My Furnace Size!

Messages
522
Location
illinois, usa
I been at this house since February 1990. About 3 years into our ownership, the house furnace broke down/ leaking combustion chamber and needed a new one. I got several quotes and prices where crazy until one guy said, I have a York Furnace for X price because the customer cancel the installation.

This York 100000 BTU was 80% efficient and close to been a lemon had several problems over the years. One factor that I did not consider was that the laundry is nearby and possibly the fumes from all the detergents could have caused some of the problems.

This furnace got replace w/ 70,000 BTU, 90% efficiency in 2005. After the installation, I kept track of the run on and off times and it was still too big. Run times where only
about 5 minutes on. We change the orifice on the burners to the smaller size and change to a thermostat that I could control the furnace run times 3 per hour.

These changes gave me a 12 run time and 12 off time at -9F this morning. The furnace still too big but what factors or choices do we have where the weather can be this extreme in some cases. Last time when cold vortex, the run times where 20 min on and 3 min off.

1. Furnace should be a 2 stage preferably one where stages are 10,000 BTU difference or more
2. Under size the furnace and provide some back up heat during extremes
3. Run a blower door test to get an actual leakage number for the house: This number is needed on J-calculation for furnace size
4. Rheem has the Cadillac in the industry w/ a 100% variable gas valve that uses sensors: Outside temp, return temp, supply temp and adjusts the gas valve based on need.

These are just my personal observation.
 
Messages
2,544
Location
Deep in the heart of Jersey
Another thing to factor in is , over the years most furnaces or boilers lose efficiency over years of use. So if you buy the bare minimum to heat your home, after 20 years it will have to run longer to produce the desired temperature. So the cost of operation goes up. Like you said insulation is cheaper on the long run, and makes your home more comfortable. Another thing to keep in mind is, if you ever plan on adding onto your existing home, buying the bare minimum won't let you expand the heating system without replacing what you have. Having some reserve capacity isn't a real bad thing.,,
 

Zee09

Site Donor 2021
Messages
4,813
Location
Fairhill Maryland
Another thing to factor in is , over the years most furnaces or boilers lose efficiency over years of use. So if you buy the bare minimum to heat your home, after 20 years it will have to run longer to produce the desired temperature. So the cost of operation goes up. Like you said insulation is cheaper on the long run, and makes your home more comfortable. Another thing to keep in mind is, if you ever plan on adding onto your existing home, buying the bare minimum won't let you expand the heating system without replacing what you have. Having some reserve capacity isn't a real bad thing.,,
Most installers would disagree. However I agree and every furnace I ever replaced ( same exact size unit) never heats
as good as the old one. lol but true and the efficiency has to be minor because I never see the savings.
My oil oil furnace went belly up after 18 years and it would rapidly heat the house.
New one is superior in every way and full of features and it just slowly heats up the place...
I hear the same from many others,
 
Messages
6,101
Location
Texas Hill Country
My thought is this. An 80 percent furnace is dirt cheap, I would just put one in, and size it how the installer recommends.

Probably a $1500 change-out, especially if you have no AC.

If the old system was short cycling there is something wrong with it or the duct work is not proper. I would fix that it sounds like a duct work issue not a furnace issue. Also could be too restrictive of a filter. Try a fiberglass filter instead of a big pleated one.

The only way I would consider a high efficiency furnace is if Illinois has some program to incentivy it. They might, when I lived in New York they had all kinds of rebate programs.
 
Messages
5,369
Location
down in the park
I have a 1 GPH furnace which works out to 120 -140k BTU depending on which gallons used. It runs first time for about 30 minutes, then just a minute or 2 per hour. 150 gallons of water in the system.

The furnace was plaqced in 1973, the burner on it was replaced in the 80s. It owes me nothing and I use about 300 gallons of fuel per year. I will keep this unit going as long as I can even though it's more than big enough to heat 2 houses. They don't make'm to last 50 years anymore
 
Messages
404
Location
Minnesota
The new thermostats are also very sensitive. Some have an adjustment for swing temp. Some with no adjustment are set for .25 - .5 degree swing. This can contribute to short cycling, especially without a 2 stage blower.
 

LazyDog

Thread starter
Messages
522
Location
illinois, usa
The new thermostats are also very sensitive. Some have an adjustment for swing temp. Some with no adjustment are set for .25 - .5 degree swing. This can contribute to short cycling, especially without a 2 stage blower.
You are correct, in my mom's hydronic system I install a very simple thermostat where it gave 1.5 degrees difference real happy.
 
Messages
5,148
Location
Ohio
You sure don't want to screw with the air orfice of the burners. Too much air is just as bad as not enough air into the burners.You set them while observing the flame/color/flutter and leave them alone. All you did is lessen its burning efficiency by fiddling with the intake air of the burners and making the furnace work harder. That's why it's running longer. Pay for a clean and check to have a qualified technician check out the furnace before you get into trouble.
If you added insulation or upgraded windows, that will lessen the load on your furnace making it cycle more often.
 
Last edited:
Messages
10,529
Location
MA
You need to do a Manual J calculation. There's a few free ones out there. Basically most furnaces are oversized and most installers won't do the calculation and just throw in the same sized or bigger system. Eventually you get one that's way oversized. I've done a few on some of my units and they have 100,000 worth of BTUs even though those are only 75% efficient so really 75k BTUs. The most generous I was able to do with the manual J calculations just called for 40k, really ranged from 30-40k. Needed 40k at night with -10 degree weather which rarely happened. On the flip side, the place would warm up really quick. That might be other reason it's oversized.
 

LazyDog

Thread starter
Messages
522
Location
illinois, usa
You need to do a Manual J calculation. There's a few free ones out there. Basically most furnaces are oversized and most installers won't do the calculation and just throw in the same sized or bigger system. Eventually you get one that's way oversized. I've done a few on some of my units and they have 100,000 worth of BTUs even though those are only 75% efficient so really 75k BTUs. The most generous I was able to do with the manual J calculations just called for 40k, really ranged from 30-40k. Needed 40k at night with -10 degree weather which rarely happened. On the flip side, the place would warm up really quick. That might be other reason it's oversized.
I do agree mots installers just put what comes out and never do a proper air flow. They normally set the AC on the high speed and in some cases they used a Y tap for AC and heat at high speed on the blower.
 

LazyDog

Thread starter
Messages
522
Location
illinois, usa
You sure don't want to screw with the air orfice of the burners. Too much air is just as bad as not enough air into the burners.You set them while observing the flame/color/flutter and leave them alone. All you did is lessen its burning efficiency by fiddling with the intake air of the burners and making the furnace work harder. That's why it's running longer. Pay for a clean and check to have a qualified technician check out the furnace before you get into trouble.
If you added insulation or upgraded windows, that will lessen the load on your furnace making it cycle more often.
Does not what I did: The company told us and sold us the smaller jets for the furnace.
 
Messages
5,148
Location
Ohio
Does not what I did: The company told us and sold us the smaller jets for the furnace.
All you are doing is decreasing gas flow into the burners, basically the same thing. You are just making the furnace work harder. The burners are designed to accept a given volume of gas into them. Seems to me, your thermostat is where you should be looking at, maybe replacing.
 
Messages
17,067
Location
Silicon Valley
I think most 2 stage furnaces I see have low stage set to 60% or so of the high stage. They likely design that for a reason like condensation or simplicity. Personally I am a bit concern about variable in small step because that usually means complicated motor control for the blower and draft inducer, and that can lead to reliability problems (in inverters and power circuitry). Since most of the time a visit by the HVAC guy will wipe out all the energy savings, I'd rather not having to deal with that.

A boring 2 stages furnace seems optimal in moderate climate, you can always use other means to heat the house up in occasional extreme weather in addition to the furnace on full blast.
 
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