable speed. I did have an attic fan added when the roof was re-roofed. Only one return upstairs. Big problem. In a new house under construction from Ryan Homes, I saw insulated flexible duct routed through the attic. I imagine that could be back-fitted into my house. I really preferred the all metal, all inside the envelope ductwork that I saw at Cambridge Homes.

Looks like the choice is between throwing away the cooling part of the system and buying a new house. I prefer the new house, but then I'd have to find a buyer for mine. Bye-Bye Carrier. I'll talk to a Trane dealer next time.

Thanks again for the many quick responses.

[beenthere]

There is NO 2 stage 3.5 ton made.
So he didn't switch or write the wrong mod number, it doesn't exist.

Hope your wife is happy with her choice.

good luck getting her to ok another OD unit.

[smokin68]

At this stage a whole house dehumidifier is your best solution IMO.

Why do a load calc?

Why size a unit?

Why make sure it operates correctly?


Indoor air quality,energy consumption,and system longetivity are the main reasons I can think of. Now that IAQ can't be achieved with your oversized system, it needs help, a dehumidifier.

JMO.

[bb]

Any chance of you adding-on a large mother-in-law suite ?

[Kevin O'Neill]

bb,

I got lots of gas. My co-workers say it is OK if I send you some. :D :D

[ckone180]

ckone180, it was my wife and the salesman who thought they were smarter than the load-calc.



I hate to hear that, not the part about the salesman upsizing, but the fact your wife could convince you of a mechanical item! LOL.

Don't worry, my wife would too, if I asked for her opinion.

[energy_rater_La]

Why not try a stand alone dehumidifier located in your conditioned space?
Removing humidity down to acceptable level (RH 50% in my hot humid climate) will remove 'hidden' heat & make house temsp seem cooler.
For a couple of hundred bucks, Lowe's & HD carry EnergyStar dehumidifiers.
One stand alone dehumdifier will dehumidify 2,000 sq ft.

We have solved a lot of these issues with this simple install.
If you later want to upgrade to a build in dehumidifier these are the guys to tell you which one & why.
But first see if additional dehumidification is the answer for you.

Best of luck.