13 SEER Condenser with 10 SEER Evaporator


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Old 07-19-08, 01:59 PM
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Question 13 SEER Condenser with 10 SEER Evaporator

Hey, guys. I'm trying to learn about this air conditioning stuff.

I'm in Houston, had a 13 year old Rheem split system, 4 Ton and 10 SEER. It wasn't able to keep the house cooled, a guy came out and said the condenser was bad, the fins on the coil were falling off. He also cleaned the coil on the evaporator.

So he puts in a new Rheem "Value Line" 13AJA48 13 SEER 4 ton condenser. It works better, but it still can't get the temp below 78. He comes back out and says the return vent is too small, it's 12x36. It does pull air pretty hard, but it was adequate in past years when the system was working well.

So I start doing research and find out it's bad to have a compressor coil larger than the evaporator coil - I guess that's what I have now? So do I need to have the evaporator coil replaced or would a thermal expansion valve (TXV) feeding the evaporator be enough? What's my cheapest fix?

Thanks for your help,
Matt
 
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Old 07-19-08, 03:27 PM
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These are merely suggestions, use your own judgment, & that of the local contractors.
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First, a lot of the heat-gain issues need to be effectively dealt with!

Second, the duct system needs to be thoroughly sealed & everything sized for optimal airflow. (Enlarge that Return & more!)

Third, after doing the above, with that SEER mismatch, I would experiment with different CFM airflows between 350 per/ton to 450 per/ton, & use what works best.

Of course, the condenser & evaporator SEER Ratings should be matched.

The new 4-ton 13-seer condenser will no doubt have a smaller BTUH compressor than the old 4-ton 10-seer.

If a proper sized TXV were installed on you 10-seer E-Coil, it might work reasonable well, especially if you are dealing with humidity problems.

If you don't want to spend the money & do everything possible in the correct sequence, it may get the temp & humidity in your comfort zone.

In real world conditions, high SEER is not the whole story, do what you can afford & don't worry about it. HVAC RETIRED - udarrell
 
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Old 07-20-08, 06:06 AM
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I Would Absolutely Replace the 10-seer Coil

In prior post, I was trying to hard to save you upfront money! That could be very costly in the long-run.

Ignore prior post, & do what is right for the equipment & for efficiency, install a matching E-Coil with a TXV!

Concerning heat pumps, the charge level is different for heating mode than cooling mode on mismatched systems. Again, with a HP it must always be a matched coil system.

HVAC RETIRED - udarrell
 
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Old 07-20-08, 03:19 PM
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Thanks for your input, udarrell.

It's not a heat pump, we've got a gas furnace.

Humidity isn't a problem now, the problem is the AC runs continuously but can't get the temp below 78-79 in the day.

Any ballpark on the cost of a new coil and TXV? That is if I can get them without replacing the whole air handler.

Thanks,
Matt
 
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Old 07-20-08, 03:42 PM
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Retired

Originally Posted by HVAC RETIRED
These are merely suggestions, use your own judgment, & that of the local contractors.
----------
First, a lot of the heat-gain issues need to be effectively dealt with!

Second, the duct system needs to be thoroughly sealed & everything sized for optimal airflow. (Enlarge that Return & more!)

Third, after doing the above, with that SEER mismatch, I would experiment with different CFM airflows between 350 per/ton to 450 per/ton, & use what works best.

Of course, the condenser & evaporator SEER Ratings should be matched.

The new 4-ton 13-seer condenser will no doubt have a smaller BTUH compressor than the old 4-ton 10-seer.

If a proper sized TXV were installed on you 10-seer E-Coil, it might work reasonable well, especially if you are dealing with humidity problems.

If you don't want to spend the money & do everything possible in the correct sequence, it may get the temp & humidity in your comfort zone.

In real world conditions, high SEER is not the whole story, do what you can afford & don't worry about it. HVAC RETIRED - udarrell
It wont work!

*A little research on this subject will answer this question in detail - You can start with A recent RSES study. Changing the expansion valve (the only positive thing you can do) at best will get you 60% of the intended capacity. The only answer is to match seer ratings of both the outside unit, and the a coil. Clearly, if asked the manufactures of Trane, York, Lennox...and so on, they will say to you - It Wont Work Period. The issue here is physical size and capacity. You cant adjust that!
 
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Old 07-20-08, 06:33 PM
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The Real Deal is Absolutely Right!

The Real Deal is Absolutely Right!

I went a little soft-headed on my first post, I corrected that on my second post.

I have seen too many matched systems that produced about half their nominal BTUH Rating, - due to bad installs, etc.

At least, the matched systems can be made to work by the real pros.

The new higher SEER units have a lot more refrigerant, plus coil capacity, which is impossible to completely compensate for.

If the vapor Superheat can't be kept within tolerances, there is also a possibility of damage to the compressor due to too hot a discharge temp over 225-F & compressor oil breakdown.

Way to go "Real Deal," we cannot be too softhearted with customers, - it has to be done Right from the get-go even if they think they can't afford it!

Were I yet in business, I personally would not have sold him the condenser & installed it, without a matched E-coil with a TEV metering device on it.

HVAC RETIRED -udarrell
 
 

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