# Bryant 90i Plus delivers Little Heat



## alex_ny (Dec 18, 2008)

My Bryant 90i Plus (355MAV, 60kBTU) is delivering too little heat this winter season. I was not suspicious until the outside temperature dipped
below freezing. A 3degF jump in temperature increase in the morning (requested by the programmable thermostat) takes about 4-6 hours to achieve. I very much suspect that the blower speed is too low for the high-heat mode. The furnace controller does not display any errors in normal operation. The microprocessor passes all self tests without any errors. The blower speeds up during the self test. (There is at least a 'low'and a 'hgh'speed.) - The furnace has no problem with the 3degF heat requests, if in emergency mode [and, thus, running at max speed]. Since the blower can run at maximum speed, why does the controller not regulate the blower to higher/highest blowr speeds. - I would also say that the air speed out of the registers is lower than last year.
Is there anybody who can provide some diagnostic insight into this furnace problem? I would like to keep the repair cost to a minimum by exchanging the correct component the first time around. Thanks for any advise.


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## SKIP4661 (Dec 3, 2008)

I assume the air filter and a/c evaporator are clean. Is the high fire stage brought on by the processor or the thermostat? Is the furnace even going into high fire? If it's controlled by the thermostat, the offset between the first and second stage may be such that it doesn't go into high fire. If controlled by the processor , could be the processor or gas valve.


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## alex_ny (Dec 18, 2008)

Thanks for your quick reply. The air filter is new (8 weeks, changed at the beginning of heating season). It appears to make no difference whether the thermostat or the furnace controller is requesting the high-heat. Either way the heat output appears low. The second stage valve pulls through and it is energized with 24V.(audible klicking of the the valve selenoid). - I changed the gas valve, since I suspected it just like you do. I will check the A/C evaporator (heat exchanger) for cleanliness. [That would be quite some back pressure in the ducts, but this might fool the microcontroller in lowering the maximum blower speed during its startup calibration cycle.]


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## SKIP4661 (Dec 3, 2008)

There may be a dip switch on the processor marked emergency heat. If there is, you can switch it to emergency. This will run the furnace in high fire and blower on high speed. Did you check the high fire manifold gas pressure to make sure it is correct. If this furnace is propane, both high and low pressures will be low with a new valve.
They come regulated for natural gas from the factory and need to be adjusted for propane.


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## alex_ny (Dec 18, 2008)

There is indeed a bank of dip-switches on the controller board and that is exactly how I operate the furnace currently, in emergency heat at high-heat and maximum blower speed. This is also the operating mode that made me more suspicious about the blower speed than the gas valve/flow. One thing remains for sure: I will check the A/C-evaporator as you pointed out previously. - By the way, the gas valve came with instructions from Carrier and made no mention of any adjustments for gas type. I am operating the furnace on natural gas supplied by the utility company.


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## beenthere (Oct 11, 2008)

Did you change the board settings comfort this summer.
In comfort, it doesn't run the blower at high speed.

Read the install manual, to make the proper settings.

Its not, and was not the gas valve. Or it wouldn't have made a difference in emegency mode/setting.

Problem now, is you don't know if the new gas valve is set to the correct pressure, unless you used a manometer to check it.


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## alex_ny (Dec 18, 2008)

I did not change the settings for "CF" and "AC" [ continuous fan, air condition] during the summer. The setting remain at the highest allowed setting for the configuration (60kBTU). The gas valve exchange did not change the overall behaviour of the furnace whether in "normal mode" or ine "emergency mode". Heat output is too little under "normal operation". Only in emergency mode (max blower speed) is the heat output more than sufficient. The 3degF in the morning is easily achived in 15-30min.


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## beenthere (Oct 11, 2008)

Did it work ok in normal mode last winter.


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## alex_ny (Dec 18, 2008)

YES, the furnace worked very well last season and produced the required heat achieving the 3degF increase every morning easily within 15-30 min.


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## alex_ny (Dec 18, 2008)

Skip4661,
I checked the A/C-evaporator/heat exchanger and it looks clean. Now, I am really running out of ideas and begin to suspect the calibration cycle the microprocessor goes through. According to the manual (58MVP, HK42FZ012), the blower increases speed after the low pressure switch closes. The micro measures the RPM at the time the high pressure switch closes and uses this RPM for the high-heat mode. If the high-pressure sensor is faulty (but not broken!) and/or the RPM-feedback to the micro is faulty, the blower speed will remain too low without any errors indicated on the board. Which one is it? the high-pressure switch or the controller? - Any thoughts?


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## SKIP4661 (Dec 3, 2008)

Are you getting any error codes when the unit goes into high fire? If the pressure switch failed to close you would get an error code.


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## alex_ny (Dec 18, 2008)

Skip 4661, 
There are no errors. [... and as I wrote the controller self check exercises all components except the gas valve correctly and without any errors.] - That's why I wrote the pressure switch is possibly "faulty" (closes at incorrect high-pressure) "but not broken". Similarly, the micro gets the RPM of the motor when the high-pressure switch closes, but it might not be the correct/actual one (because something is faulty in the RPM feedback). 
PS: During the self check, the micro controls the blower to a 'low' and 'high' setting for the correct time.


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## SKIP4661 (Dec 3, 2008)

Have found moisture in the pressure switches before. You can remove them and see if you can shake anything out.


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## alex_ny (Dec 18, 2008)

Good idea and I checked that. All the pressure tubing is very clean and free of any moisture. In my downdraft configuration the pressure switches are at the highest point of all the sensing tubing.


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## alex_ny (Dec 18, 2008)

Beenthere & SKIP4661
(and everyone else who is stumbling across this thread).
The problem with my BRYANT 90i plus (355MAV with HK42FZ012 controller, 60kBTU) persists. It still delivers too little heat in "normal control mode" namely 1st and 2nd stage appear to be active, but the output heat is too little to keep the house at comfortable temperatures when the outside temperature dips below freezing. [The house was kept warm very well last winter season and there were no physocal changes done to the system.] - There is no error code. The controller passes the self test without any error codes. Since the self test does not actuate the gas valve, I originally suspected that the HIGH-heat selenoid of the gas valve does not open although 24VAC are present and the selenoid klicks. A new gas valve did not make a difference! I changed the controller board without making a difference either! - I verified that the HIGH-heat mode indeed uses about twice as much gas (about 1 cf/min) as the LOW heat mode (about .4 cf) by observing the gas meter. The temperature increase across the heat exchanger confirmed what I already knew observing the home temperature: In normal control mode, I measure a temperature differential for both LOW- and HIGH-heat of about 50F which is at the very low end of the temperature range for the HIGH-mode and clearly below the temperature range for the LOW-mode specified on the furnace label. - I have no idea what is wrong with the furnace except that I still believe that the blower speed(s) are too low certainly for the HIGH-heat mode. 
I am surviving the winterseason by putting the controller in "emergency heat" that runs the blower and inducer at maximum speeds and keeps the house comfortably warm. The gas consumption is just the same as in the HIGH-heat mode (about 1cf/min) and I measure a temperature differential of about 100F across the heat exchanger. That certainly heats the home, just the blower is a little noiser.
If anybody has any brilliant idea what else I can do, I appreciate your posting.


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## beenthere (Oct 11, 2008)

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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## alex_ny (Dec 18, 2008)

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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## beenthere (Oct 11, 2008)

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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## alex_ny (Dec 18, 2008)

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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## sgthvac (Apr 15, 2007)

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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## alex_ny (Dec 18, 2008)

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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## bob22 (May 28, 2008)

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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## JohnH1 (Jan 9, 2009)

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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## alex_ny (Dec 18, 2008)

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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## alex_ny (Dec 18, 2008)

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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## JohnH1 (Jan 9, 2009)

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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## alex_ny (Dec 18, 2008)

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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## alex_ny (Dec 18, 2008)

*.... and the SOLUTION:*

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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## beenthere (Oct 11, 2008)

Glad to hear its taken car of now.


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## JohnH1 (Jan 9, 2009)

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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## alex_ny (Dec 18, 2008)

Dear JohnH1,
The blockage of the secondary heat exchanger is internal to it. Quite a bit of iron oxide (rust) seems to have discolored the white plastic of the drain trap. Since the inducer wheel fell off its shaft about 14 months ago, because its sheet metal collar was completely rusted and had desintegrated, the debris from it had the same color and did not make me suspicious. Some of the heat exchanger have a polycarbonate coating on the inside that can desintegrate, peel off, flake off and add to the blockage of the trap. I did clean the trap last year and at the beginning of the troubleshooting (leading to this thread) and found little black/gray particles in it. Maybe the particles came from the heat exchanger?


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