Make old furnace more efficient?

[travelover]

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 characteristics of a modern furnace (80% and above) are a lot different than your comfy old low efficiency furnace. You are going to end up with far more airflow at a lower air temperature, (temperature rise) and this my involve a lot of ductwork changes to end up with an acceptable 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 barely 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 conditioning.

Thanks for the input. My duct work is all steel and I've metal taped it at every joint that is accessible. I'll pick the installer's brain for improvements, though. I think I have a good handle on my heat loss numbers.

It does look like I'll need to install a chimney liner for my hot water heater.

 

[travelover]

Perhaps you could buy a home warranty from someone like American Home Shield and hope that it fails beyond repair within the next year or so.
Our previous home came with a 2 year version, and sure enough, they got to buy me a new air handler. All they would pay for was a Goodman. It was only slightly better than the broken unit.
Normally I'd never buy a warranty or use the lottery as a retirement plan, but your case sounds like it might be worth a gamble. The warranty that is.

That is an interesting approach, but like I said, I've been anticipating its demise for 20 long years. With my luck, it would last just until the warranty ran out.

 

[travelover]

.................What happened at the furnace manufacturer: They found a way to increase the efficiency 2%. Maybe they did the little reflector/baffle thing I suggested (decreasing the overall system efficiency of the HVAC system, but increasing the measured efficiency of the furnace). Or maybe they made the heat exchanger out of slightly thinner material to increase the heat transfer to the air. Sure, the heat exchanger might burn out and put the furnace in a landfill in 1/2 the time, but they met the government's "efficiency standard.".............

I'm sure there are some interesting discussions going on at the furnace manufacturers right now. Overnight their 94% furnaces just got discounted by 30%. They are probably scrambling to install upgrade kits on all the in-stock furnaces to bump them up to 95% and relabeling them as such.

 

[samclem]

It does look like I'll need to install a chimney liner for my hot water heater.

Of course, when you have the furnace replaced you'll have new PVC ducts installed (for incoming air and exhaust). If you think you might go with a high efficiency water heater (more energy credits!) and if the WH is placed near the furnace, think it all through as to where the ducts for each will run and where hey will terminate (i.e. don't let the "easy" routing for the furnace ducts make things much harder when you do the WH).

Both installers I contacted wanted to run the intake/exhaust for the furnace right through the side wall of the house. That would have been okay, but we had future plans for the area outside that wall. Instead, I ran both through the interior of a wall cavity, combined them in the attic to the single 3" concentric vent, and ran them through the roof. All the sawing and crawilng through the attic would have cost a fortune if I'd hired it out. I was a little concerned about condensation (cold outside air in the intake pipe getting below the dewpoint inside the wall and getting drips on the outside), so I wrapped that pipe with thin (1/4") adhesive foam pipe insulation. I haven't had any problems. Maybe the foam was overkill.

 

[travelover]

Of course, when you have the furnace replaced you'll have new PVC ducts installed (for incoming air and exhaust). If you think you might go with a high efficiency water heater (more energy credits!) and if the WH is placed near the furnace, think it all through as to where the ducts for each will run and where hey will terminate (i.e. don't let the "easy" routing for the furnace ducts make things much harder when you do the WH).

....<snip>........

Good point. Water heater is 20 years old (I do wring the last life out of appliances). I think I'll stay with a conventional gas tank type water heater and continue vent it up the chimney. In order to get into rebate efficiency, you need to go tankless or with a really expensive condensing water heater. With just DW and I, water heating is a small expense.

Any thought on chimney liners - aluminum vs stainless for this application?

 

[samclem]

. . . I think I'll stay with a conventional gas tank type water heater and continue vent it up the chimney. In order to get into rebate efficiency, you need to go tankless or with a really expensive condensing water heater. With just DW and I, water heating is a small expense.

Any thought on chimney liners - aluminum vs stainless for this application?

I had to do the same evaluation a few years ago, we went with a "conventional" (60% efficient) gas water heater. Tankless were too expensive and I wasn't confident of the longevity. The payback period on the condensing gas storage (tank) systems wasn't attractive for us, especially when considering the potential for increased maintenance. I suppose today the government subsidy might tip the balance the other way for some folks.

I also needed to re-line my chimney for the gas water heater. I did it myself, and it was fairly starightforward. This flexible vent product made the whole thing easy/possible as a DIY project. I used regular straight B-Vent sections and elbows from the WH to near the chimney base. Then, I lowered 25' of straight B-vent down the chimney shaft, on the end was 4' of this flex duct. I put a cord through the inside dangling down and was able to snag the cord and pull it through the hole where the thimble is. DW pulled on the cord as I slowly lowered the pipe, and it worked great with the flex the duct gently curved 90 degrees to nearly horizontal. I connect the flex to the straight portion from the WH. Installed a cover and strain-relief over the top of the chimney tile and a cap on my new Wh chimney--Done. This was not a hard DIY project, but I had to order the flex vent online. I suppose you could even use the flex pipe for the whole job (except where you go through a wall) , but it would be more expensive and I think the corrugations would reduce the speed of the exhaust gas flow and increase the amount of inner surface area, possibly leading to increased chance of condensation inside the pipe. Regardless of whether you use smooth or corrugated, I'd recommend spending a little more and going with double-wall (Type-B) pipe if you are going any distance at all. It will keep the exhaust gases warmer which helps ensure a good draw and also reduces the chance of condensation inside the pipe.

Oh--back to your question. I think the inner pipe is aluminum, the outer pipe is galvanized (I wonder how they join them. I doubt they use silver solder). I'm away from home, so I can't check.

 

[travelover]

..........<snip>..........Oh--back to your question. I think the inner pipe is aluminum, the outer pipe is galvanized (I wonder how they join them. I doubt they use silver solder). I'm away from home, so I can't check.

Thanks. I see kits from $100 to $500 and I'm just trying to figure out what I really need. I know that a wood or oil furnace needs stainless. Building inspector says I need to bump it up to 4" from 3" and only requirement is that it is UL listed.

Z-Flex Aluminum Chimney Liner Kit For Gas 4 In. X 25 Ft.

 

[jazz4cash]

I may have missed it.....is the unit located in conditioned space? If it is, I would be tempted to build an enclosure and route outside air to to it. Colder, denser air supports combustion and you save by not having the furnace consume the air you just paid to heat up. I am thinking of doing this to our 80+% unit located in the basement.

 

[ERD50]

I may have missed it.....is the unit located in conditioned space? If it is, I would be tempted to build an enclosure and route outside air to to it. Colder, denser air supports combustion and you save by not having the furnace consume the air you just paid to heat up. I am thinking of doing this to our 80+% unit located in the basement.

I've considered the enclosure around our furnace and water heater and pump in outside air. I know this goes against conventional thinking, but....

I'm not convinced it helps, unless you are drawing air in through living spaces, which might make you feel drafty. Our furnace is in the basement, and I suspect there are plenty of leaks to bring in combustion air.

So yes, the furnace is dumping out air that was heated which seems like a waste. But, if you bring in outside air, that air is cold and it has to take longer to heat things up inside your furnace. Seems like that is a wash, but I can't really convince myself one way or the other.

It would be hard to measure, if we were expecting outside air to increase overall efficiency by say 5%, then my theory would say that if it is a wash, the temperature differential in the furnace duct would be 5% less, and 5% of a 60F delta is just 3F in a 120F duct. Measurable with good equipment, but you'd need to be pretty careful to reduce errors.

Or maybe it is just the square root of 5%? Total heat energy increases as a square of the temperature delta, right? And my gas bill reflects energy, not temperature? So maybe just 1.5F out of ~ 120F?

I've just never seen anything convincing on this, just hand-waves. We talked about it a while back:

http://www.early-retirement.org/for...ting-oil-to-natural-gas-30652.html#post567022

Now, if you do this in combination with sealing up the house tighter, I could see the combined effect helping. Again, my heating bill is ~ $1000 (plus a few hundred in electricity for the fan, I think). I'm not going to invest too much for a 5% annual savings. I'm not trying to stimulate the economy, I want to save money (and the environment if I can do both).

-ERD50

 

[travelover]

I may have missed it.....is the unit located in conditioned space? If it is, I would be tempted to build an enclosure and route outside air to to it. Colder, denser air supports combustion and you save by not having the furnace consume the air you just paid to heat up. I am thinking of doing this to our 80+% unit located in the basement.

I think I'm convinced to get 95% efficient unit to get the 30% Federal tax credit, which will have a cold air intake.

 

[jazz4cash]

I think I'm convinced to get 95% efficient unit to get the 30% Federal tax credit, which will have a cold air intake.

I agree. Our house is 9 years old and furnace installer claimed the next upgrade in efficiency was the same unit with plumbing added for outside air to the combustion chamber. My point was to increase efficience of an older existing furnace by "borrowing" a portion of the new technology.

 

[jazz4cash]

I've considered the enclosure around our furnace and water heater and pump in outside air. I know this goes against conventional thinking, but....

I'm not convinced it helps, unless you are drawing air in through living spaces, which might make you feel drafty. Our furnace is in the basement, and I suspect there are plenty of leaks to bring in combustion air.

So yes, the furnace is dumping out air that was heated which seems like a waste. But, if you bring in outside air, that air is cold and it has to take longer to heat things up inside your furnace. Seems like that is a wash, but I can't really convince myself one way or the other.

It would be hard to measure, if we were expecting outside air to increase overall efficiency by say 5%, then my theory would say that if it is a wash, the temperature differential in the furnace duct would be 5% less, and 5% of a 60F delta is just 3F in a 120F duct. Measurable with good equipment, but you'd need to be pretty careful to reduce errors.

Or maybe it is just the square root of 5%? Total heat energy increases as a square of the temperature delta, right? And my gas bill reflects energy, not temperature? So maybe just 1.5F out of ~ 120F?

I've just never seen anything convincing on this, just hand-waves. We talked about it a while back:

http://www.early-retirement.org/for...ting-oil-to-natural-gas-30652.html#post567022

Now, if you do this in combination with sealing up the house tighter, I could see the combined effect helping. Again, my heating bill is ~ $1000 (plus a few hundred in electricity for the fan, I think). I'm not going to invest too much for a 5% annual savings. I'm not trying to stimulate the economy, I want to save money (and the environment if I can do both).

-ERD50

Keep in mind your furnace is sucking a large amount of air into the combustion chamber. Some of this is air you have already paid to heat. Since the furnace is sucking in air for combustion, the negative pressure inside the home will draw more cold air in through whatever cracks you have in the structure. Cold dense air from the outside also burns more efficiently. Look under the hood of your car and you will see that air is routed from the grill area into the engine rather than being drawn from around the engine itself. If you heating bill is low and your home is comfortable, any cost/effort put into an upgade has a long payback/low benefit.

 

[ERD50]

Keep in mind your furnace is sucking a large amount of air into the combustion chamber. Some of this is air you have already paid to heat. Since the furnace is sucking in air for combustion, the negative pressure inside the home will draw more cold air in through whatever cracks you have in the structure. Cold dense air from the outside also burns more efficiently. Look under the hood of your car and you will see that air is routed from the grill area into the engine rather than being drawn from around the engine itself.

Oh, I understand all that. I'm still not certain how much net benefit there is. Specifically, if there would be enough net benefit for me to build a closet around my furnace and pipe in outside air.

Hmmm, one "real world measure" might be available. If we could find as close to apples-apples on a line of furnaces, and see how the efficiency ratings compare when going from a "free air" model to a "sealed combustion/outside air" model. The problem is, they might change several things at once.

IIRC, when I bought my furnace ~ 13 years ago, it was ~ 90% eff, and the sealed units were only ~ 92-93? I think you had to go "pulse" to get up ~ 94-95? So if my memory is close, probably only a couple % points advantage for sealed. Not enough to motivate me to change, or pay much more for the feature. For new construction, if it saves the cost of a chimney, it might make more economic sense.

I'm not certain the ICE analogy is 100% applicable. Yes, you can get more power by pushing dense air in, but I think the cylinder pressure is based on differentials of in/out heat (0F in and 600F out for example, gives the same force on the piston as 100F in and 700F out). But they still want the oF air, because they can burn more gas with more air, and get more power out with the same size engine. That analogy is probably flawed also, but something like that. In a furnace I just want max heat out. An ICE throws most of its heat away.

-ERD50

 

[jazz4cash]

.....get more power out with the same size engine.
-ERD50

I think the analogy is reasonable....given your statement above, it seems you agree. Nine yrs ago our furnace installer claimed a ~3% efficiency rating improvement for the same unit with outside air for combustion, so it's not a huge improvement, but a little here and a little there adds up. I believe that does not include the full benefit if the furnace is in conditioned space because it is does not account for the cannibalized air.

 

[ERD50]

Yes, when they rate the efficiency, I'm not sure they are taking into account the total efficiency of the "system" (cold air entering the house), or just of the furnace.

On the ICE engine analogy, don't confuse "power" with "efficiency". I was referring to power. Might still be a flawed analogy on my part though. -ERD50

 

[travelover]

..................

Any thought on chimney liners - aluminum vs stainless for this application?

I'll take advantage of this discussion being bumped to ask this question again. Anyone do a chimney lining with an aluminum kit for a gas appliance?