[key1cc]

I had my first Blower door test and Combustion analysis test done today as part of a NJ state sponsored energy Audit. I have nothing but praise for the BPI accredited Pro that did the work (and it is not because he offered me a job...twice , Thanks HVAC-Talk for my education in this area. Best $125 I ever spent for 4 solid hours of work. If it was not state sponsored it would cost much more...If you live in NJ I reccomend you take advantage of this opportunity.

Ok, now lets get dow to business. I will start with the combustion analysis since I have participated in some very intense and excellent threads on the subject of whether 1st or 2nd stage is more efficient.

The tech used a Bacharach combustion analyzer and checked both 1st and 2nd stage heating operation. Oh before I forget, I have a 100k BTU Trane XV95 furnace and a 4 ton XL15i AC with matched 5 ton ADP coil.

Here are the numbers for 1st stage:
Stack temp = 90
O2 = 8.4
CO2= 7.1
ambient temp = 72
CO= 5 ppm
Efficiency = 98.3

Here are the stage 2 numbers
Stack temp = 105
O2 = 8.3
CO2 = 7.2
ambient = 74
CO= 4 ppm
efficiency = 95.0

I know the O2 numbers is not as low as 6.0 like the great Jim Davis likes to shoot for but what do you think?

I have to run for about 45 minutes will be back to post more data.

Key1

 

[beenthere]

Is ambient your basement temp, or your combustion air temp?

 

[bdivell]

not 100 % but i am pretty sure o2 can be from 5 to 9 range i would have to look in my books/notes....but i'd say looks good ... have to be happy with those readings

 

[fenian]

I'm not sure jim puts weight into those efficiency numbers given he uses a temp rise times cfm equation. If he's online he'll speak for himself.

out of curiosity do you have the cfm/ temp rise for both stages?

Congrats on the positive experience with the audit.

 

[key1cc]

Is ambient your basement temp, or your combustion air temp?

It was 77 degrees here today and we had the basement windows open for a while so it could have been the basement temp but I am not sure. The reading was taken right off of the meter while the probe was inserted in the pvc exhaust pipe. I will receive the full comprehensive report in about a week.
Key1

 

[key1cc]

I'm not sure jim puts weight into those efficiency numbers given he uses a temp rise times cfm equation. If he's online he'll speak for himself.

out of curiosity do you have the cfm/ temp rise for both stages?

Congrats on the positive experience with the audit.

not 100 % but i am pretty sure o2 can be from 5 to 9 range i would have to look in my books/notes....but i'd say looks good ... have to be happy with those readings

Thanks for the feedback. The CFM for stage 1 is 1000 CFM and the ESP is 0.43 and the max temp rise is 61F

The CFM for stage 2 is ~1500 esp = 0.98, temp rise ?

Key1

 

[key1cc]

Now for the blower door test data.
According to BAS (building airflow standard) and MVG (minimum ventilation guidlines) the standard airflow number for a property the size of mine is 2864.
The actual value obtained with the test was 3400. I am not clear on this but I beleive it means my leak rate is the difference between the two numbers. He used the Minneapolis Blower Door machine and set if for -50 pascals. The machine put the house under negative pressure then we went around with a hand held smoke generator to indentify the leaks. I was suprised at the leakage due to the outlets and light switches on interior walls as well as exterior. My comprehensive report is due in a week.
Key1

 

[sktn77a]

Jim will probably say those numbers are meaningless as they don't define the heat delivery to the system. However, given the first law of thermodynamics, I'd say that they fly in the face of all those people who claimed the low stage efficiency was as low as 50%!

 

[paul42]

Now for the blower door test data.
According to BAS (building airflow standard) and MVG (minimum ventilation guidlines) the standard airflow number for a property the size of mine is 2864.
The actual value obtained with the test was 3400. I am not clear on this but I beleive it means my leak rate is the difference between the two numbers. He used the Minneapolis Blower Door machine and set if for -50 pascals. The machine put the house under negative pressure then we went around with a hand held smoke generator to indentify the leaks. I was suprised at the leakage due to the outlets and light switches on interior walls as well as exterior. My comprehensive report is due in a week.
Key1

drywall is very flat, standard construction 2x4's are not. The inevitable result is lots of small cracks and air leaks, between the top plate and the drywall at the ceiling to attic plane.

 

[mbarson]

The lower O2 reading on second stage prove that second stage is more efficient but there is still too much excess air. Those efficiency numbers you posted are not correct as they are calculated efficiency not burner efficiency or delivered efficiency. The truth is that with latent losses, excess oxygen, and flue temp losses the actual efficiency will never be over 90%. If you get your O2 down to 6% you may up the efficiency to 105% or better! Your blower door data does not give you any useful information by itself. You need to correlate it with the volume and floor area of the house. This will tell you your ACH50 and can be converted to ACHnat to show how leaky the house is. You will most likely get these numbers in your report.

 

[key1cc]

drywall is very flat, standard construction 2x4's are not. The inevitable result is lots of small cracks and air leaks, between the top plate and the drywall at the ceiling to attic plane.

Agreed. And in addition, the bottom plate (rim joist?) and the foundation in the basement. These are typically large sources or air leakage.
Key1

 

[motoguy128]

Jim will probably say those numbers are meaningless as they don't define the heat delivery to the system. However, given the first law of thermodynamics, I'd say that they fly in the face of all those people who claimed the low stage efficiency was as low as 50%!

It rediculous to even think for a second that the 1st stage on a condensing furnce is 50%. Why? Because if you're condensing water vapor out of the flue gases, you're at least 85% or greater or have really high air flow rates. Second, everyone's bill with a 2 stage furnace that runs in low stage most of the time, would be really high.


I think higher efficincy on 1st stage is logical. You basically have an oversized heat exchanger.

If I stick a bigger radiator in my car, and bypass the thermostat, what happens? the discharge water temp out of the radiator will be lower at the same engine load and airlfow rate.

So the only way it would be less efficient is if there was proportionally less airflow on low stage. I believe there is proportionally more airflow, because the temp rise is usually lower on 1st stage than 2nd. On my XV80, for example, the 1st stage airlfow is only 75% of 2nd stage, but the BTU's are 65% of 1st stage. So the rate temp rise is 6 degrees lower.


Good to see that there's some data to prove what makes sense logically.

 

[key1cc]

Your blower door data does not give you any useful information by itself. You need to correlate it with the volume and floor area of the house. This will tell you your ACH50 and can be converted to ACHnat to show how leaky the house is. You will most likely get these numbers in your report.

I spent some time yesterday reviewing my notes and I beleive I have a better understanding of the data.
First my living space of 2800 sq.ft is added to the basement space of 1300 sq. ft for a total of 4100 sq. ft.. next a few calculations are done that considers my stories above grade (2) and my location (NJ) and the number of residents in my house (3) and the height of my walls (8ft). The result is 191 CFM ventilation is required for my building.Then that gets converted to the "minimum CFM50" by multiplying by a factor based on my location (in the U.S. or Canada) and the height correction for my 2 story home.

After all of this we get a minimum CFM50 requirement of 2864 as I listed in a earlier post. Next my actual reading was done with a house negative pressure set to -50 pascals. The result was 3400 CFM. If the result would have been less than 2864 CFM they would reccomend a fresh air intake. If the result would have been significantly lower (below 70% of the CFM50 requirement) then they would insist a fresh air intake is installed.

Since my value was higher (3400) than the requirement (2864) then there is room for me to reduce my leakage without a fresh air intake as long as I don't take the value below (2864).

Key1

 

[sktn77a]

It rediculous to even think for a second that the 1st stage on a condensing furnce is 50%. Why? Because if you're condensing water vapor out of the flue gases, you're at least 85% or greater or have really high air flow rates. Second, everyone's bill with a 2 stage furnace that runs in low stage most of the time, would be really high.

Absolutely! And this was the argument used back then, but endless debate obfuscated logic!

 

[TMH58]

CO2 levels must be below 10. I find it a little hard to believe you are getting 7 ppm CO. That is very good levels. In most states, below 100 is acceptable. Of course, if it were over 40, I would be looking at flame impingement,alignment,etc.

 

[CincyHVAC]

Here are the numbers for 1st stage:
Stack temp = 90
O2 = 8.4
CO2= 7.1
ambient temp = 72
CO= 5 ppm
Efficiency = 98.3

Here are the stage 2 numbers
Stack temp = 105
O2 = 8.3
CO2 = 7.2
ambient = 74
CO= 4 ppm
efficiency = 95.0

I know the O2 numbers is not as low as 6.0 like the great Jim Davis likes to shoot for but what do you think?

Key1

You are possibly under-fired. Your stack temps are too low. They should be between 120* and 140*. Your O2 readings are closer to the 9% range also indicating that you could be under-fired. Your CO readings as low as they are also indicating a possible low-fire situation.

Sounds to me, this furnace could use a little tweaking.

 

[key1cc]

You are possibly under-fired. Your stack temps are too low. They should be between 120* and 140*. Your O2 readings are closer to the 9% range also indicating that you could be under-fired. Your CO readings as low as they are also indicating a possible low-fire situation.

Sounds to me, this furnace could use a little tweaking.

Now thats the kind of feedback I was hoping for.
When I "clocked" my gas meter ( by turning off all gas appliances and only letting the furnace run), I realized my manifold gas pressure could use adjusting because I only clocked 60,000 btu incoming on low fire and 89,000 btu incoming on high fire. My capacaity should be 65k and 100k incoming and 95k and 61750 k out.

Is it worthwhile to have the manifold pressures tweaked up a bit?

Key1

 

[TMH58]

It is a good idea to set your manifold pressures to reccommended factory setitings. Most manufacturers have a low btu gas setting and a high btu gas setting. You should never set manifold pressures higher than factory reccomendations. Also, the only way to check true AFUE ratings is in a lab. The ratings you read with a combustion analyser are usually lower. Alot of factors come in to play like vent pipe size and length, btu content, burner alignment,etc.

 

[beenthere]

Depends on what your temp rise is to teh CFM of air.

 

[key1cc]

Depends on what your temp rise is to teh CFM of air.

Good point.

I already have a 61F temp rise with a range of 35-65.

...would not want to exceed the 65 and have the limit switch start tripping.

Key1