HVAC Question - Upper Temp Limit

[Tripp11]

To the HVAC experts:

We had installed a Bryant 80x Preferred Plus furnace a year or so ago. In the evenings, our thermostat is set down to 66 degrees and by the time we wake up it calls for 70 degrees; however, by the time I'm walking out the door, it's only up to 67-68 degrees.

Our old furnace (15 years old) would run and run and run until it reached the desired temperature; however, this new unit will run for 10 minutes and then the burners will shut off for 3-5 minutes. Then they will fire up again and burn for 10 minutes, and then shut off for 3-5 minutes. Meanwhile the fan continues to run.

So, to my inexperienced mind, it seems like the furnace is hitting some upper temp limit. My guess is that is not normal and should be looked at ASAP?

TIA.

 

[jkaetz]

Check the filter. If it is not getting enough airflow over the burners, they will shut off to keep from overheating and then fire up again after the temp has come down. This is usually because of a clogged filter. Can be other reasons as well. You can pull the filter to see if that is the issue but don't run without one for an extended period. Dirt blown through the burners and evaporator coil is bad news.

 

[Tripp11]

Replaced filter this past weekend. It is conceivable something else could be restricting airflow. I will do a little more checking tonight.

 

[daedrian]

If you are using a high micron filter, like one labeled as micro-allergen, even a new one can restrict the air flow a lot. Might still be a good idea to run it without the filter and see what happens.

 

[remauto1187]

If the filter is blocking the furnace intake enough to cause a hi limit to be reached the furnace SHOULD shutdown and go into alarm. (Never restarting until alarm is cleared). See if you have any diagnostic alarms appearing. (Red LED on circuit board) The diagnostic flashing led list of alarms are usually on a stiker on the inside of the front cover of the furnace. Keep in mind that there is a safety switch that will shutdown furnace when you pull the front cover that covers the heat exchanger/burner.

 

[Buck69]

If the blower is a belt drive, check that the belt is tight. The drive pulley may need adjusted as well

 

[Tripp11]

Thanks for all the help! I will check all of the above. Much appreciated.

 

[looney2ns]

Any return air vent's or supply registers blocked/closed?

 

[Tripp11]

Well, much to my surprise, here's what I think it was...

Two weeks ago, wife bought an in room humidifier for our bedroom. Even though I replaced the filter this past weekend (actually two weekends ago), the filter has a white coating of mineral dust all over it. So, perhaps, in 7-10 days time, that humidifier is sending these fine mineral particles up into the return air and onto my furnace filter, effectively clogging the filter.
Is that even possible so quickly? That's the only thing that has changed though.

So, another filter replacement and leaving off the humidifier - and we'll see what happens.

To answer the question above, no return air vents or supply registers blocked/closed.

 

[jkaetz]

Well, much to my surprise, here's what I think it was...

Two weeks ago, wife bought an in room humidifier for our bedroom. Even though I replaced the filter this past weekend (actually two weekends ago), the filter has a white coating of mineral dust all over it. So, perhaps, in 7-10 days time, that humidifier is sending these fine mineral particles up into the return air and onto my furnace filter, effectively clogging the filter.
Is that even possible so quickly? That's the only thing that has changed though.

So, another filter replacement and leaving off the humidifier - and we'll see what happens.

To answer the question above, no return air vents or supply registers blocked/closed.

Is the humidifier an ultrasonic type or does it simply have a sponge type filter that wicks water up and into the path of a fan? If found out the ultrasonic type humidifier is bad news with our water. Everything in our bedroom was coated in mineral dust and it had also clogged the furnace filter. I could take the filter outside and smack it to get a brilliant dust cloud.

 

[Mr. Habib]

Try distilled water in the humidifier.

 

[Tripp11]

Is the humidifier an ultrasonic type or does it simply have a sponge type filter that wicks water up and into the path of a fan? If found out the ultrasonic type humidifier is bad news with our water. Everything in our bedroom was coated in mineral dust and it had also clogged the furnace filter. I could take the filter outside and smack it to get a brilliant dust cloud.

It was one of these....wife and her addiction to QVC. Don't get me started...

http://www.qvc.com/Air-Innovations-Ultrasonic-Clean-Mist-Humidifier-with-Ceramic-Filter-Search-Results.product.V32459.html?sc=V32459-SRCH&cm_sp=VIEWPOSITION-_-1-_-V32459&catentryImage=http://images-p.qvc.com/is/image/v/59/v32459.001?$uslarge$

 

[jkaetz]

It was one of these....wife and her addiction to QVC. Don't get me started...

http://www.qvc.com/Air-Innovations-Ultrasonic-Clean-Mist-Humidifier-with-Ceramic-Filter-Search-Results.product.V32459.html?sc=V32459-SRCH&cm_sp=VIEWPOSITION-_-1-_-V32459&catentryImage=http://images-p.qvc.com/is/image/v/59/v32459.001?$uslarge$

From my experience above, this is almost certainly your problem. Using distilled water should work as well, but will get pricey if you use the humidifier a lot. I ended up getting a whole house humidifier and haven't had a problem since.

 

[churchmouse]

Do the in duct whole house humidifier. Also make sure the furnace is not over fired. I have found several new units running to high manifold pressure and over firing. On long runs like coming off night set back and making a 4* climb in these severe weather conditions they run too hot. Add a partially plugged filter to this equation and you will hit the heat-X high temp limit. This causes an added 5 minute delay after the limit closes before the burners will re-fire.

Discharge air temps should run between 110* to 115* at the registers. This is just an average but if the Manifold pressure is set at 3.5" of water column at the manifold you should see these temps.

 

[sig-guy]

If the filter is blocking the furnace intake enough to cause a hi limit to be reached the furnace SHOULD shutdown and go into alarm. (Never restarting until alarm is cleared). See if you have any diagnostic alarms appearing. (Red LED on circuit board) The diagnostic flashing led list of alarms are usually on a stiker on the inside of the front cover of the furnace. Keep in mind that there is a safety switch that will shutdown furnace when you pull the front cover that covers the heat exchanger/burner.

What furnace are you thinking of that lockouts on a tripped limit? This is not the normal proceedure.
The safety switch you are referring to is on the blower door, not the heat exchanger/burner door.

 

[sig-guy]

If the blower is a belt drive, check that the belt is tight. The drive pulley may need adjusted as well

A year old residential furnace isn't going to have a belt driven blower.

 

[sig-guy]

Two weeks ago, wife bought an in room humidifier for our bedroom. Even though I replaced the filter this past weekend (actually two weekends ago), the filter has a white coating of mineral dust all over it. So, perhaps, in 7-10 days time, that humidifier is sending these fine mineral particles up into the return air and onto my furnace filter, effectively clogging the filter.
Is that even possible so quickly?

Yes, it's possible. I've saw a 20x25x5 media filter plugged in less than 30 days by one of these machines!

 

[ultra...good]

Amazon.com - Aprilaire Humidifier, 120V Whole House Humidifier w/ Auto Digital Control .75 Gallons/hr, Model Aprilaire 700

I put one of these humidifiers in last fall when I replaced my furnace. There was a lot of wiring involved with the outdoor sensor and then to the furnace for the blower control, but was worth it as the humidifier is set it and forget it. Always keeps a maximum amount of humidity in the air given an outside temperature. Change the water panel once at the beginning of the year and that is all.

Another reason it is good for me is this unit will cycle the blower once an hour to check for humidity and if it is low it will add to the air. I use wood burners so the furnace may not call for heat for hours but it will still be dry. Not anymore. I also piped the unit into a hot water supply to aid in evaporation.