alex_ny

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.

SKIP4661

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.

alex_ny

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.]

SKIP4661

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.

alex_ny

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.

beenthere

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.

alex_ny

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.

beenthere

Did it work ok in normal mode last winter.

alex_ny

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

alex_ny

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?

SKIP4661

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.

alex_ny

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.

SKIP4661

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

alex_ny

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.

alex_ny

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.