Residential System/Duct Matching

[Rob1964]

I have a good handle on the cooling load of my home, 38,000BTUH-40,000BTUH. It is an older house with most likely undersized ductwork (designed for heating only, it is a forced air gas furnace). My choices were a 3 or 3 1/2 ton unit, I'm going with the 3 1/2. The Contractor is going to put in a 4 ton coil, says I'd be better off with the larger coil. Given the fact that the refrigeration is slightly oversized, as opposed to undersized, is it better to have an oversized coil with the possibility that airflow will be restricted due to an undersized duct system? The new furnace will be an American Standard with a 4 ton rating "Airflow in Tons". These ratings only come in full ton increments, the unit I'm getting also is available in 5 ton, identical, but comes with a larger motor. I'm guessing the tech adjusts the airflow in the field.

 

[njnick]

Having the oversized coil with extra surface area will increase the contact time the refrigerant has with the circulating air stream. The extra surface area will help because of the reduced air quantity.

 

[willard3]

If you are taking any fresh air or have a high latent load in the house, the chances of the evaporator coil frosting are higher with an oversized coil.

Coil frosting will reduce airflow.

 

[Rob1964]

It sounds like it is probably a good idea to get an oversized coil for this situation. A large coil will also have a lower pressure drop thru it and that is a good thing, given the ductwork is already undersized. My installing tech says the oversized coil will help me get the 3.5 tons out of the condensing unit given my ductwork is working against me airflow-wise. There are alot of homes in mid Atlantic with old duct systems designed strictly for heating and are therefore undersized.

 

[AbbyNormal]

An "Oversized" evaporator will run at a warmer saturated suction temperature and will result in less latent cooling.

Since a larger coil will run warmer, why would it be more prone to frosting?

Furnaces as small as 75,000 BTU/hr input will have over sized blowers for 4 tons of air or 1600 CFM at 0.5" ESP, and this 1/2 inch would include the cooling coil.

A high speed tap on such a furnace would be 1600 CFM, perhaps the installing tech could use a medium speed tap to get 1400 CFM or whatever your system calcualtes out at.

Old furnaces were probably designed around a 90F temperature rise so unless the old furnace was about a 200,000 input, it may be tough to move 1400 CFM. Possibly have to modify some ductwork and or go with an ECM blower motor.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[Rob1964]

The supply ductwork off the furnace is bullheaded (19"x19") and has two taps, one at 7"x19" (.92sf) and one at 8"x14" (.77sf). The return to the unit is at 24"x30" (3.3sf) The duct runs are quite short, though.

I'm still a bit confused about the advantages of a larger coil in this situation. Since I'm on the higher side in terms of unit size and the unit is not going to be running as often as a result, do I really want a coil that is less efficient at removing water? Also, the tight ductwork will most likely result in less than ideal air flow, wouldn't I want the air to make up for that by being colder? Seems like what is being said is that the air will not be colder, but warmer than if I put in a 3 1/2 ton evaporator. I do not have any signficant latent loads in the house and not much fresh air either.

The new coil is about 50% larger than the 3 ton coil from the old system (25 years old). The condensing unit is also a hell of a lot bigger than I was expecting, I guess the higher SEER units need all that area for efficiency.

 

[Rob1964]

My new system is up and running. Getting 56F leaving air temperature with 76F coming in. A/C fan is on max.

 

[AbbyNormal]

I did not realize you had AC already. Was 3 tons insufficient?

The power you pay for to run that compressor is proportional to the pressure difference between the low suction pressure in the evaporator and the high head pressure in the condenser.

The coils get bigger in the high SEER equipment. A bigger evaporator grabs heat at a higher suction pressure (hence higher suction temperature) and a bigger condensing coil allows heat to be rejected to the outdoors at a lower temperature and therefore a lower pressure.

With suction pressures rising and head pressures falling the compressor does less work and uses less electricity.

The warmer evaporator coil means less moisture is condensed out when air contacts the coil.

A new 3 ton system will have more sensible capcity and less dehumdifiying capacity compared to the old system. Over sizing evaporator coils will further increase the suction pressure and decrease dehumidification.

What is your exact sensible and latent load and how does it match up to the sensible and latent capacity of what is about to be installed?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

A little late to be asking if it was already installed

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[Rob1964]

It's not too late to ask, as what I'm trying to do is get a handle on understanding what is happening.

My old system had a 33,000 BTUH condensing unit mated with a 36,000 BTUH coil w/ TXV. The coil dates from 1962, was a Bryant. The Carrier condensing unit was put in 1982 and was changed today to an American Standard 13 SEER AC/80% two speed gas furnace. The new condensing unit is 3.5 ton and the coil was upsized to 4 ton by the Contractor (at no charge, said it was only $30), as I've stated. The furnace is 80,000BTUH and has 4 ton air flow. My old furnace was 96,000BTU out installed in 1959 and AC was not in the house at that time, so the ductwork was designed for heat. I have since added 500sf of living space, so I figured the additional 9,000 BTUH would come in handy.

The system has been running since noon, it is an 84 F day and my first impression is that it is not cycling too much and is doing fine at removing humidity. I lived with the old system for 15 years and had a pretty good feel for the air output and I don't think it is much higher than it used to be, so my airflows are on the low side. I'm thinking of tapping into the discharge plenum with a new run to the new living space, it didn't get enough air now and still doesn't, temp there is 2-3 F lower than the rest of the house.