Just not enough air?


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Old 11-28-12, 10:44 AM
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Just not enough air?

Moved into a home this summer, built in 1953. The gas furnace is in the attic.

Home is 1250 sq feet. There is one return vent in the bedroom hallway, measures 16x20.

The LR/DR has two round vents in the ceiling. Each of the 3 BR have one, kitchen has 2 and bathroom has one. When all vents are open, the airflow is barely noticable. Because of that, when we moved into the house in the summer, it would not cool down. Now in the winter, the house does not heat up. What we do is close all the vents except the one in the LR/DR and that room warms up quickly. I can also have one BR open and still warm those two rooms. Any more than that and the quantity of air is minimal and the room does not heat up.

The filter in the return is new. I went into the attic and I cannot detect any leaks at the points where the flex vents connect to the furnace. The model is Janitrol GMP075-3

Is the unit simply not big enough for my house? Does it need more than one return to get the quantity of air it needs? What else could be causing this? I had a furnace tuneup and cleaning and everything is working properly.
 
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Old 11-28-12, 11:51 AM
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Take the cover off the burner compartment of the furnace.

Observe the sequence of events that occurs at the furnace when you turn up the thermostat to 80 degrees and post that sequence of events in order and in detail.

Do the burners stay on or do they shut off, with the fan that circulates air around the house continuing to run, and the burners later turning on and off repeatedly while the fan continues to run?

In short, you need to describe the operation of the furnace in sequence and in detail.
 
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Old 11-28-12, 12:06 PM
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the burners remain on the whole time when the fan is on. when the thermostat reaches the desired temp, the burners turn off, fan remains on for another minute or two and then turn off.
 
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Old 11-28-12, 01:30 PM
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Well ---- the furnace appears to be operating normally. I was expecting you to report that the burners were shutting off without reaching the thermostat setting, which would have indicated that the furnace was overheating and the burners were shutting off because of that.

Do you have natural gas or propane?

Obtain the BTU input of the furnace off the rating plate in the burner compartment of the furnace and report that here.

Where are you located and how big is your house?

Are you in a housing development with many similar sized dwellings, by chance?
 
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Old 11-28-12, 01:43 PM
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Hi,

It's natural gas. I live in Indiana and the house is 1250 sq feet. Its not in a development, just a regular house. The issue is not the temperature of the air that comes out of the vent, it is quite hot. The issue is the quantity of air. If I have more than 2-3 registers open, the air is so minimal, the system will not heat (or cool in the summer) The unit is 75k 80%
 
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Old 11-28-12, 03:43 PM
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The speed of the fan probably needs to be increased. Usually that's pretty easy to do.

However, you can go too far in increasing the fan speed. The acceptable heat rise for the furnace is listed on the rating plate in the burner compartment of the furnace.

The heat rise is measure while the furnace burners and fan are operating. The heat rise is the temperature of the outgoing air minus the temperature of the air going into the furnace.

The temperatures should be measured right at the furnace, but to give an example:

suppose the temperature measured at the return air duct is 65 degrees and the warm air temperature at the warm air duct closest to the furnace is 120 degrees. The heat rise is 55 degrees.

The rated furnace heat rise is a range of temperatures, which might be 60-80 degrees. The idea would be to select a fan speed than maintains a temperature with that range. A higher fan speed produces a cooler heat rise, a lower fan speed produces a higher heat rise.

You might want to use a GOOD QUALITY thermometer to measure the heat rise and compare that with the heat rise on the furnace rating plate.
 
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Old 11-28-12, 05:21 PM
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I have never moved into a house that had a clean evaporator coil.

You might take a look at yours while you are checking the unit.
I'd guess the rated heat rise is 35 to 65 degrees. If yours is at the top of the range or exceeding the rated heat rise it might point to a dirty coil as well.











 
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Old 11-28-12, 05:34 PM
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That's a very good point, Houston!


Better to check on what might be plugging up the air flow than speeding up the fan.

But doing a good job of checking on that is likely to be difficult for a homeowner to do.
 
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Old 11-29-12, 06:35 AM
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Check the blower wheel for dust and dirt.

Use fiberglass filters in the return only - 16x20 is small for a 75000 btu furnace.

You may not be able to access the evap coil.

What kind of ductwork do you have? If flex, check for kinks and sags.

Is the furnace horizontally installed?

---------------
Normally the default fan speed for cooling is the highest and you had the same problem in the summer, so I doubt that increasing the speed will solve the problem.
 
 

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