First floor system might be oversized

[545GAlady]

I have a question about our first floor system. We have a 2012 Trane XR15 3.5 ton H/P matched up with a 2013 Lennox SL280v 90,000 BTU input two-stage variable-speed furnace with a 4 ton blower. The first floor is 2,200 sq. ft. I don't understand why the tech did a load calculation that came up to 3.62 and the current ductwork can't handle 1400 cfm. So basically the furnace runs around 1060 cfm the first 7 minutes, then ramps up to 1220 cfm. Both installers said we are fine on return air, which I HIGHLY doubt it. You can hear a loud whining noise when the blower is at the higher speed. We called a tech out and he lowered blower speed due to the fact our current ductwork can't handle 1400 cfm (He said current ductwork can hold around 1150 cfm's on the supply side and 1080 on the return side) and to help solve the noise issue. The system still cools and heats great even with the blower speed set at the current speed. Question is, can too little airflow hurt the performance of the system? What can be done to solve the issues? Thanks.

[545GAlady]

So I'm really guessing that the system is slightly oversized. I also need to mention that the XR15 has a loud start up outside. The installers who installed our upstairs unit a few weeks ago said the XR15 might need a hard start kit. To add more information, we have 4 return branches connected to the ductboard. Two of them are 10", one is 12", and one is 6". We plan to call a tech out to look at everything including the ductwork. But for now, what are your opinions?

[beenthere]

Return is under sized. So supply may be also. 1220 is ok for 3.5 tons. Will remove more moisture then 1400 CFM will.

[545GAlady]

Thanks beenthere. On the supply side, airflow is very weak in the registers that are the farthest away. I know this can be common. I suspect the layout of ductwork is very poor (welcome to the south, where flex is everywhere). What do you think? Even though you said 1250 cfm is ok for 3.5 tons, should the tech still increase the blower speed slightly?

[545GAlady]

I meant to say 1220 cfm for the last post.

[beenthere]

Is the air blowing out real strong at the supplies closest to the air handler? If so, speeding up the blower will increase air flow at them as well as the furtherest ones.

Is it possible to increase the size of the supply runs that are the weakest air flow.

[545GAlady]

Is the air blowing out real strong at the supplies closest to the air handler? If so, speeding up the blower will increase air flow at them as well as the furtherest ones.

Is it possible to increase the size of the supply runs that are the weakest air flow.

Yes, the air is pretty strong at the supplies closest to the furnace. The return air issue will be fixed will soon and the ductwork will be check to make sure it is fully stretched out. It is possible to increase some of those weaker supply runs (glad we have drop ceiling in the basement rooms to be able to access the ducts).

[beenthere]

What your describing is a typical under sized duct system, both return and supply.

[545GAlady]

What your describing is a typical under sized duct system, both return and supply.

Yup, that pretty much explains it. It's a shame that none of the technicians we've had in our house ever mentioned anything about the ductwork except measuring the size. *sigh* Looks like we need to call somebody out from another company to inspect the ductwork.

[545GAlady]

I have one last question about the system. Since we have a Honeywell Visionpro IAQ thermostat, I am wanting the dehumidification to be controlled by the variable speed blower motor. Apparently the installers said they connected an extra wire for that, but the blower isn't ramping down to 80% of the speed. The extra wire is connected to the DS terminal on the furnace control board and it says "dehumidification signal" in the furnace manual. I noticed there's another terminal which is DH that says "Dehumidification ouput communicating thermostat only" in the manual. That terminal has nothing connected to it. Did the installers set this up wrong? Hopefully whoever we can call to come out can fix everything.

[beenthere]

Probably didn't set up the IAQ right.

[545GAlady]

Probably didn't set up the IAQ right.

It looks like the installers set the stat to "whole house dehumidifier" because the blower runs during no cool calls when there's a call for dehumidification. I think it has to do with something with the connections on the EIM.

[udarrell]

What is the design & layout of the main duct?

On the main trunk, every couple of take-off runs or a number of feet, it should transition to a smaller trunk to keep static pressure up which helps the airflow to the distant rooms.

Might be able to have dampers installed on the closer branch duct take-offs to force more airflow to the distant registers.

If the Supply ducts & Return duct system, along with the RA filter areas are sized large enough, with a 4-Ton blower it should be able to deliver the right amount of air to those distant rooms...

Should have had a load-calc performed, before sizing the A/C!

[beenthere]

Incorrect set up on the IAQ.

[545GAlady]

What is the design & layout of the main duct?

On the main trunk, every couple of take-off runs or a number of feet, it should transition to a smaller trunk to keep static pressure up which helps the airflow to the distant rooms.

Might be able to have dampers installed on the closer branch duct take-offs to force more airflow to the distant registers.

If the Supply ducts & Return duct system, along with the RA filter areas are sized large enough, with a 4-Ton blower it should be able to deliver the right amount of air to those distant rooms...

Should have had a load-calc performed, before sizing the A/C!

There's more than one trunk coming out of the ductboard. One is large (main one), and there's three smaller ones. The main one has ducts coming off of it.

Dampers are something I've been thinking about for a while.

The company who installed the XR15 said they did load calculation, but I don't exactly if they actually did one or not. Contractor from company who installed furnace did do a load calculation, but he used a "wheel" to drive around the first floor to calculate size.

[545GAlady]

Incorrect set up on the IAQ.

That's most likely what I'm thinking.

[energy_rater_La]

I use a rolling tape measure.
what he was doing was measuring the house to
determine how many sq ft.
ideally he would have measured windows, distance
to overhang of roof, size of overhang and orientation
of each window. also amount of shade if applicable.

I love my rolling tape measure, makes these
measurements fast. still need tape measure..but
not to measure perimeter.

best of luck.

[motoguy128]

It' too late now, but I question the quality of their load calculation. Its probably not too far off depending on you shading and window size, number and location. THe thing abotu GA is that it is fairly hot and humid, but not extremely hot and humid, but does have a very long cooling season and short winter.

Being a little short on airlfow isn't a bad thing in your climate. 350 CFM per ton is a good nominal airflow target anyway, that what your getting at 1225. 1060 for 5 minutes too is also OK when it first starts, and should be OK for dehumidify on demand.


I think overall you'd be better off with a 3 ton system sicne that's all you ductwork can manage. I wodner how accurate some of the number and "guesses" were that they used on that load calculation.

[545GAlady]

It' too late now, but I question the quality of their load calculation. Its probably not too far off depending on you shading and window size, number and location. THe thing abotu GA is that it is fairly hot and humid, but not extremely hot and humid, but does have a very long cooling season and short winter.

Being a little short on airlfow isn't a bad thing in your climate. 350 CFM per ton is a good nominal airflow target anyway, that what your getting at 1225. 1060 for 5 minutes too is also OK when it first starts, and should be OK for dehumidify on demand.


I think overall you'd be better off with a 3 ton system sicne that's all you ductwork can manage. I wodner how accurate some of the number and "guesses" were that they used on that load calculation.

I can't recall exactly if they checked window size, numbers, etc. All I remember is them checking square footage. We are gonna call someone from a company we've never used before (they seem to have excellent reviews) and we shall see what he finds. I'll give an update once that happens. Thanks for your information.

[energy_rater_La]

sizing by sq ft isn't sizing it is an old rule of thumb sizing.
paying for an accurate load calc is the first step to having
a properly sized unit. sized for your specific house, not just
xxx tons for xxx sq ft.

best of luck.