400 CFM across the evaporator, unless you have some serious duct leaks I vote no not acceptable, will it work can't say!

 

how did you measure cfm of registers?

 

I believe the tech used a manometer. It was box-shaped device with a meter and tapered fabric hood. He placed the hood over each register to read the CFM.

 

Not ideal, however it would not surprise me a bit. Yes, it would still function. Nine hundred CFM on a three ton system is probably closer to the norm than any of us would want to believe.

 

Ditto. Found this online:

In 1996 a study commissioned by Arizona Public Service Co., the utility investigated total airflow in newly constructed houses in Phoenix. The study revealed that:

The airflow in 14% of the tested homes was at 90% of nominal airflow (360 cfm/ton).
39% of the homes tested at 80% of nominal airflow (320 cfm/ton).
14% tested at 70% airflow (280 cfm/ton).
7% tested at 60% of nominal airflow (240 cfm/ton).

I suppose, like everything else in life, there's the ideal, what it's supposed to be . . . then there's the realworld.

 

Well, of course 350-CFM per/ton is usually close to the minimum even in high humidity conditions.

First, check the ducts for air leaks & that the blower wheel blades & evaporator are clean; plus that the blower is set on the correct speed.

The indoor evaporator can only absorb the heat that is flowing through it; when there is insufficient airflow the system will be operating way below the Rated Btuh of the equipment.

At 50% indoor or lower relative humidity the system may not satisfy the room TH, though it would with proper airflow.

I like 400-CFM per/ton or more; even in humid 'high' latent 'total load' conditions, though the sensible is higher at design high-load conditions on high-speed it moves a lot more air through the evaporator coil, - & I find that my window units bring the humidity down even faster on the highest fan-speed setting!

That means they are doing more total heat removal on the highest speed. One of the main reasons for slowing the blower speed is 'to get longer runtimes during less than near design load-temp' conditions.

Yes, 300-FPM or lower is very common & 'one of the many reasons' why the A/C doesn't do its job right.

 

350-400 allowed +- 15% Of design on actual measured airflow

 

Let's talk about the system. Is it single stage or two stage? Variable speed inside? How did you test the cfm coming out? Flow hood, cheap cfm meter etc???

Depending on the equipment it might be working just fine? I am curious how you checked the cfm to the whole house?

Flow hood is the best. Next one would need to look at the settings on the system to see what it's set up for. Speed taps or dip swicths may not be set up right for your system and is only allowing x amount of cfm.

Once this is taking into account for then you can look at the systems performance data and see what 900 cfm means in regards to staic presure and amount of Btu's it is providing at that cfm.

 

It's a single stage (Trane XR14/XT80 furnace/blower). The tech used a flow-hood and did mention the motor/CFM could be adjusted. However, was just perusing the XT80 manual and didn't see any mention of any kind of adjustment. Might have overlooked it.

 

If your looking for 75^F, 50%RH inside, you need a 50^F cooling coil temp. With long runs, you will the desired 55^F indoor dew point you need. Measuring air flow is difficult at best. Measuring the coil temp is a matter of reading the gauges and adjusting the air flow.
Regards TB

 

A 5-Ton system measured delivered supply CFM of 1640 times the measured enthalpy change of ‘ONLY’ 4.15, times the total BTU constant of 4.5 to find the system delivered BTU of 30,627.

They then checked the manufacturer’s engineering data for identifying under the current operating conditions of 76 entering wet-bulb and 85 degree dry-bulb outdoor that the equipment is rated for 56,200 BTU. The CSER (Cooling System Efficiency Ratio) came in at 54%.- A Rob Falke article.

A typical enthalpy change through a typical system at 400 CFM per ton is around a 6.3 to 6.5 enthalpy change. The above 5-Ton was only 328-CFM per ton of cooling...

The 3-Ton unit 'here' at only 900-CFM; that's only 300-CFM per/Ton; @ a low enthalpy change of 4.15 that's 16,808-Btuh; a 1.5-Ton system.

Even at an enthalpy change of 6 it would be 24,300-Btuh or 2-Ton capacity.

 

our contractor locator is one of the best ways to assure a system gets installed correctly. the techs that frequent this site and maintain membership get access to so much training, and assistance, we learn a lot and get customers proper installations.
another good way is asking friends and family, neighbors and such who they've had great service from.

it's just crazy how little the majority of contractors out there know about airflow and ductwork...

 

The fact they even checked is a good sign. Now you need to find out where it went. Duct blaster, blower door tests etc. Check with your local utility many times they'll offer energy audits for free or reduced cost, it benefits you and them.

 

Did they ever check the return to see what its taking in?

 

if they wrote down the cfm for each supply,
then you should be able to determine the ducts
that leak.
size of duct & cfm delivered.

ditto on both setting of fan speed
& return leakage.

I love a flow hood, it can tell you so much.
I've always though the flow hood was as useful
as a duct blaster in determining duct leakage.
if the powers that be would allow it as a testing
method, I'd trade in my duct blaster in a heartbeat.

 

Yes a flow hood can tell you a lot on whats going on with a system and the ductwork. While I think a duct blaster is a great tool I would have to agree with you on flow hood vs duct blaster for determining leakage. I do believe in a new home without the system running yet a duct blaster is better to determine leakage though.

The trick to a flow hood is to understand your duct system and the equipment it paired with. Example: you have a simple 1500 square foot home single story with new metal pletum R-6, with (7) new 8" flex drops running a spider system in the center of the home with all drops less then 25' taking into account 4% comparison loss. All new 10x10x8 boxes for proper air flow deliver to each room. The equipment in the home sized properly in this example is a Trane 2.5 ton a/c unit. The ductwork could handle up 1120-1190 cfm at 0.5 static pressure.

Measuring the cfm on the example home listed above =1000-1050 cfm which = 0.5 static pressure via the Trane performance date table on that unit. So in this case I have very little duct loss. Now lets take that same system knowing all the above and measure each drop supply all but one is supplying 160-170 cfm the other one is only supplying 75 cfm. Odds are that duct is either came out of the pletum or leaking, damper is shut off 50%, or that drop has been compressed. Its all about what equipment you have, what size ductwork and having the date on all listed.

The only time I believe the duct blaster would be better then a regular flow hood in a existing home is when you have extended plentum as a flow hood can only tell us each supply drop to a room but can't tell us if that trunk line is leaking other then some of the drops will have decreased air flow out of them. You give me performance data, a flow hood, duct cal. and the time to review the system then I can find duct leakage or let you know what the system is doing in a allowed amount of time.