My wife and I bought a rather old house this summer and discovered this winter that the furnace doesn't really work all that well. I have looked at it as much as my extremely limited knowledge has let me, so figured I'd ask here. We noticed that it was getting worse and worse as winter went on, with the weather warming up not really helping matters. I finally checked the filter and found it to be horribly dirty, so I replaced that immediately, without much help.

The furnace runs. I have confirmed that it lights and burns, and the blower blows. I can see and hear all that. However, the vents on the first floor of the house blow barely warm air and hardly any amount of air. The vents on the second floor actually seem to blow slightly stronger, but blow cool-to-cold air. The furnace also seems to only run for 4 minutes(almost exactly) at a time before shutting off.

It seems like something serious is wrong, and it is probably at the end of it's life, but I wanted to get see if there was anything else I could check before having a furnace repair person out to look at it.

 

Welcome to the forums.

Does your furnace have an evaporator coil in it for air conditioning ?
If it does..... it's most likely plugged up and will need to be professionally cleaned.

 

Given that there is central air it most likely does, yes.

I checked out the furnace a bit more and the model number is CMPB075U3A. The lighter LED apparently is also blinking with a fault code of 3, which from the manual says:

Pressure switch circuit was still sensed as
open 30 seconds after the inducer was
energized. System is in 5-minute delay
mode, with inducer off. After 5-minute delay,
new ignition sequence is initiated.
NOTE: SmartValve On/Off switch in OFF
position during a call for heat generates this
flash code.

1. Move SmartValve control switch to ON
2. Inducer and inducer wiring.
3. Low line voltage power supply.
4. Airflow proving switch operation, tubing, wiring.
5. Obstructions or restrictions in appliance air intake or
exhaust flue system that prevent proper combustion air
flow.
6. Other devices (such as auxiliary limits) in the airflow
proving switch circuit.

Does that match up with a plugged up evaporator coil?

 

Also check the blower wheel for dirt build up.

 

If the furnace ran for 4 minutes...... you shouldn't be getting three flashes. Three flashes is a start up problem based on draft inducer pressure issues.

I was just looking at the ducane troubleshooting list. They have multiple troubles mixed together. In other words.... a three flash code should refer to the inducer blower only. It doesn't. It says to check inducer and main blower. A four flash code is an overheat problem. Its says to check blower fan, etc. and flame rollout switch. Those two problems are not connected.... just like a draft inducer problem is not related to a main blower problem.

If you are getting poor airflow then you'll need to address that first. Clean blower wheel like mbk3 suggested. You confirmed you have central air and I'm leaning towards a clogged cooling coil.

 

There could be several problems with this furnace if it's been neglected over the last 12 years - plugged evap coil and/or secondary heat exchanger, underfired or overfired, cycling on high limit switch.

You should call a pro to thoroughly clean, inspect, and tune this furnace. (especially since you just bought the place - if there's a problem such as a cracked heat exchanger, you may be able to get a settlement from the previous owner)

 

Given the amount of potential problems, and the fact that I'd rather not break anything more than it is trying to fix it, I'm going to have our home warranty people dispatch a furnace pro out to take a look at it. I mainly posted here to see if there was anything simple I could have checked to potentially fix it.

 

Home warranty will only do the minimum to get the furnace running - not necessarily properly.

Use a competent, independent contractor who isn't bound by a crooked insurance company.