Typical Furnace Cycle Times

[pepper3245]

What is a "typical" cycle time for a furnace setup?
Our current setup (soon to be replaced with HP and new furnace) is an 80k BTU (output) in a 2500 sq foot 25yo house. The thermostat (Lux TX500 located in hallway at center of house) shuts the furnace down from 9am to 5pm. At 5pm the furnace (single stage) turns on to a setpoint of 19C which takes around 1 hr to reach. So from 6pm to 10pm the furnace does not come back up. Outside temp approx 4C-6C at 10pm.
Using the HVAC-Calc sw, the calculated loss is 54k BTU/h.
So I'm pretty sure our current furnace is way oversized. Is the culprit the furnace or possibly the thermostat?

[4l530]

Originally posted by pepper3245
Is the culprit the furnace or possibly the thermostat?

Either one or both. Not really enough info to say. Are you saying that after recovery is complete, the furnace never comes back on the rest of the night?

Are you sure it's not an 80k Btuh input?

[jerrygr]

Been asked that question many, many, many times when doing service calls. The answer: There is not one. Everyone's comfort zone ( temp to be comfortable, heat and cool) is differant. A difference in temp settings controls equals run time. More run time( higher cost), lower run time( lower cost) heat or cool. There is no constant. Now you know why thermostat are adjustable.

[pepper3245]

Furnace is a 100K 80%. The furnace may come back on at 10:30pm but the thermostat is programmed to turn it off at 11pm. The thermostat is programmed to fire the furnace from 7:30am-9am and 5pm-11pm. From what I see the furnace will fire 1C below SP and turn off 1C (or was it 2C?) above SP.

[cem-bsee]

are you concerned that the house is not to SP sooner?
what time do you get home?

I would state that your furn is oversized --
at the normal lowest temp expected for your area, the furn should run all the time -- even at the recommended design temp -- [which is ___ ?]

seems like the t-stat doing its job ok

[pepper3245]

I'm home around 6-6:30pm so the furnace may or may not be on. I'm comfortable with the ramp up to the SP, I'm just not sure of a 80kBTU/80% is more suitable (as backup to the HP) for this house.
The temp did get into the -8C to -10C last month, after reaching the SP it MAY have ran for 60-90mins (no more than 20 mins/cycle) between 7 and 11pm.

The furnace was definitely replaced by the previous owner 6-7 years ago, maybe they replaced their 100k (40-60%?) with another 100k (80%)?

I don't know what the design temp is.

[cem-bsee]

my 1983 manN calls for 19F design there -- -7C

such should be shown on your heat load calc --

so, if your furn was sized for decent effeciency, it should have been running longer ea cycle --

[beenthere]

A 80,000 input 80% furnace will output 64,000 btu's, 10,000 above your load calc, so it would be appropriate for your load calc.

It will have a longer run time during recovery then your current furnace.

But, get a real thermostat, for your new furnace.

[deejoe]

Here we go again ( lol)
as long as you are comfortable (heatwise) what would you rather have,....
a furnace that runs constantly?....
or one that runs about 6 or 7 minutes a couple of times within an hour..
Do you think there is any comparison in fuel consumption?,.... lol..do the math.

[beenthere]

Here we go again ( lol)
as long as you are comfortable (heatwise) what would you rather have,....
a furnace that runs constantly?....
or one that runs about 6 or 7 minutes a couple of times within an hour..
Do you think there is any comparison in fuel consumption?,.... lol..do the math.

I have my oil furnace reduced from its 1 GPH firing rate to .65 GPH.

It runs real long now. And uses alot less oil a year then it did at 1 GPH.

I even switched back and forth between the nozzles or like days.
And the reduced firing rate always uses less oil.
With the 1 GPH nozzle, I can heat my place to 90 when its - 20 outside.
With the .65, about 73 at 0.

I did do the math. And over sizing doesn't save fuel.

[deejoe]

I have my oil furnace reduced from its 1 GPH firing rate to .65 GPH.

It runs real long now. And uses alot less oil a year then it did at 1 GPH.
I did do the math. And over sizing doesn't save fuel.

__________________________________________________ __________
duh! You are a real math whiz (lol)
how long did it take you to figure out that a .65 nozzle running for six minutes will burn less fuel than a 1.0 nozzle running for six minutes.

[beenthere]

__________________________________________________ __________
duh! You are a real math whiz (lol)
how long did it take you to figure out that a .65 nozzle running for six minutes will burn less fuel than a 1.0 nozzle running for six minutes.

And where did I say it runs the same amount of time.

Learn to read.

My burner run time is about 25% longer with the ,65, then it was with the 1 gallion nozzle.

Yes, it runs longer, and uses less fuel.

[trane]

Your wasting your time Beenthere. He knows way more than than we do