Dear HVAC-TALk AOP. Thank you for sharing your expertise!
We live in Denver, CO in a 1930s brick tudor style home at 3,450 sq ft. The basement is about 80% finished with 946 living space. The 1st floor footprint is 1,500 and the 2nd story is about 1,00 sq ft. The basement and unfinished crawlspace has some insulation along with the attic. Otherwise the 1st and 2nd story walls are just brick. There are a normal amount of windows and glass patio door. Given the age of the home, I would say it is not that air-tight or well insulated.
We just removed the old steam boiler and cast iron radiators and replaced with a forced air system. The installer quoted a 40K unit in the attic for the 2nd story. Then an 80K unit for the basement and 1st floor. The 2nd story unit is working great and seems to be the right size furnace. The issue is the 1st floor.
There are 9 heat registers on the first floor and 6 in the basement being fed by the 80K unit. During the recent artic blast the unit has been running continually for about 72 hours. The 1st floor has been at 66-68 degrees and will not reach the 70 degrees that the thermostat was set at. I've confirmed that this is not a clogged or dirty air filter. The unit is not short cycling. It seems to just be undersized.
So I go to double check the model it is a Rheam R92TA0701317MSA at 70K BTU. All the quotes I received had recommended an 80K high-efficiency furnace. For some reason, the installers here at the last minute went with 70K. It is COVID and a lot of things are sold out. So not sure if that has anything to do with it.
I guess this is the classic question of sizing the unit for the average temps or the lows. In Denver, we get zero degree weather at least 3-5 days a year. There are 21 days where the temps are below 32 for the full 24 hours. The average highs in Jan can be 47 and the lows 20s. So this below zero weather is limited to just a few days of cold fronts.
However, it seems crazy that my unit is at practical capacity at 66 degrees during these cold fronts. Also, this switch-up to a 70K unit is frustrating. I don't know how much a 10K BTU difference would make.
Any thoughts?
We live in Denver, CO in a 1930s brick tudor style home at 3,450 sq ft. The basement is about 80% finished with 946 living space. The 1st floor footprint is 1,500 and the 2nd story is about 1,00 sq ft. The basement and unfinished crawlspace has some insulation along with the attic. Otherwise the 1st and 2nd story walls are just brick. There are a normal amount of windows and glass patio door. Given the age of the home, I would say it is not that air-tight or well insulated.
We just removed the old steam boiler and cast iron radiators and replaced with a forced air system. The installer quoted a 40K unit in the attic for the 2nd story. Then an 80K unit for the basement and 1st floor. The 2nd story unit is working great and seems to be the right size furnace. The issue is the 1st floor.
There are 9 heat registers on the first floor and 6 in the basement being fed by the 80K unit. During the recent artic blast the unit has been running continually for about 72 hours. The 1st floor has been at 66-68 degrees and will not reach the 70 degrees that the thermostat was set at. I've confirmed that this is not a clogged or dirty air filter. The unit is not short cycling. It seems to just be undersized.
So I go to double check the model it is a Rheam R92TA0701317MSA at 70K BTU. All the quotes I received had recommended an 80K high-efficiency furnace. For some reason, the installers here at the last minute went with 70K. It is COVID and a lot of things are sold out. So not sure if that has anything to do with it.
I guess this is the classic question of sizing the unit for the average temps or the lows. In Denver, we get zero degree weather at least 3-5 days a year. There are 21 days where the temps are below 32 for the full 24 hours. The average highs in Jan can be 47 and the lows 20s. So this below zero weather is limited to just a few days of cold fronts.
However, it seems crazy that my unit is at practical capacity at 66 degrees during these cold fronts. Also, this switch-up to a 70K unit is frustrating. I don't know how much a 10K BTU difference would make.
Any thoughts?
source https://hvac-talk.com/vbb/showthread.php?2226649-Artic-Blast-Undersized-Furnace-or-just-really-cold&goto=newpost
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