One stage verses Two stage furnaces

[Jimmy Higgins]

My furnace is getting on in the years. Upgrading to it back in '05 from the old chugger helped reduce my gas usage a lot. I really like the idea of a Heat Pump, however, the electric costs of a heat pump will be higher than gas. I've already insulated the home and it is about as green as that gets, so going gas verses electric on a gas/coal grid won't make much of a difference.

So I was checking the furnace situation online and saw there are staged furnaces. Websites are out there suggesting a two stage 80% furnace will be as cost efficient as a one stage 90% furnace. But for the life of me, I can't understand why that could possibly be. Heating X CY of air requires Y energy. That math doesn't change based on the amount of time you take to do it. Are the losses that great when it fires up?

 

[bilby]

Furnaces heat both the air inside the building, and the flue gasses that are dumped outside (unless you like carbon monoxide poisoning).

Presumably a two stage furnace is better because the flue gasses are colder.

I know nothing about furnaces, but basic thermodynamics suggests that this must be the case, if they genuinely are more efficient.

 

[Jimmy Higgins]

I'm not doubting the veracity of a 90% furnace. It was the claim that a two staged 80% could be more efficient than a 90% furnace that I was questioning.

I did some more internet'ing on this and the claim is likely only viable for large homes and small apartments. Places where some portions of the home get the heat more actively and are warmer than the cooler portions of the building. It is less about heating an area is it about heating all of the air in the house, which isn't as equal across the house, so a two cycle can address the portions of the home that aren't heating as quickly without blasting out the people in the warmer rooms.

I'd question whether the furnace is sized right in that case.

 

[steve_bank]

Two-stage heating means the furnace has two levels of heat output: high for cold winter days and low for milder days. Since the low setting is adequate to meet household heating demands 80% of the time, a two-stage unit runs for longer periods and provides more even heat distribution.

Advantages And Disadvantages Of A Two-Stage Furnace | LG Home Comfort

What is a 2 stage furnace and how does it work? Click here to learn all that and more as we uncover the advantages and disadvantages of these popular systems.

lghomecomfort.ca

Comparing Energy Costs and Long-Term Savings

A single stage furnace operates at full power when there is a need for warmth. It blasts out as much hot air as possible, which means that during the coldest months, it disperses the most amount of energy it can. This makes single-stage furnaces cost more on electric bills.

So what is a two-stage furnace doing differently? It comes down to having two settings that allow for more precision in how it responds to temperature changes. These allow it to gradually increase the heat from a lower temperature, leading to lower energy bills while keeping your space more consistently heated.

2 Stage Furnace Advantages

What is a two-stage furnace bringing to your home? Here are six benefits:
1. Low-Maintenance Electronic Commutated Motors (ECMs)

ECMs (electronic commutated motors) are a main feature of a two-stage furnace. Since these furnaces cycle on and off less frequently, the system has less stress. This means your furnace will have fewer breakdowns and an extended operating life.
2. Improved Energy Efficiency

Two-stage furnaces are well known for their energy-efficient features. By running at low capacity most of the time, they use much less energy than single-stage furnaces while maintaining good air circulation and heat output.
3. Reduced Operating Costs

Two-stage furnaces use less fuel and power, reducing operating costs over time. They also provide more consistent temperatures throughout your home without wasting energy or overworking your HVAC system. Plus, you won’t have to worry about repairs or frequent furnace replacement costs since they aren’t easily damaged.
4. Quieter Operation

Two-stage furnaces are quieter than single-stage furnaces due to their slower operating speeds and ECMs. Single-stage furnaces produce loud noises when they turn on and off. But with a two-stage furnace, these noises are much less noticeable and will not disrupt your day as much as they would with a single-stage furnace.
5. More Even Temperatures

Rather than producing large temperature fluctuations throughout your house, the two-stage system will keep temperatures steady. This provides added comfort and reduces cold spots due to drafts from windows or doors, all while keeping energy costs down.
6. Better Indoor Air Quality

The improved airflow from 2-stage furnaces can help keep dust and other irritants from circulating through your living space. This can help improve indoor air quality and reduce allergy symptoms for your family.

The 80% is not a system efficiency.

The typical heater is 'bang bang.' when it turns on it goes full blast. When temperature falls below a low limit it goes on full blast until an upper limit is reached. The difference between the two limits is called a hysteria band, it prevents the heater fro constantly turning on and off. Repertoire overshot occurs so excess energy is used.

Sounds like 2 stage may be some kind of proportional control. The amount of heat and forced hot air is prepositional to the temperature difference. That could re3ult in ore even temperatures around a house.

Rate of heat transfer is proportional to temperature difference. If the blower has a low flow rate the heater has to run longer to heat up a house when it is very cold outside. A large blower with a higher flow rate when the outside temp is lower may result in wasting energy.

 

[Loren Pechtel]

Two-stage heating means the furnace has two levels of heat output: high for cold winter days and low for milder days. Since the low setting is adequate to meet household heating demands 80% of the time, a two-stage unit runs for longer periods and provides more even heat distribution.

Advantages And Disadvantages Of A Two-Stage Furnace | LG Home Comfort

What is a 2 stage furnace and how does it work? Click here to learn all that and more as we uncover the advantages and disadvantages of these popular systems.

lghomecomfort.ca

Comparing Energy Costs and Long-Term Savings

A single stage furnace operates at full power when there is a need for warmth. It blasts out as much hot air as possible, which means that during the coldest months, it disperses the most amount of energy it can. This makes single-stage furnaces cost more on electric bills.

So what is a two-stage furnace doing differently? It comes down to having two settings that allow for more precision in how it responds to temperature changes. These allow it to gradually increase the heat from a lower temperature, leading to lower energy bills while keeping your space more consistently heated.

2 Stage Furnace Advantages

What is a two-stage furnace bringing to your home? Here are six benefits:
1. Low-Maintenance Electronic Commutated Motors (ECMs)

ECMs (electronic commutated motors) are a main feature of a two-stage furnace. Since these furnaces cycle on and off less frequently, the system has less stress. This means your furnace will have fewer breakdowns and an extended operating life.
2. Improved Energy Efficiency

Two-stage furnaces are well known for their energy-efficient features. By running at low capacity most of the time, they use much less energy than single-stage furnaces while maintaining good air circulation and heat output.
3. Reduced Operating Costs

Two-stage furnaces use less fuel and power, reducing operating costs over time. They also provide more consistent temperatures throughout your home without wasting energy or overworking your HVAC system. Plus, you won’t have to worry about repairs or frequent furnace replacement costs since they aren’t easily damaged.
4. Quieter Operation

Two-stage furnaces are quieter than single-stage furnaces due to their slower operating speeds and ECMs. Single-stage furnaces produce loud noises when they turn on and off. But with a two-stage furnace, these noises are much less noticeable and will not disrupt your day as much as they would with a single-stage furnace.
5. More Even Temperatures

Rather than producing large temperature fluctuations throughout your house, the two-stage system will keep temperatures steady. This provides added comfort and reduces cold spots due to drafts from windows or doors, all while keeping energy costs down.
6. Better Indoor Air Quality

The improved airflow from 2-stage furnaces can help keep dust and other irritants from circulating through your living space. This can help improve indoor air quality and reduce allergy symptoms for your family.

The 80% is not a system efficiency.

The typical heater is 'bang bang.' when it turns on it goes full blast. When temperature falls below a low limit it goes on full blast until an upper limit is reached. The difference between the two limits is called a hysteria band, it prevents the heater fro constantly turning on and off. Repertoire overshot occurs so excess energy is used.

Sounds like 2 stage may be some kind of proportional control. The amount of heat and forced hot air is prepositional to the temperature difference. That could re3ult in ore even temperatures around a house.

Rate of heat transfer is proportional to temperature difference. If the blower has a low flow rate the heater has to run longer to heat up a house when it is very cold outside. A large blower with a higher flow rate when the outside temp is lower may result in wasting energy.

Yup, and the warmer the air going through the ducts the more energy will be lost in the process. I do not know the economics of it but in general running at a lower power level is advantageous.

 

[bilby]

Yup, and the warmer the air going through the ducts the more energy will be lost in the process.

Lost to where? If the ducts are heating the house via "losses", what is really being lost?

The heat coming from the floor and the vents, rather than just the vents, seems like it could actually be more comfortable for the occupants of the house.

 

[Tharmas]

Yup, and the warmer the air going through the ducts the more energy will be lost in the process.

Lost to where? If the ducts are heating the house via "losses", what is really being lost?

The heat coming from the floor and the vents, rather than just the vents, seems like it could actually be more comfortable for the occupants of the house.

That assumes you have a basement where the heating unit is installed. Very few houses have basements where I live, and in our case the heating unit is in an attic space, so lost heat warms the attic and the outside world. Fortunately, our ducts are insulated.

 

[thebeave]

Two-stage heating means the furnace has two levels of heat output: high for cold winter days and low for milder days. Since the low setting is adequate to meet household heating demands 80% of the time, a two-stage unit runs for longer periods and provides more even heat distribution.

Advantages And Disadvantages Of A Two-Stage Furnace | LG Home Comfort

What is a 2 stage furnace and how does it work? Click here to learn all that and more as we uncover the advantages and disadvantages of these popular systems.

lghomecomfort.ca

Comparing Energy Costs and Long-Term Savings

A single stage furnace operates at full power when there is a need for warmth. It blasts out as much hot air as possible, which means that during the coldest months, it disperses the most amount of energy it can. This makes single-stage furnaces cost more on electric bills.

So what is a two-stage furnace doing differently? It comes down to having two settings that allow for more precision in how it responds to temperature changes. These allow it to gradually increase the heat from a lower temperature, leading to lower energy bills while keeping your space more consistently heated.

2 Stage Furnace Advantages

What is a two-stage furnace bringing to your home? Here are six benefits:
1. Low-Maintenance Electronic Commutated Motors (ECMs)

ECMs (electronic commutated motors) are a main feature of a two-stage furnace. Since these furnaces cycle on and off less frequently, the system has less stress. This means your furnace will have fewer breakdowns and an extended operating life.
2. Improved Energy Efficiency

Two-stage furnaces are well known for their energy-efficient features. By running at low capacity most of the time, they use much less energy than single-stage furnaces while maintaining good air circulation and heat output.
3. Reduced Operating Costs

Two-stage furnaces use less fuel and power, reducing operating costs over time. They also provide more consistent temperatures throughout your home without wasting energy or overworking your HVAC system. Plus, you won’t have to worry about repairs or frequent furnace replacement costs since they aren’t easily damaged.
4. Quieter Operation

Two-stage furnaces are quieter than single-stage furnaces due to their slower operating speeds and ECMs. Single-stage furnaces produce loud noises when they turn on and off. But with a two-stage furnace, these noises are much less noticeable and will not disrupt your day as much as they would with a single-stage furnace.
5. More Even Temperatures

Rather than producing large temperature fluctuations throughout your house, the two-stage system will keep temperatures steady. This provides added comfort and reduces cold spots due to drafts from windows or doors, all while keeping energy costs down.
6. Better Indoor Air Quality

The improved airflow from 2-stage furnaces can help keep dust and other irritants from circulating through your living space. This can help improve indoor air quality and reduce allergy symptoms for your family.

The 80% is not a system efficiency.

The typical heater is 'bang bang.' when it turns on it goes full blast. When temperature falls below a low limit it goes on full blast until an upper limit is reached. The difference between the two limits is called a hysteria band, it prevents the heater fro constantly turning on and off. Repertoire overshot occurs so excess energy is used.

Sounds like 2 stage may be some kind of proportional control. The amount of heat and forced hot air is prepositional to the temperature difference. That could re3ult in ore even temperatures around a house.

Rate of heat transfer is proportional to temperature difference. If the blower has a low flow rate the heater has to run longer to heat up a house when it is very cold outside. A large blower with a higher flow rate when the outside temp is lower may result in wasting energy.

Hysteria band? Do you mean hysterisis? Hysteria band is a term applied to girlfriends and such. Big difference.

 

[steve_bank]

Obviously I meant hysteria....

In general a loss is any energy that goes anywhere other than where you want it to go.


Ducts represent heat conduction loss through the walls of the duct, and frictional losses. A lot depends on whether the air flow is laminar or turbulent.

You can look up turbulent vs laminar flow and how duct dimensions affect it. The Reynold’s Number. Turbulent flow loses are proportional the square of velocity. On a cold day the more air you try and force through the duct the higher resistance the motor has to overcome.

In most HVAC duct systems, the airflow is turbulent, not laminar. Turbulent flow is characterized by chaotic mixing and swirling of the air, while laminar flow involves smooth, parallel layers of airflow.

In heat transfer at relatively low low pressure and temperature air and water are both considered incompressible fluids. Their dynamic flow characteristics in a duct or pipe are the same.

Basic model wise hot air heating is not much different than hot water heating. Pump hot water through a pipe in a room. The added step is heat transfer from the pipe to room air.

It is an older book on cooling, but it has info on heat transfer and air flow. PDFs online

Cooling Techniques for Electronic Equipment, 2nd Edition
Dave S. Steinberg

 

[thebeave]

Obviously I meant hysteria....

In general a loss is any energy that goes anywhere other than where you want it to go.


Ducts represent heat conduction loss through the walls of the duct, and frictional losses. A lot depends on whether the air flow is laminar or turbulent.

You can look up turbulent vs laminar flow and how duct dimensions affect it. The Reynold’s Number. Turbulent flow loses are proportional the square of velocity. On a cold day the more air you try and force through the duct the higher resistance the motor has to overcome.

In most HVAC duct systems, the airflow is turbulent, not laminar. Turbulent flow is characterized by chaotic mixing and swirling of the air, while laminar flow involves smooth, parallel layers of airflow.

In heat transfer at relatively low low pressure and temperature air and water are both considered incompressible fluids. Their dynamic flow characteristics in a duct or pipe are the same.

Basic model wise hot air heating is not much different than hot water heating. Pump hot water through a pipe in a room. The added step is heat transfer from the pipe to room air.

It is an older book on cooling, but it has info on heat transfer and air flow. PDFs online

Cooling Techniques for Electronic Equipment, 2nd Edition
Dave S. Steinberg

Yep. Its common these days to use insulated flexible ducting, and while more convenient and easier to install, the uneven surfaces creates much more air turbulence than rigid metal duct. Best to spend a little more and use rigid metal ducting and add insulation to it. And don't use duct tape (believe it or not)! Use metal foil tape for long lasting and leakproof joints. Over my years spent crawling around in attics and basements, I have seen many ducts partially or even fully disconnected due to old dried up duct tape (exacerbated by contractors being careless crawling around them). It does make for more comfy conditions on a cold day if you're working there but overall its a really bad idea for the homeowner's pocketbook.

 

[steve_bank]

I don't know about that, I used duct tape on leaky water pipes. ...

It is always handy to have a roll of duct tape around.

During the Apollo 13 mission, duct tape played a crucial role in keeping the three astronauts alive. It was used to create a makeshift filter system that allowed them to breathe by adapting square CO2 scrubber canisters from the command module to the circular filtration system of the lunar module. This ingenious solution, along with other available materials like plastic bags and hoses, helped to filter carbon dioxide and ensure the astronauts had breathable air for the journey home.

I wonder who decided to carry a roll of duct tape.

 

[thebeave]

I don't know about that, I used duct tape on leaky water pipes. ...

It is always handy to have a roll of duct tape around.

During the Apollo 13 mission, duct tape played a crucial role in keeping the three astronauts alive. It was used to create a makeshift filter system that allowed them to breathe by adapting square CO2 scrubber canisters from the command module to the circular filtration system of the lunar module. This ingenious solution, along with other available materials like plastic bags and hoses, helped to filter carbon dioxide and ensure the astronauts had breathable air for the journey home.

I wonder who decided to carry a roll of duct tape.

Duct tape is great stuff. Its just not good for ducts (on a long term basis). Dries out from age and heat, loses its adhesion and crumbles into dust. I'll bet the guy who thought to put a roll of duct tape on the Apollo spacecraft was a hero (one of many) on that mission. Funny how everyday objects were able to save Apollo missions. While on (or one the way to) the moon, a circuit breaker handle broke off which was crucial for blasting off the surface. Buzz had the idea to use the tip of his pen to pick at it and get it to switch. They probably would have had to swallow their secret cyanide suicide pills otherwise.

 

[Loren Pechtel]

Yup, and the warmer the air going through the ducts the more energy will be lost in the process.

Lost to where? If the ducts are heating the house via "losses", what is really being lost?

The heat coming from the floor and the vents, rather than just the vents, seems like it could actually be more comfortable for the occupants of the house.

I have never lived in a house where the air ducts did not at least partially face outside environment air. This place is actually done quite well--but still half the ducts are insulated tubes in the attic. I've never put a thermometer up there but attics typically are hotter than the environment in the summer.

 

[Loren Pechtel]

Yep. Its common these days to use insulated flexible ducting, and while more convenient and easier to install, the uneven surfaces creates much more air turbulence than rigid metal duct. Best to spend a little more and use rigid metal ducting and add insulation to it. And don't use duct tape (believe it or not)! Use metal foil tape for long lasting and leakproof joints. Over my years spent crawling around in attics and basements, I have seen many ducts partially or even fully disconnected due to old dried up duct tape (exacerbated by contractors being careless crawling around them). It does make for more comfy conditions on a cold day if you're working there but overall its a really bad idea for the homeowner's pocketbook.

One advantage of the flex stuff--sound decoupling. I do agree it would be better if it were metal at least for most of the run, though. (Although if I were ever designing a house the air handlers would be at ground level, not up in the attic.)

 

[Loren Pechtel]

I don't know about that, I used duct tape on leaky water pipes. ...

It is always handy to have a roll of duct tape around.

During the Apollo 13 mission, duct tape played a crucial role in keeping the three astronauts alive. It was used to create a makeshift filter system that allowed them to breathe by adapting square CO2 scrubber canisters from the command module to the circular filtration system of the lunar module. This ingenious solution, along with other available materials like plastic bags and hoses, helped to filter carbon dioxide and ensure the astronauts had breathable air for the journey home.

I wonder who decided to carry a roll of duct tape.

Duct tape is great stuff. Its just not good for ducts (on a long term basis). Dries out from age and heat, loses its adhesion and crumbles into dust. I'll bet the guy who thought to put a roll of duct tape on the Apollo spacecraft was a hero (one of many) on that mission. Funny how everyday objects were able to save Apollo missions. While on (or one the way to) the moon, a circuit breaker handle broke off which was crucial for blasting off the surface. Buzz had the idea to use the tip of his pen to pick at it and get it to switch. They probably would have had to swallow their secret cyanide suicide pills otherwise.

Well, it was a Space Pen!

But there's no reason they would carry cyanide. They are in a spacecraft, simply vent it to space, but not too quickly. It's completely painless, you won't even recognize it unless you've been trained to and even then you might not. (Knowing how to recognize it might give you a few seconds warning to grab your oxygen mask. Useless in space, but relevant in the world of military aviation.)

 

[bilby]

I've never put a thermometer up there but attics typically are hotter than the environment in the summer.

Do you use heating in the summer?

Where I live, the attic is fiercely hot in summer, but the rest of the house is cooled, not heated.

 

[steve_bank]

everal software solutions can model two-stage furnace systems, including DesignBuilder, Audits from Elite Software, Autodesk's CFD software, and Ansys Thermal Desktop. These tools allow for analysis of heat transfer, thermal comfort, and system performance, including the efficiency of two-stage furnaces operating at both high and low capacity settings.

There may be free software from furnace manufacturers and other sources. Try search on free cfd software.

CFD is computational fluid dynamics.

 

[Loren Pechtel]

I've never put a thermometer up there but attics typically are hotter than the environment in the summer.

Do you use heating in the summer?

Where I live, the attic is fiercely hot in summer, but the rest of the house is cooled, not heated.

Same ducts for heat and AC. The cooled air is going through ducts in a very hot space, that will lose some of the cool.

 

[bilby]

I've never put a thermometer up there but attics typically are hotter than the environment in the summer.

Do you use heating in the summer?

Where I live, the attic is fiercely hot in summer, but the rest of the house is cooled, not heated.

Same ducts for heat and AC. The cooled air is going through ducts in a very hot space, that will lose some of the cool.

None of our A/C is ducted. We have split systems in pretty much every room, so we only cool the spaces we are using.

The units can operate as heaters in reverse cycle mode too - but we almost never have cool enough weather to need it for more than a couple of nights a year.

 

[Jimmy Higgins]

I've never put a thermometer up there but attics typically are hotter than the environment in the summer.

Do you use heating in the summer?

Where I live, the attic is fiercely hot in summer, but the rest of the house is cooled, not heated.

Same ducts for heat and AC. The cooled air is going through ducts in a very hot space, that will lose some of the cool.

Florida has the AC in the attic too. Which seems really stupid. Can't make the house a little larger and put the thing both on the interior and with reasonably easy access to?

But according to this blog post, it is the best place to put the AC. They almost explain why.

???