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Old 01-02-2009, 04:59 PM   #16
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Bryant 90i Plus delivers Little Heat


First.

Increasing blower speed decreases temp rise.

So if in emergency heat, its moving more air, and the temp rise is greater, then it has to be using more gas.

Inducer speed could be slower then it should be.
Manifold pressure could be low.

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Old 01-02-2009, 05:29 PM   #17
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Bryant 90i Plus delivers Little Heat


Der Beenthere,
You wrote: First.Increasing blower speed decreases temp rise. - I do agree, yet the gas meter does not turn faster.

You wrote: So if in emergency heat, its moving more air, and the temp rise is greater, then it has to be using more gas. - I do agree, however the gas meter does not support this. So is has do be something else (in the furnace) that is not detected by the controller, since there are nor error codes.

You wrote: Inducer speed could be slower then it should be. - This is a new idea I have not checked on. The inducer wheel could be spinning on the motor shaft. Yet, while in "emergency heat", the speed is higher (or high enough) and the gas burns with more ogygen and, thus, at higher heat. [ I will check this and respond.]

You wrote: Manifold pressure could be low. - Is this really possible in view of the fact that the gas consumption is in line with expectations for a 60kBTU unit? I thought I had ruled this out (e.g. plugged/dirty orfices in the manifold) with my gas consumption measurements. Also, in "emergency heat" it generates more than enough heat.
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Old 01-02-2009, 05:46 PM   #18
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Bryant 90i Plus delivers Little Heat


With out a manometer.
The clocking of the gas meter is only a rough indicater.

The gas valve references the negative pressure in the burner box.
Higher inducer speed means a lower vacuum, which makes the gas valve put more gas into the burners.
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Old 01-03-2009, 10:23 AM   #19
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Bryant 90i Plus delivers Little Heat


Dear Beenthere,
Thanks for your thoughts. I checked the inducer wheel. It is firmly locked onto the motor shaft. No slipping of the inducer fan wheel is possible. - You wrote: The gas valve references the negative pressure in the burner box. Higher inducer speed means a lower vacuum, which makes the gas valve put more gas into the burners. - I understand your point. Since the controller calculates the inducer RPM and finds it to be within the expected range (there is no error code), this leaves just the pressure switches (malfunction but not broken) . Since the controller does NOT flash an error as long as the low/highRPM-ratio is between .55 to .90, the pressure switches might switch at almost the same [low] pressure as long as the high-switch is at least 10% higher than the low switch. This would be compatible with the rather low heat output (and measured low temperature raise) of the HIGH-heat mode despite increased gas consumption. - I will check the pressure switches against a manometer.
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Old 01-03-2009, 09:10 PM   #20
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Bryant 90i Plus delivers Little Heat


alex_ny-

From reading this thread, you stated that you had a 50-55 degree air split in low fire and less in high fire. Since you have a manometer test your gas pressure. It's been suggested several times but you resist the recommendation. Your pressure switch is not the problem if it was you would have a fault when it tried to go to high fire. The pressure switch has to close on the high side before the gas valve will increase gas pressure if it doesn't close there would be a fault. Your gas pressure should be stated on the data plate. Usually around 1.6" wc on low fire and 3.5 on high. before you test it remove your vaccum hose off the gas valve and set your high fire 1st.
If you have minimal to low air split (supply -return) it's either to much air across the heat exchanger or not enough gas supply to heat the system.
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Old 01-09-2009, 07:55 PM   #21
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Bryant 90i Plus delivers Little Heat


Dear readers and experts [e.g. sgthvac, beenthere & SKIP4661],
I appreciate the feedback and thoughts you provided in this thread, but there is still no solution to the problem. I am operating my furnace in "emergency heat mode", a dip-switch setting on the controller, that bypasses all regulations (except safety) and runs the furnace (blower and inducer at maximum speeds)
Again, I invite any suggestions or thoughts:
Here are the facts: My BRYANT 90i plus (355MAV with HK42FZ012 controller, 60kBTU, about 15 yrs old) produces little heat and does not flash any error codes. Self test is passed without any faults or errors. The furnace is a 2 stage condensating furnace of 90%+ efficiency with HSI mounted in downflow configuration. The two-stage gas valve pressure is verified with 1.5 and 3.5 inwc, correlating very well with the gas rate clocked on the meter for this 60kBTU unit, the low & high pressure switches close at .95inwc and 2.0inwc (as required). However, the temperature difference (output air minus input air) is 50F and 52F for low and high heat. According to model plate, the temperature rise should be 60F-70F and 50F-60F for low and high heat, respectively. These current readings correlate very well with the observation that the house cannot be kept warm by the furnace when the temperature is around freezing or below.
For the experts:
Under microprocessor control, in the low heat mode the inducer runs at 1500rpm producing about -1.0 inwc, in the high heat mode the inducer runs about 2800rpm producing about -2.9 inwc. Whether this is correct or not, I do not know. Does anybody know?
In emergency heat mode (at maximum speed): the inducer runs at about 4850rpm producing about -8.7inwc and a heat difference of about 100F. The heat output is just as I used to (or more; hard to say), however the furnace is a little noiser than it used too be.
QUESTION: What keeps the furnace from producing the rated heat under normal controller settings given the above? What else can I measure?
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Old 01-09-2009, 09:50 PM   #22
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Bryant 90i Plus delivers Little Heat


Alex,
First post 12/17, and you posted today with same problem. That's over 20 days. Might be time to call in a repair person unless you are enjoying this process. I can't imagine you are saving any money by now since you're running e-heat. Are you an engineer by any chance?
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Old 01-09-2009, 10:15 PM   #23
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Bryant 90i Plus delivers Little Heat


Just a thought. Durring normal operation some how could the A/C be running and then when you go into emergency heat it shuts down.
Probly wrong but it dosent hert to look.
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Old 01-10-2009, 09:00 AM   #24
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Bob22 and JohnH1,
The breaker is off for the A/C. The unit is certainly not running. - Yes, I am attending to the unit in evenings and on weekends, since it produces more than enough heat in the "emergency heat mode". I had telephone conversations with local Carrier representative and, specifically, with the Tech that would be dispatched in case of a service call. The tech suggested many of the things in this thread and is as puzzled as I am. He also confirmed that the service call would be charged by the hour and the long list of activities would have resulted in quite a bill so far. My view: I saved all money for service calls and invested in spare parts ($300 so far: gas valve and controller board). I am an experimental physicist and have/can get all the tools I need. In addition, over the past three years I was not impressed with any of the service guys coming out. In one case, I got all my money back for the visits resulting in nothing after I found the the lug of the 2nd stage on the gas valve was not seated. In all cases, the companies wanted to sell me a new unit and also rip out the temperature and zone controller (both Honeywell PC8900/W8900 and TZ4).
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Old 01-10-2009, 09:08 AM   #25
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Bryant 90i Plus delivers Little Heat


Bob22,
In my view I am saving money and the family is not freezing. The gas rate (clocked meter and measured pressure) in the "high heat mode" is exactly the same as in "emergency heat mode". (I measured this, since I was interested as well.) - Based on the measurement, I conclude that the furnace produces only in the "emergency heat mode" the rated 60kBTU, whereas in the "high heat mode" (under microprocessor control) the gas consumption is that of a 60kBTU furnace, but without the matching heat generation. To keep the house comfortable, I would have to have an additional heater in the home costing me more fuel (wood/kerosene/etc.)
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Old 01-10-2009, 12:18 PM   #26
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Bryant 90i Plus delivers Little Heat


The power should be on to the A/C unit. It has a heater in the compressor to keep the oil warm and migrating out of the compressor.

I just don't get that in both high stage and emergency the btu input is the same but yet you get more heat in emergency mode. The only other difference stated is the draft induser runs faster causing a higher negitave draft. This in itself will basicly suck more gas from the valve. So I am assuming you are getting more gas through the unit. It just dosent show up accuratyl enough by clocking the gas meter. I am not fimerler with this furnace what is the difference in the high stage an emergency heat. I assume you are putting into emergency heat at the furnace somehow and not at your zone panal?
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Old 01-10-2009, 01:54 PM   #27
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Dear JohnH1,
thanks for the tip on the A/C-unit. I actually was under the impression of switching power off in the winter so that an erroneous start of the unit would not lead to damage of the compressor. I will check into the existence of a heater in the A/C-unit. -
You are now at the "puzzling state" regarding my furnace that I find myself in over the last three weeks or so. The "emergency heat mode" is a DIP-switch setting on the controller board (HK42FZ012) that bypasses all process control functions (without compromising safety checks) and runs blower and inducer at maximum speeds. In this mode, the furnace heats the house very nicely, it is just a little bit noisier. At least in this mode do I get the BTUs out of the consumed gas.

PS: I have checked the pressure(s) in the duct work upstream and downstream of the furnace [Just in case something invisible blocks the ducts?]. All readings are within .1inwc. Only in "emergency heat mode" [max. blower speed] is the pressure on the intake -.3inwc and on the output .6inwc [before A/C-heat exchanger].
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Old 01-30-2009, 08:39 PM   #28
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Bryant 90i Plus delivers Little Heat


Dear Readers and Experts,
in the final analysis all diagnostics & symptoms of the furnace described above are correct. The low heat generation requiring the furnace to be operated in "emergency mode" (blower and inducer at maximum speed) were caused by an aged/partially blocked secondary heat exchanger first determined by visual inspection after removing the blower. The CO-readings confirmed that in "emergency mode" the readings were about 1980ppm whereas in "high heat mode" the readings were off-scale at flue temperatures of about 97F in both modes. Therefore, the combustion in "high heat"-mode was so poor that only about 50% of the rated BTU were produced despite "high heat" gas consumption, in "emergency mode" the maximum inducer speed improved the combustion at -8.7inwc so much that the furnace produced about the rated BTUs with still much too high CO-values of 1980ppm. - The confusing and puzzling part was that the furnace did not show any pressure errors typically indicating such a "bad" secondary heat exchanger and pointing to the root cause more quickly. - Since the manufacturer has a lifetime guarantie on the heat exchanger (incl. labor), the about 4 hr replacement resulted in a charge of $70 for a "diagnostic service call". The furnace runs now normal with excellent combustion producing <16ppm in CO-values.
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Old 01-30-2009, 08:48 PM   #29
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Glad to hear its taken car of now.
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Old 01-30-2009, 08:49 PM   #30
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Very interesting. Thanks for letting us know. Were you able to find out why or how the secondary heat exchanger got blocked internally?

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