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Make old furnace more efficient?
Old 02-22-2009, 02:46 PM   #1
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Make old furnace more efficient?

Here is my situation. I have a 40 year old 200,000 Btu (input) gas furnace that works fine, except is only about 60% efficient. My house loses about 32,000 BTU / hour at 5 degrees F. Iím wondering if I can improve the furnace's efficiency or if I should just quit worrying about it until the furnace dies.

A couple of possibilities to improve the efficiency are a flue vent damper and de-rating the furnace. Iím looking for some quantification of the effectiveness or possibility of these strategies. According to the Department of Energy web site:

EERE Consumer's Guide: Gas-Fired Boilers and Furnaces
Quote:
Vent dampers
The most common retrofit is the addition of a vent (or flue) damper. A vent damper prevents chimney losses by closing off a boiler's vent when the boiler isn't firing. Steam boilers benefit from vent dampers more than hot-water boilers, and bigger boilers benefit more than smaller ones. Vent dampers, however, may not be cost effective with properly sized, newer furnace models.
Quote:
Derating gas burners
Many boilers and furnaces in today's homes are oversized, particularly if you've upgraded the energy efficiency of your home. It is sometimes possible to reduce the heating capacity of your gas boiler or furnace to make it operate more efficiently by reducing the size of the gas burner orifice, and possibly also the baffles. This is a difficult process that should only be performed by a qualified technician, and in some cases, it could violate local building codes and void manufacturer's warranties. If allowed, though, the modifications should cost less than $100 and can save up to 15% of your fuel costs.
In researching the internet, I havenít been able to find any quantification of the efficiency savings for a flue damper (not even a good range) and Iíve found no one that has de-rated a furnace successfully, as alluded to in the DOE article.

Anybody out there have any experience with either of these two options?

P.S. The payback on a high efficiency furnace is at least 10 years, more if you include the lost opportunity of investing the money, so Iím passing on that for now.

Thanks for any thoughts on this subject
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Old 02-22-2009, 03:00 PM   #2
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At 5 degrees F the furnace probaly runs a good amount, so flue damper would not be much help, at warmer outside temps may save a few %. Not likely to recover the cost anytime soon.
Gas furnaces are tough to retrofit with smaller burners, not sure anybody would do it. They are sized for the combustion chamber.

I'd wait for the the thing to die. Or if ambitious, toss it this summer and get a more efficient unit. By the way 32000 BTU/hr seems like large heat loss. Unless big old house. My guess with the economy in the tank, you could get good pricing.
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Old 02-22-2009, 03:09 PM   #3
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We use a HE gas forced air system in Central OH. However, I can think of a few things that would not involve a lot of work or a lot of money, except for the last one or two:

a. Close registers in unused room and close the door.
b. Get those plastic things the direct the hot air over the floor rather than up the walls.
c. Get a "set-back" programmable thermostat and set it realistically - sweaters are inexpensive - so are those electric bed pads you place on top of the mattress (we love ours).
d. Turn off the pilot in the summer - if you have a gas fireplace that you do not use turn that one off too.
e. If you have any vaulted ceilings install and turn on a ceiling fan (low and in the "winter mode") when you are in the room.
f. Check insulation and for any air leaks (this one will cost a few $$ but the payback is faster (should be a 30% energy credit for some of this in the 2009 tax rules).
g. If all else fails (and they will not) I would consider a new HE Furnace with energy credits (if available; check tax rules and with your local gas distribution company) - while the payback may be long it will help when selling.

Remember if you can save significant utility costs it is like investing the money - return is return.
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Old 02-22-2009, 03:27 PM   #4
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Sorry, I've no experience with making these furnaces more efficient.

If your furnace is 40 years old, it's gonna crump out within a few years, so investing even $100 to get a 10% or so improvement seems inefficient.

Here's a 95% efficient 115K BTU furnace that sells for $1122. Subtract the tax credit and I think this will be below a thousand bucks. Putting it in is easy if you are handy (check local codes, and have a licensed HVAC pro sign it off for you or have him/her do it from scratch--probably a couple of hundred bucks.) This Goodman unit is a fine furnace, has a 10 year warrranty on the heat exchanger, etc. Wouldn't the payback be a lot less than 10 years if you can get this done for $1600 net total (after tax rebate) and you thereby reduce your heating costs by 35%? If you do this in the spring, the bonus is that your cranky old furnace won't die in the middle of some cold winter night--you won't be able to shop for a bargain then!

If you don't want to get the furnace now, I'd do what OAG recommends and look for any undone cost-effective ways to decrease the heat loss from your home. If you've got any ducting in unconditioned spaces, be sure it is sealed airtight and well insulated (this is a very common cause of heat loss). Any improvements on the house will pay off even after the old furnace dies. Be careful with shutting off too many registers--it can cause your furnace fan to burn out or your furnace's heat exchanger to overheat (burn though early).

Congrats on knowing your home's heat loss--99% of homeowners don't know, and most HVAC contractors I contacted didn't want to run the calcs to tell me what mine was. If you are confident in the heat loss calc, you could go with a 95% efficient furnace smaller than 115 BTU. A smaller furnace (with a smaller fan) will save a little gas, will make the house more comfortable (more even temps due to longer fan "on" times) and help your AC unit do a better job in removing humidity in the summer.
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Old 02-22-2009, 04:05 PM   #5
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Quote:
Originally Posted by ls99 View Post
At 5 degrees F the furnace probably runs a good amount, so flue damper would not be much help, at warmer outside temps may save a few %. Not likely to recover the cost anytime soon.
Gas furnaces are tough to retrofit with smaller burners, not sure anybody would do it. They are sized for the combustion chamber.
Thanks - I wondered about that

Quote:
I'd wait for the the thing to die. Or if ambitious, toss it this summer and get a more efficient unit. By the way 32000 BTU/hr seems like large heat loss. Unless big old house.
I'm surprised to hear that . It is a 2100 sq ft Colonial that has R48 insulation in attic and has been sealed pretty well
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My guess with the economy in the tank, you could get good pricing.
My thoughts, too, though payback seems like a long time
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Old 02-22-2009, 04:13 PM   #6
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Hey, with a more efficient furnace you'll only be throwing 20% of your heating bill up the chimney instead of 40%.
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Old 02-22-2009, 04:14 PM   #7
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Quote:
Originally Posted by OAG View Post
We use a HE gas forced air system in Central OH. However, I can think of a few things that would not involve a lot of work or a lot of money, except for the last one or two:
Quote:
a. Close registers in unused room and close the door.
Did that
Quote:
b. Get those plastic things the direct the hot air over the floor rather than up the walls.
Good idea
Quote:
c. Get a "set-back" programmable thermostat and set it realistically - sweaters are inexpensive - so are those electric bed pads you place on top of the mattress (we love ours).
have a setback thermostat and keep at 60 at night
Quote:
d. Turn off the pilot in the summer - if you have a gas fireplace that you do not use turn that one off too.
Pilot is off in summer
Quote:
e. If you have any vaulted ceilings install and turn on a ceiling fan (low and in the "winter mode") when you are in the room.
No vaulted ceilings
Quote:
f. Check insulation and for any air leaks (this one will cost a few $$ but the payback is faster (should be a 30% energy credit for some of this in the 2009 tax rules).
Sealed all outlets on outside walls, and ceiling fixtures, sealed and insulated rim joist, glass fireplace cover, 15 inches of fiberglass in attic
Quote:
g. If all else fails (and they will not) I would consider a new HE Furnace with energy credits (if available; check tax rules and with your local gas distribution company) - while the payback may be long it will help when selling.
Still not sold on cost ROI - only energy credit is $150 Federal - no state

Quote:
Remember if you can save significant utility costs it is like investing the money - return is return.
Yea, but -the money is invested now Thanks for all your suggestions.
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Old 02-22-2009, 04:24 PM   #8
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Originally Posted by samclem View Post
Sorry, I've no experience with making these furnaces more efficient.

If your furnace is 40 years old, it's gonna crump out within a few years, so investing even $100 to get a 10% or so improvement seems inefficient.
Right. Ironically I've been waiting for 20 years for it to die and it just keeps going.
Quote:
Here's a 95% efficient 115K BTU furnace that sells for $1122. Subtract the tax credit and I think this will be below a thousand bucks. Putting it in is easy if you are handy (check local codes, and have a licensed HVAC pro sign it off for you or have him/her do it from scratch--probably a couple of hundred bucks.) This Goodman unit is a fine furnace, has a 10 year warrranty on the heat exchanger, etc. Wouldn't the payback be a lot less than 10 years if you can get this done for $1600 net total (after tax rebate) and you thereby reduce your heating costs by 35%? If you do this in the spring, the bonus is that your cranky old furnace won't die in the middle of some cold winter night--you won't be able to shop for a bargain then!
Good point. I had not thought of installing one myself, but I'm sure I could do it. I read the thread with your and Bunny's thoughts on Goodman and I'm not adverse to using their unit.
Quote:
If you don't want to get the furnace now, I'd do what OAG recommends and look for any undone cost-effective ways to decrease the heat loss from your home. If you've got any ducting in unconditioned spaces, be sure it is sealed airtight and well insulated (this is a very common cause of heat loss). Any improvements on the house will pay off even after the old furnace dies. Be careful with shutting off too many registers--it can cause your furnace fan to burn out or your furnace's heat exchanger to overheat (burn though early).
Good thought. I've sealed and insulated it as well as is practical without new windows, etc. I only have one far room shut off
Quote:
Congrats on knowing your home's heat loss--99% of homeowners don't know, and most HVAC contractors I contacted didn't want to run the calcs to tell me what mine was. If you are confident in the heat loss calc, you could go with a 95% efficient furnace smaller than 115 BTU. A smaller furnace (with a smaller fan) will save a little gas, will make the house more comfortable (more even temps due to longer fan "on" times) and help your AC unit do a better job in removing humidity in the summer.
I did my own Manual J calculation, then cross checked it by calculating heat loss based on actual consumption last year in conjunction with actual HDD data.
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Did everything EXCEPT
Old 02-22-2009, 04:29 PM   #9
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Did everything EXCEPT

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Originally Posted by travelover View Post
Did that
Good idea
have a setback thermostat and keep at 60 at night
Pilot is off in summer
No vaulted ceilings
Sealed all outlets on outside walls, and ceiling fixtures, sealed and insulated rim joist, glass fireplace cover, 15 inches of fiberglass in attic
Still not sold on cost ROI - only energy credit is $500 Federal - no state

Yea, but -the money is invested now Thanks for all your suggestions.
Sounds like you have everything under control EXCEPT the new furnace. I forgot the name of the furnace (someone mentioned it earlier this week) that actually does not use any electricity to operate. Seems like a nice option in case you wanted to leave town for a few weeks to head south.
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Old 02-22-2009, 05:20 PM   #10
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Thanks - I wondered about that
It is a 2100 sq ft Colonial that has R48 insulation in attic and has been sealed pretty well
My thoughts, too, though payback seems like a long time
Your heat loss didn't strike me as high.

I can't find my calcs, but our house is approx 1900 sq ft 2 story ranch (sq ft includes "finishable" basement). Approx R-45 in the attic, probably R-9 effective in the top floor walls, no insulation at all (yet) in the basement walls or floor, tight double-pane windows. 90% of ductwork is in conditioned spaces. Your house is likely better insulated than mine. We live near Dayton (5700 HDD). My 69K BTU (input) 93% efficient furnace is fine, maybe even a tad big. We keep the thermostat at 69 deg upstairs, but the basement does get a lot cooler due to the lack of insulation. Even when temps go below 0 deg outside, the duty cycle of our furnace doesn't ever seem to exceed 50%. The only time the second-stage burners come ons when I bump-up the thermostat set temp in the AM.
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Old 02-22-2009, 06:02 PM   #11
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For comparison I checked my numbers.

My house is roughly 1900 square feet, pretty well insulated brick, cube shape, split level 1970s construction. In SW PA.
Oil heat.
Have an elapsed time meter connected to oil burner, thus measuring only burner ON time, in minutes. On several 0 degree days with temps in -5 to -7 range overnight, burner ON time was about 5 Hrs in several 24 hr periods.

Furnace has a .75 gal/Hr nozzle. Measured 85% efficiency this fall.
1 Gal oil=140000 BTU
@85% eficiency 1Gal= 120000 BTU output
.75 gal nozzle=90000 BTU/Hr output
5 Hrs ON time in 24 Hr period=450000 BTU output, total heat output for 24 Hrs
1hr heat replacement= 18750 BTU

So My house with 1900 square foot floor are needed 18750 Btu of heat replaced on an hourly basis.

Unless there is an error in my thinking and calculations, my house's hourly heat loss at an average of about 0 degrees F is 18750 BTU/hr.

Furnace is still grossly oversized, but due to combustion chamber limitations can not go to smaller burner. Ideally it should be a .25 gal/Hr burner. When we bought house furnace had a 1.75 Gal/Hr burner nozzle, talk about stupidity and waste.

DW keeps thermostat at 74 degrees F, a lost cause in discussions.

These numbers were taken when my woodstove was off for the test period. With woodstove running oil use use is much less.
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Old 02-22-2009, 06:24 PM   #12
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I have not run numbers in a long time, but I seem to recall that replacing a 60% eff furnace was one of those things with a good payback.

I see samclam offered a source, I'll only add that if you go for low-90's in eff, is is usually more cost effective than trying to squeeze out a few more %. You hit diminishing returns and increasing costs. I say low 90's, but that was 15 years ago, things may have changed a bit since then, but you get the point.

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Old 02-22-2009, 06:27 PM   #13
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With a 40 year old forced air furnace, I'd want a lot of high quality CO detectors.
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Old 02-22-2009, 06:46 PM   #14
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With a 40 year old forced air furnace, I'd want a lot of high quality CO detectors.
Good point - I have two. One in bedroom and one on main floor.
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Old 02-22-2009, 06:59 PM   #15
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. . . I'll only add that if you go for low-90's in eff, is is usually more cost effective than trying to squeeze out a few more %. You hit diminishing returns and increasing costs. I say low 90's, but that was 15 years ago, things may have changed a bit since then, but you get the point.

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The only problem is that you only get the tax credit if the furnace is 95% efficient. I bought a 93% efficient model before the credits were available, and I can't believe they had to do very much to gain the extra 2%. If I were in the engineering dept of the furnace mfgr, I would have put small reflectors above the burners (on the upflow models) so the radiant energy from the heat exchanger was directed back into the combustion section, thus warming the air a tiny bit more inside the furnace chasis for the same amount of fuel burned. Of course, in the REAL world, the radiant energy from the heat exchanger is "seen" by the supply ductwork, which gets hotter and helps heat the air even more effectively than this little reflector setup would, but the furnace mfgr probably doesn't get "credit" for heating that occurs inside the ductwork.

The tax credit is for 30% of the cost of the unit, so it's worth going for the 95% efficient one.
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Old 02-22-2009, 08:00 PM   #16
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....................

The tax credit is for 30% of the cost of the unit, so it's worth going for the 95% efficient one.
I see this has just changed with the stimulus package bill. It was $150, now it is 30% up to $1500
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Old 02-22-2009, 08:07 PM   #17
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I see this has just changed with the stimulus package bill. It was $150, now it is 30% up to $1500
For two years, right? It is time to pitch that old clunker and save the planet! (and your wallet).
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Old 02-22-2009, 08:38 PM   #18
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For two years, right? It is time to pitch that old clunker and save the planet! (and your wallet).
I was hoping there would be a more generous tax credit coming and now it is here. I think I'll get some estimates this spring before the cooling season kicks in.
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Old 02-22-2009, 08:53 PM   #19
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The only problem is that you only get the tax credit if the furnace is 95% efficient. I bought a 93% efficient model before the credits were available....

The tax credit is for 30% of the cost of the unit, so it's worth going for the 95% efficient one.
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I see this has just changed with the stimulus package bill. It was $150, now it is 30% up to $1500

Oh my. So the govt has to micro-manage what kind of furnace I buy? That wouldn't be so bad, except their math seems to be way, way off.

It looks like the portion of the gas bill that I can attribute to heating here in Northern IL is ~ $1000/year. The difference in that bill with a 95% furnace versus a 93% furnace would be.....

~ $22 a year. wow.

So the govt thinks there is some reason that I need to be encouraged to the tune of $1500 to buy a 95% furnace? Is that good use of our money?

When I bought my furnace, (~90%) I made the decision for myself what was cost effective. So why, oh why, is the govt throwing in some artificial numbers to alter this calculation?

This IS about the money people. The govt is spending OUR money to influence other people's financial decisions, and the end result is a decision that does not make economic sense. That is a waste of our money, and we have no say in the matter. I can't see how spending $1500 to save $22 is going to grow the economy.

Would you spend $1500 to save $22/year? Probably not. So why does the govt do it in our name?

And guess how many furnace installers are going to be offering a $5000 package so that you can "maximize" your tax savings?

So maybe this seemingly innocuous thread about furnaces needs to be moved to the " FIRE Related Political Topics ", because the govt has seen it fit to stick their noses in this aspect of our daily lives? At this rate, nothing will be non-political.

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Old 02-22-2009, 09:08 PM   #20
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When spring arrives , start looking for a qualified HVAC contractor who is hungry for business. (I was one for many years , but my business was often non-profit , and not by choice ). early spring is the slowest time of year.

A forced air furnace is only part of the system and the charistics of a modern furnace (80% and above) are a lot different than your comfy old low efficency furnace. You are going to end up with far more airflow at a lower air temperature, (temerature rise) and this my involve a lot of ductwork changes to end up with an acceptible system. Your old ducts may have a lot of leaks if 'Flex Ducts". A new furnace with more airflow will make this even worse.

The good part of this is, old flex ducts were R-3.2 or 4, and now R-8 flex duct is available if you demand it.

P.S. Do not oversize the system , just barly big enough will be more comfy than too big and cycling frequently. Two speed or variable rate furnaces are sweet , but very costly same goes for air conditionong.
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