Migrating from 80% AFUE to 90%+, vent work?

[PandaBear]

Not planning to do it yet but this has been in my mind for a while, my priority is comfort instead of saving a few hundred dollars in installation or a few tens of dollars in heating bill a month:

Currently I have a Rheem Criterion 100000 BTU furnace (24 years) + a 3 ton Carrier SEER 13 AC (7 years?) for a 1500 sqft 2 stories house (cathedral ceiling in the living room, so surface area of the house is equivalent to 2000 sqft). The AC is running as expected, constantly on in the summer and the house stays at 78F. The furnace on the other hand, is way too strong in the winter, running about 1 min on then 15 mins off, etc. The furnace vent, shared with a water heater, going through a wall between 2 bedrooms, is also pretty loud when running. I replaced a draft fan 2 years ago because it was too loud and it is much better but still audible. Also, that 1/15 cycle of 100000 BTU is very uncomfortable, like someone blowing a hair dryer at you.

So I am thinking, when this furnace die (or when we can't handle it anymore), we'll replace it with a smaller 90+ unit as small as possible to handle the 3 ton AC. With vent going to the side of the house, it will be much quieter than going between 2 bedrooms. The vent would likely travel 10-15 feet to the exterior wall instead of using existing vent so that's likely the main extra expense.

Let's say I want the smallest output furnace available, and the common wisdom of 350-400 CFM per ton of AC, will running something like a 30000 BTU or 40000 BTU 2 stage furnace be ok? I think to go from around 1070 CFM to 1600 CFM it would need a furnace with around 60-80k BTU instead of the 40-60k BTU, or go with a variable output (those 1% increment output model) furnace.

So I have these questions:

1) Would 1070 CFM / 30000 BTU furnace, be ok for a 3 ton AC? or would it ice up the evaporator? Currently it has a 1600-2000 CFM unit.

2) How much does it cost to route PVC to the side of the house? When you did the 80 to 90+ migration how much extra labor did it cost you?

 

[Dinoburner]

Generally an FHA furnace with A/C will use the highest speed available for the A/C if a multitap motor and a slower speed for heat. The t`stat has a lot to do with heat cycles unless it`s an old mechanical or mercury model. 100000 btus sounds like a lot for your area, however, the current furnace should be able to be setup better for comfort. If the fan motor is belt driven then the pulley adjustment would have to be changed between A/C - heat not convenient.

 

[HangFire]

If noise is an issue, be sure and get a modulating furnace with an ECM fan. It will spend most of its time throttled down, and it ramps up and down gradually and doesn't attract attention. Modulating also makes burner size choice more flexible. It doesn't matter if you get the 40000BTU and not the 30000 furnace, if it spends most of its time throttled down to 1/2 or 1/3 of 40000. The nice long run times keep the house evenly heated and the warm air constantly circulating.

If you do go 2 stage, ask your manufacturer and installer if the air handler size is really locked into the furnace capacity. There should be no problem putting a smaller burner in a larger air handler. (Other way around, yeah that could be a problem). The only "problem" is that the combination is not usually ordered that way. It may be a non-stocked special order. You may not find that in a Goodman but a Carrier can put it together for you. If the air handler is too small, besides icing you could also have a noise problem with higher velocity air.

 

[PandaBear]

Interestingly, the noise problem is actually not the air handler noise (it is in the garage), even the current 100000 BTU one (1600CFM) is not really noisy. The noise came from the draft / exhaust fan's air velocity and (formerly) motor bearing noise. I replaced the whining draft fan 2 years ago (still functioning well) with one as small as possible and as low rpm as possible to quiet it down. It made a significant difference, but it is still not silent, it is enough to keep someone with insomnia awake.

I've briefly though about the idea of a modulating furnace, but not sure about the reliability. I think I can trust multi-tap fan, and I think venting to the side of the house would be sufficient. Modulating furnace are all 90% correct? which means I cannot use the metal vent between the 2 bedroom anyways.

20k BTU should be sufficient, even if I hard wire the 2 stage furnace into the low stage. The typical weather we have is mid 40s to 50s evening outdoor, and 58-60 indoor, having the thermostat set to 62-64. AC on the other hand, are typically 85-98 outdoor, 82-88 indoor, and having the thermostat set to 78-80. It always take a while for the house to cool down from a hot day, if the AC is not on during the day but turn on when we get home. The 3 tons is usually constantly on for hours and the indoor temp never go lower than 78, so if I were to replace both the AC and furnace together I would definitely put in a 3.5 ton + 30k BTU.

 

[hemitom]

With your low heat load have you looked into heating with hot water???
simply put its a heat exchanger put in a air handler, hot water is piped from your hot water tank into the heat exchanger .
i'm thinking it would be a perfect option for your low heat load..

This would be taking account your hot water tank is located in the garage.

 

[JustinH]

I would not consider anything over a 80 percent furnace unless California is subsidizing high efficiency furnaces.

Also the BTU sounds way too high.

Best thing is to have a manual J done and see what the heating and cooling needs actually are.

I never had a 100k furnace even in New York.

An 80 percent furnace with half the BTU's will be dirt cheap, any rebates will be irrelevant and you will save a ton of money. Plus you don't need to pay for new exhaust duct work.

 

[PandaBear]

Originally Posted by JustinH
I would not consider anything over a 80 percent furnace unless California is subsidizing high efficiency furnaces.

Also the BTU sounds way too high.

Best thing is to have a manual J done and see what the heating and cooling needs actually are.

I never had a 100k furnace even in New York.

An 80 percent furnace with half the BTU's will be dirt cheap, any rebates will be irrelevant and you will save a ton of money. Plus you don't need to pay for new exhaust duct work.


Agree in theory, the only reason for this plan would be for comfort. The house (former model home) came with this 100000BTU beast and I added the AC about 7 years ago. We just couldn't use the heater because the heat blasted out is uncomfortable, and the draft motor and air velocity noise between the 2 bedrooms (a circular pipe flattened and go between 2 drywalls next to beds). 90+ would be a good excuse to divert the vent to the side of the house instead of up, all inside the garage, instead of the "going up and then through the bedrooms" route. So yeah, the whole point is to pay for the exhaust duct work so it is quieter. Energy efficiency is just a side effect, and since we are at it, might as well make it smaller so it is constant on instead of bursting.

With our climate we only need something really small, even too small is ok. Based on my napkin math, using AC's load as a reference (I know 3 tons is just right), 20000k-30000k BTU would be perfect.

Nobody in the area does manual J, that's a lot of extra work that contractor won't make money back on. They usually go by experience and most of them come up with similar number. i.e. my 4 quotes for the AC come with 3, 3.5, 3.5, and 4 (that guy said he will only sell the biggest AC the duct and blower can support). Based on the comfort level I can say 3 ton is absolute minimum and some customers will complain, and 4 may be too cold and some customers will complain.

So far I see that 80 and 90+ only cost about a 500-800 difference, and BTU from 30-80k seems to be within $150 of each other, am I right?

 

[PandaBear]

Originally Posted by hemitom
With your low heat load have you looked into heating with hot water???
simply put its a heat exchanger put in a air handler, hot water is piped from your hot water tank into the heat exchanger .
i'm thinking it would be a perfect option for your low heat load..

This would be taking account your hot water tank is located in the garage.


I though about the idea of a "boiler / heat exchanger" with floor board heating instead, it would cost money but the biggest problem is we will have to move quite a bit of stuff to mount a boiler, and the whole house's flooring would need to be out. It may be an option if we replace the floor before we replace the furnace.

Cost for a 1500 sqft 2/F house vs a 90+ furnace vent work?

 

[hemitom]

you can buy systems now that run off of your domestic hot water tank actually been out for a number of years, no need for a boiler , uses your existing duct work.

perfect for your low heat load demands. Looks like a normal furnace minus the gas burner heat exchanger etc. In its place is a water heat exchanger piped to your hot water tank, blower pushes air through the heat exchanger just like a normal furnace.

 

[Dinoburner]

At some point perhaps considering a heat pump/ A/C unit would make sense as most of the noisy bits are outside.

 

[PandaBear]

Hydronic air handler seems like a great idea, thanks for suggesting that.

Heat pump is not a good idea around here with tiered utility rate going from 20c/kwh baseline to almost 40c/kwh at high tier. I heard of a "friend's friend" ripping out heat pump to install natural gas furnace (1 therm around $1). Beside, I have a working AC that should way outlast the furnace.

 

[skyactiv]

I’d spend the money and get a large variable speed AC system with appropriate furnace. Probably a 60k btu 2 stage furnace. But you can’t belong to the worry wort reliability club.

 

[KrisZ]

Did you check the control board at what speed it is set to run on heat? You can probably lower that to the bare minimum the dip switches allow, it should help some for now.

As far as new furnace size, if 3 tons of AC is working fine now, then a 1200 cfm unit should do the trick. A two stage furnace would also be a good idea and these are as reliable as one stage units. You would just have to ensure it is wired properly to use only stage one for heating and only stage two for AC.

Edit:
I think all 90%+ units need a PVC intake and exhaust, not just an exhaust like 80 and below units. So budget for that.

 

[hrv]

On think that I noticed after replacing my 1008 Goodman brand HVAC with a gas furnace with a Carrier single stage HVAC and gas furnace in 2017 is that the blower motor is much stronger then the old...much more air flow...also because of the..it is a bit louder...also the HVAC line set is a little louder then the old unit with the R22...I was told that the newer ones run at a much higher pressure then the old..The HVAC is just outside my bedroom wall and the line set comes in there..

 

[BMWTurboDzl]

Not planning to do it yet but this has been in my mind for a while, my priority is comfort instead of saving a few hundred dollars in installation or a few tens of dollars in heating bill a month:

Currently I have a Rheem Criterion 100000 BTU furnace (24 years) + a 3 ton Carrier SEER 13 AC (7 years?) for a 1500 sqft 2 stories house (cathedral ceiling in the living room, so surface area of the house is equivalent to 2000 sqft). The AC is running as expected, constantly on in the summer and the house stays at 78F. The furnace on the other hand, is way too strong in the winter, running about 1 min on then 15 mins off, etc. The furnace vent, shared with a water heater, going through a wall between 2 bedrooms, is also pretty loud when running. I replaced a draft fan 2 years ago because it was too loud and it is much better but still audible. Also, that 1/15 cycle of 100000 BTU is very uncomfortable, like someone blowing a hair dryer at you.

So I am thinking, when this furnace die (or when we can't handle it anymore), we'll replace it with a smaller 90+ unit as small as possible to handle the 3 ton AC. With vent going to the side of the house, it will be much quieter than going between 2 bedrooms. The vent would likely travel 10-15 feet to the exterior wall instead of using existing vent so that's likely the main extra expense.

Let's say I want the smallest output furnace available, and the common wisdom of 350-400 CFM per ton of AC, will running something like a 30000 BTU or 40000 BTU 2 stage furnace be ok? I think to go from around 1070 CFM to 1600 CFM it would need a furnace with around 60-80k BTU instead of the 40-60k BTU, or go with a variable output (those 1% increment output model) furnace.

So I have these questions:

1) Would 1070 CFM / 30000 BTU furnace, be ok for a 3 ton AC? or would it ice up the evaporator? Currently it has a 1600-2000 CFM unit.

2) How much does it cost to route PVC to the side of the house? When you did the 80 to 90+ migration how much extra labor did it cost you?

You're going to need someone who has access to the appropriate modeling software so they can take into account house orientation, number of windows, window size/location, etc when it comes to the heating load of your house. A local energy rater who specializes in that sort of thing can help you. I'm sure they're everywhere in your area. Residential HVAC companies generally rely on "rule of thumb" or "always did it this way" kind of thinking.

 

[brianl703]

Did you check the control board at what speed it is set to run on heat? You can probably lower that to the bare minimum the dip switches allow, it should help some for now.

If his ductwork is marginal this could cause the heat exchanger to run at temps beyond normal, which at best results in shorter life and at worst results it in it tripping out on high limit.

 

[pantygnome]

When we converted to natural gas back in 2016, I went with a Carrier Infinity system. Our original oil furnace was 90K BTU...the replacement Carrier was 64K (!) BTU. And the replacement has been FAR better at heating the house.

If you go with a fully modulating heating system, it is much more comfortable and efficient running at "low and slow" as I like to call it. Slow air speed, low heat %. I think mine will range from 40% to 100% gas heat but 99% of the time its running at 40. That helps with noise, gas cost and comfort. Was it extra to go this route? Of course, but its what I wanted (I *hated* our old system because it was 100% or 0%, so the house got warm quickly, furnace turned off and then the house cooled down...wash/rinse/repeat all day and it just wastes $$$ on oil)

 

[Hillbilly Deluxe]

Attention, Attention K-Mart shoppers.

We are about to close, it's almost time for Lubby's to open for dinner at 4:30. Please get out of the store.




Also, this thread is 3 years old...

 

[cmarshall78]

You need to do a manual j load calculation. Otherwise your just making an educated guess.

 

[TmanP]

When we converted to natural gas back in 2016, I went with a Carrier Infinity system. Our original oil furnace was 90K BTU...the replacement Carrier was 64K (!) BTU. And the replacement has been FAR better at heating the house.

If you go with a fully modulating heating system, it is much more comfortable and efficient running at "low and slow" as I like to call it. Slow air speed, low heat %. I think mine will range from 40% to 100% gas heat but 99% of the time its running at 40. That helps with noise, gas cost and comfort. Was it extra to go this route? Of course, but its what I wanted (I *hated* our old system because it was 100% or 0%, so the house got warm quickly, furnace turned off and then the house cooled down...wash/rinse/repeat all day and it just wastes $$$ on oil)

We have an Infinity furnace from 2010 that has been fantastic. It’s imperceptibly quiet when running on its lowest stage, which takes care of most of the heating needs except coming out of setback. My last apartment had a single-stage Carrier Comfort (still high efficiency), and it was striking how much more noticeable its cycling and operation were than the Infinity.

I’d say cases like OP’s would be well served with a small furnace that is either modulating (expensive) or a two-stage (cheaper option, but still much less noticeable than a single-stage).

Also, the ac running continuously to reach 78 would seem less than ideal to me. Is that your preferred setpoint anyway? I’d want to be able to get the house down to 72 or so - 78 being the result of full-out running seems like it might be a bit overtaxed.