[nugentoffer]

Hello,

I was trying to diagnose if my A/C system is running optimally or not, and thought a good place to start would be to check difference between return and supply air temperatures.

I was able to get into attic and drill two holes, first hole right before air enters the blower, and the second is at where air leaves the evaporator coil. Figure these holes would also help for static pressure drop readings in the future.

Any HVAC experts here tell me what I should expect in terms of temperatures differences at these two points?

Also, is it better to do this test at return and supply registers?

Thanks for your kind advice

Location: TExas
Gas Furnace : Bryant 310AAV PSC 3/4HP
Evap Coil: ADP HD-Series 5-Ton
Mount : Horizontal Left
Single story Home

 

[supers05]

Somewhere between 12 - 20*F. Depends on airflow. If you can measure wet bulb and airflow then we could tell you how it's working.

Cheers!

 

[nugentoffer]

Hey Super,

Any suggestions to measure airflow at the handler?

 

[BayouRunner]

I've always measured at the return and a grill at the ceiling. You would be surprised how much the difference is when you measure it at a longer duct. So I usually try to measure it close to the supply. Most new equipment is around 20 degrees differential. The older stuff I have seen it as low as 10 (time to replace if you cant get it to work any better than that). Blower speeds, attic temps and other factors make a difference in temp drop as well. So these figures are not set in stone.

 

[supers05]

Anemometer.

Something like:
http://www.testo-direct.ca/images/products/cache/testo/410i/main-direct/testo_410i_.jpg

http://www.testo-direct.ca/images/products/cache/testo/405i/main-direct/testo_405i_.jpg

http://o.aolcdn.com/hss/storage/adam/fd3eb3ddb41d24227791cc67c8ffa513/IMG_5994.JPG

Cheers!

 

[supers05]

We could guess at the airflow from the model numbers of the furnace. But it won't be terribly accurate.

Cheers!

 

[nugentoffer]

ok I will.look into that Testo 405i and followup, thanks Mr.Super

 

[yuri]

15-20F in my experience. Any less than 15 and you are usually low on freon. However I have never seen the extreme attic conditions of Texas etc.

I would check it 1-2 feet downstream from the coil and subtract it from the return temp where it enters the unit.

Wash the outdoor coil first and get the freon level checked also.

Check it early in the morning B4 it gets too hot.

 

[bfletcher7]

If I may jump-in, for those of us with no visual access to the inner coil do we need to cut-out a section of sheetmetal for inspection/cleaning access, or do you compare DT values against published performance specs to make an assessment? Except, if you did that couldn't there be an airflow problem (i.e. dirty inner coil) contributing to skewed numbers? Thanks.

 

[BayouRunner]

If I may jump-in, for those of us with no visual access to the inner coil do we need to cut-out a section of sheetmetal for inspection/cleaning access, or do you compare DT values against published performance specs to make an assessment? Except, if you did that couldn't there be an airflow problem (i.e. dirty inner coil) contributing to skewed numbers? Thanks.

If pressures are good but line is extremely cold (or colder than normal) I would open up metal. Preferred method for me is an access door at that point. Makes a nice clean job. Sometimes I open up sheetmetal. It depends on the job

 

[yuri]

If I may jump-in, for those of us with no visual access to the inner coil do we need to cut-out a section of sheetmetal for inspection/cleaning access, or do you compare DT values against published performance specs to make an assessment? Except, if you did that couldn't there be an airflow problem (i.e. dirty inner coil) contributing to skewed numbers? Thanks.

You got that theory right and not a lot of people understand it.

You can be grossly low on freon but with a very dirty coil still get reasonable low side pressures but not enough cooling and it can initially fool junior techs. Happened to me in 1979.:biggrin2:

Nowadays with snake inspecto-cameras we can cut a 1" hole and stick it in and take a look at the coil. Very hard to deal with box/case coils that sit on top of a furnace. HVAC is not a easy exact science or job.:vs_no_no_no:

Visual inspection is ALWAYS the best and if you can cut a hole and use a dentist type mirror that is old school and the best.

 

[nugentoffer]

Good point about the dirty coil, as I was able to open the access door and look top down into my A-frame. It looked pretty clean, with little to no-buildup.

Since it is horizontal mount, I was thinking of drilling another hole on the bottom side of the A-frame to visually inspect the coil.

Static pressure drop across the wet coil was on the high side (.4wc) so debating if I should inspect it further.

Thoughts?

 

[yuri]

You need to inspect where the air enters the coil as dirt collects there.

 

[Bret86844]

The upstream side of the coil is where the dirt hits first, so definitely worth looking at that side.

Is 0.4" high for a residential coil? We see 0.5" to 1.0" all day long on commercial/industrial.

 

[yuri]

Not that I overly care about what the other techs say (or get upset) but I never check static pressure. Where I am we mostly have large old school ductwork and don't seem to have a lot of airflow problems. I have +30 yrs experience and can fine tune and MAKE most systems work. There are variables like fan speed/ductwork size/air filter type/air noise/humidity removal/worn or mismatched units etc and a lot of the time you have to work around those issues.

Some people like to check static and talk theory but where I am heating is most important and as long as the ductwork is big enough so the furnace is not riding the limit control and cutting out then the AC works fine.

 

[supers05]

Not that I overly care about what the other techs say (or get upset) but I never check static pressure. Where I am we mostly have large old school ductwork and don't seem to have a lot of airflow problems. I have +30 yrs experience and can fine tune and MAKE most systems work. There are variables like fan speed/ductwork size/air filter type/air noise/humidity removal/worn or mismatched units etc and a lot of the time you have to work around those issues.

Some people like to check static and talk theory but where I am heating is most important and as long as the ductwork is big enough so the furnace is not riding the limit control and cutting out then the AC works fine.

You're not alone. I'd only check it with vvt/vav systems.

Cheers!

 

[user_12345a]

Split is very, very unreliable and shouldn't be counted on imo.

Humidity, airflow impact the split. if u take it it the wrong place it can be off by several degrees.

U can get more accurate taking wetbulb return air and using a chart to see what the supply drybulb should be.

A 12 degree split can be fine if the fan is moving well in excess of 400 cfm per ton - also if the returns are at floor level and pulling in air several degrees below the set point so the relative humidity of return is higher than 50 to 55%.

18f split can be off if the airflow is low and there are other probs with the system.


------------

You're not alone. I'd only check it with vvt/vav systems.

Cheers!

What do you do when you have a 1.5 or 2 ton unit on a furnace with a 3 ton drive as is common in canada?

Leave it on high and have it not dehumidify properly? Lower the speed and guess?

 

[yuri]

I have a 2 ton unit on a 3 ton drive and no we don't guess. Don't be silly "friend". That is why we have a 4 speed blower.

Mostly we use the lowest speed (red ) for circulation and the next one for cooling and med high for heat. We also follow the superheat/subcooling charts and with Lennox they have a approach temp/method.

The TX valve can adjust and modulate. If you read the fine print on the manufacturers charts they state the values can vary +/- 5% depending on airflow and are made under ideal test conditions with clean coils and properly matched furnaces/air handlers in their test labs. Once you get out in the real world you have to make it work. That is where the static pressure theorists etc are separated from the real techs.

In the US they have different criteria for de-humidifying in some of the humid states but some are like Canada where it is so/so important. There they can have on demand de-humidifying with newer units or they use larger evap coils but I imagine it is about 25% of the market and they have workarounds.

Lennox etc does not design or tailor units for 25% of the market.

 

[supers05]

Split is very, very unreliable and shouldn't be counted on imo.

Humidity, airflow impact the split. if u take it it the wrong place it can be off by several degrees.

U can get more accurate taking wetbulb return air and using a chart to see what the supply drybulb should be.

A 12 degree split can be fine if the fan is moving well in excess of 400 cfm per ton - also if the returns are at floor level and pulling in air several degrees below the set point so the relative humidity of return is higher than 50 to 55%.

18f split can be off if the airflow is low and there are other probs with the system.


------------


What do you do when you have a 1.5 or 2 ton unit on a furnace with a 3 ton drive as is common in canada?

Leave it on high and have it not dehumidify properly? Lower the speed and guess?

Actually you're talking about delta T, not split.

Split = entering air - SST (approx coil temp)

Split determines dehumidification. So evaps for refrigeration will be much lower then comfort cooling. 10*F compared to 20-35*F


For airflow, if I think there is an issue, I use my anemometer and find actual cfm. Usually, i test heat first, which will tell me if I'm moving the rated cfm in the first place via temp rise. If my pressures are good, sh/sc good, deltaT and coil temp and amp draw are good, I don't bother with cutting holes into ductwork, 30ft above the floor.

When I get my hot wire anemometer, I'll use it much more often, as it requires much smaller access holes.

Cheers!