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Community and Q&A

2nd story addition heating

Miyeko Endy | Posted in Mechanicals on

We’re working on an addition to a 740sf 1940s home in Portland, OR – addition includes +300sf on main level and new 2nd floor (1300sf). The existing furnace is brand new high efficiency gas, but not enough capacity for the addition. The owners do not want to replace it since it is so new. We are in the process of value engineering to bring costs down, but need to evaluate how to heat the new 2nd floor. Cost is an issue, but could possibly persuade them on a good investment.
Insulation: R-49 flat ceilings/R-30 vaulted attic storage space, R-23 walls. Existing main floor is primarily 2×4 construction, so R-15 walls most likely.
Any thoughts for recommended system to heat the 2nd story in this mixed old-and-subpar / new-but-not-totally-tight envelope?
I appreciate any advice you can give – thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    It's hard to buy a furnace that is small enough for a well-insulated house. Most residential furnaces are significantly oversized.

    I would go back and carefully scrutinize the inputs on the Manual J calculation. (Someone did perform a Manual J calculation, right?) If the inputs on the Manual J are accurate and entered without fudge factors, I'll bet that the existing furnace is properly sized. If not, you should probably improve the specifications of the home's thermal envelope.

  2. GBA Editor
    Martin Holladay | | #2

    Two questions:

    1. What is the Btuh rating of the existing furnace?

    2. What is the design heat load of the house + addition according to the Manual J calculation?

  3. Expert Member
    Dana Dorsett | | #3

    A typical code-min ~2500; 2 story house in a Portland OR climate will have a design heat load of about 20,000 BTU/hr. @ +24F (the 99% outside design temp for Portland: ). While you would have some wall area that is sub-code, and possibly some windows too, it's highly unlikely that it would hit 30,000 BTU/hr with the windows & doors closed.

    If you haven't run a Manual-J (or don't have the tools & time), it's pretty easy to do a crude I=B=R calculation with a simple spreadsheet tool. If you pick 69F for your inside design temp that will give you a design delta-T of 45F (make it 50F if you want to do the math in your head. Then it's a matter of coming up with reasonable U-factors for the different construction type. For the quick & dirty U-factors, use U0.10 for any 2x4 wall area, and 0.65 for the 2x6 with R23s, U0.04 for the R30 vaults and U0.022 for the R49 areas.

    The measure up the surface area of each type (in square feet), and it's

    Area x U-factor x delta-T= heat loss.

    For example, say you have 300 square feet of 2x4 wall (after subtracting out the windows & doors). The heat loss from the 2x4 wall areas is about

    300' x U0.1 x 50F= 1500 BTU/hr

    Do the same for all the other insulated surface types, and add it up.

    For any new windows, use the manufacturers' labeled U-factor for the window. For any old wood single-panes use U 1.0, or U0.5 if they have clear-glass storm windows. Antique solid exterior doors, are about U0.5, paneled exterior doors with windows call it U0.7. Insulated new doors would be U0.25-3 unless they're something special.

    Above grade uninsulated poured foundation would run U1.0, uninsulated band joist about U0.5.

    When you add it all up, if the place leaks like a sieve, add a 30% fudge-factor. If it's pretty tight don't bother- it's "in the noise error."- the interior heat sources such as occupants, hot water heaters, & refrigerators pretty much offsets the infiltration losses of a tight house.

    Then compare the number to the output BTUs of the furnace- I'm betting it's at least 2x oversized for the true heat load if not 4x.

  4. Miyeko Endy | | #4

    Thank you so much Martin and Dana! This is great info - I'll check with the owner's about the BTUs on the furnace they have and then look at some calculations. We haven't done any of this so far because I really didn't know the first thing about it. Thanks for helping me learn a little more!

  5. Richard McGrath | | #5

    You have stated that the addition is a total of 1300 sq ft . Are there any new baths and what type of usage DHW usage do they expect ?

  6. Miyeko Endy | | #6

    I was able to check on the furnace and do some quick (very rough) heat loss calculations. Here's what I came up with:
    Furnace capacity: 57,000 btu output
    Rough heat loss: 35,000 btus/hr (this is with a large "fudge factor" and simplifications of the house massing, so would probably be less) - actually, without the infiltration fudge factor, heat loss total is closer to 18,000 - looks like I'm adding way too much for infiltration.

    So it looks like the furnace will be more than adequate - thanks for your help figuring this out! Now we're just trying to figure out where we can drop ceilings so that the air distribution can be at the floor of the 2nd floor instead of the ceiling.

    As for Richard's question, this is a good question that needs to be addressed. The house is going from 1.5 baths to 3.5. The water line will need to be upgraded from 5/8" to 1". They are also installing a 75-gallon soaker tub, so the water usage increase will be significant (I did talk them down from the 110-gallon model). The existing water heater is electric, 50-gal. The plan right now is to add a tankless water heater to cover the upstairs DHW needs (2 full baths including master suite & laundry), and keep the existing water heater for the existing 1.5 baths + kitchen. I haven't looked into the needs for this in much detail. Do you have any recommendations?

    Thank you - GBA has been such a great resource!

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