Load calc question

[mike93lx]

I am working on replacements for my 1st and 2nd floor hvac systems. 3rd floor is staying as-is.

Current systems are both 2.5 ton ac with 75k btu 80% NG furnaces. my hvac guy is suggesting 2.5 ton heat pumps and 45k btu 80% furnaces

Only lived here for a year, but cooling and heating are both adequate. On the hottest days, the ac runs all day. I don't have heating data at hand, but I don't recall the heat ever running all out and it never had a problem keeping up

I bought a license for hvac-calc and input everything for my 1st and 2nd floor. Room sizes, heights, walls, windows, insulation, ducting. I looked up data from ASHRAE on design conditions and used 105 summer temp, -5 winter temp, 134 summer grains of moisture and a high daily range. target temps are 75 in the summer, 70 in the winter, with 55% RH

if i am reading this right, the program is suggesting that i could heat and cool the entire house with a single 2.5 ton a/c and a 48k btu heater, so a pair of those should be great, right? I'm confused as the current systems' cooling performance doesn't really seem to have much headroom

any advice?

 

[PWilks]

If your house is really leaky, the “infiltration” number could be much worse than the theoretical calculator is spitting out. Air movement also has an impact on insulation effectiveness, so your loss through the building shell via conduction would also be worse.

When was the house built?

 

[mike93lx]

If your house is really leaky, the “infiltration” number could be much worse than the theoretical calculator is spitting out. Air movement also has an impact on insulation effectiveness, so your loss through the building shell via conduction would also be worse.

When was the house built?

2003. Windows are decent quality andersen and the crawl is encapsulated, but no additional air sealing has been done.

I had wanted to get a blower door done, but never got around to it, so I have no idea what the ACH figure is

 

[PWilks]

2003. Windows are decent quality andersen and the crawl is encapsulated, but no additional air sealing has been done.

I had wanted to get a blower door done, but never got around to it, so I have no idea what the ACH figure is

Couple things. First, I originally missed you saying the heating has no issue heating up. This is typically because the units are grossly oversized for “maintaining” a temp. They need to be sized for large swings in heating (bringing a house and everything in them up to temp).

You also didn’t provide how large your house is, but I suspect given that you have dedicated units per floor, we’re dealing with a large house.

If I had to guess, infiltration is probably why your AC can’t keep up. The whole hard on for building efficiency and the desire for “air tight” houses only started in the past couple decades. There’s new construction houses built today that still leak like a siv. In my neck of the woods, 3 ACH on a blower door meets code.

 

[mike93lx]

Couple things. First, I originally missed you saying the heating has no issue heating up. This is typically because the units are grossly oversized for “maintaining” a temp. They need to be sized for large swings in heating (bringing a house and everything in them up to temp).

You also didn’t provide how large your house is, but I suspect given that you have dedicated units per floor, we’re dealing with a large house.

If I had to guess, infiltration is probably why your AC can’t keep up. The whole hard on for building efficiency and the desire for “air tight” houses only started in the past couple decades. There’s new construction houses built today that still leak like a siv. In my neck of the woods, 3 ACH on a blower door meets code.

The house is about 3500 sq ft.

The A/c is keeping up fine. It runs all day on the hottest day and maintains temp. I work from home, so I hold the desired Temps basically all the time for the occupied spaces. I tend to let the unoccupied spaces drop/rise just a few degrees, so there aren't big swings that the systems need to overcome.

I sent out an email to the guy that did my crawl space about having a blower door test done. Hoping the cost is reasonable. In MA, they were included in state-sponsored efficiency programs, but it doesn't look like VA offers that.

Edit, he responded saying it would be $400 but he feels it's a waste of money to get it just to confirm the load calc.

Maybe I play it safe and upsize the first floor? Or take a chance and go smaller on both...

 

[larry4406]

Can't help you on your sizing question but I hope you are upgrading to 90+ units assuming you can get the PVC flues outside.

Also, consider the dual fuel heat pump. It uses gas as backup heat when the heat pump can't keep up/looses efficiency when the temperature gets too cold.

We recently had all of the insulation removed from the attic of our 1987 constructed home. Installed about 5 inches of spray foam on top of the dryall and truss bottom cords over the entire space, then blew in 17-19" of cellulose insulation. Amazing difference. When the attic was purged it was quite clear how leaky the house was from all of the open holes/gaps at various penetrations for wires/pipes, etc.

 

[mike93lx]

Can't help you on your sizing question but I hope you are upgrading to 90+ units assuming you can get the PVC flues outside.

Also, consider the dual fuel heat pump. It uses gas as backup heat when the heat pump can't keep up/looses efficiency when the temperature gets too cold.

We recently had all of the insulation removed from the attic of our 1987 constructed home. Installed about 5 inches of spray foam on top of the dryall and truss bottom cords over the entire space, then blew in 17-19" of cellulose insulation. Amazing difference. When the attic was purged it was quite clear how leaky the house was from all of the open holes/gaps at various penetrations for wires/pipes, etc.

We are doing dual fuel, but dealing with the exhaust was holding me back on condensing units. The layout is terrible for it. 80% is so much easier and I expect that the heat pumps will pickup enough that the added savings of a 90+ will never pay back nor be worth the install hassle.

I would love to do a dual stage or variable speed unit with a super efficient furnace, but I can't justify the cost

The attic layout is a little weird . Two bedrooms are below finished space and the other two are below unfinished attic. I am going to do some work in the attic with added insulation and a radiant barrier, but I've had out an insulation/air sealing guy a couple times and his opinion has been that he'll gladly take my money for more work, but I'll be wasting it

 

[fitter30]

Contact your electric and gas company see if they offer a energy audit that includes a blower door test.

 

[mike93lx]

Contact your electric and gas company see if they offer a energy audit that includes a blower door test.

Nothing that I can find online. Dominion has some incentives but they are incredibly low value.

 

[PWilks]

I think the short answer is, if your AC is running continuous on a really hot day, it’s really close to being sized right on the money. I would guess that the difference between what you’re seeing is due to the online calculator making some assumptions, and your leakage higher than the program assumed.

 

[mike93lx]

I think the short answer is, if your AC is running continuous on a really hot day, it’s really close to being sized right on the money. I would guess that the difference between what you’re seeing is due to the online calculator making some assumptions, and your leakage higher than the program assumed.

If I bump the ach to 1.5, it puts me right at the capacity of the current cooling setup. The heating numbers are right on the edge of what's being proposed. I'm thinking it will be prudent to upsize the first floor to allow a little headroom.

 

[PWilks]

If I bump the ach to 1.5, it puts me right at the capacity of the current cooling setup. The heating numbers are right on the edge of what's being proposed. I'm thinking it will be prudent to upsize the first floor to allow a little headroom.

I really doubt the savings between a 2.5 ton and 3 ton AC are worth it on the off chance you get a hotter than design day and you’re stuck in an 85 degree house for the entire evening.

We’ve rented for the past 5 months while our house is being built and the engineer who designed our building grossly undersized it for our unit. There’s days when our 1300 sqft apartment has been 10 degrees above set point running balls to the wall all day and never can catch up.

If you’re going to replace what you currently have, I would recommend keeping the same capacity. If you ever get around to air sealing your house better, you’ll benefit from shorter run times (hopefully longer equipment life) and save whatever you would have on the first cost between unit sizes on electricity.

 

[mike93lx]

I really doubt the savings between a 2.5 ton and 3 ton AC are worth it on the off chance you get a hotter than design day and you’re stuck in an 85 degree house for the entire evening.

We’ve rented for the past 5 months while our house is being built and the engineer who designed our building grossly undersized it for our unit. There’s days when our 1300 sqft apartment has been 10 degrees above set point running balls to the wall all day and never can catch up.

If you’re going to replace what you currently have, I would recommend keeping the same capacity. If you ever get around to air sealing your house better, you’ll benefit from shorter run times (hopefully longer equipment life) and save whatever you would have on the first cost between unit sizes on electricity.

I didn't mean the a/c. I am fine with the capacity I have.

The issue is heating. I have two 75k btu NG units and my hvac guy feels they are way oversized.

What I have now is all original and I have no confidence that sizing was really considered. An oversized heater doesn't present problems like an oversized cooling unit would. I wouldn't be surprised if the builder used the setup I have in all of the houses they built in the area.

 

[jar944]

FWIW I'm running one (of the two) 3 ton units and a 80k furnace for 5700 conditioned sqft a bit north of you. Two 3 ton units are grossly oversized for my house as just the one cools fine except for the 100 degree days.

 

[mike93lx]

FWIW I'm running one (of the two) 3 ton units and a 80k furnace for 5700 conditioned sqft a bit north of you. Two 3 ton units are grossly oversized for my house as just the one cools fine except for the 100 degree days.

The 2.5 ton a/c's are definitely the right size for my house as-is. If I conditioned the attic, better insulated the crawl and did a bunch of air sealing, maybe those could go down, but none of that is in the cards for now. A single unit would never get it done

Heating is the big question.

 

[PWilks]

The 2.5 ton a/c's are definitely the right size for my house as-is. If I conditioned the attic, better insulated the crawl and did a bunch of air sealing, maybe those could go down, but none of that is in the cards for now. A single unit would never get it done

Heating is the big question.

I would personally put in two units at 60k. When you mess with the ACH, what heating output does it give when the ACH makes the cooling load equal to your existing unit size, if that makes sense

 

[mike93lx]

I would personally put in two units at 60k. When you mess with the ACH, what heating output does it give when the ACH makes the cooling load equal to your existing unit size, if that makes sense

82k at negative 5 degrees ambient, 70 degree interior temp

 

[mike93lx]

Just found some interesting data.

This is from my 2nd floor nest. January had the only days this past winter with heating over 5 hrs on any individual day. Those two days at just under 8 hrs were about 23 degrees out. So maybe not the best gauge of design size?

I'm going to plug my 1st floor nest back in and see what it shows

 

[jar944]

The 2.5 ton a/c's are definitely the right size for my house as-is. If I conditioned the attic, better insulated the crawl and did a bunch of air sealing, maybe those could go down, but none of that is in the cards for now. A single unit would never get it done

Heating is the big question.

I think this place leaks like a sieve as far as ACH (2007 k hovanin construction) I guess considering how well just running one unit cools, maybe it doesn't leak as bad as I thought.

My 80% 80k furnace runs a lot, almost continuously on cold winter nights (days have significant solar gain.) It's so much that guests comment about it never shutting off.

 

[mike93lx]

Here's January on my first floor

If we assume the run time as all during the day, then it ran for about half of the time

 

[mike93lx]

I think this place leaks like a sieve as far as ACH (2007 k hovanin construction) I guess considering how well just running one unit cools, maybe it doesn't leak as bad as I thought.

My 80% 80k furnace runs a lot, almost continuously on cold winter nights (days have significant solar gain.) It's so much that guests comment about it never shutting off.

My units are reasonably well "hidden", other than to my son. The 2nd floor unit is right above his bedroom and the 1st floor is right below. But neither are particularly loud.

This nest data and the load calc is leading me to something in the 60-75k range for the first floor and dropping the second floor to 45k

 

[PWilks]

I’m just really shocked that you’re getting that kind of runtime (heat loss) with only a 50 degree delta.

My parents in the suburbs of Minneapolis have a 120k 80% furnace that’s pushing 20 years and they see maybe 50% runtime even on our coldest windy days.

 

[mike93lx]

I’m just really shocked that you’re getting that kind of runtime (heat loss) with only a 50 degree delta.

My parents in the suburbs of Minneapolis have a 120k 80% furnace that’s pushing 20 years and they see maybe 50% runtime even on our coldest windy days.

Have you looked at the data though? This is for the whole day, including when we are sleeping, so without it, I'd guess run time was higher. Most of those days are 4-6 hours run time for each floor

 

[danski0224]

The house is about 3500 sq ft.

The A/c is keeping up fine. It runs all day on the hottest day and maintains temp. I work from home, so I hold the desired Temps basically all the time for the occupied spaces. I tend to let the unoccupied spaces drop/rise just a few degrees, so there aren't big swings that the systems need to overcome.

I sent out an email to the guy that did my crawl space about having a blower door test done. Hoping the cost is reasonable. In MA, they were included in state-sponsored efficiency programs, but it doesn't look like VA offers that.

Edit, he responded saying it would be $400 but he feels it's a waste of money to get it just to confirm the load calc.

Maybe I play it safe and upsize the first floor? Or take a chance and go smaller on both...

By definition, if the piece of equipment is sized correctly, it will operate all of the time at design temperature and maintain the designed setpoint.

This also means that when the outdoor temperature is greater than the design temperature, the equipment will still operate all of the time, but the conditioned space will warm up or cool down accordingly.

Knowing the infiltration value will get the load calcs in line.

That data (the blower door process) will also point to places that may be able to be sealed up effectively, which will save money long term. Not all flaws/leaks can be corrected effectively.

There is (allegedly) a fudge factor built into the Manual J system.

The problem with running things right on the line of calculated sizes crops up when the assumptions are wrong. The biggie is building leakage.

The quality of the construction of your windows doesn't mean squat. What matters is how they are attached to the building and the rough opening. This is stuff that you can't see without removing trim, but the blower door, smoke pen and thermal camera can see. I have seen daylight under a window after removing interior trim.

Right behind that is improperly installed ductwork. This encompasses bad design/installation with respect to airflow (square throat elbows without turning vanes, cap and tap connections, etc.) and ductwork that is undersized (the 6" round residential duct does NOT flow 100 CFM like many assume).

Figuring out the enthalpy of your current AC system will tell you what the actual operating capacity is. Improper refrigerant charge, wrong metering device and poor airflow will all kill enthalpy. Less than $1000.00 of "smart probes" and a phone/tablet app can generate this data in less than 10 minutes.

I recall a Honeywell system that required a laptop and was well over $10k.

But, even if the HVAC tech knows how to do all of this stuff, the general public does NOT want to pay for it. I have someone that regularly ******* about comfort issues on the second floor, and at the same time, refuses to pay for additional diagnostics. So, nothing gets done.

So, the cheap fix is to pick the next size up from the load calc and hope for the best when the results are "close" to an equipment size break. The big problem here is if the ductwork is undersized for the existing system, it will be much worse for the bigger system. Static pressure and horsepower changes are not linear.

Energy costs from comfort heating and cooling only consume ~45%-~55% of all annual energy dollars, so there's no sense in doing it right

 

[yeldogt]

I have said this before ...... my new place in PA is over 4000sf and is heated with a single 60k boiler. And -- it is a converted church with 14' 25' and some 35' ceilings. Plus some exposed stone walls ...... the difference is the spray foam. 3T will keep it cool -- except the loft where there is a mini. I also have a hidden mini that will blow out the kitchen when I have a party.

Air infiltration can be as much of a problem or more than insulation -- especially with units in unconditioned spaces. Not only do they suffer from just being in hot or cold areas of the house -- the blowers are creating pressure imbalances that can move unconditioned air into and push conditioned air out of the house.

When your house was built there were few small capacity heaters .... manufactures made what sold. HVAC installers went big with unit and small with ductwork. Now there are at least some smaller heaters available ... energy audits and specific codes have opened a market for them.

Remember --- single speed AC units (condensers) come in 1/2 ton -- but the air handlers and related equipment is normally full ton with dip switches for the blowers.

If any of your current heaters are duel stage with control boards -- they will have some logic built in that the nest will screw with.

As in another post. Why not wait for all the new stuff coming and the rebates and other related things you can take advantage of ? A fully modulating furnace and multi speed condenser for the biggest part of the house will add a lot of comfort .. yes you may pay 5/6k more for it.

The vast majority of HVAC people have never lived in a house with great equipment. The -- this is what I have in my house reply is often said. The owners all have the best stuff ...

 

[PWilks]

Right behind that is improperly installed ductwork. This encompasses bad design/installation with respect to airflow (square throat elbows without turning vanes, cap and tap connections, etc.) and ductwork that is undersized (the 6" round residential duct does NOT flow 100 CFM like many assume).

You went on a long tangent here, but a 6” Home Depot round will easily do 100 cfm if you don’t put a bunch of garbage flex duct somewhere in the run. A decently manufactured 6” round that is “average” in smoothness can easily do 100 cfm at less than .08” h2o / 100’ of duct. For a 10’ straight run off of a bathroom fan, this is less than a hundredth of an inch of static.

I suggest that @mike93lx should just stick with what he’s at on the cooling side capacity, and 60k or larger on the heating side.

Infiltration is the issue and if you aren’t able to currently address it, it’s better to be conservative in your equipment sizing.

 

[mike93lx]

I have a lot of flex. Other than the main trunk, it's all flex.

I decided to do 2.5 ton heat pumps with a ~70k furnace on the 1st floor and a ~45k furnace on the second. Returns will be upgraded but I am leaving ducting alone for now. I'm going to do some work in the attic with insulation and a radiant barrier, but I would like to chase some air sealing soon as well

 

[jlv03]

You mention a heat pump in the first post as part of the HVAC upgrade. I replaced my first floor unit a few years ago and went with a heat pump and a two stage gas furnace (96%). I didn't understand the need for 2 stages of gas with the heat pump, but it was my understanding that it came along with the package to get the variable speed blower that I really wanted. Turns out the two stage is nice for the defrost cycle, as it cranks up to the high stage when the outside unit is defrosting, preventing a huge blast of cold air out of the vents.

If you do downsize don't discount considering a two stage as well (even though you will ultimately have 3 stages once you factor in the heat pump).

 

[yeldogt]

You mention a heat pump in the first post as part of the HVAC upgrade. I replaced my first floor unit a few years ago and went with a heat pump and a two stage gas furnace (96%). I didn't understand the need for 2 stages of gas with the heat pump, but it was my understanding that it came along with the package to get the variable speed blower that I really wanted. Turns out the two stage is nice for the defrost cycle, as it cranks up to the high stage when the outside unit is defrosting, preventing a huge blast of cold air out of the vents.

If you do downsize don't discount considering a two stage as well (even though you will ultimately have 3 stages once you factor in the heat pump).

Most systems don't run the heat pump and the gas furnace at the same time. This will over heat the coils of the evaporator. Duel fuel is using one or the another based on what fuel is cheaper based on outside temp. Heat pumps can be less expensive than NG when it warmer out and one has cheap electric rates. Some will go on for a short time when defrosting

Multi stage furnaces are very common today -- many are fully modulating. variable speed blowers can match the furnace output and provide just enough heat

 

[mike93lx]

I'm not doing a variable speed or multistage setup, and as much as I would like to go with a super efficient condensing furnace (I really do want to cut back gas usage), the install and cost were two big road blocks. The equipment location in the house and how the venting would work was really unappealing.

I am doing a simple, single stage, dual fuel setup. Hopefully it all works well

 

[yeldogt]

I'm not doing a variable speed or multistage setup, and as much as I would like to go with a super efficient condensing furnace (I really do want to cut back gas usage), the install and cost were two big road blocks. The equipment location in the house and how the venting would work was really unappealing.

I am doing a simple, single stage, dual fuel setup. Hopefully it all works well

If you have to replace something .... IE your AC unit that keeps tripping. You have to replace it ... I get that -- although the need has now past for the season.

There are a lot of changes coming in the HVAC world and with supply changes and a still tight market things are currently expensive. I really do think we will be in a better market next year -- product and labor. Lots of new products.

You have only been in the house for a year and with your SF -- something is up because you have three units where in heat mode -- one should get you the temps. You will be spending a lot of money for very little upside (maybe some quieter condenser)

Replacing like equipment typically has no payback other then you now have new equipment

 

[mike93lx]

If you have to replace something .... IE your AC unit that keeps tripping. You have to replace it ... I get that -- although the need has now past for the season.

There are a lot of changes coming in the HVAC world and with supply changes and a still tight market things are currently expensive. I really do think we will be in a better market next year -- product and labor. Lots of new products.

You have only been in the house for a year and with your SF -- something is up because you have three units where in heat mode -- one should get you the temps. You will be spending a lot of money for very little upside (maybe some quieter condenser)

Replacing like equipment typically has no payback other then you now have new equipment

I don't want the latest and fanciest. I want simple, reliable and some reduced gas usage from the heat pumps.

The first floor hasn't tripped since I swapped the nest, so it appears that was the culprit and I am definitely not past this season's need for cooling.

Maybe materials will drop next year, but wha Tia also coming next year is a increase in efficiency requirements, which will drop off these less expensive systems. If I wait til next year, I am confident that it would not be cheaper and would almost certainly be more expensive.

I have 20 year old hvac that is running at fla. It might last many more years, it might fail next year. I am comfortable with giving up on a few more years of life to control my downtime (knowing that new doesn't guarantee reliability)

 

[yeldogt]

I don't want the latest and fanciest. I want simple, reliable and some reduced gas usage from the heat pumps.

The first floor hasn't tripped since I swapped the nest, so it appears that was the culprit and I am definitely not past this season's need for cooling.

Maybe materials will drop next year, but wha Tia also coming next year is a increase in efficiency requirements, which will drop off these less expensive systems. If I wait til next year, I am confident that it would not be cheaper and would almost certainly be more expensive.

I have 20 year old hvac that is running at fla. It might last many more years, it might fail next year. I am comfortable with giving up on a few more years of life to control my downtime (knowing that new doesn't guarantee reliability)

Well -- you are going to have a sizable expense replacing two systems that may or may not need to be replaced .... with more that one in the house -- even if you are unlucky and it dies -- you get it replaced. People get systems replaced all the time.

Seems the attic one is the most problematic ...

I have alway found that living in a house for a while is a good idea ..... if you fix the leaks and add insulation in the future you will be dropping the load needed.

As I said in another thread -- I bought a vacation property with older equipment spring of 2020. Going down there next month for a few weeks. I did price out systems with a local guy just in case one dies --- but ... I want to do a bit more looking around as I'm not positive there is not a better way

 

[mike93lx]

Well -- you are going to have a sizable expense replacing two systems that may or may not need to be replaced .... with more that one in the house -- even if you are unlucky and it dies -- you get it replaced. People get systems replaced all the time.

Seems the attic one is the most problematic ...

I have alway found that living in a house for a while is a good idea ..... if you fix the leaks and add insulation in the future you will be dropping the load needed.

As I said in another thread -- I bought a vacation property with older equipment spring of 2020. Going down there next month for a few weeks. I did price out systems with a local guy just in case one dies --- but ... I want to do a bit more looking around as I'm not positive there is not a better way

The attic one hasn't given me any problems.

Replacing anything before it fails is not ideal from a pure cost perspective and I don't know why that seems to be such a controversial stance. Do you run everything to failure? Of course not.

I work from home and live in a climate where a/c is critical, IMO. Worrying about an a/c issue is not something i want to deal with.

 

[yeldogt]

The attic one hasn't given me any problems.

Replacing anything before it fails is not ideal from a pure cost perspective and I don't know why that seems to be such a controversial stance. Do you run everything to failure? Of course not.

I work from home and live in a climate where a/c is critical, IMO. Worrying about an a/c issue is not something i want to deal with.

I thought you had a thread on the attic one? Maybe one for your office (not you?)

I'm proactive .. sure. It seems AC units either die in 8 years or rust away. Have never had an old one die ... one leaked. Most times the heater goes and the whole thing gets replaced

 

[mike93lx]

I thought you had a thread on the attic one? Maybe one for your office (not you?)

I'm proactive .. sure. It seems AC units either die in 8 years or rust away. Have never had an old one die ... one leaked. Most times the heater goes and the whole thing gets replaced

It's the first floor unit that was tripping the breaker. That furnace/coil is in the garage. The second floor and third floor furnaces/coils are in the attic.

I have never owned a central air system, so I'm learning this as I go. The furnaces are all original as well and are being replaced.

 

[yeldogt]

The first class on AC tells you ------- most of the east coast w/ humidity you don't want to oversize AC -- humidity is a key factor in comfort .. and in partial use capacity.

80% furnaces are very simple devices -- normally the heat exchanger fails. Some have more of a history of going -- not hard to tell. Leaks from the AC units can hurt them. Not much to them ....

Units in the garage have the same issue as an unconditioned attic ... not as bad as far as heat/ cool -- but they change the pressure in the house if the ducts leak. Make sure sealed well.

 

[danski0224]

I don't want the latest and fanciest. I want simple, reliable and some reduced gas usage from the heat pumps.

Heat pumps trade gas usage for electric usage.

As the outdoor temperature reaches the thermal balance point, the heat pump will be "on" all of the time. It also delivers cooler air than fossil fuel or electric strip heat. Note that the thermal balance point may not be the same as the economic balance point. This needs to be checked regularly as utility costs change.

Some people do NOT like the constant fan, some do not like the cooler discharge temperature.

Properly sized ducts and an ECM blower will save money over a regular blower motor.

A variable speed fan (which is also ECM) can overcome some ductwork shortcomings to deliver the set airflow, to a point. The other edge on that sword is because the variable speed motor can adjust RPM to maintain set airflow, it draws more electricity as it cranks RPM to overcome poor ductwork.

A plain old multi tap ECM motor cannot adjust speed. As static pressure goes up, airflow drops. I'm not sure if it is possible to get a furnace without an ECM blower motor any longer.

Given the life expectancy of HVAC equipment, going with a two stage 80% AFUE unit with a constant airflow variable speed motor will be the best choice for the expected lifespan. You will also gain significant comfort benefits on the cooling side if your thermostat supports it, from the variable speed motor.

The cost difference between an otherwise identical 80% AFUE and a 92%+ AFUE installation based on perceived fuel savings alone will NEVER pay off.

The two stage furnace is for comfort and longer run times on the lower stage in the shoulder seasons to even out indoor temperatures. It will NOT save money on fuel (at least no one should promise that).

For the most part, residential heating and cooling equipment is a commodity. Outside of new things manufacturers try to save them money and increase their profits, the equipment has lots of the same stuff inside across brands. Failures can usually be traced to poor installation practices much more often than being bad out of the box.