Installing a new furnace in a new built 35x30x8 1,050 sq ft single floor house. One heating supply company told me I needed a gas 45,000 BTU single stage furnace, another heating supply house told me I needed a gas 70,000 BTU 2 stage furnace. I have no idea which heating supply company is right, any help would be appreciated, thanks...

High Eff. Lennox G26Q3-75 - installed in 1997. I am struggling with getting my house up to temperature. It has something to do with the pressure sensor i believe. Seems to be 2 symptoms at the moment. I let the house go down to about 60F during the night, then, when the thermostat calls for heat: 1) if i just let the furnace take intake air from in the house (I only do this temporarily for testing) it will act normally for a few cycles (burner will stay on for 10-15 minutes before shutting off). Then the burner will only run for about 3 or 4 minuers at a shot. The blower motor pretty much never shuts off unless i lower the thermostat enougth to shut down the call for heat. 2) On quite a few occasions, with the intake line all in place, taking air from outside, the inducer motor will take 3 or 4 tries to start. I have checked the exhaust and intake lines. All free and clear. Have cleaned out the AC evaporator, but cleaning it didn't change anything. Thanks in advance for any advice folks.