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Old 07-16-2018, 10:01 PM   #31
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Re: Replacing my HVAC unit


Quote:
Originally Posted by user_12345a View Post
90%+ does have other advantages, like sealed combustion when done as 2-pipe and less noise from the burners/inducer. Either there's a burner box or the access panel is sealed off.

Time to go read up on this type of gas furnace.
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Old 07-17-2018, 01:04 PM   #32
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Re: Replacing my HVAC unit


There's a few points i'd like to make

1. a gas furnace (80% or 95%) does not lose efficiency running on stage 1 vs stage 2.
In fact, some are slightly more efficient on stage 1 due to running at a lower delta T. you can actually make an 80% furnace more efficient by increasing the air flow to the point of condensing in the heat exchanger, but then they rust out and fail. the nameplate rating is a very specific test under certain operating conditions.

the average furnace/AC is oversized, so running in stage 1 might run long enough to get to nameplate efficiency before it cycles off. longer runtimes generally = closer to rated efficiency

a 95% furnace MUST maintain condensing operation (effectively its efficiency) or the discharge gas temp will exceed the rating of plastic components, and it will shut down on a safety trip.



1a. the point of 2 stage/variable systems is to match the load. it's like the gas pedal in your car. would you want to drive down the freeway flooring it, then turning off the engine, or holding your foot where you need it to keep driving steadily? this is the difference between on/off (single stage) and multi stage gear.

my favorite benefit of stage 1 on my furnace (40/60k 80%), is how QUIET it is in stage one. it's basically inaudible. I have a thermostat that saves logs of runtime and temp sensors (honeywell prestige), and on the coldest days of the year, it runs stage 1 100% overnight, and cycles stage 2 on and off. at no point in time does stage 2 run 100% for a 24 hour period.

given the age of my furnace (15 years or so), the fact that it's an 80% unit, and the previous owners removed ductwork during remodels, and i need new windows/more insulation, i may end up installing a smaller furnace down the road.


2. you may still be able to install a condensing furnace, even if you can't go out the closest wall. obviously you have HOA issues to deal with, but plenty of people in my neighborhood used their chimney as a chase for the PVC. so long as you slope and size the pipe correctly, you can run quite a distance with PVC. you don't have to have the "ugly pipes" associated with high efficiency systems, they make more pleasing concentric, decorative bulkhead vents that you could use.

3. be aware that AC systems are "nominal" sizing. with the same indoor setpoint, a 24000 BTU system has more capacity when it's 75F out than when it's 105F out. most are actually smaller than rated at 105F (i.e. a 24,000 BTU unit may only be 19,000 BTU
it is possible that a newer, higher SEER system may also have a bigger outdoor coil vs an older unit, which may effectively give it more capacity vs your old system, even though the rating is the same. you'll have to look at the performance curves for the equipment to find this out.


4. yes, multi stage A/C systems aren't sold in half sizes. keep in mind that you get 2/3 capacity on low, and full capacity on high. a 4 ton would get you 2.6 tons on low, 4 tons on high. a 3 ton would get you 2 tons low, 3 tons high. if your ductwork isn't big enough to flow 4 tons, then you'll get extra noise and lower efficiency - this also means you wont get 4 tons of cooling, but you will have paid for it.

personally I'd strongly consider the 3 ton two stage. running longer on stage 1 will help keep humidity in check, even with a bigger coil (all the more reason NOT to oversize it). it will also be slightly quieter if someone bothers to set the airflow properly.

you can probably make up that half ton by investing in the envelope (improving insulation, air sealing, low-E windows, planting a tree that blocks sun, closing a shade on the sunny side of the house, etc). remember, you pay for envelope upgrades ONCE, you pay to run your heat/AC every time you use it.

for example, i have a toy thermal camera that plugs into my phone, and i found this upstairs - that's money being wasted!


my poorly insulated 1950s house has an old single stage 2 ton, 10 SEER, 700sqft/ton (excluding finished basement), and maintains setpoint so long as it's not over 95F out and full sun. beyond that it creeps up about 2F in the room the thermostat is located in (which has inadequate supply ducts). given that I need new windows upstairs, and two rooms don't have supply ducts at all, I see no logical reason i would consider a larger system. given how bad my ductwork is i doubt i'm getting all two tons, even though I maintain it (cleaning the outdoor coil regularly, keeping my indoor air filter in good shape, making sure registers are open, etc). if I replace it I'll likely get a 2 stage 2 ton.
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Old 07-17-2018, 02:19 PM   #33
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Re: Replacing my HVAC unit


Quote:
Originally Posted by u3b3rg33k View Post
...
Thanks for your time and your in-depth reply.

--

"a gas furnace (80% or 95%) does not lose efficiency running on stage 1"

That's interesting. Would you mind explaining why this is? Mechanical system are just higher efficiency when it's not running at 100% of their rated rating? If that's the case, then does it make sense to buy a system that is 30% bigger than the capability of the duct work and always run it at 70% capacity?

--

"a 95% furnace MUST maintain condensing operation (effectively its efficiency) or the discharge gas temp will exceed the rating of plastic components, and it will shut down on a safety trip."

Would you mind explain a bit more about the "maintain condensing operation..." part? Looking over the Trane A/C unit offerings, all 80% units are NON-condensing. All 90% units are CONDENSING. Does this mean that in order to get to 90%+ efficiency the entire heat workflow is more complicated? Are there more parts in a 90%+ efficiency? If so, does it mean that the 90%+ can break down more frequenly, and maybe have a shorter life span compare to 80%? Does 90% unit of the same size as the 80% unit cost more?

--

With regards to HOA issue and the 90% gas furnace, yeah, I'm really looking into this issue. Going to be talking to the consultants again.

--

I live in Virginia USA. Annual median temperature range is 87*, and 26* in January.

I only really use the A/C during 2 months out of the year. Our comfortable temperature is 77*. Thus I am planing to only get a 1-stage A/C and a 2-stage gas furnace. Does it make sense to you to go with a 1-stage A/C for my specific need?

--

"personally I'd strongly consider the 3 ton two stage."

Yeah it really sucks and confusing when different consultant comes out and give different estimate. I have about 10 comes out and the majority of them just size the system the same as the current system 3.5 ton. They don't even bother doing their own load calculation. The two that does do load calculation. One come back with 3.5 ton, and the other come back with 3 ton. So based on only those two numbers, there's no way to say which one has the correct load calculation. If I had more consultant doing their own load calculation, I would have a better idea of what my house would required.


---

I also have a FLIR. Cost me $400. At that price, it's no toy for me. :-)
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Old 07-17-2018, 03:22 PM   #34
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Re: Replacing my HVAC unit


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Originally Posted by 012abc View Post
Thanks for your time and your in-depth reply.

--

"a gas furnace (80% or 95%) does not lose efficiency running on stage 1"

That's interesting. Would you mind explaining why this is? Mechanical system are just higher efficiency when it's not running at 100% of their rated rating? If that's the case, then does it make sense to buy a system that is 30% bigger than the capability of the duct work and always run it at 70% capacity?
I think the question is why would it change? barring very small changes in efficiency for fans running at different speeds, the fuel consumption (BTU in) to work done (hot air out, BTU out) ratio shouldn't change.

when my 40/60k BTU input 80% system runs at stage 1, I expect to get 32kBTU/hr delivered. at 60k input, i expect to get 48k delivered.

if I had a 40k 80% single stage gas furnace, i would expect to get 32kBTU/hr delivered.
if I had a 60K 80% single stage gas furnace, i would expect to get 48kBTU/hr delivered.

if either of those furnaces delivered less than their rated delivery, something is wrong with the furnace and it needs repair. if mine does not deliver those numbers in the respective stages, then something is wrong with mine and it needs repair.

all I have is both of those furnaces in one box.

it never makes sense to buy a bigger system than your ductwork can support, because it will cost more up front and you would have to lock out stage 2. in effect, you would have heavily overpaid for a single stage system rated for the lower output.

does that answer your question?



Quote:
Originally Posted by 012abc View Post
"a 95% furnace MUST maintain condensing operation (effectively its efficiency) or the discharge gas temp will exceed the rating of plastic components, and it will shut down on a safety trip."

Quote:
Originally Posted by 012abc View Post
Would you mind explain a bit more about the "maintain condensing operation..." part? Looking over the Trane A/C unit offerings, all 80% units are NON-condensing. All 90% units are CONDENSING. Does this mean that in order to get to 90%+ efficiency the entire heat workflow is more complicated? Are there more parts in a 90%+ efficiency? If so, does it mean that the 90%+ can break down more frequenly, and maybe have a shorter life span compare to 80%? Does 90% unit of the same size as the 80% unit cost more?
my point is that if you're burning fuel, the energy has to go somewhere. if you reduce the efficiency of a furnace, that means you're delivering less energy to the home, which means you're putting less energy into the air going through the furnace.
if you're burning 60kBTU of fuel at 95% efficiency, 57kBTU gets delivered to the ductwork. if you drop that to 85%, 51kBTU gets delivered to the ductwork. that 9kBTU has to go out the exhaust pipe.
I don't think you can get 9kBTU out a PVC pipe with the fan that comes on the furnace (remember it isn't going to suddenly start moving more air), and 9k BTU is roughly two hair dryers on high (4-5kBTU each). once you have enough heat in the exhaust gasses, they will be too hot to condense the moisture out, which means you're now putting even hotter air through parts not rated for that temperature. the safety switches will turn it off before this point.


Quote:
Originally Posted by 012abc View Post
With regards to HOA issue and the 90% gas furnace, yeah, I'm really looking into this issue. Going to be talking to the consultants again.
Quote:
Originally Posted by 012abc View Post
I live in Virginia USA. Annual median temperature range is 87*, and 26* in January.

I only really use the A/C during 2 months out of the year. Our comfortable temperature is 77*. Thus I am planing to only get a 1-stage A/C and a 2-stage gas furnace. Does it make sense to you to go with a 1-stage A/C for my specific need?
that depends on your requirements and the price. if you want to not hate being indoors, and save money, sure. if the price of a 2 stage is marginally more, maybe not. that's up to you. given that you already have a 2 stage gas furnace, you probably already have a 2 stage thermostat, and the associated wiring in place to operate it as a 2 stage unit. so the only additional cost is the outdoor unit and wire running to it. that brings the price down.

Quote:
Originally Posted by 012abc View Post
--

"personally I'd strongly consider the 3 ton two stage."

Yeah it really sucks and confusing when different consultant comes out and give different estimate. I have about 10 comes out and the majority of them just size the system the same as the current system 3.5 ton. They don't even bother doing their own load calculation. The two that does do load calculation. One come back with 3.5 ton, and the other come back with 3 ton. So based on only those two numbers, there's no way to say which one has the correct load calculation. If I had more consultant doing their own load calculation, I would have a better idea of what my house would required.
you can do your own load calc, and/or look at your energy bill average change for those months. figure out how much time it runs vs average temperature and you can get an idea of how much it runs per hour on average.

Quote:
Originally Posted by 012abc View Post
I also have a FLIR. Cost me $400. At that price, it's no toy for me. :-)
[/quote]
it's a toy in that i don't really need it, but it sure is fun to have. it's probably paid for itself by now in energy savings.
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Old 07-17-2018, 04:05 PM   #35
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Re: Replacing my HVAC unit


You can run into problems getting airflow to the top floor with the 2-stage or variable capacity a/c so be careful. Properly sized 1-stage can do a pretty good job and run long cycles.

The 90%+ furnaces have the burners at the top, a secondary heat exchanger and condensate handling parts - trap, tubing, etc. They don't have more mechanical/electronic stuff to bread.

The secondary heat exchanger is at the bottom where the cool return air enters and most of the remaining heat gets transferred to the air.

They're called condensing because the exhaust gets cooled down to the point where the moisture produced by combustion condenses into a liquid.

In doing so, the latent heat in the moisture is released and goes towards heating the house.

This is old technology, came out in the 1980s.
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Old 07-17-2018, 05:50 PM   #36
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Re: Replacing my HVAC unit


A gas furnace running in first stage IS less efficient than when it is running in second stage. Even if it has a VS inducer to reduce the O2 in the combusted gas.


Part of the heat transfer process in the furnace from the gas flame to the heat exchanger is from radiant heat from the gas flame. You lose more of the radiant heat than increasing the blower speed can/could ever regain.


Saving from a 2 stage furnace comes from being able to set the thermostat a degree or 2 lower and still be comfortable as you were at the higher setting with the old single stage furnace.
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Old 07-17-2018, 05:53 PM   #37
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Re: Replacing my HVAC unit


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Originally Posted by user_12345a View Post
...
Ok so after doing some research, my initial conclusion for my home is to NOT get a 90%+ efficiency gas furnace and just stick with 80% efficiency. Below are my reasons. Please correct me if I am wrong.

---

Benefits:

- 10 - 15% more efficient when it comes to converting burn gas to heat the home. Gas where I live isn't that expensive. I pay $140 during the coldest month (February) of the year.

- Air is less dry compare to a 80%.

---

Drawbacks:

- Purchase cost of 90% is higher compare to a 80% of a similar sized unit.

- One time cost to run two PVC pipes to the rear of my home. Cut up some dry wall ceiling. Cost to patch up drywall ceiling. Gotta submit application for HOA approval. Not that big of a deal, but more work.

- More parts. Thus system is more complex. Thus more stuff are likely to go wrong. Thus requiring more maintenance.

- Requires more maintenance. I spoke with a HVAC consultant that he said the 90% needs to be fix / maintain more often compare to a 80%.

- 2 heat exchangers on the 90% instead of 1 on the 80%. When the heat exchanger the cracks, it's just best to replace the entire gas furnace because the cost of replacing the cracked heat exchanger is high. So having 2 heat exchangers would increase the likely hood that a heat exchanger would be cracked. If one of the two heat exchanger is cracked, it's best to replace the entire gas furnace.

---

So to summarize, the cost (time and money) for a 90% is not worth getting over a 80% because you will never make up that cost throughout the lifetime of the system.
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Old 07-17-2018, 05:59 PM   #38
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Re: Replacing my HVAC unit


Quote:
I spoke with a HVAC consultant that he said the 90% needs to be fix / maintain more often compare to a 80%.

A plain Jane 90% plus furnace doesn't require anymore frequent maintenance than an 80%. You have a trap that should be cleaned once a year, that an 80% doesn't have.
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Old 07-17-2018, 06:16 PM   #39
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Re: Replacing my HVAC unit


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A gas furnace running in first stage IS less efficient than when it is running in second stage. Even if it has a VS inducer to reduce the O2 in the combusted gas.


Part of the heat transfer process in the furnace from the gas flame to the heat exchanger is from radiant heat from the gas flame. You lose more of the radiant heat than increasing the blower speed can/could ever regain.


Saving from a 2 stage furnace comes from being able to set the thermostat a degree or 2 lower and still be comfortable as you were at the higher setting with the old single stage furnace.
I have questions:
can you put up some pudding to bear out your claims of reduced efficiency? like a DOE study? mfg data?
what is the efficiency of an 80% furnace in stage 1? are you saying it's under 80%?
what is the efficiency of an 80% furnace in stage 2? more than 80%?

why would a smaller flame result in lower efficiency of radiant heat transfer to the inner wall of a heat exchanger? you have a BETTER surface area to volume ratio with what is now an oversized heat exchanger AND longer dwell time for the combustion gasses. basically the same tricks they pull to get a higher SEER rating from an AC.

please note that I have made no claim that a 2 stage furnace will save any money.

I also don't believe the inducer fan is there to reduce O2 in the gas - that's what tuning the main valve is for. the inducer fan is there because inshot burners require assisted draft.
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Old 07-17-2018, 06:23 PM   #40
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Re: Replacing my HVAC unit


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I have questions:
can you put up some pudding to bear out your claims of reduced efficiency? like a DOE study? mfg data?
what is the efficiency of an 80% furnace in stage 1? are you saying it's under 80%?
what is the efficiency of an 80% furnace in stage 2? more than 80%?

why would a smaller flame result in lower efficiency of radiant heat transfer to the inner wall of a heat exchanger? you have a BETTER surface area to volume ratio with what is now an oversized heat exchanger AND longer dwell time for the combustion gasses. basically the same tricks they pull to get a higher SEER rating from an AC.

please note that I have made no claim that a 2 stage furnace will save any money.

Testing them is how I know. Using a combustion analyzer and clocking the gas meter.


While you have greater heat exchanger surface, you don't have a lot of radiant heat, to transfer. The flame being much smaller, means that there is a greater distance for the little bit of radiant you have to travel. Which allows for more or the excess combustion air to cool the heat exchanger down some.


A 80% in first stage may only be 86% efficient, depending on brand/manufacturers design. But 80 % in second stage.
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Old 07-17-2018, 06:34 PM   #41
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Re: Replacing my HVAC unit


I think you made a typo there?

as for excess air, there shouldn't be more excess air in stage 1 than in stage 2. that would tell me it's been set up incorrectly.
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Old 07-17-2018, 08:51 PM   #42
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Re: Replacing my HVAC unit


for fun i pulled the plate off and looked and look at the plate actually on my furnace:
high is 66k in, 54k out, for 81.81(repeating)% efficiency
low is 45k in, 36k out, for 80.0% efficiency

so it appears you are right, but not much difference there.

i definitely wouldn't replace mine with a 35k output single stage just to try to gain 0.8% efficiency.
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Old 07-17-2018, 09:40 PM   #43
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Re: Replacing my HVAC unit


The 2-stage is most likely less efficient on low fire. The ratings are one thing, actual performance in the field another.

Also, there's steady state and annual efficiency. Annual is what matters.

So while low fire may be slightly less efficient than high at steady state, the longer cycles, less cycling may offset the difference.

You won't get 95% seasonal out of at 95% afue if the cycles are very short.

Also, you're only getting the actual rated BTU per hour if the orifices are actually correct for your local natural gas.

I don't have experience, but there are people who say a lot of furnaces are under-fired from the factory at rated pressure and the supplied orifices.

Having the fuel input wrong messes up the fuel to air ratio, the extra air cools the flame and you get a smaller flame. First stage on a 2-stage furnace does have a worse fuel to air ratio and a smaller flame too, so yah there's gotta be efficiency loss. But likely under 5%.

Just viewing some combustion testing vids on you tube, you can see how low fire has more excess air.

The modulating furnaces are the worst.

---------
"- 10 - 15% more efficient when it comes to converting burn gas to heat the home. Gas where I live isn't that expensive. I pay $140 during the coldest month (February) of the year."

If you go from 80% to 96%, you'll save closer to to 16 or 17%. You have to do the math, looking at existing efficiency and dividing, subtracting result from 1, times 100.

You can't just do straight subtraction.

Natural gas is dirt cheap now and will rise over the life of the furnace. Do not calculate savings based on today's gas prices.

Quote:
- Purchase cost of 90% is higher compare to a 80% of a similar sized unit.

- One time cost to run two PVC pipes to the rear of my home. Cut up some dry wall ceiling. Cost to patch up drywall ceiling. Gotta submit application for HOA approval. Not that big of a deal, but more work.

- More parts. Thus system is more complex. Thus more stuff are likely to go wrong. Thus requiring more maintenance.

- Requires more maintenance. I spoke with a HVAC consultant that he said the 90% needs to be fix / maintain more often compare to a 80%.

- 2 heat exchangers on the 90% instead of 1 on the 80%. When the heat exchanger the cracks, it's just best to replace the entire gas furnace because the cost of replacing the cracked heat exchanger is high. So having 2 heat exchangers would increase the likely hood that a heat exchanger would be cracked. If one of the two heat exchanger is cracked, it's best to replace the entire gas furnace.
The only real issue I see is having to open up the wall/ceiling.

You should know that 80% is pretty much obsolete. They're made for warm climates and applications with furnace in unconditioned space.

The extra parts are not mechanical or electrical. The secondary heat exchanger is stainless steel tube and has fins.

They aren't exposed to the intense heat directly from the flame so don't crack. by nature of the design being small tubes, they don't crack.

The only extra maintenance is dealing with the condensate trap during regular maintenance visits. Otherwise, they're the same.

If you're trying to keep repair costs to a minimum, don't get a 2-stage furnace, they have more expensive parts and an extra pressure switch to fail.

Get a single stage psc motor 90%+ furnace, this will cost less to maintain than a 2-stage/variable 80% condensing furnace.

It's easier for the contractor to connect an existing b-vent or chimney pipe than run now pvc pipes. They may come up with a lot of dumb bs excuses to not put high efficiency.

With the exception of tough to vent/drain applications and attic installs, I think contractors pushing mid efficiency, making excuses need to be kicked to the curb.
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Last edited by user_12345a; 07-17-2018 at 09:45 PM.
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Old 07-18-2018, 02:31 AM   #44
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Re: Replacing my HVAC unit


Quote:
Originally Posted by user_12345a View Post
...
Thank you so much again. I learn at least one new thing with every single one of your reply!

--

"You should know that 80% is pretty much obsolete."

Yup. I didn't know that.

--

"They may come up with a lot of dumb bs excuses to not put high efficiency."

Why is it in their best interest to not put in high efficiency system? Doesn't make any sense to me. I do pay the HVAC companies more for higher efficiency system. I do pay the HVAC companies more for the annual maintenance of these higher end system.
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Old 07-18-2018, 10:56 AM   #45
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Re: Replacing my HVAC unit


I just know where I am they used to make excuses - more complex, will last as long, it's more expensive, etc. All non-sense. People do what they're used to (and what's convenient - like not having to run exhaust in intake pipes, faster install) and make excuses to rationalize.

The condensing furnaces have a lower cost of ownership if you look at fuel costs over 20+ years. Same basic technology but with the secondary heat exchanger.

Safer and quieter too with sealed combustion, doesn't dry out the house by having the combustion air be replaced by outdoor air.

Now mid efficiency units aren't even legal to import into canada any more.

They almost phased them out in northern us but I believe it was blocked -> https://patch.com/illinois/plainfiel...fici07d8f49130
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