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*You could ballpark the CFM using the static test & the air handler’s graph. You could measure the CFM delivered to each room with a hood Alnor Balometer, it’s usually the best instrument to use, but not cheap. Anemometers are reasonably priced & should be used for checking velocity from diffusers.

Measuring the air velocity from diffuser’s is a bit tricky because you should use the diffuser mfg’ers data which you should always have with you.You can usually get the diffuser mfrs data, say its a 1.5-Ton system that already has 6″rd branch duct runs, to achieve enough CFM airflow, you need close to 700-FPM velocity in the ducts.

I would want to use a diffuser with a little more free open sq.ft area than the 6″ duct  which area is 0.19635-sq.ft., say middle of the room in the ceiling; Hart & Cooley 2-way curved blade 12×6 has Ak .235-sq.ft. Free-Air-Area delivering 140-CFM at 600-FPM - diffuser face velocity.

This would help lower the velocity of the duct through the diffuser & reduce air noise.Throw is 7.5-ft toward each wall. Terminal velocity at the occupant level is 75-FPM.

If one diffuser face-velocity were to yield 135-CFM times 5 outlet runs - yields 675-CFM *X’s 26.666-BTUH per CFM = <18,000-BTUH, - that’s 450-CFM per-ton of cooling means less BTUH per one CFM.

Whatever CFM you need for that room or area, divide the Sq.Ft. free-air-area into the requires CFM to get the FPM velocity.Say we’re using 450-cfm/ton of airflow; 12,000-BTUH / 450= 26.6666-BTUH per CFM.

You need 3600-BTUH for that room, 3600 / 26.666 that’s 135-CFM / by .235-sq.ft. diffuser area is only 574-fpm face velocity.  Using 400-cfm per/ton / 12000-BTUH is 30-BTUH per CFM. 

 Taking the manifold gage head pressure & gage condensing temperature is very important data. Coupled with a condenser air discharge temp-reading, if the condenser gage pres/temp is too high compared to the TH reading, there may be non-condensibles in the system. 

Also, there is a legitimate formula I use to determine the operating BTUH it is delivering at all the data taken.

All the mfg’ers ought to list the condenser temp-split (it varies with EER & SEER) just like they list the indoor split, it is valuable trouble shooting info.

You can also use the condenser temp-split (it contains both Latent & sensible heat) combined with the indoor data to plot the indoor CFM. I was never good at math, but those equations have to balance, & they do work!

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Quick Check for Sizing Units & Ductwork to Achieve Adequate Airflow Actually, even on service calls where there are cooling problems the ductwork should have a quick Manual D performed.

Then take the ESP static pressure & compare to blower graph or chart, also take the FPM duct velocity.Then do a quick estimate of airflow per equipment tonnage.

To find area of a round duct; Duct diam is 7″; 7″X7″= 49-sq.ins., X’s .7854 = 38.04845-sq.ins divided/ by 144= 0.2672541-sq.ft. area X’s FPM Velocity 600-FPM = 160.35246-CFM X30 = 4,810.5738 each 7″ run X’s 6 branch runs = 28,863-BTUH, or airflow for 2.4-ton.That would also be good for 2-ton; at 550-FPM velocity X’s 0.2672541= 147-CFM X 30 = 4,410-BTUH each run X 6-runs = airflow for 26,460-BTUH.Never sell units requiring more airflow than the duct system & Blower will support! - udarrell

Low side line is sweating good, high side line luke warm. Temp coming out of vents are 69 degrees.

I removed the entire inside system outside and cleaned the coils with evap cleaner. cleaned Blower Motor and Squirrel cage.

Replaced it.

Evacuated the system and re-charged system.

Low side would not rise above 70 lbs pressure, but high side continued to climb to 350 lbs pressure still not cooling sufficiently.

Check ducting for leaks, all looks good.

Do you have any Idea what it could be. Rodger Touchette

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Copy the data chart in the prior post, & fill out as much as you can.

NEED: Outdoor temp; indoor temp & Relative humidity.

Approximate airflow.

A shot in the dark, it sounds like a partial restriction somewhere in he refrigerant circuit.

I more information to make any meaningful judgments.

No actual data, only a shot in the dark guesses.

A liquid line, including metering device, restriction would show high head & low suction pressure.

If there is a liquid line restriction somewhere you may have overcharged the system causing the high head pressure.

- udarrell

I came across this site last night, after searching the web for answers. We just installed a TRANE 16Seer on our 2100 sf home. Compared to last year, at this time, my bill has doubled. Last year during summer (with 2 MasterCools installed, one running 24 hours, the other about 10 hours), my bill was under $200. This year, with our new HVAC, in addition to new windows all around the house, my bill was over $300.
What am I doing wrong? - Carmen
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Hi Carmen,

I am new to blogs & blogging. My web site is my main stay.

I do not have enough information to do any kind of analysis, however, it could be a poor install & setup of the equipment, refrigerant lines & the ductwork system.

Your unit is costing you a fortune, the first thing to look for is hot air getting into the Return Air duct system, that would overload the evaporator & condenser & run your costs way up. What is the F-Temperature rise/split from the outdoor condenser above the outdoor temperature. it appears that you live in a dry climate.

So, your unit is costing you a fortune, I need to have the temperatures below, - that you can easily take for me:

“Copy only the data portions & paste in your Word-Processor, make a couple hard-copies, then send me what you take, & get as much of this data as your Tech will provide for you.” You can delete the (You &/or Tech) before you make hard copies for your Tech.

This is very important: Always tell the tech that you want all the data he takes, clearly recorded with your billing!

I can run the temperatures data you take with an accurate CFM Airflow & ball-park the Nominal BTUH your new A/C is delivering, at those conditions.

TROUBLE SHOOTING DATA - (You or Your) HVAC Tech

For my information concerning summer design & your climate, provide your state & nearest large city __________
TONNAGE of Unit or outdoor condenser model number: ___________ You
TXV or orifice metering device? _______You or Tech, - not real critical to this analysis

Outdoor condenser’s discharge-air-temperature _____You
Out-Door temperature: ____You
Condenser air Temp  split _____You

Indoor Total-ESP - External Static Pressure  _______Tech
All Supply Air velocities in FPM: ___Tech
I’ll do the math for the total CFM:

Suction line pressure ___Tech
Suction-Vapor line temperature: ___Tech
Saturated Suction Temp – Gage - Suction Saturated Temperature. ___Tech
Superheat ______Tech

Head pressure _______Tech
SCT – Gage - Saturated Condensing Temp _______Tech
Small Liquid line - temperature: _______You
Subcooling: _______
Return Air temp DB &, WB or %RH: _______You
Supply Air temp DB &, WB or %RH: _______You

With the information data below and an accurate CFM airflow + sea-level height, I can ball-park its BTUH output
Return Air temp DB &, WB or %RH: ____ ____You
Supply Air temp DB &, WB or %RH: ____ ____You
Take a cotton cloth & wet with room temperature water, wrap it around the thermometer & use it for the wet bulb temps above.

Except for the Pressures, Superheat & subcooling you can take most of the other Temp readings for me.

My advice: to every user, get a low cost humidity gage at a local hardware store & see what the percentage-RH is in your home, I can cross-reference that to the wet bulb depression so you could use the humidity gage instead of wet or use both.

If you want to save money you will take the time when it is hot & warm in the house to take all this temperature data!

Please Respond, ASAP.

Anyone is welcome to use the Data Collection for use as they see fit!

“Copy only the data portions & paste in your Word-Processor, make a couple hard-copies, then send me what you take, & get as much of this data as your Tech will provide for you.”

My monthly electric bills run $50.00 or less, & I keep very cool, in a 1937 farm home, even in 104-F Heat Index!

Are your electric bills too high?

Visit my many HVAC web pages at:

http://www.udarrell.com/airconditioning-sizing.html

For Techs a Trouble Shooting chart, for HVAC Techs & HVAC User’s a Service Data Chart to check BTUH Performance:

http://www.udarrell.com/ac-trouble-shooting-chart.html

I am here to help you Conserve Energy & save a lot on your monthly utility bills.

A high percentage of HVAC systems are operating way under their Rated Efficiency!

Visit the HVAC links at the bottom of the pages.

Edit: 08/03/08, Carmen, your initial post is in the unnoticeable uncategorized comment section!

udarrell - Darrell, Retired HVAC Tech & Contractor from mid 1970’s

Feel free to post about anything related to HVAC and/or Energy Conservation.

There are many areas where your air conditioner or heat pump may be losing efficiency, together we can achieve optimal efficiency of operation.

This is valuable free information being offered to you so you can save on your energy usage costs, & practice Energy Conservation for America. Visit my many Energy Saving HVAC web pages at:

http://www.udarrell.com/airconditioning-sizing.html

For Techs a Trouble Shooting chart, for HVAC Techs & HVAC User’s a Service Data Chart to check BTUH Performance:

http://www.udarrell.com/ac-trouble-shooting-chart.html

I am here to help you Conserve Energy & save a lot on your monthly utility bills.

A high percentage of HVAC systems ar operating way under their Rated Efficiency!
Practice Energy Conservation it Pays Big Dividends forn you & for your country.

Visit the HVAC links at the bottom of the pages.

Darrell Udelhoven - udarrell.com