Just right, or oversized?

[walnuttree]

So, tell me if I have this right....

A brand that lists their 45,000 BTU 80% furnace actually has an output of
36,0000 BTUs? So if my load cal for my home calls for 32,000 BTU's of heat,
this would be just the perfect size?

[JoeSix]

Trane makes (or did two years ago) a two-stage 40K BTU 80% furnace that would match your heat load perfectly on high, while only putting out about 20K BTU's on low, very nice for not so cold days.

One of the other nice things about a two-stage is that it accommodates slight oversizing better. You still want to get it right, but as long as the low stage is well under the design load, it will work fine.

[nashobasales]

Sounds right to me. Oh yeah and what he said.:D ;)

[permafrost12345]

So, tell me if I have this right....

A brand that lists their 45,000 BTU 80% furnace actually has an output of
36,0000 BTUs? So if my load cal for my home calls for 32,000 BTU's of heat,
this would be just the perfect size?

Thought you cleared that up in your other two threads u started yesterday?

[Hvacman2]

I think mr. walnut is looking to get a single stage heater instead of a two stage. It sounds like he can't run another thermostat wire to accomodate a two stage thermostat so he thinks it won't work correctly, contrary to what most people in the know have been telling him. Not trying to mock you mr walnut, it's just that your overthinking your new system and your not listening to what seasoned contractors are telling you. You can use a single stage or two stage heater with a single t'stat and be fine either way. However, if we keep having these mild winters, ( and they are forcasting mild winters for the east), you would be better off with a two stage heater. It's all about how you spend your money. You either buy more toys, or you give it to the utiility man. It's your call.

[amd]

I think mr. walnut is looking to get a single stage heater instead of a two stage. It sounds like he can't run another thermostat wire to accomodate a two stage thermostat so he thinks it won't work correctly, contrary to what most people in the know have been telling him. Not trying to mock you mr walnut, it's just that your overthinking your new system and your not listening to what seasoned contractors are telling you. You can use a single stage or two stage heater with a single t'stat and be fine either way. However, if we keep having these mild winters, ( and they are forcasting mild winters for the east), you would be better off with a two stage heater. It's all about how you spend your money. You either buy more toys, or you give it to the utiility man. It's your call.


For the record, two stage furnaces in theory don't consume less gas than their single stage counterparts. The feature is there for comfort.

There's no point of connecting a two stage unit to a single stage t-stat when it'll continuously cycle between low, high, and off.

Why someone would spend $4000+ on a nice piece of equipment and control it with a $30 homecheapo t-stat is beyond me.

[walnuttree]

Hey Mister amd, I do NOT have a 30.00 homecheapo stat! I have recently the
Honeywell new touch screen stat!
Every contractor I had in here all tells me I DONT not need a 2 stage stat ,and with my home, my currently stat will work fine, and mostly run on low
stage!

[hvaclover]

You were advised in three other threads about size. What are you unclear about?

[pecmsg]

Every contractor I had in here all tells me I DONT not need a 2 stage stat ,and with my home, my currently stat will work fine, and mostly run on low
stage!

Then why do you keep asking? Buy what ever you want.

[ampulman]

So, tell me if I have this right....

A brand that lists their 45,000 BTU 80% furnace actually has an output of
36,0000 BTUs? So if my load cal for my home calls for 32,000 BTU's of heat,
this would be just the perfect size?

Hey guys: Not butting in, but I'd like your input on a point that has me concerned.

So far, the only way I could get a heat calc was to do it myself. My loss came to 60,000 BTU. According to the author (of the software), one should allow a safety factor of 20 or 25%.

One furnace that I am considering, a Rheem Mod 75,000 unit (upflow), has a high stage output of 72000 which would be exactly 20% oversize at design. My understanding of the furnace is that it would probably not operate at full capacity except maybe for recovery from setback (11 degrees). While this hasn't been the coldest winter in memory, we had a cold spell which was 4 degrees below design, and the Mod could handle a difference of 10+ degrees below design.

I had originally considered two 80,000 units but because excess sizing, and of blower specs, I'm leaning toward the Mod.

Since we normally keep our occupied temperaure at 66 degrees, it would be necessary to lower the setting in the event of temperatures in excess of 10 degrees below design. In looking at the discussion here, I haven't seen any reference to a safety factor and am wondering what emphasis you put on this, or to put it another way: how much safety is reasonable?

AM

[hvaclover]

Don't hijack the thread ampulman.

[tpa-fl]

I have recently the Honeywell new touch screen stat! Every contractor I had in here all tells me I DONT not need a 2 stage stat ,and with my home, my currently stat will work fine, and mostly run on low
stage!

If it's the retail version of the stat, it won't support 2-stage operation. Even 1 of the 4 VisionPro models available doesn't support 2-stage operation. If you've got a 2-stage furnace, you NEED a 2-stage stat for it to work properly and both the furnace and stat need to be configured to run as 2-stage, controlled by the stat.

You can run a 2-stage furnace off a single-stage stat, but you're practically negating the value of a 2-stage furnace at that point. You buy 2-stage for comfort. An arbitrary timer isn't going to do anything for comfort, but a 2-stage thermostat will.

[beenthere]

Every contractor I had in here all tells me I DONT not need a 2 stage stat ,and with my home, my currently stat will work fine, and mostly run on low
stage!


They're just telling you what you want to hear.

2 stage furnaces can work with a single stage thermostat.

If you really want the benefits of a 2 stage furnace, then you NEED a 2 stage thermostat.

[walnuttree]

Yeah, that may be true....but isnt a 2 stage furnace with a single
stage stat STILL much better that a single stage furnace with a single
stage stat??

[tpa-fl]

Debatable. That's like buying a Bentley or Corvette and trying to fit tires for a Chevy Nova on it. It'll move forward, but don't expect it to do what it's capable of.

Again, same answer as we've been telling you. Get a true 2-stage stat, have it wired to the furnace to do 2-stage operation from the stat, and have both the furnace and stat configured to let the stat do the staging.

But, it's your money. If you want to spend money on a 2-stage system and not use it, that's your choice, your money.

[hvaclover]

the short answer is no. Th two stagw will work better w/ two stafe stat because it will control the stages where a single stage stat cannot

[beenthere]

Yeah, that may be true....but isnt a 2 stage furnace with a single
stage stat STILL much better that a single stage furnace with a single
stage stat??


This is why I said they are just telling you what you want to hear.

You don't want to hear, or believe that you should get a 2 stage stat for your 2 stage furnace.

The salesman figured out you didn't want to hear it, so he told you what he knew you wanted to hear, instead of risking losing the sale.

[Hvacman2]

Hey guys remember that mr walnut cannot run the t stat wires needed to support a two stage stat. He says he lives in a condo and you know you cannot run new wires without making holes in the house and he doesn't want to. Also, a single stage furnace would be oversized for his condo. Now knowing this, how could you say that he should not go for the two stage furnace. So in this particular situation, he should !!!!!

[beenthere]

Hey guys remember that mr walnut cannot run the t stat wires needed to support a two stage stat. He says he lives in a condo and you know you cannot run new wires without making holes in the house and he doesn't want to.



Honeywell IAQ, only need 3 wires from the stat to the module.

[tpa-fl]

Hey guys remember that mr walnut cannot run the t stat wires needed to support a two stage stat. He says he lives in a condo and you know you cannot run new wires without making holes in the house and he doesn't want to. Also, a single stage furnace would be oversized for his condo. Now knowing this, how could you say that he should not go for the two stage furnace. So in this particular situation, he should !!!!!

A good sparky (electrician) can pull wires with minimal, possibly no additional holes. NO, he should NOT go with a 2-stage furnace in this case -- a single stage furnace with the PROPER capacity is what he needs, OR a 2-stage furnace which is reasonably matched to his situation. If I'm going to oversize a system, 2-stage is the way to do it, BUT, the actual load should be somewhere inbetween the two stages, preferably near the upper output.

There's also the add-a-wire thingies, which can be made on the cheap if you know a little bit about electronics.

[JoeSix]

Yeah, that may be true....but isnt a 2 stage furnace with a single
stage stat STILL much better that a single stage furnace with a single
stage stat??

You really need a two-stage stat. A White-Rogers 1F81 is a good reasonably priced model.

With a single stage stat, the two stage furnace will (1) run on low for a preset amount of time, and then (2) switch to high until the thermostat demand is met, and then (3) switch off. This negates much of the benefit of the two stage furnace.

With the two-stage stat, the furnace will run only on low unless it cannot meet the thermostat demand, in which case it will switch to high until it gets close to the thermostat set point.

When the heat load exceeds what the furnace can provide on low, it will run on low for a while, switch to high, then switch back to low, then back to high, providing continuous, efficient and comfortable operation. The duration of low and high will shift more toward high as it gets colder outside.

Meanwhile, with a single stage stat, the furnace will always cycle off unless you exceed the heat load of the high stage (very rare -- furnaces are often rediculously oversized).