Furnace install evaluation requested

[jmlcolorado]

Okay folks, im about 50% done installing my furnace in the garage and just wanted some input to make sure im not missing something.

I picked up a a 80% 125k BTU furnace. I have framed up a closet in the corner of the garage with a platform about 20" off the floor. My plan is to use the area under the platform as the return air plenum. I'll install a door with a 16x24 grill for air to enter.
The furnace will set up on top of the paltform. Ill build a supply air plenum on top and duct out 4 8" ducts with grills for the supply into the garage.
In a perfect world, i would chase the supply plenum into the attic and run a trunk up there feeding about 8 small grills around the ceiling. The problem is because of the placement of the furnace, i dont have much room in the attic because im at the edge of the house where the trusses sit on the exterior walls. Until i figure a better way to get it into the attic, this system will have to work.
how much better would it make it heat if i had it ducted into the attic?

Combustion air will come from 2 8" fresh air vents from outside. 1 at the top of the closet and 1 about 12" above the platform.
I'm having a bit of a hard time figuring out how to seal the 2 spaces in the closet between the return and the main area above the platform.

I have just wired a dedicated 15amp circuit for the furnace. my cutoff switch is in a 3 gang box with my light so i can turn the furnace off/on as needed.

I have attached a few photos better showing my plan.
1: Rough framing inspection by the boss, Rocky
2: Initial framing and outside layer installed. Need inside rocked and all taped.
3: Proposed return air plenum.
4: New circuit roughed in.
5: Rough drawing of the plan to see how it all works.

 

[HVAC Phil]

Just put a 2x4 in frame sealing off the area between the return and closet base, this would be called a "firestop". Only having 4 8" supplies is not gonna be enough outlet air with a furnace that huge, it will ride limit. You have to get rid of the air, it will always go off on limit and eventually crack the heat exchanger. I would try to run the trunk into the attic, with the heat runs that close to the return, it will recirculate the air, won't get the air accross the garage. Why such a big furnace? You could heat a 2500 square foot house with that.

 

[6t7gto]

With your design of the closet, how do plan to service the furnace?

 

[Cars&Classic]

I am just learning the furnace stuff so dont take my word, but someone else may not have noticed, if I am intrepreting it right his cold air return in in the bottom section, I thought it had to be atleast 18" above the floor? (I sell houses and have had several fail inspection for this) but maybe it is just a local code?

 

[jmlcolorado]

Just put a 2x4 in frame sealing off the area between the return and closet base, this would be called a "firestop". Only having 4 8" supplies is not gonna be enough outlet air with a furnace that huge, it will ride limit. You have to get rid of the air, it will always go off on limit and eventually crack the heat exchanger. I would try to run the trunk into the attic, with the heat runs that close to the return, it will recirculate the air, won't get the air accross the garage. Why such a big furnace? You could heat a 2500 square foot house with that.

Thats what i was afraid of. Guess ill try to get the supply trunk into the attic. Thats going to be a challenge
The only reason im using a unit so large is because i got it for free. I had a 100k furnace rigged for 2 years in my rental and it worked great with a sheet metal cap and 2 8" elbows off the top directing air. Seemed to work great for 2 years with no issues but im trying to make this one right
Would it help at all if i change the fan speed higher to help rid the hot air?

With your design of the closet, how do plan to service the furnace?

Where the furnace is now, its just sitting in there. I will turn the furnace to have the door pointing straight out the closet in its final location.

I am just learning the furnace stuff so dont take my word, but someone else may not have noticed, if I am intrepreting it right his cold air return in in the bottom section, I thought it had to be atleast 18" above the floor? (I sell houses and have had several fail inspection for this) but maybe it is just a local code?

My understanding was that the reason for having it up off the floor was to keep the burners from sucking in fumes and chemicals heavier then air. I understand the return ***** air, not subject to direct flame so its okay at the floor. I could be wrong though.

 

[JimL]

That thing probably has a 5ton drive... 2000cfm 8" run is 200 cfm.

16x24 grill isn't going to be big enough either. It will sing like crazy pulling that much air through it.

Big furnace. Could heat a 4000 square foot home easily with it.

 

[jmlcolorado]

That thing probably has a 5ton drive... 2000cfm 8" run is 200 cfm.

16x24 grill isn't going to be big enough either. It will sing like crazy pulling that much air through it.

Big furnace. Could heat a 4000 square foot home easily with it.

Correction, it's a 100k btu. Still huge for a garage but it's what I have

The reason I was going to use a 16x24 return grill is because the opening in the bottom of the furnace is 15x24 so I fighure I would match the return grill to the furnace. After all, I can't pull more air through a larger grill if the furnace hole is only so big.

I'm starting to think ill need to build a drop soffit at the edge of the garage. This would be storing a trunk line that's 16x20 (that's the size of the supply hole. In the top of the furnace). This will run about 10 feet before transferring to a plenum into the attic which will be the same size.

It sounds like this thing pumps some serious air so I will up the number of supply grills from 4 to 10.
What size duct should I use to go to the ceiling grills if I use 10 of them? If 8" runs are 200cfm, should I use 10 8" round ducts?

 

[jmlcolorado]

BTW, this furnace will eventually heat the garage addition as Well as the original garage. The addition will be an additional 690 sq ft onto the already 460 sq ft.

 

[Bronson]

Find some scrap Celotex and cut a piece to lay on the concrete inside the return air cavity. Also, cut some pieces and nail to Your framing in that area. This will make the blower operation a little quieter when You are heating. An old HVAC Guy taught Me that trick, and it works.

 

[jmlcolorado]

Find some scrap Celotex and cut a piece to lay on the concrete inside the return air cavity. Also, cut some pieces and nail to Your framing in that area. This will make the blower operation a little quieter when You are heating. An old HVAC Guy taught Me that trick, and it works.

Fantastic thanks for the advice!
I will be sheet rocking the entire return plenum below anyways, but if I can make it quiter too, I'm all for it!
Thanks!

 

[Provincial]

I know you can put smaller nozzles in an oil furnace to lengthen the cycle time. Can gas furnaces be recalibrated to put out fewer btu's also?

Run a duct from the furnace to a point where you can penetrate up to the attic and continue out into the room. You can build a soffit with 2x4's and drywall to hide it and put lights under it. Unless you have a hip roof, you should be able to run the duct along one of the walls in the photos, and near the walls you ususally can live with the lower ceiling height. A wide, shallow duct will give more headroom.

 

[Charles (in GA)]

It won't do any good to elevate the furnace 20" off the floor and then start sucking air at floor level. You need a 20" high solid box, and then frame a return plenum on top of that, then the furnace on top of that.

Charles

 

[jmlcolorado]

Man, this thing Is becoming a PITA. Almost enough to make me switch to a HawtDog and use the closet for the compressor

Thinking about the above posts, I've been thinking,
My furnace is 100k btu, which is a 4 ton right? (25,000 btu=1 ton, 100,000 btu=4 ton).
The specs of the blower of my furnace is 11x7", with the settings as follows:
Med Lo-1005 cfm
Med Hi-1150 cfm
Hi-1300 cfm

SO if a 20x16 return grill is 585cfm and a 20x20 cfm return grill is 720, I would need 2, 20x20 return grills ( 1440) to supply enough air right?
If this is true, how can the furnace **** 1300 cfm on the high setting if the hole in the furnace itself is only 20x16!?!?

Let's say I install 2, 20x20 return grills to match the return cfm of the furnace, I would need to match the supply cfm to the return too right?
If an 8" round duct = 245 cfm and a 10" round = 440 cfm, then in theory, 1 vertical trunk with 4 10" ducts would allow 1760 cfm to exit.

The manual for the furnace says "4 ton requires 1, 18" (? Cfm )OR 2, 14" ( 1950 cfm)OR 2, 12" and 1 10" (1870cfm ) supply. I imagine this corresponds to the speed of the fan in the furnace.

My head says that 4 10" supply's will suffice, at 1760 cfm if the fan is on low.

Now this is only talking about designing supply ducts to allow enough cfm's for the heat exchanger to operate properly. This does not talk about placement of said ducts which is a totally different conversation, I think.

Am I on the right track here?

 

[jmlcolorado]

It won't do any good to elevate the furnace 20" off the floor and then start sucking air at floor level. You need a 20" high solid box, and then frame a return plenum on top of that, then the furnace on top of that.

Charles

I was under the impression the regulation to elevate the furnace was to get the burners away from fumes of a garage.
I see water heaters on platforms all the time in garages just to get the burner off the ground.
The return is a totally different use of air. The return on the ground will only suckminnmore dust and debris causing the filter to clog faster.

But as mentioned earlier, I'm a rookie which is why I'm here asking questions.

 

[JimL]

Man, this thing Is becoming a PITA. Almost enough to make me switch to a HawtDog and use the closet for the compressor

Thinking about the above posts, I've been thinking,
My furnace is 100k btu, which is a 4 ton right? (25,000 btu=1 ton, 100,000 btu=4 ton).
The specs of the blower of my furnace is 11x7", with the settings as follows:
Med Lo-1005 cfm
Med Hi-1150 cfm
Hi-1300 cfm

SO if a 20x16 return grill is 585cfm and a 20x20 cfm return grill is 720, I would need 2, 20x20 return grills ( 1440) to supply enough air right?
If this is true, how can the furnace **** 1300 cfm on the high setting if the hole in the furnace itself is only 20x16!?!? Big difference pulling air through an open hole and a grill.

Let's say I install 2, 20x20 return grills to match the return cfm of the furnace, I would need to match the supply cfm to the return too right?
If an 8" round duct = 245 cfm and a 10" round = 440 cfm, then in theory, 1 vertical trunk with 4 10" ducts would allow 1760 cfm to exit.

The manual for the furnace says "4 ton requires 1, 18" (? Cfm )OR 2, 14" ( 1950 cfm)OR 2, 12" and 1 10" (1870cfm ) supply. I imagine this corresponds to the speed of the fan in the furnace.

My head says that 4 10" supply's will suffice, at 1760 cfm if the fan is on low.

Now this is only talking about designing supply ducts to allow enough cfm's for the heat exchanger to operate properly. This does not talk about placement of said ducts which is a totally different conversation, I think.

Am I on the right track here?

We do quite a few garage furnaces. If it was me i would have hung it off the wall laying horizontal.

I think your going a bit overboard. Frame in some more grill openings on the sides of your closet down low. Have a plenum made that goes to the ceiling. Come off one side or the front with a 12x24 or 10x30 going through the wall of the closet, laying on its side. Put a register with adjustable louvers on it and roll with it. The air will circulate fine. We have done similar setups with one unit in a 80x120x18 building and it works fine.


Here is a couple we did in garages, first one we just put a big elbow on the top letting it blow the length of the garage. Second one just had a plenum with a couple registers cut in.

 

[jmlcolorado]

We do quite a few garage furnaces. If it was me i would have hung it off the wall laying horizontal.

I think your going a bit overboard. Frame in some more grill openings on the sides of your closet down low. Have a plenum made that goes to the ceiling. Come off one side or the front with a 12x24 or 10x30 going through the wall of the closet, laying on its side. Put a register with adjustable louvers on it and roll with it. The air will circulate fine. We have done similar setups with one unit in a 80x120x18 building and it works fine.


Here is a couple we did in garages, first one we just put a big elbow on the top letting it blow the length of the garage. Second one just had a plenum with a couple registers cut in.

I very well might be going overboard on this install
In my rental house I had a 70's furnace literally running on an extension cord. Heck, I didn't even have a return plenum, just ran the thing with the blower cover open sitting on a shelf. The supply was just a cap with 2 8" elbows blowing heat where I wanted it
It ran great for 2 years with no problems except it was UGLY, and far from code.

I really wish I could put this in the attic but it's too tall. According to the manual, it says it's up flow only. But I never says I CAN'T set it horizontally.

I guess I'm just trying to make sence in my head why this is supposed to be this way or that. I enjoy crash courses in new stuff

 

[JimL]

What kind of furnace is it? Model number

 

[jmlcolorado]

Rheem RGDG-10EAMER

 

[Orangestang]

If it has a ribbon style burner(kinda like a gas grill) then it can only be used as a upflow. If it has inshot burners it can be laid down. I had an older furnace in my garage that sat on the floor had the return up high. and a short piece for a supply,I keep my gas can in the shed as I was always careful of what I did when it came to using chemicals.Was the furnace to code? Probably not but it worked.

 

[mrobins297aaa]

How big is your garage?
unless its huge, just run the plenum up and tap a 22x14 duct (assuming 1300cfm) in the plenum and bring it right thru the wall with a grille. Believe me it will not short circuit that much if at all. Your making this way too much work.

the code is the burners have to be 18" above the floor.

if you still feel like driving yourself nuts with this, the formula for sizing ductwork is:
A x V = CFM
A= area is sq ft
V= velocity of the air, standard residental design is 900 fpm (feet per minute)
So ie, if your fan is 1300 cfm and the vel = 900 the formula looks like 1300/900 = 1.44 sq.ft
to get the duct size you convert sq.ft to sq. inches so 1.44 x 144 = 208 sq. inches
you can use any combo of h to w that multiplies out to 208.

17.33 x 12 = 208, or 20.8 x10 and so on.
I use 22 x14 because your going to put a grille on there and it probably has a free area of only about 70% of the total. the blades take up space.

 

[mrobins297aaa]

also there are 12,000 BTU's in a ton.........and it pertains to cooling not heating

one more thing the industry standard is 400 cfm of air flow per ton of cooling.

so 3 tons = 1200 cfm
4 tons = 1600 cfm
20tons = 8000 cfm

 

[jmlcolorado]

How big is your garage?
unless its huge, just run the plenum up and tap a 22x14 duct (assuming 1300cfm) in the plenum and bring it right thru the wall with a grille. Believe me it will not short circuit that much if at all. Your making this way too much work.

I gotta say, I really like it when someone says I'm working to hard to do such an easy task

also there are 12,000 BTU's in a ton.........and it pertains to cooling not heating

one more thing the industry standard is 400 cfm of air flow per ton of cooling.

so 3 tons = 1200 cfm
4 tons = 1600 cfm
20tons = 8000 cfm

Thats kinda what I thought, and I don't ever plan on installing an Ac condenser for the garage if I can't handle the few hot days in Colorado, I don't belong here since there are places MUCH worse off for that

 

[KyleQ]

It seems that you made this way more complicated then you needed to. This is what my father recently built, he had it on the floor before we knocked out the middle wall, has worked flawlessly for 20+ years.


I am going to hang a furnace on the wall and run ducting in the same way down one side.

 

[jmlcolorado]

It seems that you made this way more complicated then you needed to. This is what my father recently built, he had it on the floor before we knocked out the middle wall, has worked flawlessly for 20+ years.


I am going to hang a furnace on the wall and run ducting in the same way down one side.

Hey I like that. That would even dress up nice with a soffit built around the duct with grills with adjustable louvers.
I might work off that idea

 

[jvitez]

Big furnace. Could heat a 4000 square foot home easily with it.

My previous house was 2600 sq ft with full basement, so heated 4100 sq ft in our Siberian climate. House was reasonably EE with R40 ceiling, R20 in 2x6 walls, R12 for the basement walls, and triple pane casement wooden windows, but nothing fancier than that. The house came with a 150,000 btu 80% forced air furnace. Waaaaaaaaay oversized. Your 100k furnace will not be running at peak efficiency.

 

[jmlcolorado]

My previous house was 2600 sq ft with full basement, so heated 4100 sq ft in our Siberian climate. House was reasonably EE with R40 ceiling, R20 in 2x6 walls, R12 for the basement walls, and triple pane casement wooden windows, but nothing fancier than that. The house came with a 150,000 btu 80% forced air furnace. Waaaaaaaaay oversized. Your 100k furnace will not be running at peak efficiency.

I am fully aware the furnace is oversized and will not be running at maximum efficiancy. But as I stated earlier, this is what I have and I must make due. Would you tell a homeless guy that his box is under insulated? Of course not, he's fully aware of that and is just working with what he has
The garage has drywall everywhere but no insulation in the attic. The walls are only 2x4. It has a 32" man door with NO weather stripping or even a threshold. PLUS is has a half lite single pane window in it. The garage door is original to the house (70's) made of cracking wood, and also has no weather stripping. The 3030 window in the east wall isn't helping much either.

If I were trying to achieve maximum efficancy, I would pony up for a nice R14 insulated metal garage door with full weather stripping, a new fiberglass insulated exterior rated door, insulate the attic to R30, remove the single hung window completely, even rip up and pour a new concrete slab that's not heaving in the middle, to allow the garage to seal all the way across the bottom. But unfortunately, I don't have $15k to throw at the house right now and it's December and all my projects are sitting stagnate and getting cold.

I hope I'm not comming off as a......whatever, but that's not the advice I'm looking for.

I really don't want to come off as defensive (though I am) but myself and almost every other member that come on this board is seeking advice and knowledge from other DIY'ers to learn something and get a sense of accomplishment in the end. "your furnace is too big" doesn't learn me anything new

Rant completed.

 

[HVAC Phil]

There is a way to cut back the output of the furnace....

 

[jmlcolorado]

There is a way to cut back the output of the furnace....

Would doing that help the life of the furnace in the long run?
How do you do that? Smaller orifice?

 

[HVAC Phil]

Would doing that help the life of the furnace in the long run?
How do you do that? Smaller orifice?

It would cut the required air delivery to properly cool the heat exchanger due to lack of supply air/return air. You could plug the end burner(pipe plug in gas manifold), assuming you have 4 burners, would cut it back 25k. Is it a pilot or HSI? If HSI, you will have to relocate the flame sensor to the "new" last burner.

 

[jmlcolorado]

It would cut the required air delivery to properly cool the heat exchanger due to lack of supply air/return air. You could plug the end burner(pipe plug in gas manifold), assuming you have 4 burners, would cut it back 25k. Is it a pilot or HSI? If HSI, you will have to relocate the flame sensor to the "new" last burner.

Its HSI. Could i plug the first burner and leave the flame sensor in its current place at the last burner?
Would i need to slow the fan speed to compensate or would it be okay on the factory setting?

 

[HVAC Phil]

Its HSI. Could i plug the first burner and leave the flame sensor in its current place at the last burner?
Would i need to slow the fan speed to compensate or would it be okay on the factory setting?

You could plug the 1st burner, but you have to make sure on how the igniter is mounted for ignition, that's why the last is usually best to plug. Remounting the flame sensor is no biggie, just mount with a zip screw. I would run the fan on low speed(red wire). Now the return you have will be adequate, 4- 8" runs will be fine right off the plenum since the static pressure will be higher. Now this will work flawlessly for you.

 

[Mike007]

Don't plug off any of the burners. It causes uneven heating of the exchanger and can cause premature exchanger failure. There is no way to properly de-rate most modern furnaces.

 

[HVAC Phil]

I wouldn't worry about it. Like he said, it's a garage. I wouldn't de-rate a furnace that was running 24/7 for the actual house, i would install the correct one. I did the same thing for a friend that was tight on cash from a furnace i pulled from a house, been working fine for him over 10yrs. Heat in his garage for min investment. I'd rather de-rate it than see him running on limit due to lack of proper air flow for 100k BTU.

 

[jvitez]

I really don't want to come off as defensive (though I am)
Rant completed.

Glad I could help.

For anyone else reading this, I thought someone could find the information useful that in my personal experience, referencing the comment about heating a 4000 sq ft house, that this range of furnace size is too big, and therefore will not be running at max efficiency. But if it's what you've got, it's better than nothing. I hear you.

My attempt at giving my experience cdertainly generated a surprising long reply. Kind of like banging the sore thumb it seems. Anyway, good luck with your install, and enjoy a warm, even if less efficient, garage.

 

[jmlcolorado]

Glad I could help.

For anyone else reading this, I thought someone could find the information useful that in my personal experience, referencing the comment about heating a 4000 sq ft house, that this range of furnace size is too big, and therefore will not be running at max efficiency. But if it's what you've got, it's better than nothing. I hear you.

My attempt at giving my experience cdertainly generated a surprising long reply. Kind of like banging the sore thumb it seems. Anyway, good luck with your install, and enjoy a warm, even if less efficient, garage.

I should apologize. I think you might have sent that reply at a bad time of day
I spend some serious time reading posts on a number of different forums and I'm always seeing replays that absolutely zero positive input to the OP's question. Not that your didn't add some good information.

I really appreciate all the input from everyone on this board. I probably read threads 80% more then I post, mostly cause there is SO much info, a guy can usually find an answer if he looks hard enough.

BUT, it's much more fun when all the info pouring in pertains to my particular situation.

Once again, my apologizes.

 

[jvitez]

Apology accepted. I appreciate your reply. Respect and politeness is all too rare nowadays, especially on internet forums.

Let us know how it all pans out. With a relatively leaky.not-so-well-insulated garage, your oversized furnace might not be that oversized after all. Warm is better than cold, especially when using tools!

 

[jmlcolorado]

Well has is run, b vent is set. I've fired the furnace with no supply plenum installed and it heats the garage from 40 to 65 in about 7 minutes. It stays off for about 15 before turning on for 5-6 to hold the temp.
I have the inside of the closet all sheetrocked and fire taped as well as the outside. Now I just have to figure out how to make a suPply plenum.
It'll take 2 sheets of 36x48 to make the box. Then I'll steal KyleQ's idea and run 1 8" duct out each side. With 3 supply grills each. That will spread the heat pretty evenly across the whole garage.
I won't be cutting the outpuT any. I'll just live with what it is. It doesn't seem to be short cycling.
This summer I will defianatly be insulating the attic. I came out last night and the 12" of snow on the roof was melting away rather fast and dripping from the frozen gutters
Came out to a nice sheet of ice on the driveway from thE same condition.
Everything is completely up to code (or will be once 100% completed)
Pretty excited to have this project finished so I can move on to the others.