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Newer Kenmore 5 ton Gas Pack on home we just bought. Induced draft airflow question

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#41 ·
Also... unless you live in an arid climate, an ac system that is oversized can lead to several very serious health and building structure issues.
 
#42 ·
Your thermostat may be calling for second stage the entire time, when you set it X degrees above current temp. Finish heat call in high heat mode, is a common setting of many thermostats.

First stage/low fire is often less efficient then high fire/second stage.

So after 12 minutes you were at steady state efficiency.
 
#44 ·
N8engnr, you have several technical posts to establish your knowledge. Apply for Pro status and take this to the Pro Residential area.

I got my chops busted pretty good when I came to this forum earlier this year. However, once these members got to know me a little, and I showed that I truly wanted to learn from them they started helping me out tremendously.
 
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#45 ·
Why don't you apply for pro membership?

PHM
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n8engr said:
Dean, I am a Stationary Engineer, not a Mechanical Engineer. I have been in my current position with the same major hosp chain for over 25 years. I have worked at a few facilities and I am not "new to HVAC work". I work on a lot of different types of equip and systems besides HVAC, but I have worked on plenty of HVAC systems, I just don't have experience on residential furnaces like this, other than changing a roll out switch, gas valve or other minor repairs in previous homes. I've had a Universal CFC license for 20 years and I have been turning wrenches and going to school since I joined the AF at 17 and worked on the SR-71 and U-2 as a Jet Eng Mech in the late 70's. I have an A&P license and several other licenses including Certified Energy Mgr certificate. I don't think I 'know better than everyone else', but I do think I know more than you give me credit for. I also think engineers with college degrees often get a bad rap. I have friends that have BSME's and believe me that doesn't make them inept at hands-on repairs or intelligent improvisation. I have been a mfrs rep in a previous job and I know that often the difference between a manufacturing design and a design improvement simply comes down to COST. The HVAC market is very competitive, because something hasn't been done to improve a system's efficiency isn't always due to the fact that it can't be done (because the "geniuses" that designed it would build it that way if it was possible), it just may not be a smart cost-point product and would not be as profitable for them to produce. I regularly change out the cheap/crappy components that new equip comes with because it is worth it to me to have that valve, pressure switch, sight glass, whatever, last seven years before it needs repair or replacement instead of two. A manufacturer can't build equip with those kinds of extra costs, especially these days, they would simply not be able to compete in the market.

So...If I chose to go to a shop that makes exhaust pipes for semi trucks and had them bend a helix out of 3 or 4" SS pipe so that I could have a "heat exchanger" that I could put a couple of feet from my furnace in the return duct to route high temp exh air into the from the top (sealed of course from the return duct airflow) and let it blow out the bottom of the duct to the ground (along with whatever condensate rolled down the pipe), that would probably cost me $200 to do myself, but I'm guessing it would probably pay for itself within a couple of years. That isn't a cost that a mfr would spend. It doesn't mean its an "unthinkable thing to do", it just means that it wouldn't be a wise manufacturing choice. Also, while it might be practical in my installation/location dropping two feet into the ground, it probably wouldn't work in many other install/locs. There is room for improvement on virtually ANY system.

Today I brought home a Bacharach PCA3 combustion analyzer that I use to tune boilers on natural gas and fuel oil at work.
"De-tuning" also called "de-rating", means to limit the full firing rate of a boiler. This is often done when setting up a new boiler because it is often not safe to run it to 100% of the rated max firing rate depending on the installation/location. It is also done, I have done it myself with factory blessing, to reduce the firing rate on an existing boiler to avoid damage to an old refractory or because the boiler can no longer make NOX limits at higher firing rates. It is not the opposite of "tuning your car"(really??). I was looking at possibly detuning this furnace because I agree with some of the contributors to this string that this furnace may be oversized for this house. However, an oversized A.C. system is not necessarily a bad thing and...again, I did not want to just throw out this almost new system just to improve heating efficiency. So I was thinking that if I can get the firing rate down with proportionately lower fuel air mixture, it might be able to be operated like a properly sized furnace (I realize this may mean slowing down the blower motor and several other steps, I did get that).

As it turns out, while doing the combustion analysis, I now think that this 2 stage furnace is only firing in the second stage.
I deliberately let the house cool to about 62' before running the combustion check after work today. The set point was 67' when the unit started. The system never changed its firing rate even when it was at set point about 20 mins later. Just to check, I set the S.P. up to 77' to see if it would stage up, it did not.
Ironically (based on all the input I heard about how inefficient this unit would be until it was firing for at least 10 mins), Its peak efficiency of 80.5% was reached within about 2 minutes of starting up. I took consecutive readings every 5 mins after that and efficiency kept dropped steadily; dropping to about 79.2% after 20 mins. EGT was about 410' initially, climbing to 420'. Initial readings: O2 6.8%, CO 5 PPM, Eff 80.0%, CO2 8.0% T-Stk 409', T-Air (ambient) 54.5', *EA 42.8%, NOX 35PPM.
10 mins later (12 mins firing) O2 6.7%, CO 5 PPM, Eff 79.5%, CO2 8.0%, T-Stk 420', T-Air 50.8', *EA 41.9%, NOX 35PPM.
*EA (Exh Air) percent is MUCH higher than any fuel burning device that I have worked on. The closest thing that I have to this unit is a Fulton 9.5 HP steam boiler that we use for steam sterilizers, I have 10 of these in my Plant. The EA is usually less than 10% during stack analysis/tuning on those boilers.

I started this thread asking what I should be expecting to see for induced draft airflow from a unit like this and only one contributor actually had pertinent, detailed info pertaining to that question. I still think the combustion airflow is much too high, but I haven't measured it yet. I have a flow hood at work for air balance checks, but its not the right instrument for this airflow and my anemometer is broken. My impression from experience measuring LOTS of supplies and returns is that it is flowing at least a couple of hundred CFM: exhaust is seriously blowing out of this port, but I am going to find a way to measure it.

Based on what I saw today, I am going to have the installer come out and verify that this unit is running in stage one heating mode, since it is not changing. The pkg unit and thermostat along with ductwork in aprox 1000 sf of additions were installed by the Sears subcontractor, so if it is only running in stage two heating that seems to me it would explain the high gas usage and short cycling and there isn't much of any excuse for it not being set up right, since they did the whole job including specifying and wiring the Sensi T-stat. This was all done for the previous owners, who sold the house shortly after completion of all of the renovations.
Click to expand...
 
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Discussion starter · #46 ·
I will apply for Pro membership, if that is allowed now. When I first joined a few days ago it looked like participating in theis Newbie forum for a certain amt of time/ # of posts is required.

I will see what the Sears tech says when they come out. It looks to me like it wasn't set up properly in the first place.
I was wrong, the unit is not 6 mos old, its 18 mis old, so the labor warranty is over, I may have a battle trying to get them to admit the problem goes back to install.

I agree that the O2 and CO levels are good, thise are similar to targets that I use on boilers, but the Excess Air seems crazy high at 42%: that a lot of BTUs being wasted to atmosphere-unless these units are much different thsn anything that I've seen before.

Thus is a 115000 BTU unit, so the numbers that I was given for a generic 120000 unit for ballpark airflow: 68 CFM, 450 FPM should be a good comparison. I plan on measuring the exh airflow before the tech comes out.
I am sure that it is at least double that volume.

Efficiency is not bad where it is, but its just firing too hard. I'm not sure why firing at a lower rate, if the unit is oversized, is not an obvious solution?
If it's never running in Stage 1 heating mode and it is short cycling, whatever the efficiency difference is (75 vs 80%?), it still seems to me that running at the lower firing rate will significantly lower the gas bill and drastically reduce the short cycling.

Thank you all for your input through this string.
 
Discussion starter · #47 ·
BTW, whoever said that I said the heater raised the temp 10 degrees in 10 mins misunderstood my post. The heater raised the indoor temp from 62 to 67 (5 degs) in a little under 20 mins. I raised the set point to 77 for a few mins after it got there to see if the unit was in Stg 1 and would stage up to Stg 2 with a larger diff from set pt. It did not, and it only got up ti 69' during those few minutes.

Sorry for the misspellings, using my phone and can't see what I'm writing!
 
#48 ·
Excess air , co2 and eff are just calculations based on the three things the analyzer actually reads, O2,CO and temperature. Since the O2 and CO are in line but the temperature is not you're dealing with an air flow problem.
Derating will make your temperature go down but also make your O2 and excess air rise. For every 1℅ increase in O2 above what you now have you'll lose an additional 1℅ effeciency.
 
#50 ·
Maybe you should verify the gas meter is on the up and up?
 
Discussion starter · #54 · (Edited by Moderator)
Hi Guys. It was what I suspected. The unit was wired wrong from installation. The 1 yr Sears warr was up, but when the rains finally stopped and I opened the unit and saw that the installer hooked the wht W1and blk W2 wire to only one stat wire and did the same with the yel Y1 and pink Y2. I called Sears svc out and they agreed to have a different installer come out and pull the correct wires (8 conductor) back to the stat and wire it correctly, gratis.
The stat was a two stg htg and clg stat. The pkg unit was also two srage htg and clg. Thecstat was even prigrammed for two stg htg and clg. The installer only pulled a 6 conductor wire and then lazily (why should he care its not his utility bill!) wired the unit to run in high htg and cooling only! Nice guy! The people that sold us the house and paid $ for this installation in 2015 obviously wasted several hundred bucks on excess gas and electricity last year. We wasted a couple of hundred ourselves (moved in in Nov) in part of Nov and all of Dec before I saw what was going on! The unit runs fine now. Flue gas airflow is less than half of what it was in high heat. No more short cycling. No more heating the whoe neighborhood with the exhayst from this unit! Thanks for the advice from thise who offered constructive tips Hopefully no one that uses this site would think of doing what this subcontractir did!
 
#55 ·
Glad you got it sorted out. Sometimes we go round and round trying to think of complicated problems when the only problem is an improper jumper wire or a defrost jumper on the wrong pins.
 
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#56 ·
Food for thought:

O2 - 6.8% = Flame temperature of 2650 degrees Flue temp of 420 degrees 2650 minus 420 = 2230 degrees transferred

Let's de-tune
O2 - 9% = Flame temperature of 2350 degrees Flue temp of 350 degrees 2350 minus 350 = 2025 degrees transferred.

That is why it should be called de-tuning because you make and transfer less and less btus as you de-rate.

Flue temperature is a bit high but as previously mentioned that sounds more like an airflow problem. Speed up blower and measure flue temp. If it goes down you improved. If it goes up we did the wrong thing. Slow it down and observe the same thing. If the Delta T on this gas pack is over 72 degrees the blower is too slow. If it is less than 62 degrees the blower is to fast.

You also made a wonderful discovery while testing with your combustion analyzer. You learned that efficiency in heating goes down the longer it runs not up! The faster we can heat and get the unit to shutdown the more efficient. As long as the cycle is 4-5 minutes minimum you are good to go. This was the biggest problem I found on commercial and industrial equipment. Everyone was told to keep it running to save money. That was a fairy tale started because of flame failures that no one knew how to fix and it was costing a fortune in energy usage. Because an A/C unit has to dehumidify short cycling might not be good unless you live in Phoenix. Yes there is an increase in inductive load on A/C but not running load.
 
#57 ·
Huh...short cycling is now good. Who woulda thunk it?
 
#58 ·
hurtinhvac said:
Huh...short cycling is now good. Who woulda thunk it?
Click to expand...
Higher efficiency but having to maintain the indoor temp at 72 to feel comfortable, uses more gas then lower efficiency and only needing to maintain 70 in the house to feel comfortable.
 
#59 ·
Doing the job and cycling off has never been a bad thing. What does an A/C do on a 75 degree day when the t-stat is set for 72 degrees? I don't believe anyone would ever let refrigerant out of an A/C unit because it was oversized, or would they? It will prevent short cycling! In the heating mode cycling on and off every 4 to 5 minutes is not short cycling. Running for 10 minutes is a sign of poor efficiency and performance. People have been making up stories like this for years to cover up defects they don't want to admit to.

hurtinhvac said:
Huh...short cycling is now good. Who woulda thunk it?
Click to expand...
 
#60 ·
Well, n8engr's system might give us a little anecdotal information. He has several months worth of gas bills running on high heat only. Now, he can report back to let us know if staged heating turned out to be more or less expensive, and if he noticed any increase in comfort.

One case doesn't make a complete argument, but it will give us something to think about. Low stage on combustion furnaces rarely shows efficient combustion.

I have heard arguments about more heat and cool cycles fatiguing joints in heat exchangers sooner.

Any thoughts on that?
 
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Jim Davis
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