5T coil with 3T condenser - will it work??

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Old 09-12-06, 02:47 PM
DGF
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5T coil with 3T condenser - will it work??

I just found this site - what an amazing amount of invaluable info - wish I had seen it earlier!

Anyway, my somewhat weird situation is: I am having an AC system retrofitted to my existing forced air heating installation. The system was originally prepped to take AC, but the condenser was never hooked up. On inspection, it seems that I have a 5 ton Carrier CE3A coil already installed, and a 3 ton Goodman GSC14 (14 SEER) condenser sitting on an adjacent pad, but not hooked up. My furnace is a variable speed 'Day & Night' 90i 355MAV unit.

Predictably I guess, the original HVAC guy is now nowhere to be found, so I called another installer who tells me that there is no way the 5 ton coil will work with the 3 ton condenser - is this correct? I don't mind some loss of efficiency or slightly raised operating costs, but will this combo flat-out not work?

Based on other advice and on multiple sizing exercises I believe that the 3 ton condenser is sized correctly, and the furnace delivers nicely in the winter. If I understand correctly, swapping the coil for a smaller unit could restrict my CFM throughput, potentially decreasing heating capacity in winter - is this a valid concern?

This new guy tells me that I should either downgrade the coil to 3 tons, or replace the condenser with a 5 ton model - matching the units obviously makes sense, but as I already have the 3 ton unit sitting outside and a fairly good quality coil installed, I'm trying to understand how best to move forward.

Thanks in advance for any words of wisdom!

- Dave
 
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Old 09-12-06, 02:56 PM
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Wink

For sure get one or the other. You cant do what you have now Have some one run a AC load on the home for what size you need there. For a 3 ton AC you need about 1200 CFM. What is the furnace cfm ?????

ED
 
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Old 09-12-06, 03:46 PM
DGF
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Thanks for such an amazingly fast response!

I have an old print-out with what looks like load calculations on, I have no idea how it was derived, but it has room volumes, window sizings, R-factors, etc. I'm unsure if this is the type of thing that you were refering to, but the bottom-line seems to indicate a 3 ton AC requirement.

Regarding the furnace CFM - I believe it is currently running at 2000 CFM. Cutting back to 1200 CFM for the 3T coil will presumably give a large drop in my heating abilities. This is the root of my dilemma.

So, from your initial response it would seem that my current coil/condenser combo is a no-go. Is it that it just won't work, or will just it work badly? [I asume that the 2 ton difference is too big as I've seen other posts where smaller deltas seem ok].

Regards,

- Dave
 
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Old 09-12-06, 04:09 PM
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You could set your furnace to run at a slower speed for A/C than for heat. You would need to find the cfm for all the speed taps on the furnace and see if you are able to reduce it down to an acceptable level.

I don't see why the coil wouldn't work with the correct sized piston or TXV. You can always go larger inside but not outside. I am currently running a 4ton furnace and evaporator with a 3-ton condenser and it works great.
 
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Old 09-12-06, 07:24 PM
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Coil

My biggest concern would be if the condensing unit is R410a, will the coil stand the higher pressures? As far as will it work? I'd bet that 5 ton coil is not much bigger than today's 3 ton coils. Like CovTiger said, as long as the correct txv or piston (I prefer the txv) is used it should work fine.
 
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Old 09-13-06, 05:42 AM
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The gsc14 series condensers are r-22. So no problem there. I think it would probably work if you can slow down that blower in cooling mode. If it were mine, I would hook it up and see what kind of results are obtained. You really have nothing to loose, the condenser has to be hooked up anyway as it would be impractical to put in a 5 ton condenser when only 3 is needed. Besides, a new 3 ton coil is cheaper than a new 5 ton condenser. But again, I think it has a good shot at working.

Let us know what happens.
 
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Old 09-13-06, 09:27 AM
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Thanks to all for the valuable feedback.

Jim's post did a great job of summarizing all the input - I do now think I will give my current combo a try (+ txv), and see if the results are acceptable. If not, I'll down size the coil.

The 90i Plus furnace appears to be extremely flexible with all aspects of its configuration, so setting the dip switches to throttle things back during the summer months is pretty easily done.

Thanks again for the help - you guys rock!

- Dave
 
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Old 09-13-06, 03:58 PM
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I agree with the TXV, make sure the TXV is rated at 3 tons, and have the dealer set up the dipswitch to make the blower match then 3 tons.

What type of t-stat do you have?
 
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Old 09-14-06, 06:25 PM
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Hi Jay

Thanks for the further verification.

My thermostat is a middle-of-the-road White-Rodgers model - given my situation, should I be looking for any particular features/capabilities?

- Dave
 
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Old 09-14-06, 06:34 PM
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If you don't have a two stage t-stat, I would look into that to get the full comfort out of a two stage furnace. or if you want to control the furnace speed and what not from the t-stat, look into the evolution t-stat.

http://www.bryant.com/products/controls/evolution.shtml
 
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Old 09-15-06, 08:54 PM
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Thanks for the great url - I now realize that I've only been using a subset of my 90i's capabilities by not having a two-stage t-stat!

I searched, but couldn't find a related wiring diagram for the Evolution - any idea how many individual wires I would need to utilize its full capabilities?

Thanks again for all the guidance.

- Dave
 
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Old 09-16-06, 04:44 AM
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I have not been around these type of control, I think I recall reading some where sayint it uses 4 wire, and your furnace board should have hooks up for that. Not the R-W-Y-G where a a normal T_stat goes to.
 
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