60,000 vs 80,000 BTU furnace
One contractor offered a 2 stage 80,000 gas furnace and when I asked him (the other quotations I have are for a 2 stage 60,000 gas furnace) why he said: "The furnace that we are going to install will run most of the time on low fire, which is around 50,000 BTU's. We recommend this furnace over a 60,000 BTU because the 60,000 will have to run a lot longer because it will be running on low fire which is around 35,000 BTU."
I liked him a lot; he saw the chimney flue loose (and the house inspector had mentioned it in the report) and he fixed it right away (he just pushed it properly in place). I wonder whether the other contractors saw this 'issue'. So I would have given him the contract even if a few hundred $ more expensive.
But now, I do not know what to believe. Were all the other contractors wrong by offering a 60,000 furnace?
It depends whether or not you need 60 or 80k based on where you live.
How did these estimators come up with these numbers?
Originally Posted by syndicated
Someone that oversizes sounds like someone who will use a single stage thermostat and time to high constantly. When are you more comfy on a cold day, heat on or off? On. You don't turn the heat off in your car, you just turn it down. Why not have the same comfort at home. Get the right sized furnace and if it runs near non stop on low in bitter weather, you will be very comfortable.
Tell us where you are located & what major city you live near; & square footage of your home.
The required furnace sizing depends on a lot of factors; but in moderate climates, a 60,000-Btuh Input 95% furnace should easily handle a mid-sized home that is rather energy efficient.
If a two stage that 35,000-Btuh will be handling the heat loss a vast majority of the time.
However, you need an accurate room by room heat-loss calc to size properly according to the winter design in your location.
So should I go with a 2 stage 60,000 furnace?
Originally Posted by BaldLoonie
around 1,700 square foot
Originally Posted by udarrell
near the Vancouver area (BC), moderate climate
For Vancouver? Go with the 60,000 btu for sure, unless your house is old and poorly insulated.
If you were east of kelowna, then I'd say the 80.
I would also advise you to purchase the model with the variable speed fan, and get the best warranty possible.
"The furnace that we are going to install will run most of the time on low fire, which is around 50,000 BTU's. We recommend this furnace over a 60,000 BTU because the 60,000 will have to run a lot longer because it will be running on low fire which is around 35,000 BTU."
This is exactly what you want properly sized furnace or boiler to do. Cycling on and off is akin to stop and go traffic. Matching the heat load to the heat loss in real time will keep you more comfortable and burn less fuel.
Start with a proper heat load and use the right size.
Take your new friend out for a beer and tell him the error of his ways.
if 60K is enough to handle the heat load at it's highest (coldest temp outside) then an 80 is WAAAAY oversized. your ductwork is likely not sized to handle the airflow needed to feed this furnace, which will result in condensation buildup in the heat exchanger which will result in rapidly failed heat exchanger. now, if your house actually needs 70K, I'd go for the 80, which will have some cycling under low fire, and still have the flame to handle the bitter cold nights that will occasionally hit.
it all depends on an accurate load design calc of your house.
The TRUE highest cost system is the system not installed properly...
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Originally Posted by CurbYourEnthusiasm
The contractor makes that sound like it's a bad thing. The perfect HVAC system runs continously and delivery the exact amount of heating or cooling needed to match the heat loss or gain of the space, with exception to the proper use of setbacks. ALL OTHER DESIGN are a compromise. Run times do not wear out equipmetn nearly as much as cycling on and off.
IF you only need a 60k and install a 80k, on low fire, the 80k will still run < 50% of the time 90% of the heating season. A 60k furnace will run continously almost 1/3 of the time.
Furnaces are slightly more effcient on high stage than low stage. So are most heat pumps in heating mode...FWIM.
If I was a betting man, (I'm not) without knowing anything about your home, I bet even 40k BTU's would maintain temp in your house at design conditions if you have a 60k installed right now.
Finally, an 80k will need at least 1200CFM on high stage, and will still deliver 800CFM. In comparision, a 60k furnace needs only 900CFM minimum on hi stage and 600CMF on lo stage. 1/2 hte airlfow will use 1/4 hte electricity to deliver, so even if it runs twice as long, you will use less electricity. 1/2 the airflow will also be 1/2 ro 1/3 the noise level on the same ductwork. In most homes, low stage is almost silent. Lo stage is typically 37,000 on most 60k 2 stage furnaced. Longer cycles give more even termpatures and if you have and temeprature imbalances in your home, a larger unit will make it worse.
I would be asking, whether you need a 60k furnace? Maybe a proper load calculation says a 45k will be plenty if you don't have a large AC unit.
Which do you want, a noisy furnace that short cycles, or a furnace that delivers nice even tmepratures and runs a long time, extending equipment life and reducing energy use?
Do you use more gas driving a constant 60mph in the right lane in heavy traffic, or in the left lane going 80mph, then 45, then 80, then 45 and in both cases you arrive at your destination at almsot the same time, meaning it's the same average speed, same distance covered. In the first scenerio, you only needed a 50HP engine, in the 2nd you need 150+HP to "keep up" with traffic.