BTUs per sq ft tells nothing. Don't let anybody size based on that. I'd wait for the 80K rather than oversize. An oversized 2 size is just like an expensive single stage. I know, I have one.

Wait til Dan sees this, he'll work with you on getting a calc done or another member has a program he offers to use.

We just got in a S9V2 80K 4 ton blower a few minutes ago. May just be your distributor not stocking enough. Ours has had few outages and not for long.

 

40-45 BTU per square foot is ridiculous, even for the Chicago climate. That's a rule of thumb from literally 50+ years ago. The only way you might need an 80K furnace for a 2200 square foot house there is if you have single pane windows without storms and ZERO insulation. My house is a similar size to yours, and while it's had some improvements over the years like basic double pane windows and insulation, it's an antique that wouldn't meet even meet today's bare minimum building codes, let alone qualify as super-insulated. I have a 60K BTU modulating furnace. When it got to -10 degrees a few years ago, which is well under the design temperature for your area, my furnace still never went above 80% modulation (without setpoint changes).

You need your heat load calculated with a Manual J load calculation. Plenty of information about that and tools out there. You can also approximate the load you need based on your fuel bills. Here's a tutorial on how to do that: https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new. Honestly I feel the latter is more reliable if you don't know all the precise details about the house like insulation values, etc. and aren't willing to find them out. You can do either calculation yourself if you're willing to geek out and crunch some numbers. "Ballpark" manual J's still tend to oversize significantly in my experience working with engineers that homeowners have hired on high-end projects, but are still way better than a wild guess or a rule of thumb like your contractor seems to be doing.

Obviously I've never been to your house, but unless it's extremely unusual I can pretty much guarantee that if you install an 80K 2 stage furnace, it will only ever need to run on low stage (wasting the extra money you paid for the 2 stages) and it will STILL be grossly oversized (wasting your fuel bills). Roughly what % of the time does your current furnace run when it's below zero out? When properly sized for your climate, it should be running pretty much constantly at those temps.

 

Okay, I gave this a try. Also looked into my Ecobee data to see what happened in the winter months.

Ecobee data: January and February 2021 I had my thermostat set at a constant 72F. My current furnace is a 2 stage controlled by the furnace. The 2nd stage kicks in after 7 1/2 minutes of 1 stage running. The data shows that my furnace ran anywhere between 10-20 minutes with breaks of 10-20 minute in between runs. The max temperature difference that I recorded was -1.4F in between cycles. From a comfort perspective I feel like the temperature inside the house has always been pretty comfortable and the data suggests that I was not dealing with short cycling.

Now to the head load calculation that I was pointed to.
From Jan 16th to Feb 16th I used 311 therms. My furnace is 80% efficient so that means that heat delivered to ducts was 249 therms. However I removed 15 therms due to a gas stove and water heater (I got the 15 from the summer months). That leads me to about 236 therms.

Did the math from degree days using a base of 65F, that ended up at 1571. The found the 99% design temperature for the same location I used on degree days and that was 1.4F.

The numbers are 23,600,000 / 1571 = 15022 degree-day. That means 15022 / 24 = 626 degree-hour. Since the design temp is 1.4F and the base is 65F I ended up with the difference of 63.6F. That multiplied with the degree-hour of 626 ends up 39,800 BTU. Then doing the sizing by multiplying by 1.4 ends up at 55,700 BTU.

My house is average when it comes to insulation. I fixed pretty much most of the points where cold could get in without any barriers. Last winter was pretty comfortable in pretty much every room. I guess based on the numbers I made I should not really go for 100K BTU since that's gonna be way to oversized. But now I am not sure thou I'd go lower than 80K BTU.

 

If you have a decent enough idea of your house material used you can do a load calculation yourself in the meantime for starters, here’s a free one.

Are you stuck on American Standard ( is this the only brand this Contractor carries? ) as there are many other brand models that once you find out what size you need for A/C and Heat you can get other bids.

https://www.google.com/url?sa=t&rct...kOBr0QFnoECBUQAQ&url=https://www.loadcalc.net/&usg=AOvVaw2tUJpstnwxqi0c65CNPb1r

 

Pretty much stuck on American Standard or Trane. So far all the quotes on Trane were ridiculously more expensive for a reason I cannot understand since they're basically the same thing. I also got some Lennox quotes and the price for those quotes was good but I am not really sold on Lennox anymore these days.

 

Are you changing the A/C? What size is it? The S9V2 80K has a 3 ton and a 4 ton blower model.

 

Currently I have a 3 ton and has worked out well. I figured that part with the contractor and feel fine about it. My issue was with just the 100K BTU vs 80K just because they don't have it in inventory. Usually those claims are very suspicious and make me trust the contractor less which then leads me to trying to figure out on my own information from unbiased people.

 

Yeah I don't buy those excuses for not doing a Manual J. He just doesn't want to take the time (which to be fair is typical). Insulation declining in R-value over time depends on the material, and can be estimated. Similarly, air leakage can be measured or estimated and is one of the factors included in Manual J. It is true that it's harder to do an accurate Manual J for an older home than a new build, but it's way better than a guess. 30 BTU per square foot is still just a guess!

The point of design temperatures is to balance those really cold days with max comfort the rest of the time. There's no point sizing a furnace for extremely rare events. It's not like as soon as you go below design temp, the furnace stops working - it just may have to run constantly, or you may drop a degree below the setpoint that rare day of the year. If worst comes to worst, you can even use a space heater. It's OK, and not worth sacrificing comfort and efficiency for the much more frequent mild days. Also consider that the lowest temps and zero solar gain (ie. worst case scenario) happen early in the morning, when you're probably asleep and your thermostat is set back anyway (furnaces are usually sized for a 70 degree setpoint).

 

Alright I went ahead and did a Manual J Load calculation myself using CoolCalc. That's much easier to use than loadcalc.net. I think most of the information that I had to fill was simple but I had a few selections where I was not sure if I picked the right stuff.

I've landed on 30K BTU cooling and 72K BTU heating. That makes me think that the 3 tons AC I selected should work and maybe that a 60K BTU 96% furnace might be too small?

Here are a few things about my house:
- 2 story home with encapsulated crawls space and unfinished basement. the crawl space is connected to the basement
- my attic has insulation on the floor joists
- the furnace is located in the basement and all ducts inside the basement are not insulated. There are two vents in the basements that are usually not kept open, also no returns in the basement. There's a large branch in the basement which splits into 2, one for the first floor and one for the second floor. The first floor branch then splits for about 4-5 vents. The branch that goes up into the attic splits into 5 for 4 bedrooms and 1 bath. The ducts in the attic are insulated with probably the lowest rated insulation (not very thick).
- I have 2 large returns on the first floor and then 4 smaller returns in each of the bedrooms. The returns on second floor are done via the floor joists. The returns on first floor via ducts.

Here are some of the questionable selections I made on CoolCalc
- Ceiling Type: ceiling under attic or attic knee wall
- Primary floor type: Floor over enclosed unconditioned crawl space or basement
- Dimensions: I did a block load where I created the dimensions for the whole first floor and the second floor. I did not include the basement in there and I have not created separate rooms.
- Duct system: This was the hardest. I selected "Above and below conditioned space" with R2 insulation value. Cool calc then generated 2 duct systems. The one for the basement where I selected "Location = unconditioned basement" and "supply geometry: trunk and branch" and "percentage of ducts that run in unconditioned space: 100%". The duct system for the attic is as follows: "location = attic", "supply geometry = trunk and branch", "return geometry = trunk and branch", "R-value = R-2", "percentage of ducts that run in unconditioned space: 100%".

My issue right now is that if I change the R-value for the duct systems from R-2 to R-6 the heating BTUs go all the way to 87K. Considering that the basement ducts are not insulated but they also are not in a fully unconditioned space I am not sure what R-value to set for them. The ones in the attic kinda make sense to be on R-2 since the insulation they have looks pretty old and pretty thin.

I've posted all this info in the hope that someone that has more experience with the Manual J Load calc can spot maybe mistakes that I made.

 

I am changing my HVAC system and been getting some quotes from a few companies.

My current furnace is a 110k BTU 80% Lennox 2 stage variable blower.
I live near Chicago and my house is about 2200 sq ft.

Most companies quoted me for a 2 stage variable speed blower 80K BTU 96%.

I have selected a company that works with American Standard and they are telling me that
they can only install a 2 stage variable speed blower 100K BTU 96% furnace
because the 80K BTU 96% are not available in inventory at all.

I have not had anyone do a manual J calc yet.

All I was able to find is that for the zone that I live in
I need about 40-45 BTU / sq ft. <<< Damn Old misrepresentation of current Reality

RESIDENCE BUILT 19 _ _ ?

Chicago
2,200 Sq Ft
1,100 Sq Ft / Floor + Basement ( Entirely underground)
1994 __ R-13 Walls; R-35 Ceiling, Double pane Windows, Well Sealed ( < 0.35 A.C.H. Natural)
___ W.A.G. 43,000 - 52,000 BTU/HR

LOAD CALC to follow.
------------------------

40- 45 BTU/ HR- SqFt applies to houses built pre-1960.
A house < 25 years old will have a heat loss 30% to 50% of a house built PRE- 1955.

MANY US. Regions
Any house built With Decent QUALITY since ~ 1995 is likely close to 20 BTU/HR - SQ FT.
High Performance Houses built in the last few years might be 15 BTU/HR -SQ FT.
_ High Performance _: Better than IECC 2018

So, if one wish to use AN OLD RULE-of-Thumb, they may be off by ~ 200% to 300%.

Of course, any rule-of-thumb is only a ball-park (+/- 20+ %) value.
The issue is the estimate should be accurate within 10% to achieve an appropriate equipment selection.

Analysis of energy use is an important check on calculations regarding older houses.

 

The house was built in 1967. Basement is unfinished and uninsulated ducts run through it. Basement is 6ft underground with about 2 ft above ground. The basement still has the original single pane windows that are really leaky. First floor is about 1200 sq ft and second floor about 1000 sq ft. The ceiling is on 2x8 joists and the insulation goes a bit over the joists so I think the R value is closer to R-28. All windows were replaced like 8-10 years ago with double pane glass.

One thing to note is that I plan to finish the basement in the next 2-3 years which will add about 500-600 sq ft to the area that needs heated.

You mentioned that analysis of energy use is important. Someone else pointed me to something like this and the result i got was ~40K BTU/hour. I also did a Manual J Load Calc to the best of my abilities which pointed me to ~72K BTU. However, I am not sure I selected the right values when it came to the duct system.

Put some more details about the house in my previous post but I'll copy it again

- 2 story home with encapsulated crawls space and unfinished basement. the crawl space is connected to the basement
- my attic has insulation on the floor joists
- the furnace is located in the basement and all ducts inside the basement are not insulated. There are two vents in the basements that are usually not kept open, also no returns in the basement. There's a large branch in the basement which splits into 2, one for the first floor and one for the second floor. The first floor branch then splits for about 4-5 vents. The branch that goes up into the attic splits into 5 for 4 bedrooms and 1 bath. The ducts in the attic are insulated with probably the lowest rated insulation (not very thick).
- I have 2 large returns on the first floor and then 4 smaller returns in each of the bedrooms. The returns on second floor are done via the floor joists. The returns on first floor via ducts.

 

That basement gets a separate load loss calculation. It’s surrounded by the ground so shouldn’t see much below 30*

I would recommend I’f at all possible installing in floor radiant heat.