Inadequate evaporator


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Old 08-07-14, 11:28 PM
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Inadequate evaporator

My house which is large was originally set up with a gas heater and swamp cooler air conditioning. Some years ago it was converted to refrigeration AC by adding an evaporator to one of the furnaces and an outside condenser unit. I made some measurements on the unit today and my conclusion is that the evaporator is too small for the condenser.

This is my conclusion.

I'm wondering if I could install a second evaporator on the other furnace and extract some more cold into the house to better utilize the capacity of the condenser?
 

Last edited by Houston204; 08-08-14 at 06:13 AM. Reason: no refrigerant pressure advice allowed
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Old 08-08-14, 05:00 AM
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15 deg of sub cooling and a 25 deg delta T? You're removing quite a bit of heat there my friend. I would be very happy with those numbers.
 
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Old 08-08-14, 06:17 AM
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A second condenser and evaporator coil on a differant furnace would be a good idea.
 
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Old 08-08-14, 08:17 AM
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tom63: 15 deg of sub cooling and a 25 deg delta T? You're removing quite a bit of heat there my friend. I would be very happy with those numbers.
What tonnage is the condenser & what CFM is the indoor airflow.
What %RH is the indoor return air?
What large city do you live near to?
 
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Old 08-08-14, 10:23 AM
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tomf63
15 deg of sub cooling and a 25 deg delta T? You're removing quite a bit of heat there my friend. I would be very happy with those numbers.



Agreed, but it is a big house and with a superheat of only 3 degrees, I think I could be moving more heat into the condenser with a second evaporator.


Houston204
A second condenser and evaporator coil on a differant furnace would be a good idea.



Yes, I've never done one before. Was wondering if I could just parallel a standard evaporator, or if I had to use some sort of heat exchanger without an expansion valve in series

HVAC RETIRED

What tonnage is the condenser & what CFM is the indoor airflow.
What %RH is the indoor return air?
What large city do you live near to?


condenser is about ten years old, pretty sure it's 5 tons. evaporator is on an 80k btu airease unit also about 10 years old... not sure how many cfm it can push. humidity was 45 per cent according to my wifes weather station. The indoor wet bulb on my SMAN4 showed 52 degrees... didn't figure the humidity. I'm in the desert near Salt Lake City. Typically pretty dry here.
 
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Old 08-08-14, 10:52 AM
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condenser is about ten years old, pretty sure it's 5 tons. evaporator is on an 80k btu airease unit also about 10 years old... not sure how many cfm it can push. humidity was 45 per cent according to my wife's weather station. The indoor wet bulb on my SMAN4 showed 52 degrees... didn't figure the humidity. I'm in the desert near Salt Lake City. Typically pretty dry here.
It is very difficult to get 2000- or 2200-CFM airflow in a residential duct system.
Usually the static pressure is real high & airflow drops to that of a 3.5-Ton system.

I'm guessing only around 1450-CFM * 25°F temp-drop * 1.1 is 27.5 * 1450-CFM is 39,875-Btuh sensible; * 1.20 latent is 47,850-Btuh total or perhaps 4-ton performance. (I could be wrong!)

Depends on how low the % of relative humidity is indoors, whether the airflow is that low...
 
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Old 08-08-14, 03:00 PM
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Yes, that was my conclusion (that the current evaporator system is incapable of delivering the heat load to the condenser), which is why I'm trying to figure out how to add a second evaporator. If I do it in parallel, my concern is what happens if the second unit fan shuts off and starts feeding liquid refrigerant into the return line along with the evaporated refrigerant from the first unit. Will the superheat of the first unit be sufficient to evaporate the liquid refrigerant before it gets to the compressor? I'm guessing I will have to add a second contacter in series operated by the 24 volts from the second furnace to shut down the compressor if both furnaces aren't blowing
.
 
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Old 08-09-14, 02:59 PM
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I've decided that a second contacter is a bad idea. I'm going to try to find a 24v solenoid valve that can shut off the refrigerant at the high side of the second evaporator. I think that would be a better way to solve the problem.
 
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Old 08-10-14, 05:44 AM
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You can't put 2 evap coils on a single condenser. Coil and condenser must match.
You've got to add a 2nd system, air handler and condenser.
 
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Old 08-12-14, 02:30 PM
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tomf63
You can't put 2 evap coils on a single condenser. Coil and condenser must match.
You've got to add a 2nd system, air handler and condenser.


can't as in physically impossible, illegal, or morally wrong?
 
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Old 08-12-14, 03:41 PM
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It's not physically impossible, it's not illegal, but it would be morally wrong to tell you to do it. It just won't work.

But best of luck if you try to do it.
 
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Old 08-12-14, 04:31 PM
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You can't put 2 evap coils on a single condenser. Coil and condenser must match.
You've got to add a 2nd system, air handler and condenser.

can't as in physically impossible, illegal, or morally wrong?
Can't as in mechanically wrong. Coil and condenser must match. A 5 ton condenser must have a 5 ton coil. You can't simply add a coil and split the refrigeration lines, and you sure can't put them in series either. Refrigeration doesn't work that way.
 
 

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