need heat rise specs for TempStar F/G96VTN furnace
Hi all,
Trying to troubleshoot a fairly new TempStar F/G96VTN furnace, running on natural gas. The installation/service manual says that the temperature rise limits are on the "furnace rating plate", which I cannot find. The original installer is of no help because I had to threaten to sue him for the bad installation (I collected).
At least in high-heat (stage 2) mode, the furnace shuts down before the thermostat stops calling for heat (i.e, the LuxPro P722Uc thermostat is still flashing "HEAT 2"), eventually restarting on its own. I suspect that it is hitting some sort of internal temperature limit, and want to see what the temperature rise is.
BTW, when operating on low-heat (stage 1) last season, the furnace cycled on and off approximately 3 times per hour - nice. But operation on stage 2 was usually limited to 5 minutes or so, before it shut down and eventually restarted, perhaps 6-8 times per hour. It was annoying, so I "disabled" stage 2.
The system is in a 1600 sq. ft. ranch style house located in Western New York State.
The ID plate is on the inside wall near the burner. It is illegal to remove it.
If you have a new furnace and it's missing.... there is a problem there.
It does sound like it's short cycling from overheating. It's probably too big for the existing ductwork.
Must be a serious issue as the install manual says the variable speed blower should keep the furnace from overheating in each stage.
I did find the specs, not near the burners though, but on the inside of the lower (blower/air distribution) cover. They are 30-60 degF for low and 40-70 degF for high.
I understand your concern about the ducting, but I doubt it is that. Quick history:
1) The house was originally about 1200 sq. ft., built in the 60's with the oil furnace approximately centered in the basement, with 8x14" main trunk lines running north and south from it, and various 6" round branch lines feeding the various rooms. There was a central air return of some sort, but I don't recall the exact details. BTW, we're at about 1300' elevation.
2) I bought the house in 1993, and had a gas (at first propane, later natural gas) furnace installed. I also had an addition built that increased the size to around 1600 sq. ft. I had them zone the two original main trunk lines, plus add a third zone (8x10" main trunk line to two 8" round branch lines) for the addition. The central return air ducting that they installed was quite "Rube Goldberg". However, except that the zoning never really worked correctly, the furnace did okay until 2020, when it started having control issues.
3) I called an hvac tech to solve the furnace control issues, and he convinced me that the heat exchangers were cracked too and that I needed a new furnace. He sort of had me "over the barrel" (it was last December), and I agreed to have him install a new two-stage furnace. But I wanted the new furnace to be about 6' south of where it was to better use my shop space, so it was "slid" that distance along the existing ductwork. See the attached photos of the marvelous sheet metal work he did. The first photo is looking south into the plenum and shows the butchered south main trunk, and to the left the very undersized hole for the third zone.
The second photo shows the piece of that south duct removed, exactly as it was.
The already "Rube Goldberg" air return system was also butchered, but I won't bother you with the details of that.
4) When I had the issues with the short-cycling on stage 2 last December-January, I too suspected high resistance. But I measured less than 0.5" W.C. static pressure drop in the ductwork (on stage 1; never tested on stage 2).
5) I have just about finished a complete redo of the main trunk lines, eliminating the zoning dampers and the third zone main trunk line completely, and replacing them with 8x16" ductwork. Also added a couple of new branch lines, so now there are ten 6" round take-offs plus two 8" round take-offs. Right now my great local hvac/sheet-metal shop is fabricating a new custom air return duct, which will flow far better than the earlier one. So I am currently running the system with just a filter on the side of the furnace as an air return (i.e., very free), and all the registers open. I will test the static pressure drop again as well as measuring the temperature rise, and I'll do the pressure and temperature tests on both stage 1 and stage 2 modes, and report the results here. But bottom line is that I think the pressure drop is pretty low with the new ductwork.
Question: I have read that when doing the temperature rise test, to be sure that the thermometer doesn't "see" the glow of the heat exchanger. I have an a/c evaporator on top of the furnace; is that enough to block the view, or should the thermometer go "around the corner" into one of the main trunk lines rather than in the plenum?
I took some measurements for both temperature rise and total static pressure drop. The temperature rise values are less than precise as all I had was an 8" immersion thermometer with a 1-3/4" dial that reads from 50 to 300 degF, so pretty hard to read (at least with my senior eyes). As I said before, the return air is just drawn out of the basement directly into the furnace right now (will have new custom air return installed in a week or two). What I didn't state before was that the plenum is not tightly sealed to the top of the evaporator yet (will be soon), so with some leakage there, the return air temperature could be rising a bit during the run. Next time I'll add a thermometer by the air inlet.
The ambient temperature was 68 degF when I started. Using an immersion point centered on the plenum about 5" above the evaporator, I got readings as high as 148 degF on low heat, and 156 degF on high heat, so definitely on the high side of the heat rise range, unless of course the thermometer is seeing the glow of the heat exchangers through the evaporator. When I moved the thermometer to another hole that was higher and much closer to the edge of the plenum, the numbers dropped considerably, about 20 degF.
As I had expected, the total static pressure drop readings were very low, in the range of 0.1" W.C., almost unreadable on the manometer.
So it seems to me that the short cycling issue is more related to temperature rise than to total static pressure drop. Although I'm a retired mechanical engineer and an experienced DIYer, this is my first foray into the hvac world, and I'm not sure I want to be screwing around with modifying gas pressure settings or orifice sizes, but I'm always willing to learn. Of course it is a bad time of year to be trying to get a reputable tech to look at it too.
Any advice is welcomed, including addition tools/gauges to get.
If I'm reading this correctly you are taking temperature rise and TESP checks on a system that lacks complete supply/return ductwork. At this stage I think you'll find that any results you get are going to be less than accurate. The temperature rise is very dependent on airflow which will be changing after the addition of the return air duct. The installation manual requires that the blower door be installed to measure temperature rise and lacking a return air duct you essentially have an open blower. Total external static pressure will also require the total ductwork to factor in it's resistance. Even after you finish the ductwork and take new measurements I would strongly suggest more research before altering anything gas related. You have a relatively complex system with many other ways of making adjustments. Good luck, I'm sure you get a handle on it. BTW, digital HVAC thermometers are easier on the eyes.
Thanks for your sage advice Fastback! You are correct about the incompleteness of the system. But as I stated earlier in my long-winded posts, the system was short-cycling on stage 2 last winter with a far more restrictive system, and then exhibited the same immediately even with the open system, so I suspect that it isn't going to go away with the new, custom cold air return ductwork I'm having made.
I don't mind investing in a decent digital HVAC thermometer. Would like one that has multiple inputs, and that I could leave the sensors in place "forever". What would you recommend?
Glancing at the installation manual for your system I saw lots of DIP switch adjustments. I'm sure with a complete review of it you'll be able to improve the short-cycling issue.
You're sounding like a tool junkie, -I share that addiction-. As far as thermometer, I keep a "Fieldpiece" folding unit in my pocket. Never had a need or desire for a forever mount.
Thanks again fastback. I'm aware of the DIP switches, although most of them are for A/C, which I don't have yet except for the evaporator coil. The factory settings are that only SW1-4 and SW1-7 are on. SW1-7 is just for blower off delay. SW1-4 is the "comfort/efficiency adjust", and I will try turning it off as when on it decreases low heat air flow 7% and decreases high heat air flow by 10%, so that could certainly cause higher temperature rise and hence possibly short cycling. Good call! I currently have SW1-2 on as it sets the furnace to low heat only.
Tool junkie: I "resemble that remark". I've had my sports car / automotive / racing phase, my machinist phase (part of the former), my carpentry phase, and am now into my woodworking phase (Festool, et. al.). Now that I looked into it, you're pointing me towards a Fieldpiece phase. : ) I just hope when I die, my wife doesn't sell all my tools for what I told her I spent on them.
"A thousand pardons" for not getting back to you helpful folks sooner. It took many months longer than I had expected for the hvac shop to finish the return ductwork, and longer yet for me to get everything installed correctly.
In any case getting the return working properly has solved the short cycling, and I have just completed a 30 minute run on stage two! The only problem I have now with the more direct return is that the return grille is noisier than we'd like; but I'll start a new thread to address that. Measuring the heat rise (a bit off center now), I get around 37 degF on stage one and around 49 degF on stage two, so towards the lower end of the spec.
Again many thanks to PJMax/Pete and Fastback (btw, I did get the Fieldpiece digital thermometer that you recommended - nice) for your great help.
15+ year old Lennox gas furnace/central AC packaged unit.
Furnace finally gave out, presumably the heat exchanger because CO alarm inside the house went off, gas company called, valve shut off at the gas line.
Considering replacing the heat exchanger instead of a complete new unit out of economic reasons. 2 winters ago I had to replace the ignition board, and saw that this particular unit has just 2 burners.
Does that mean it needs to match to a heat exchanger that specifically has 2 clamshells?
Hope someone can spot what I've missed. The issue started after igniter replacement. When the gas valve turns on, sometimes ignition occurs, sometimes not. Ignition may fail on a first or 2nd strike, then ignite on the 3rd. Other times the furnace trips out. When I reset the controls, same thing repeats. In effort to resolve, have confirmed the igniter is properly positioned, confirmed that safety circuits and sequencing are correct. Confirmed gas manifold pressure is correct (3.57 WC). Burners and manifold were removed and fully cleaned (including orifices). When the unit is attempting to start, It seems as if an inverse air current may be sometimes preventing gas from flowing though the burner tube to the igniter. If I "help" the gas flow by blowing gently toward the burner tube (toward the igniter) as the gas turns on, ignition occurs perfectly every time. It seems that gently blowing air in the correct direction allows the gas to overcome whatever may otherwise prevent it from passing correctly through the tube. Since it's a high efficiency furnace with enclosed burner box, I can't imagine where the spurious airflow may be originate (or is it an incorrect diagnosis?)
09-22-21, 08:01 PM
