Replaced my A/C this weekend, results of downsizing by 1/3.

[KevinAdv]

I would certainly check the refrigerant charge I always do on a split system no matter what the length of the line set is

[54regcab]

I would think if much attic air was leaking into/out of the ductwork a 2 ton A/C wouldn't be able to keep a 75-77 degree inside temperature on a 100 degree day with a 1600sqft house. Either that or the "500sqft per ton" rule so commonly used in our area needs to die immediately for homes under 20yrs old. Part of me thinks builders oversize to cover up crappy installation practices.

The duct was "Sized for 1200CFM", but I don't think it ever got that much. 2 return of 12" diameter flex, one is about 20' long, the other 10' long. I don't see those hitting 600CFM each. I do think I'm really getting about 900CFM according to the blower curve in the furnace specs.

On 100 degree afternoons the suction line is warm enough that it doesn't sweat, but when temps get below 95 it sweats. Below 90 and it feels cold. Discharge air temperatures drop dramatically as outdoor temperatures drop. At 100+ ambient I'm surprised the volume of hot air coming out of the 2 ton condenser, seems to be twice what a typical 2 ton condenser does.

[jtrammel]

What r value is the duct wrap? If its 4.2 it will help a good deal to change to r8 with duct in attic, while you're at it you can shmear some pooky on all the joints to seal it up

[54regcab]

What r value is the duct wrap? If its 4.2 it will help a good deal to change to r8 with duct in attic, while you're at it you can shmear some pooky on all the joints to seal it up

It's standard R-6, some pooky wouldn't hurt it though. I figure if the ductwork was that bad it wouldn't be able to keep up @ 800sqft per ton. Most units around here are sized 500sqft per ton...

[jtrammel]

It's standard R-6, some pooky wouldn't hurt it though. I figure if the ductwork was that bad it wouldn't be able to keep up @ 800sqft per ton. Most units around here are sized 500sqft per ton...

I dont think its leaking badly but 99% of duct systems leak, lots leaks 25-50% of capacity of system. It may keep up even better and cycle on a 100f day at 76f set point even more disproving the rule of thumb and all you need is a bucket o mastic. Some of the worst leaks and easiest to fix are the boot to floor/ceiling/wall connections.

[beenthere]

Recheck your SC on the next 95 degree you get a chance to.

[motoguy128]

If the duct was sized for 1200 cfm and now you have 800cfm the velocity is lower so the cooled air stays in the ducts longer and gains more heat from the attic, this could be the reason for the 14f drop vs 18f drop.

Very good point. Plus you still have the larger surface areas of the slightly oversized ducts. On the flip side, if well sealed and instulated, the reduced static pressure can spot you as much even 0.5 SEER over a restrictive system with a VS blower.

My ductwork for example is fairly oversized now that I went from 3.5 to 2 tons and the blower isn't far from minimum RPM on low speed fan (50% of heating airflow). But if my attic wasn't conditioned, I suspect I'd see a large amount of heat gain and loss from the ductwork.


This brings up a good point. On attic installations, it even more improtant to minimize total length of ductwork, the number of supplies and returns, and using proper sizing to meet those objectives. Even better reason for attic installation in vented unconditioned attics to be banned. Just spend the money, give up 3' of closet space and stick ductwork in the floor and walls. But I suspect I'm preaching to the choir here.

[palmeri]

I would have liked to know what your Total enthalpy was (supply and return) on the old system, compared to the new system. If you have a accurate way of calculating your air flow, you can tell exactly what BTU's your system is putting out.

[54regcab]

I've considered getting the ductwork out of the attic, or least reducing the amount of surface area. I've already made significant reductions in surface area buy combining ducts and using y connectors at the end instead of every register having it's own duct from the trunk. To further reduce surface area would mean moving registers and patching drywall, I could move the vents closer to the air handler instead of having the ductwork go to the perimeter of the house. I have 10' ceiling in the living room, kitchen and master bedroom. 8' ceilings in the hallway, could do sidewall registers. Not sure how much I'd gain or if it's even worth the effort.

Another consideration is replacing the 1/3HP blower motor that draws 350W on low speed with a 1/5HP motor that would draw 200W on high speed. Saving 150W would improve the system EER about 1 point. It would take 1,000KWH of power savings to pay for the motor. I figure a 3 year payback time on the new motor, on the fence if it's worth the effort. I suppose if I got the motor for the right price it could be worth it.

[beenthere]

The 1/5 HP motor probably wouldn't be strong enough to give you the same air flow.

Take an accurate temp (not an IR thermometer) reading of the temp in the plenum, and then one at a couple of the supply registers. So you know how much temp delta your duct system really has, before you change things that won't help.

You might be surprised how much difference that 6 ounces you didn't add for the 10 foot line set length over 15 foot can make.

[jtrammel]

The 1/5 HP motor probably wouldn't be strong enough to give you the same air flow.

Take an accurate temp (not an IR thermometer) reading of the temp in the plenum, and then one at a couple of the supply registers. So you know how much temp delta your duct system really has, before you change things that won't help.

You might be surprised how much difference that 6 ounces you didn't add for the 10 foot line set length over 15 foot can make.

X2

[54regcab]

Would you weigh in 6 oz or charge by subcooling?

[drife678]

should tell you manufacturers requirement for subcooling on data plate on outdoor unit. obviously you would charge it per subcooling. who knows what the actually put in at the factory......

[gravity]

i would do both. just to see if the factory is right. i always verify my subcooling.

[gravity]

this is also a good thread. thanks for taking the time to do this

[ga-hvac-tech]

Recheck your SC on the next 95 degree you get a chance to.

Agree with this! Assuming ductwork is not too restrictive (high static pressure), TXV coils seem to work better if one sets the SC right up to factory specs, rather than pressure charts.

[jtrammel]

I always weigh in and verify with subcooling

[RoBoTeq]

Most likely too much air and/or duct infiltration not dehumidifying as well as is possible. The low delta-t is a direct indicator of too much air.

Possible cures; VS blower set for 600-700 cfm of air and programmed for dehumidification, an enthalpy controlled evap coil by-pass or a volume damper in the main supply trunk closed to create enough SP to drop air volume to below 800 cfm.

[SBKold]

How is it doing now that its heating?

Pull the specs on that compressor and id bet its a 19000kbtu.

[54regcab]

Most likely too much air and/or duct infiltration not dehumidifying as well as is possible. The low delta-t is a direct indicator of too much air.

Possible cures; VS blower set for 600-700 cfm of air and programmed for dehumidification, an enthalpy controlled evap coil by-pass or a volume damper in the main supply trunk closed to create enough SP to drop air volume to below 800 cfm.

I'll look into these solutions come next summer. A simple volume damper may be enough to do the trick.