What size furnace do I need? 60,000 BTUH or 80,000 BTUH?

[VFRguy]

I would greatly apprecaite some feedback on the correct furnace size for my installation? House in Seattle is 1994, 3205 square feet, 3 story, including a walk out basement, R19 wall insulation, R26 in ceiling, dual pane windows, and the basement is always cooler than the rest of the house.

I have been presented with two furnace options, both of which are 96%variable speed 2 stage furnaces: first option is a 60k BTUH furnace @ 58k hi / 37.7k low output, or second option is an 80k BTUH furnace @ 78k hi/50k low output.

My BTUH total heat loss is 48k, using 72 degree indoor temperature and 23 degree outdoor. Does the 60k unit leave me with enough extra BTUH in reserve? My concern is, what if I let my indoor temperature at night drop to 64 degrees on a real cold night, will the 60k furnace have enough BTUH in reserve to heat my house back up fast enough to a cozy 72 in the morning?

Do I go with the 60k or 80k unit?

Thanks.

[dan sw fl]

I would greatly apprecaite some feedback on the correct furnace size for my installation?

House in Seattle is 1994, 3205 square feet, 3 story, including a walk out basement, R19 wall insulation, R26 in ceiling, dual pane windows,

I have been presented with two furnace options, both of which are 96%variable speed 2 stage furnaces:
___ first option is a 60k BTUH furnace @ 58k hi / 37.7k low output, or
second option is an 80k BTUH furnace @ 78k hi/ 50k low output.

My BTUH total heat loss is 48k, using 72 degree indoor temperature and 23 degree outdoor. Does the 60k unit leave me with enough extra BTUH in reserve?

My concern is, what if I let my indoor temperature at night drop to 64 degrees on a real cold night, will the 60k furnace have enough BTUH
in reserve to heat my house back up fast enough to a cozy 72 in the morning?

Thanks.

60/ 57/ 37.7 K bTU/Hr furnace will provide the most Comfort.

Heat load will normally be about 35,000 BTU/hr given that Seattle does not go below 38'F very often (34'F in comparison to design dT of 49'F)
You would likely find that Low Stage heat on 60K furnace would be sufficient more than 2/3rd of the time.

Set-back recovery might be "learned" as 6 hours IF the temperature actually dropped to 67'F.
Therefore, the 'recovery mode' would start at 1 am for a 72'F - 7 am setpoint.
Thinking about something that might happen _once every other year_ is not the method I would use to select a furnace.

[tigerdunes]

Assuming the accuracy of a good load calc, the 60 K model should be just fine.

Dealer may want to evaluate basement and add additional supply(s) for more BTUs. You mentioned basement was always cooler which is quite typical.

What if anything are you doing about AC? With the 60 K furnace model that has a 3 ton rated blower, you would be limited to no larger than a 3 ton AC condenser.

It should be noted though the importance of having a good two stage thermostat that will control furnace staging rather than a timer on the control board. Otherwise you will always be on high stage unnecessarily. Not a detail to overlook.

A pet peeve of mine for homeowners to spend extra $$$ on a nice two stage var speed furnace and then for a dealer to place the wrong and cheaper thermostat that shortchanges the best operation of that furnace.

IMO
Good Luck!

[duckman06]

Assuming the accuracy of a good load calc, the 60 K model should be just fine.

Dealer may want to evaluate basement and add additional supply(s) for more BTUs. You mentioned basement was always cooler which is quite typical.

What if anything are you doing about AC? With the 60 K furnace model that has a 3 ton rated blower, you would be limited to no larger than a 3 ton AC condenser.

It should be noted though the importance of having a good two stage thermostat that will control furnace staging rather than a timer on the control board. Otherwise you will always be on high stage unnecessarily. Not a detail to overlook.

A pet peeve of mine for homeowners to spend extra $$$ on a nice two stage var speed furnace and then for a dealer to place the wrong and cheaper thermostat that shortchanges the best operation of that furnace.

IMO
Good Luck!

A great point as the OP would need to investigate. As I agree he or she will
Be limited to what size furnace they can install if the a/c is over (3) tons. Good catch.

To the OP you will need to find out what size a/c unit you have and go from there as tigerdunes pointed out you might be asking about the 60k furnace for no reason if the a/c unit is bigger then (3) ton. The furnace will have to have enough air flow for the a/c unit to work right if over (3) tons on a/c!

As far as your load numbers go if correct I would Chosse the 60k in 96% furnace and you should be fine. You will need a good t-stat to control the staging of the heat as I agree a timer is not the best way. If your a/c unit is bigger then (3) tons you will have to look at 80k furnace but still will need a good t-stat to control heating stages instead of timers.

Please keep us posted on the size of the a/c unit and what your dession you make.

[tigerdunes]

Thanks for the comment and support Duckman.

Not certain if OP even plans on AC but he needs to think about this.

One other thought and suggestion. I would recommend a good whole house media filter cabinet at time of your install. Changeout is very easy and normally filters last up to a year.

IMO
Good Luck!

[duckman06]

Thanks for the comment and support Duckman.

Not certain if OP even plans on AC but he needs to think about this.

One other thought and suggestion. I would recommend a good whole house media filter cabinet at time of your install. Changeout is very easy and normally filters last up to a year.

IMO
Good Luck!

No need for thanks but you are welcome. That is what I love about this site/fourm, professinal people who take the time to help customers and techs with problems they face everyday.

Also I agree with a 5" whole house media filter. A good way to protect the furnace and coil from dust, dirt, etc... along with helping to filter the air better to help with Indoor air Quality.

[SkyHeating]

If you look at the actual CFM of many of the variable speed blowers on a 60K furnace they will provide airflow for up to 3.5 ton AC which covers a lot of houses in seattle/portland.

I would put in the 60K furnace, I am working with a homeowner in Portland who is going through the same thing and the 60K really is the best option for them. Stop worrying about one day where the temperature might be 18 degrees once every few years and worry about operating efficiently and comfortably the rest of the time.

[dan sw fl]

SKY HEATING KNOWS BEST.!

OR
ANY1 Who Cares
to discover that Cooling Degree Days are ~200 and
Heating Degree Days are ~ 5,000 ... depending on specific NW locale.

OCTOBER TO MAY = HEATING

30 days < 32'F
_2 days > 90'F

http://www.climate-zone.com/climate/...a-tac-airport/

Seattle Sea-tac Airport
Temperature Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
Avg. Temperature 40.1 43.5 45.6 49.2 55.1 60.9 65.2 65.5 60.6 52.8 45.3 40.5 52.0
Avg. Max Temperature 45.0 49.5 52.7 57.2 63.9 69.9 75.2 75.2 69.3 59.7 50.5 45.1 59.4
Avg. Min Temperature 35.2 37.4 38.5 41.2 46.3 51.9 55.2 55.7 51.9 45.8 40.1 35.8 44.6

Days with Max Temp of 90 F or Higher 0.0 0.0 0.0 0.0 < 0.5 < 0.5 1.0 1.0 < 0.5 0.0 0.0 0.0 2.0
Days with Min Temp Below Freezing 10.0 6.0 3.0 < 0.5 0.0 0.0 0.0 0.0 0.0 < 0.5 3.0 9.0 30.0

Seattle Sea-tac Airport
Heating and Cooling Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
Heating Degree Days 772 602 601 474 307 144 58.0 65.0 156 378 591 760 4908
Cooling Degree Days 0.0 0.0 0.0 0.0 0.0 21.0 64.0 81.0 24.0 0.0 0.0 0.0 190

[dan sw fl]

MIN 24'F MAX 91'F for the last 8,760 hours in Seattle

http://www.wunderground.com/history/...q_statename=NA

[VFRguy]

Feeling much better now in making the right decision with the right contractor. Thank you all for your responses! Great forum!!

[tigerdunes]

VFR

Pay attention to my recommendation on best thermostat selection and addition of media filter cabinet.

IMO

[VFRguy]

VFR

Pay attention to my recommendation on best thermostat selection and addition of media filter cabinet.

IMO

I consistently have been recommended the Honeywell VisionPro 8000. Is that the kind of thermostat you are referring to?

I do have an electronic air filter, that the same thing as a media filter cabinet?

[tigerdunes]

There are many models within the 8000 series of Honeywell stats, only a few are true two stg thermostats that actually control the staging rather than the timer on the control board. You need to get the specific model number and perhaps a Honeywell thermostat expert can confirm that yours is a true two stage model. It is a big deal or else you will be running on high stage all the time when unnecessary.

IMO