Spend $700 more for 94% efficient furnace to save $60/year in natural gas?

[Kolesar]

This is a concise primer on furnace technology, too!

We are replacing our furnace that is a vintage oil-fired furnace that was modified to run on natural gas (methane). There are two furnace options: an 80% efficient furnace or a 94% "condensing" furnace. A condensing furnace recovers the latent heat of evaporation from the exhaust gasses by "forcing" the water to condense out of the exhaust gasses in a polypropylene-coated second heat exchanger.

We manage our home energy by keeping the temperature at 68F and using a programmable thermostat to make it 62F when away. We did a huge project to insulate the ceilings, but the walls are not accessible. I figure we bought 68 MCF (million cubic feet) of methane last year, so our heating bill was only $544. An 80% efficient furnace would save us $136/year in methane.

I had guessed that money would be better spent buying a high-efficiency furnace than paying someone to insulate the walls with cellulose or foam. However, my calculations predict a mere $60/year savings when purchasing the condensing furnace.

A condensing furnace requires the installation of an outside air intake to get these efficiencies. It also requires installing a drain for the condensate. Either furnace will require a chimney liner, so that is a wash. Condensing furnaces have more parts to fail. I have a hunch that the condensing furnace is more persnickety and harder to keep "tuned up" (which is necessary to keep the efficiencies). One service call could consume two years' savings.

The condensing furnace will reduce greenhouse gasses. I have not yet figured how many pounds of CO2 would be emitted by consuming an extra 8MCF of methane. At the moment, I think that GHG emissions would be the one compelling reason to buy a condensing furnace.

Please advise me on my ethical dilemna :)

[billbuckhead]

1. That's at today's prices of natural gas

It's not coming down.

[Kolesar]

2. Good point!

I could see methane doubling to $15/MCF during the life of this furnace. Even yet, that would be in the "out years" and the investment would not pay off (as a purely financial analysis)

[brokensymmetry]

6. Perhaps you're being an optimist.

I could see methane doubling to $15/MCF during the life of this furnace. Even yet, that would be in the "out years" and the investment would not pay off (as a purely financial analysis)

North America is distinctly short on natural gas. Canada is sending us a lot - they're required to under NAFTA - but their new field (Mackenzie?) is largely being used to produce oil from the northern tar sands deposits. Their supplies seem inadequate to supply the U.S. with what's needed.

Mexico is keeping all their gas. Presently, they flare off a lot, but they want to use their own natural gas, so that's not much help.

LNG takes a long lead time, and it may be at least 5 years before we can get the needed tankers built to transport the stuff; and since usage will continue to increase, it may not be possible to build enough tankers to keep up with demand.

And that means that natural gas may go way, way up....

[Champion Jack]

3. Have you looked into blown in cellulose?

"I had guessed that money would be better spent buying a high-efficiency furnace than paying someone to insulate the walls with cellulose or foam."

=======
I just insulated my 125 year old farm house with it. It was simple, messy, but simple.

[Kolesar]

5. Cellulose is the best for old walls

Installers can blow it in with force so that it does not settle.

Foam has some big problems. Polyisocyanurate expands and will push your wallboard (or lath) apart. Tri-polymer usually "shrinks back", creating air gaps where the heat leaks to the outside. If you spend $4000 for that, you will never recover your investment.

BTW, my calculations show that the walls are the house's major heat loss in this house. 25% of the predicted heat loss would go through the walls.

We still have wall treatments as an option in the future.

[IADEMO2004]

4. Get the best, We have no regrets after 10 years

We had a Carrier installed and the only service it has needed outside routine checkups is the condensate drain plugged. A trip to the kitchen sink for a wash and it was goo to go. Our furnace vents out through the wall and not the roof so I don't know why you would need a liner for the 94%. The humidity in the house will stay higher so no insulation in the walls may be a problem.

[OffWithTheirHeads]

7. Been in the business for over 35 years

1. The new condensing furnaces only need one exhaust vent made of pvc.
2. The flue dosn't have to go up your chimney (no liner necessary).
3. If you live where it snows, buy the good one.
4. Sounds like the installer is not familiar with the product. Get several estimates.

Must leave for work now. P.M. me if you need more info and I will try to help after work tonight.
Bud

[TlalocW]

8. There's a video put out by a guy

In the burbs around Washington, D.C., who never pays more than $15 a month to the utility companies for his energy needs. Along with the requisite solar paneling, the most interesting thing he does is have a corn-burning stove that effectively heats the entire house. He and a group of others have bought a silo that they fill with corn, and anyone in the group can go there with a big trashcan and fill it up for several weeks worth of fuel.

My friend, Doug, lives out there and showed me the video one Xmas. Go corn.

TlalocW

[Kolesar]

9. I bought the high efficiency furnace

We recalculated the heat loads. The furnace we needed was only $550 more (initial cost). It will still not pay itself off unless gas prices triple shortly. However, we are reducing GHG emissions!

[NNadir]

10. I have been there and done that. I bought the high efficiency furnace

for ethical reasons, but was surprised to find that I actually saved far more than I expected. In fact, my fuel bill was cut in half, but that was comparing it to an old oil furnace that was much less efficeint than advertised.

Please keep in mind that your $60 figure is based on fuel costs NOW, not in a future, where energy demand will climb.

[DemSigns]

11. Heat Loss!

Make sure your heating professionals do an accurate heat loss before sizing a furnace for your home. An oversized furnace can drastically reduce efficiency. Since you have possibly drastically reduced the heat loss of your home with all the insulating you may need a much smaller furnace than you currently have which will save you money up front. Don't work with a contractor that uses the three finger metheod of sizing a furnace or boiler. That is where they use three fingers to wipe the dust off nameplate on the equipment to see what BTU/Hr it is and quote you the same or nearly the same size equipment.

You can get a good idea of what BTU/Hr your new furnace should be by looking at your winter gas bills to figure out your Therms-per-degree-day usage. Divide that by 24 to get your Therms-per-degree-hour usage. Then multiply that by the number of degrees below 70 degrees that is your typical maximum winter low temperature to get maximum Therms-per-hour needed. Multiply that by 100,000 to get the Maximum BTUS/Hr you need for your furnace. This number is that actual number of BTUS/Hr that your current furnace needs to keep that you comfortable. Your new furnace may be more efficent so you could take some percentage off to arrive at the right size BTU/Hr furnace for your home.

The venting and intake would probably be through one hole in the wall with a concentric vent that has an inner exhaust pipe and outer ring for intake. Way better than lining a chimney if you have good spot on the wall or ceiling for the vent.

The condensate is acidic and there may codes governing how it is drained away. Often a neutralizer is used before it goes in the drain.

If you "furnace" is actually a boiler, let us know because their are other issues to consider.

[Kolesar]

14. We only paid ~$500 last year to heat the house

We bought 78 MCF of methane last season (780 therms). Of that, I figure 140 Therms were for hot water and cooking.

640 Therms/last season
640/5700 degree days => .11 Therms/deg.day
.11/24 => .0046 Therms/deg.hour
.0046 * 70 degree => .33 Therms/hour required
.33 * 10000 BTU/Therm => 33,000 BTU/hour furnace required

That's small for a 1600 square foot house with no wall insulation.

I developed a heat loss calculation sheet myself, borrowed one from a friend, and had the heating contractor do an estimate. All three of those came up with about 57,000 BTU/hour required.

We are real cheapskates when it comes to heat. Since we had been setting the thermostat back to 56 degrees for half the day, we have not been consuming the heat that 5700 degree days in Ohio would predict.

This year we will be keeping it warmer because of our geriatric dog gets chilled easily.

[DemSigns]

20. Good analysis. Your hot water and cooking gas

estimate might be a little high. We use less than 20 therms per month with five people gas stove and gas dryer. Heat loss calcs generally overshoot what is really needed is what I have seen. What BTU furnace did you go with?

[Kolesar]

21. Output: 56,000 BTU low-fire and 75,000 high fire, I recall

There is something wrong with www.bryant.com , I cannot pull up the data sheet for the Preferred line of furnaces.

The next step down would be 37,000 low fire and 56,000 high fire. Bryant has increments of 20,000 BTU for input/high-fire on their furnaces. My choices would be 60,000 BTU input or 80,000 BTU input.

Bryant is the same product as Carrier, by the way.

[are_we_united_yet]

12. Not everything is

measured in dollars.

[AZCat]

13. Here's a calculation on lbs. of CO2 reduction

1. The chemical formula for methane is CH4. The simplest form of combustion of methane is:

CH4 + 2*O2 -> CO2 + 2*H2O

2. Every molecule of methane used results in a molecule of carbon dioxide produced. Since the molecular weight of methane is 16.04 and the molecular weight of 44.01, there are 44.01/16.04 (or approximately 2.74) times as many pounds of carbon dioxide produced as pounds of methane consumed.

3. 8 MCF of methane is a volumetric measurement (in thousands of cubic feet) of the gas at 60 degrees Fahrenheit and 14.7 psi. Methane has a density of (roughly) 0.042 lbs/ft^3 at those conditions, so 8 MCF is 336 pounds of methane.

4. Using our numbers from section 2, using 8 MCF less of methane means that the carbon dioxide produced will shrink by about 920 pounds (half a ton).


I would appreciate it if someone would check my numbers, especially in section 3. All my thermodynamic data for gasses is in SI, so I may have fumbled the conversion to english units.

[trotsky]

15. We just put in a high-efficiency furnace in August.

Haven't given it much of a workout yet, but we also got new windows this past summer, so I am expecting a huge reduction in my gas bill this winter.

Like another poster said, I don't know why they told you that a 94% efficient furnace would need a chimney liner too. Mine has two 2-inch PVC pipes to the outside, one for combustion air, one for exhaust. There is no hookup to the chimney, nor would that work! That's because the air leaving a high-efficiency furnace is only about 90 degrees (compared to the 300-400 degree air leaving a regular furnace), and so it's not hot enough to vent its way up a chimney.

[Kolesar]

16. Yes, the chimney liner data was confusing as I presented it

I was grossly presenting that either system would need some sort of venting (a chimney liner for the regular furnace, or an intake/exhaust for the condensing furnace) and considered it to be a "wash". That is an oversimplification. The dealer did say that the condensing furnace could be exhausted up the stack. Recall that the furnace uses an air pump to provide oxygen to the flame.

Since then, I compared notes with the dealer who said the intake/exhaust was included in the condensing furnace quote, which made the condensing furnace more attractive. We also reviewed the heat calculations and determined that we could buy a $150 cheaper condensing furnace. The pay back period is still too long for the condensing furnace and I don't know what to expect for repair costs. I am betting (pessimistically) that gas costs are going to more than double and buying the condensing furnace because it is better for the environment. They are installing it next Friday.

[trotsky]

17. I don't think you'll regret it.

Just on the few cold days we've had so far here in Minnesota, the heat is MUCH more even and consistent than it ever was with our old furnace/windows. (Old furnace was 20 years old, old windows were original, meaning over 35 years old.)

Will your furnace have the DC variable speed fan? It uses a trickle of electricity compared to a regular AC high speed fan, and when run constantly at its lowest speed keeps the air circulating in your house. So you end up saving more there, for electricity with the fan as well as not having "hot spots" in the house.

[Kolesar]

18. We bought the "two speed" furnace

It has one speed for heating and one speed for cooling (we are not buying air conditioning, btw). Our dealer explained that the installer will set the motor windings so that we get the optimum airflow through the house to suit our ductwork. The Bryant 90i furnace with variable speed does claim $80-$100/year of fan electricity use vs. $625 for a "typical older furnace". I don't know if that data assumes one will be using air conditioning. The literature could be clearer (!).

[trotsky]

19. Hey, that's the exact model of furnace I got!

The Bryant 90i. Great machine. You will like it. It's very quiet - I can't even tell when it fires up. When it's not actively heating, the fan will slow to the lower speed (as long as you keep your thermostat set to the fan "always on" position).

One thing they told me when they installed mine: I have a programmable thermostat too, and they said that because the furnace is "intelligent," it monitors the amount of heat it needs to make to maintain your house at the set temperature. That way it knows if/when it will need to kick in to the higher stage of heating (thus consuming more fuel). A programmable thermostat can confuse the system because when it comes time to heat up again, it will immediately draw the full power and thus be less efficient. I hope that makes sense. I guess the summary is that keeping your house set to one temperature all the time will even out the energy usage and will ultimately be lower than the extra boost needed to bring it up after it cools way down.