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I recently purchased a 1998 Buccaneer 16x80 that has been completely gutted and redone with new appliances and new central heat and A/C but when the temp drops at night the max temp in the house gets to 68 degrees when the digital thermostat is set on 72 and the furnace runs all night wide open keeping the house around 68-70 degrees but never to 72. In the morning when it starts warming up outside the furnace will get the house to 72 then kick off as it should. There is an old vent left in the furnace closet that is open to outside and it sucks in mostly air from that vent instead of the return vent on the furnace. The people I bought the house from are supposed to be coming back out to plug this vent, will it solve my problem. The register temp currently varies from 95 to 105 degrees depending on the outside temp.

No, sounds like some of the elements are not turning on.

Thanks for the input. I called the air company that installed the new unit and they insist that 95-105 is a good temp at the register, but I think it should at least be a little bit hotter it being a 15kw - 53,000 btu furnace.

Oh be careful. If your unit is running that much continually and the elements ARE working, you will be in for a VERY unpleasant surprise when your power bill comes. How cold is it where you're located?

I would block off that vent. The air discharge temperature is low by probably 20-30 degrees, but sucking in very cold outside air could be the cause.

Edit: I should add that temperature rise is accurately measured within about 24" downstream of the furnace. I don't recall if our furnace (E3EB-017H) has a sticker specifying temperature rise, but the way to check proper operation is output temperature (24" of furnace) minus (-) return air temperature. The difference gives you the temperature rise. If the heating elements are both working, the only way to increase or decrease the output temperature is by adjusting the blower speed via the appropriate wire tap on the relay.