View Full Version : Duct tech was out....
11-21-2006, 08:00 AM
My last thread was about how our oil company insists I need an (expensive) A-coil cleaning because there is "very little airflow" thru our ducts and its causing the furnace to trip on high limit.
I had a HVAC tech come out to check the ductwork (after the oil co. admitted they couldnt do it). I dont have all of the numbers with me, but basically he found the system to be fairly balanced, and the coil may need a quick cleaning, but not enough to warrant the high limit switch kicking on. He measured the pressures at various points in the system, and came up with .35" WC right after the blower, .4" WC above the heat exchanger right before the coil, and .2" WC after the coil. It sounded like the .2" loss from the coil was close to normal.
However, my furnace is still tripping on the high limit (it did so while he was working on it, but it was a 70 degree day, he was thinking some of the high limit trip was because the return air wasnt very cool). He also noticed that, somewhere during one of our yearly services, one of the techs switched from a .65 GPH nozzle to a .75 GPH nozzle. He suggested that perhaps the burner is now over-fired for our system (I believe the ductwork is undersized), and to have our service tech change out the nozzle (back to a .65 GPH) at the next annual service (not until Dec 9). The furnace itself will run 85K BTU at .65 GPH, and 93K BTU at .75 GPH. Ive done an HVAC-CALC on the house (Im sure I didnt do it perfectly, but Im sure its close) and it said the house needs 90K BTU. Naturally, right in between the two nozzles (although there is no rating on the plate for the furnace, could a .70GPH be used?)
Does this sound like the correct remedy?
I think it makes more sense at least (I feel no difference in air flow from years previous to this year, coming out of the supply registers..........so the coil cant be *that* bad, can it?) It was a colder night last night, and I still caught the furnace tripping on high limit, which means the "warm" return air from the day the tech checked it out was NOT the culprit. I was also not asking the furnace to work very hard last night, just to maintain 69 degrees on a t-stat with a 1.0 degree differential.
We still plan on having the coil cleaned in the spring, but the HVAC tech suggested just a quick in-place cleaning, because I had seen the coil itself and there wasnt anything really caked onto it (basically looks just like a car radiator).
Thanks in advance.
Post the brand and model# of the coil,and someone may have the factory listed pressure drop pf the coil.
The pressure drop increases as the cfms increase,and if someone has the specs,they'll let you know.
11-21-2006, 09:28 AM
has anyone even checked to see if the limit is bad
11-21-2006, 09:31 AM
First of all i would check to see if the "rise" is within the units rating range. ,usually between 45-75 F.(return air temp near blower and supply air temp at plenum/ main takeoff.
Also check out the blower speed, it could be too low for that nozzle.
You also could try a .65 nozzle (for all the time it takes)and see how this performs.
The main issue before trying any of these suggestions is to ascertain that the coil and filter,etc are reasonably clean.
11-21-2006, 09:40 AM
OK, replying to the replies in order :)
I dont have the model # of the coil. Its an Armstrong 10 SEER 3 ton coil, but I dont have the exact model (the manual is at home, I can post it once I find it).
My oil co. actually suggested a bad fan switch to begin with. The tech didnt have the correct one in his truck, but once he opened up the plenum and turned on the blower, he surmised that it was an airflow issue (he said he couldnt feel much air at the top of the coil.........if you think about it, will there really be any airflow right above the peak of the coil? That area is not open to let air thru). I can feel plenty of air thru the supply registers in the house, but I suppose it doesnt mean its not an airflow issue.
The temp rise was within range (I believe the HVAC tech said there was a 70 degree rise). Blower is hard-wired to high speed for both cooling and heating (Im wondering if this ended up happening when someone changed to a .75 nozzle and all of a sudden needed more airflow). Filter replaced before I had anyone come out and look, put in a rock catcher just to be sure the filter was not the cause. I saw the coil myself when the oil tech had the plenum open......it may have been a little dirty, but it wasnt what they (or I) was expecting.......we were assuming there would be a layer of dust, dirt, etc. But again it just looked like a car radiator.....nice and clean. He did a light test, said he could see some light thru the coil. But again, that was the oil tech, who even admitted it isnt there speciality to figure out airflow issues. I didnt have the HVAC tech open up the plenum, because he asked me if the coil was caked, etc and I told him exactly what I saw from the oil tech. That in conjunction with the "normal" pressure drop led him to believe it was *not* a coil issue (although the coil could use a quick cleaning).
Just talked to my oil co, they said their computers show a .65 GPH nozzle. The service sticker on my furnace shows a .75 GPH nozzle. I wonder if they really did overfire it, either by accident or just because they didnt have the right nozzle with them.
11-21-2006, 11:03 AM
Replacing the nozzle with the "one you have in the truck" is too common.
It sounds to me like it is A: the limit switch ( has anyone checked to see at what temp it opens?)
or B: the nozzle size (has either tech done an effeciency and smoke test?).
My 2 cents.
11-21-2006, 11:09 AM
No one checked to verify the operation of the fan limit switch. Since the oil co. tech suggested a replacement, I may call the oil co. and tell them to bring one along during the annual maintenance. Judging from the feel of the ductwork (I know, a guess at best), it felt much hotter than it usually does. I dont know if its a valid way of guesstimating, but it *feels* like the air is staying too hot in the ductwork. In conjunction with the fan limit switch now kicking off, *something* may be making the whole system run hot. The HVAC tech didnt bother checking the switch, because the return air was so warm (naturally the day he came out it had to be like 70 degrees outside/in my house).
Efficiency and smoke test are done during our annual maintenance. I called a little late this year, so we didnt get an appt. until December (!!!!). I *might* be able to get them to come out and replace the fan limit early, but they wouldnt do the PM until the scheduled date (I asked the first tech, considering he had to vacuum soot out of the flue after he removed it, but he wouldnt do the whole PM).
11-21-2006, 12:36 PM
why was the nozzle changed?!?!?! i know that doing this can cause problems. ask the tech whom comes out to show you the data that says what nozzle is needed compared to the new nozzle and then why it was changed. also ask what the oil pressure is [should be 100 PSI if i remember correctly]
11-21-2006, 12:44 PM
Yes, I plan on being near the tech when they (finally) come out for my service. Im thinking it was a "I dont have any more .65 GPH nozzles, but hey the plate on the furnace says it can take a .75GPH nozzle" kinda thing. Ive used 2 oil companies, this is the 2nd. The first used a .65 GPH nozzle originally, then someone switched it to a .75, and I guess they realized what they did wrong because the 4 services after that was always a .65 GPH nozzle.
If they switch back to .65 GPH, I think the limit thing might go away, but I also think they might need to back down the blower speed, to keep the temp rise in range. Im also wondering if someone upped the blower speed, then figured since temp rise wasnt in range, add the bigger nozzle. Which would probably be fine, if my ducts werent undersized.
11-21-2006, 01:42 PM
We replace our system this summer because the AC wasn't cooling the upstairs. When the new system was being installed, I could see the underside of the A coil which had a layer of carpet fuzz, cat hair, and other dirt that was 1/8 to 1/4 inch thick. If there was access to the underside of the coil, that crud could have been removed without dismantling the entire system. However, when everything is assembled, there is no way to clean the underside mechanically. I do not think cleaning agents sprayed from above will dislodge the matted stuff. Hopefully your system as adequate access. Good luck.
11-21-2006, 02:14 PM
even this old man can cut thru the side of a plenum to gain access to clean a coil -- but then, my air handler is in the crawl --
are the rooms within 3F just after blower shutdown?
the furn manual should state blower cfm & temp rise --
if not, email the mfgr--
one could get a special nozzle -- not hard to ream a hole to be larger
tinknocker service tech
11-21-2006, 05:28 PM
with the blower on high for both heat and cool tells me there has been a problem with overfiring at .65. Then to increase to .75 just adds to the already existing problem
Did any one chenk the blower wheel and motor and see how much dirt is caked on them. Most airflo problems start there
Have the blower cleaned or at least checked and down size the nozzel back to where it was. See if that helps
11-21-2006, 06:18 PM
I *think* the blower was upped to high for heating when they switched over to the .75 nozzle. I remember when we first moved in, the heat would run longer and not blow so hard, then I remember the oil tech mentioning he had to up the blower speed at another maintenance.
The blower has been looked at (but not removed completely). The wheel is clean from what everyone can see.......again, the complete blower cleaning is part of the annual maintenance that we'll get in December.
The fan control on the furnace doesn't seem to allow for 2 different speeds depending on cooling or heating. So I'm wondering if one of the techs just chose "high" to make sure it works OK for cooling, and thus heating is "along for the ride" so to speak.
11-21-2006, 06:39 PM
If the limit is tripping, you are not moving enough air or are way overfired. The furnace can handle the .75 nozzle. and you gave us indication that the supply static was .4" wc. Unless your return is .2" or less, you are lacking airflow.
The pressure drop across the coil is acceptable IF you are moving enough air in the first place. If you all the sudden start moving the right amount of air, it may climb significantly... who knows.
You have a duct problem that needs to be resolved. You might get away with switching to the .65 nozzle but then again, just because the limit doesnt trip doesnt mean you dont have a problem.
11-21-2006, 07:12 PM
Reading the paperwork, we have .35" WC on the return side (he actually lists it as a negative, Im assuming that means vacuum/sucking), .4" before the a-coil (listed as a positive number, so Im assuming thats blowing). So it indeed seems like the return pressure is too high (?) My (sorta educated) guess is that the returns are too small, the blower is pulling too much air, or there is a restriction somewhere in the return(s). Or a combination of them.
I can almost 100% eliminate restriction in the return air, because we never had this issue before and I know I havent dumped anything large into the returns, and none of the openings are blocked.
I suppose it could be too little return air, but again never had this issue before. If thats the case, can I just add return ducting? I actually wanted to have one added to the basement to aid in dehumidifying during the summer.
Is it possible that the whole system was never setup to run heating on high? and as such is pulling too much pressure from the returns?
11-23-2006, 11:49 PM
I had a similar problem with my carrier 85K furnace which has a .75 GPH nozzle and from what you reported, your system ,if properly designed, should have worked with that nozzle. (Your ductwork was too small for this nozzle since I see that your supply pressure is .2" and suction at the intake plenum is .35 at lower than design flow.)
My techs who looked at my system had no equipment and first arbitrarily replaced the limit switch with no improvement;a second tech "determined" it was the coil and then gained limited access to my coil and did a vacuum cleaning which was not too effective. So I bought a manometer and did my own checking and found that my filter was part of the problem and after I put in a more porous one the problem went away. I could still use a coil cleaning but won't spend the money ($300)
Let me tell you about your system which is similar to mine.
1)You have a .2" drop across the coil at probably 3/4 the design flow; you could easily show a drop there of .35" under the actual design cfm.You are probaably getting 900 cfm when you should be getting at least 1200 cfm at high RPM
2) Your ESP(external static pressure)is at least .35 +.4"= .75 plus the drop across the filter.Since the mfr spec the flow rate at .5" esp (corresponding to 1200Cfm in my case) and degrades substantially thereafter, yours is more like 900 cfm , I would say you have a substantial flow problem for the .75 GPH nozle(and don't waste your money with a limit switch change) that would be corrected by a proper although costly coil cleaning, but with a .65GPH nozzle. You MUST go back to the .65GPH nozzle and underfire the furnace and buy time for the cleaning. As I mentioned, your ductwork is also underdesigned at .2" at the low flow but don't be quick to fix it.
If I were you, I would first:
1) get a .65 GPH nozzle, a must
You should immediately get a reduction of ,65/.75 of the temp difference which may allow you to stop right there and do NOTHING more. My preliminary estimate is that is
all you need.
But, if you can for free or very little money:
2) see if you could get an elcheapo vacuum cleaning from an access panel like I did. ( after no cost to me from 2nd tech when I insisted)).You probably dont need a full blown cleaning or any cleaning now.
3)run the filter test I mentioned(no filter) or get the tech to give you the drop across the filter ( should be less than .05") and replace with more porous type if so determined.
4)Now after this, fire it up and watch the limit temperature after a long time and if it approaches 200 degrees;if you still have a problem
remeasure the drop across the coil to see what kind of cleaning job was performed, if any.
As a final comment, if your heat loss calcs are right, you may have some trouble keeping your house warm at the design outside temp, since , as you know, at .65GPH you have a proportional reduction in heating capacity plus the added inefficiency of an underfired furnace.
11-24-2006, 06:04 AM
Thanks for the information!
I agree that the house might be "harder" to maintain in regards to heat level, but Ive always been under the impression that oil furnaces are more efficient the *longer* they run. Going from a .75 to .65 nozzle should provide the heat to the house, it will just run longer (and on design days, it would run constantly, which I always thought is the goal of a "good" system). Honestly, with forced hot air, I like it MUUUCH better when it runs longer, I just feel a lot more comfortable with the longer flow of warm air. The fan speed may need to be dropped with the .65 nozzle though, I have a feeling the temp rise wont be in spec, and the easy way out for the tech would be to put back the .75 nozzle.......but thats why I plan on being there if he attempts to do that.
Either way, the .65 nozzle is going back in when I get my annual service. Im currently doing an experiment with our return grilles to see if one is restricting return airflow (I had changed it over the summer after doing some painting....didnt think it made a difference, but at this point I guess you never know).
BTW, any suggestions on the *correct* remedy? If the ducts are undersized, any tricks of the trade to fix it when we finally get to replace this furnace? My original intention was a 2-stage oil replacement (at this point that means Thermopride), but judging from the ductwork Im not sure if the 2 stages makes the system better or worse than it currently is. I assume variable speed fan at minimum would help a little, barring any major work to fix the ducts.
11-24-2006, 07:39 AM
You may not realize that the air flow has dropped over the years because it has been a gradual drop off.
It may have been close to tripping the limit for years now, and just started to do so now.
Get the coil cleaned.
Go back to the .65 nozzle.
It didn't go off on high limit because it was 70 in your house.
The correct fix, is to have your coil cleaned, and then recheck the static of your duct work, and resize if its still high.
Until thats done, its all arm chair guesses.
11-24-2006, 08:46 AM
OK, Ill get the coil cleaned ASAP (at this point, I think it wont be until Spring.....too cold out to fire everything up after recharging the system).
The .65 nozzle will be installed on Dec 11. I might try to see if I can get the oil co out sooner for the annual service, because of the nozzle debacle.
11-24-2006, 10:49 AM
It sounds like you have an upflow furnace. Did anyone check the heat exchanger side of the coil? The top, or air outlet side almost always looks clean. The dirt etc collects on the heat exchanger side. They might be able to get a good visual if they remove the blower and look up through the furnace at the coil surface. Actually cleaning it thoroughly is more work, but seeing it from the bottom is sometimes possible.
If the coil needs cleaning get it cleaned now. Everything except the A/C restarting and final charge adjustment can be done no matter the outdoor temp.
11-24-2006, 11:34 AM
The original oil co. tech opened the plenum and pulled the cover off the end of the A-coil. We could see inside and outside, and didn't see a caking of any sort. No obvious dust bunnies, lint, etc. The best he could do was to shine a light thru the inside and try to see outside. He said he saw some light, so its not completely blocked, but even he wasnt sure how much light he *should* see.
I think Ill just make the call and have them clean it. Unfortunately they couldnt give me a solid estimate on cost, so I hope it doesnt turn out to be something they decide to break my bank with.
11-24-2006, 12:03 PM
i dont know if it helps but you arent going to see much light either way. the air is not meant to flow straight thru the coil. modern coils force the air to weave a little. the air will go from right to left and back to front. this is done so that the air stays in the coil longer as this provides more heat transfer and more efficiency. your coil may very well be dirty deep down inside or it may not. like mentioned, have it cleaned as its one thing on the list to eliminate possibilites of the problem.
11-24-2006, 04:01 PM
Im actually curious.......if the A-coil sits above the heat exchanger, and the air is blown from the bottom up (I have an upflow furnace), why/how does the air actually go thru the coil, instead of just going around the coil? I didnt see anything sealing the coil and drain tray to the sides of the plenum, so how does it work?
Or is the lack of a "sealer" there to provide airflow in the case that the coil gets clogged?
I scheduled the cleaning.....oil co says they prefer to clean it without removing it completely, mostly because the tech noted it wasnt in horrible shape.
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