Denver Furnace Sizing Help

[marklouisville]

Would appreciate help with a choice between an 80,000 or 100,000 Btu Bryant Evolution 987M furnace (97%AFUE)
Original furnace is a 140,000 BTU 80%.
Location is Louisville, Colorado (suburb of Denver). 1 Degree Winter Design temp.
Home was built 1991 front faces south, is 4,000 sq. ft. total 2650 on 1st/2nd floor with unfinished walkout basement.
New vinyl double pane windows/doors, R40 attic insulation, fireplace is a gas insert with outside combustion air - we've had a blower door test and sealed most of the large infiltration.
A/C is a 4 ton unit 21" plenum plan to stay with this compressor and coil
I performed a manual J, used HVAC-CALC came out to 80,000 heat loss - that includes sizing the walkout basement for heating.
The two Bryant choices net to 69,000 and 86,000 Btu respectively after altitude and efficiency corrections.
I've been reading this forum for a while now and love the content - its where I got the idea to do the manual J myself.
Thanks for the help in advance!

[dan sw fl]

What's your Monthly gas usage over the last 18 months?

Did anyone determine efficiency in the last few years?

[marklouisville]

800 to 900 annual therms, Monthly ranges from 13 to 150.
We run the home cool - stat is usually set to 66, Probably will run 68 with the more efficient furnace.
Blower door test was run before the window, insulation and fireplace improvements.

[dan sw fl]

800 to 900 annual therms,
Monthly ranges from 13 to 150.

We run the home cool - stat is usually set to 66,Probably will run 68 with the more efficient furnace.

http://www.climate-zone.com/climate/...lorado/denver/

150 Therms / month at average monthly temperature of
29.o'F/ 1080 HDD { 85% efficiency assumed for existing furnace}
indicates
37,000 BTU/Hr is sufficient Furnace Capacity to heat at 0'F.
______________ ______________________
Confidence in my Heat Loss estimates seems warranted IMHO
when I calculate them two different ways and they are within 5%.

I am certainly at a loss for words,
in trying to understand how you calculated 80,000 BTU/HR Heat Loss using HVAC CALC
and knowing the MAXIMUM use of Natural Gas is ONLY 150 Therms.

You must have done one hell of a job making those thermal envelop improvements.

Air Change per Hour (ACH) must be < 0.35.

Do you know what your furnace run times are when it's < 20'F outside?
i.e. 10 minutes ON, 17 minutes OFF.

Approximate
BASEMENT DIFFERENTIAL TEMPERATURE
would be
66'F - 44'F SOIL = 22'F.

[dan sw fl]

NOTE: I have Not directly included the basement area in _Q = U * A * dT_ heat loss estimate of 37,000 BTU/Hr.

[marklouisville]

Thanks, Dan
Sorry, we have not done another blower door test yet.
I went back and checked HVAC-CALC and caught a few errors
I'm right at 70K BTU - its a walkout basement so there is about 20K heat loss there, about 30K for the first floor and 20K for the second floor..
The fireplace seems high at 4K btu since its an insert and is now a sealed unit.
Note that my 20 year old Rheem is 140K BTU input but onlydelivers 87K BTU due to efficiency loss (78%) and altitude derating
BTW, if memory serves me, that unit never runs full time - even when its -5 out. The duty cycle on the coldest days is about 10 minutes on 10 minutes off
Thx
Mark

[dan sw fl]

Thanks, Dan
Sorry, we have not done another blower door test yet.
I went back and checked HVAC-CALC and caught a few errors
I'm right at 70K BTU - its a walkout basement so there is about 20K heat loss there, about 30K for the first floor and 20K for the second floor..
The fireplace seems high at 4K btu since its an insert and is now a sealed unit.
Note that my 20 year old Rheem is 140K BTU input but only delivers 87K BTU due to efficiency loss (78%) and altitude derating
BTW, if memory serves me, that unit never runs full time - even when its -5 out. The duty cycle on the Coldest days is about 10 minutes on 10 minutes off
Thx
Mark

Definitely, a SIGNIFICANT INCREASE > 40,000 total due to basement exposed to air.
That's one added reason why review of gas use is a common sense evaluation of ones' load analysis.

If the furnace runs half the time with an actual 87,000 Output,
the heat loss calc should indicate < ~ 44,000 BTU/HR

Calcs ought to be interpreted as about +/- 12% (1/8 th)

I am not conversant with those known, significant high altitude derates.

[marklouisville]

I think that we are getting closer:
The basement is not currently finished or heated - but am including this in load calc.
We have been at a set point of 66 with an aggressive setback of 62 during day working periods and nites.
The modulating 80K BTU unit seems to be close especially if we never finish the basement however I'm leaning to the 100K Btu unit as it will have a 20% safety factor.
Thx
Mark

[motoguy128]

With a modulating, the 100k will defeat much of the benefits. stick with the 80k. A larger home like yours has a lot of mass and will cool off relatively slow making it easy to ride through colder morning.

[marklouisville]

Thanks Motoguy.
Just to your point.....its snowing today and 32F outside.
I raised the set point to 68F and set it on hold - inside temperatures range from 67-70
At this temperature the house seems to be steady state - the old furnace has been off for an hour already.
I'm planning to check the duty cycles tonight when the temperatures dip into the lower 20's.
Thx,
Mark

[udarrell]

I wear warmer clothes during the heating season so I can use a wider temp-spread-differential...

I use a LUXPRO PSP511LCa programmable SWING temp-differential thermostat & set it on around #6, which provides around +/- 15-minutes of runtime per cycle. The ACE ATX 1500 4115176 is another good low cost thermostat, with both Saturday & Sunday individual settings.

A furnace loses efficiency when it short-cycles & has to reheat the heat exchanger too many times per hour.

My propane furnace has 57,000-Btuh output or 950-Btuh per minute, I kept track of the Btuh used in the coldest subzero weather & it never got close to 30,000-BTUH.

My old former Oil furnace was rated at 140,000-Btuh with a 1-GPH nozzle, however, it came with an .85 gal per hour nozzle or 119,000 Input at .80% the output when new was about 95,200-Btuh output. They run a long time before the blower kicks on, therefore, they need longer runtime cycles to improve their delivered efficiency. Short-cycling is a real energy waster especially using oil heating...None of it for me; ever!

[darctangent]

Isn't the contractor offering a Manual J as part of their work?

Furnace sizing is not the province of homeowners, I would not accept the engineering of anyone else (homeowner or not) unless I was very familiar with them and their work.

If the contractor does not offer a TRUE manual J (HVAC-Calc is NOT a ACCA recognized software, therefore not a Manual J) It would call into question all the rest of their training and dedication to produce a quality result for you. You will pay for the result you get, you might as well get a good one.

[motoguy128]

In a major metro area, it should be easy to find a contractor that can do a load calc. Very competitive markets there.