Correct sizing

[martik]

I currently have a 120k btu 30 year old upflow gas furnace. The contractor did a heat load calc and came up with 30k for a 2400 sqft house on the northwest coast - design temp 20F.
The house has been upgraded, R40 ceiling, double low-e argon windows and air sealed to 4.5 ACH. This seems too low to me but then my old furnace never ran more that 10-15mins/hr.
I am also concerned that the blower on the new furnace may not be powerful enough. It's 3 speeds are rated between 800-1350CFM at .1 ESP

[karsthuntr]

What is your ?

30,000btu is 1,000 cfm and the blower pushes 1350cfm. That is okay. (On a A/C or H/P)

Are you putting in a heat pump? Are you putting in A/C with new gas furnace? Will it be 2 stage? What is the heat gain? What is the heat loss? Does it get colder than 20deg? Are you zoning the system? What is the name of your dog?

[martik]

No ac, no heatpump, no zones, no dog just 14 heat registers.

[BaldLoonie]

Without a dog, I'd be worried about that sizing. Dogs put out heat to supplement the furnace's output.

Actually, the 120K 30 year old probably has a blower similar to today's 50K furnaces! Nowdays, you won't find a 120K without a 4-5 ton blower but back then it wasn't unusual to find a big furnace that would move around 1200 CFM.

[cem-bsee]

my 1974 elec furn moves 2100+ cfm! [ per label & as measured]

[martik]

I'd really appreciate a response about the sizing change from 120k to 30k

[BaldLoonie]

If the heat loss says 30,000 BTU will do it, then get a furnace close to that size. You'll likely end up 20-30% larger as there aren't many furnaces out there that small anyway. You tightened up the house, insulated the he#$ out of it, great windows. You won't need much heat now.

[martik]

Thanks for your help. It's hard to believe that those few improvements would result in 1/4 the heating capacity!

[fat bob]

Did you really mean 4.5 ACH? With 8 foot ceilings that 86,400 cubic feet of leakage per hour? You could do wing design testing.

[Gunnery Sergeant Hartman]

Originally posted by martik
Thanks for your help. It's hard to believe that those few improvements would result in 1/4 the heating capacity!

That furnace was probably 2X too big to begin with...

[johnl45]

They don't make a 30K but I wouldn't worry even if the unit were 100% oversized. Being heating only it won't make much difference in energy usage. Just make sure the blower can supply the necessary CFM of air to each and every room.

[martik]

Originally posted by fat bob
Did you really mean 4.5 ACH? With 8 foot ceilings that 86,400 cubic feet of leakage per hour? You could do wing design testing.

The energy test report says it was originally 10.85 ACH @50Pa which was equal to approx a 468 sq inch hole.
After the upgrades it went down to 4.5

[martik]

Originally posted by johnl45
They don't make a 30K but I wouldn't worry even if the unit were 100% oversized. Being heating only it won't make much difference in energy usage. Just make sure the blower can supply the necessary CFM of air to each and every room.

The CFM on a 40k is between 800-1300 at .1 esp. As I said, I have 14 registers with a maximum run of 20ft and ducts of 8x14. The 4 in the basement are all closed off.
I would need to go up to an 80k to get any increase in CFM
which is between 1100 and 1800.

[fat bob]

Originally posted by martik

The energy test report says it was originally 10.85 ACH @50Pa which was equal to approx a 468 sq inch hole.
After the upgrades it went down to 4.5 [/B]

Never mind. Sounds like you had a blower door test. Tight houses can get well under 1 ACH, but that's without a blower forcing air out.

[fl1]

If 30,000 btus is correct,and you get the closet size to it,the cfms reqired ,for each room ,will not exceed the cfms of that unit,period.

More cfms will only be required if you need more btus.

Your ducts are larger than needed,so with VS fan no problem.Standard fan ,you may need a damper in the main trunk to raise the static,to keep the cfm down,so you get a proper temperature rise.

Main thing is to be sure the load calculation is correct,sounds like it could be.

Good idea to calculate the cooling and get a furnace that can handle the tonnage ,if someone ever wants to add cooling.