Gas furnace not as hot as it used to be

[Dato Da]

My gas furnace had its pressure switches replaced by a professional about two months ago. I am not sure if it's related to the switch replacement, but the furnace doesn't seem to put out as much heat as it used to. As a result, it takes a lot longer to heat up the house. Furthermore, the house cools down faster than before. That gets the furnace to work on very short breaks. Here are my questions to all the experts in the forum. Thanks in advance for your insights.

1. It seems to me that when the furnace/fan is not on, I can still feel a little bit of cold draft coming out of the registers. Is there some valve in the furnace that's supposed to open to bring in fresh air when the furnace runs but shut off otherwise? I wonder if that valve is open all the time, causing the draft and the more rapid cool-down?
2. Is it possible that the problem is caused by incorrectly sized pressure switches?
3. What are the other common causes of a furnace putting out less heat?
4. I notice that the registers seem to put out most heat when the furnace stops while the fan is still going (that lasts about 90 seconds). Is that expected? Or is that an indication of an issue?

 

[Fitter30]

Pressure switches are only for proving that the draft motor is running correctly for the burner to light. There normally not a damper to let outside air in unless you have either hrv or erv. Gas furnaces normal operation blower off, burner starts, heats the heat exchanger ,60 to 90 seconds blower starts, thermostat satisfies burner shut off blower runs for 1 or 2 minutes shuts off to get most of the heat out of the heat exchanger. Like to know the brand and model numver of tour thermostat but the front cover has to come off to get the number unless you have the manual. Know where the burner section of the furnace is take the cover off usually they need to be pushed straight up 1/2" pulled towards u. Want the btu input not the output . Label can be on either side.

 

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[John Gayewski]

No outside air doesn't come into your ductwork only on the fire side of your heat exchanger, there's no way changing a pressure switch could let any outside air into your ducts.

Your house cannot be cooling faster than it was before as this has nothing to do with your furnace the rate of cooling after it's been up to temp is only related to your house.