Regular Guest
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?
Professional Member
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?Originally posted by pepper3245
Is the culprit the furnace or possibly the thermostat?
Are you sure it's not an 80k Btuh input?
Last edited by 4l530; 02-13-2007 at 06:07 PM.
Regular Guest
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.
Regular Guest
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.
Professional Member
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
harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!
Regular Guest
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.
Professional Member
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 --
harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!
God Speed, Jim. We'll miss you - 2-26-23
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.
Regular Guest
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.
God Speed, Jim. We'll miss you - 2-26-23
Originally Posted by 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 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.
Regular Guest
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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.
God Speed, Jim. We'll miss you - 2-26-23
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.
Professional Member
Your wasting your time Beenthere. He knows way more than than we do
Regular Guest
I mostly read these and don't post, but I'm amazed that this guy still doesn't get it. Just for fun and because there's nothing much doing, a .65 gph nozzle will have to run longer than the 1.0 gph nozzle to obtain the same amount of BTU.
It will cycle less, and run longer, to obtain the same btus over time.
The savings comes from the more constant run and less startups. We're running our Thermopride with a .5 gph nozzle and it's very economical over longer run times compared with a larger nozzle install.
But as trane just said, this one can go on for days.
Regular Guest
There is no doubt that staging a strong furnace (running at less than 100% total BTU Capacity), either artificially or as manuctured, will make it consume less fuel over the same period of time to achieve the same indoor temps, ceterus paribus.
I am about to get a 2-stage furnace. While I see the advantages, I am not ready to hit the Kool-Aid as hard as some of you guys.
Yes, the furnace uses less gas. However, this relies on the blower running for longer. Doesn't this mean the system uses more electricity?
So, please humor me a moment and tell me what the cost of running a blower motor in a furnace is compared to the cost of the saved fuel consumption.
Professional Member*
It depends, I did a little research on the electricity question myself, I just replaced a older furnace with a mod. Checkout http://www.gamapower.org/
You can compare specs from old to new. There are obviously variables but it is a good reference. The new efficient motors use so much less it becomes almost irrelevant though.
The point I think some are missing is that steady state efficiency is different than AFUE. If the furnace starts at 40% and reaches 80% in 8 minutes then turns off the average eff. is 60%
God Speed, Jim. We'll miss you - 2-26-23
Your now bringing in a 2 stage furnace, which is not what we are talking about.
DJ wants everyone to put in a furnace twice the size it needs to save fuel.
2 stage furnaces are for comfort, not fuel savings.
Although the fan will run longer, the electricity it uses will generate heat, which will also be put into your house.
So the longer blower run time is not a complete lost of money.
Since my HX doesn't warm up as fast as with a 1.0 nozzle, the blower doesn't start as quick. It also doesn't get as hot, so it shuts off alittle sooner then it did with the 1 gallion nozzle. So my blower run time is not a big money loser, for saving on the heating bill.
Your gas furnace will operate slightly different since its desgned for 2 stages.
Regular Guest
[QUOTE=dave davis;1377045]I mostly read these and don't post, but I'm amazed that this guy still doesn't get it. Just for fun and because there's nothing much doing, a .65 gph nozzle will have to run longer than the 1.0 gph nozzle to obtain the same amount of BTU
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oh, But I do get it.It would seem that you not only do not exceed at math , you do not read too well either. In my previous post I said that BOTH the .65 and 1.0 nozzles run for the SAME 6 minutes.The smaller nozzle runs the same time, so it uses LESS fuel.
Get it ,dude?
A note of interest for Dave the rave .Heat rise specifications have a lot to do with nozzle selection. could that be your reson to go the smaller nozzle.
I know so.
Regular Guest
_________________________________________________Your wasting your time Beenthere. He knows way more than than we do
at last, a correct statement by trane.
now you are on the right track, trane
Regular Guest
[QUOTE=beenthere;1377392]Your now bringing in a 2 stage furnace, which is not what we are talking about.
DJ wants everyone to put in a furnace twice the size it needs to save fuel.
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as usual, another wrong assumption by 'beenthere'(or is it.... not 'been anywhere')
]I,m just saying to put in a furnace that is approx 15-20% over your heat loss calculations. You will be glad you did.
twice the size to save fuel ha ha.....lol.. ha, ha,... next joke pleeeezse
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