New Furnace Can't Bring the Apartment Up to Reasonable Temperature

A few days ago in an apartment I own the 18-year old Trane xB 90% 40,000 BTU single stage furnace failed. I had a HVAC company come out and determine the unit had a cracked heat exchanger and failed inducer motor. Given the age of the unit and the cost and timing of repair vs replace, I opted to have the unit replaced in kind with a new (model# PG92MSAA36040A) Payne 92% 40,000 BTU Single Stage Gas Furnace.

The unit resides in the basement and feeds the 2nd floor apartment. Over the last 15 years the tenant has been there she has never complained about the heat. But the temperature has been unusually cold recently for an extended period of time. For the last week we have had temperatures ranging from lows of -7 to highs of 23 degrees. It is normal for us to get below zero for a day or two all winter, possibly 2-3 days, but not over the course of a whole week. Highs in winter generally range from 20 to 40 degrees.

Since replacing the new unit is unable to get the apartment up to a acceptable temperature, at first couldn't get the apartment above 62 degrees. The company returned and made adjustments, increasing the gas flow, etc. and was able to get the temperature up to 63 degrees. This time they also took measurements of the apartment and inspected the duct work. The size of the furnace its correct for the apartment but the apartment only has 4 supply ducts and poor return system, some of which is shared with the apartment below. In addition, the supply closest to the thermostat is shard between two rooms. The building is over a hundred years old and while has newer windows and doors, does not have any insulation. They returned again and increased the blower speed which only gained a one degree to 64. The unit has run constantly the last few days to be able to maintain this temperature. The new furnace is producing real heat, with temperature out of it around 120 ant the unit and 85-90 out of the registers so it appears to be working properly.

Of course none of the conditions of the ducts were reviewed prior to the install as the plan was just a one-for-one replacement of the furnace. As the owner I was expecting a unit able to support the apartment as the old one did, but it just....doesn't.

I am looking for your opinions as to why the new unit doesn't perform as the old one did. How did a unit with similar specifications do so much better? Is this a unique situation given the furnace was replaced during the abnormal low temperatures we are experiencing in the area? The forecast shows highs in the mid 20's to 30 in the next few days. Should I wait to see how the new unit performs then?

Do you think the furnace will maintain 68-72 degrees once the outside temperature rises? Even if it does, as the customer should I accept that this new furnace is unable to support 70 degrees within the unit when the temp drops into the teens?