Author Topic: Furnace questions - how to analyze options  (Read 2980 times)

G-dog

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Furnace questions - how to analyze options
« on: December 02, 2015, 07:00:58 PM »
We just had a repair done, and the tech said the control board may be going out (light flickers). He brought up replacing the furnace and noted that our current furnace is bigger than needed for the house. The furnace is almost 20 years old, so newer models are more energy efficient.

I'd like to calculate the option of fixing (control board) or replacing:

Current furnace is 1996 Lennox 75000 BTU gas furnace
Control board costs about $290 + labor, etc.
We likely need a 45000 BTU furnace for our 1000 sq ft split level

I assume that a 45000 BTU wil use less energy? Is that correct? Saves gas or electric or both?

Any idea how much more efficient a new model is vs. a circa 1996 model?

Quoted cost was about $4900, there will be a tax rebate which he said would bring the cost down to about $2900. How long to recoup the cost in savings on my energy bill ( gas savings, electricity savings)? I just need some formula, I can plug in the costs..

What would you do? Fix or replace? Supposedly, tax incentives will decrease in 2016 (re: more energy efficient appliance). I may also have a rebate I could get from our energy company.

brotatochip

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Re: Furnace questions - how to analyze options
« Reply #1 on: December 02, 2015, 08:46:35 PM »
You can purchase the control board online at a better price and replace it  yourself.  There are diy videos and how-to's on just about anything these days.  Your furnace is still working...replace it when it is unrepairable or unsafe.

zolotiyeruki

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Re: Furnace questions - how to analyze options
« Reply #2 on: December 03, 2015, 05:20:23 AM »
The inefficiency from being oversized is not as big a deal as you may think.  That 75,000 number is the maximum *capacity*.  It doesn't mean that your old furnace is actually *using* that much energy.  The efficiency of the furnace itself is far more important.  If it's an 80% furnace and a new one would be 95%, you'd be getting ((95-80)/80) = 18.75% more heat out for the same amount of gas put in.  Or put another way, you'd be using 16% less gas to get the same heat.

So take your annual heating bill, multiply it by 0.16, and then divide the cost of the furnace by that number.  That will give you a break-even point.  Or, in one equation:

Code: [Select]
                (cost of new furnace)
Payout(years) =  ---------------------
            heating bill * (1-(old eff/new eff))

Pooplips

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Re: Furnace questions - how to analyze options
« Reply #3 on: December 03, 2015, 05:38:44 AM »
My dad is an HVAC contractor and I work for him all through high school and college so I know jsut enough to be dagerous.

Control boards are easy to change. Take one wire off the old; put it on matching spot on the new. Repeat until all wires are attached to the new.

As for the size of your furnace, how well insulated is your house? Have you made upgrades to insulation, windows, etc. All those things go into the calculation for your BTU needs. Also, a 1996 furnace is most likely an 80% eff furnace. That was the base model around that time. Now you can get 92% for more of a base model up to 96-98% for high efficiency. High efficiency furnaces have VFD's and more complex control boards which can make repairs pricy.

G-dog

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Re: Furnace questions - how to analyze options
« Reply #4 on: December 03, 2015, 12:37:35 PM »
I have finally dug up some info on our furnace. What we have is a Lennox Elite G26 75000 BTU (not sure if it is Q3 or Q4). It has an igniter and is supposed to be pretty efficient at 92 AFUE (92.3 AFUE I think).

So the efficiency rating is really not bad considering the age of the unit. This model series has been discontinued, and is not highly rated based on some info I've seen online. Mostly people complain about constant repairs. That has not been our experience, which makes me wonder if a problem happened in later versions of this product.

Thank you for the advice and formulas! I suspect our system is not optimized (proper # and placement of ducts, duct work, heat capacity, blowers, etc.), but we don't have a lot of complaints so far so I plan to keep fixing it!

The repairman is here now, from what I am hearing, it may not be (just) the control board...... It is one thing to replace parts, but I am not in a good position to diagnose issues...

Thegoblinchief

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Re: Furnace questions - how to analyze options
« Reply #5 on: December 04, 2015, 09:55:09 AM »
We've gone the Theseus' ship route with our furnace. With the exception of the burner and control board, nearly every part is pretty new despite the unit being 20 years old. Even the heat exchanger was replaced, under warranty, which is the single most expensive part ($1000 with labor but we were charged $0). The effective age of the furnace if all the components are averaged is probably now 7-10 years old.

Can't remember if ours is 80 or 85% efficient, but our heating cost is about $1,000 in the average cold season. I'm going from memory and that's probably on the high side.

So even a 10% efficiency gain would take 50+ years to break even unless gas prices go up drastically.

I'd keep fixing it

G-dog

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Re: Furnace questions - how to analyze options
« Reply #6 on: December 04, 2015, 12:59:30 PM »
Since this is a discontinued model, Lennox now longer produces the parts. So, needed to get a universal control board that should fit. The trouble I heard was likely due to the repairman checking that the board was working correctly with all the elements and was mapped properly.

$363 to fix, got a bit of a break (no trip fee) since they were just here. Less than 10% of the cost of a new furnace. It had the original control board.

I always lean toward fixing vs. replacing, if for now other reason than reducing landfill waste. But I realize that this is a bias that may lead me to overlook a problem or not appreciate a benefit.

We have not added insulation to the house. Our biggest energy losses are likely windows and doors. Our house was built in 1968 - not sure how well houses were insulated back then.

chops

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Re: Furnace questions - how to analyze options
« Reply #7 on: December 04, 2015, 02:19:06 PM »
G-dog,

Houses were minimally insulated prior to the oil embargos of the 1970s - so I would suggest you look into insulating if you expect to stay in the home through the rest of your retirement. 

Not sure who your energy provider is, but over here in Cedar Rapids we have MidAmerican and they provide a 70% rebate back up to $1k on attic insulation, and an additional 70% up to $1k on wall insulation.  That could help!

Good luck

 - Chops


G-dog

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Re: Furnace questions - how to analyze options
« Reply #8 on: December 04, 2015, 03:00:36 PM »
G-dog,

Houses were minimally insulated prior to the oil embargos of the 1970s - so I would suggest you look into insulating if you expect to stay in the home through the rest of your retirement. 

Not sure who your energy provider is, but over here in Cedar Rapids we have MidAmerican and they provide a 70% rebate back up to $1k on attic insulation, and an additional 70% up to $1k on wall insulation.  That could help!

Good luck

 - Chops
We have MidAmerican also. They are pretty good re:rebates.

FIRE me

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Re: Furnace questions - how to analyze options
« Reply #9 on: December 05, 2015, 07:55:06 PM »
Since this is a discontinued model, Lennox now longer produces the parts. So, needed to get a universal control board that should fit. The trouble I heard was likely due to the repairman checking that the board was working correctly with all the elements and was mapped properly.

$363 to fix, got a bit of a break (no trip fee) since they were just here. Less than 10% of the cost of a new furnace. It had the original control board.

I always lean toward fixing vs. replacing, if for now other reason than reducing landfill waste. But I realize that this is a bias that may lead me to overlook a problem or not appreciate a benefit.

We have not added insulation to the house. Our biggest energy losses are likely windows and doors. Our house was built in 1968 - not sure how well houses were insulated back then.

I have a 1998 model Ruud furnace that is no longer manufactured. But when I Google the model number, lots of replacement parts (mainboard, draft inducer, pressure switch, roll out switch, igniter, flame sensor, gas valve) come up from a large selection of suppliers. Both new old stock Ruud parts, and third party replacement parts.

I'd be really surprised if parts are not readily available for a popular brand like Lennox. Especially a 20 year old Lennox, for which parts demand will be strong.