View Full Version : New home: furnace in garage
01-26-2005, 05:58 PM
Frist of all, my aplogies if this has been addressed -- I tried search and did not find anything.
We are in the midst of building a new 3000 sq ft ICF home here in Dallas and it will have 2 gas furnaces. One in the front, and one for the back vaulted area w/ 2nd story studio and room over the garage. We have had an energy audit, etc. of the design by an engineering firm specializing in such things so I am not concerned about insulation, zoning, tonnage etc.
What I am seeking more info on is the placement of the 2nd zone's gas furnace -- our GC is planning on placing it in the unconditioned garage. It will be in an enclosure suspended from the 8 foot ceiling there, against the corner of the back wall next to the interior wall of the attached garage.
I would appreciate it if anyone could relate to me the do's and don'ts of such an installation in a garage, as it's new to me -- I come from the land of basements! Things such as air supply -- is it just from the garage? Plus, are there any issues with "garage smells" we need to be concerned with entering the house?
I am a bit of a knowledge junky and like to be fully informed with what we are sinking a siginificant amount of money into.
Thanks so much!
01-26-2005, 06:11 PM
Obviously I'm not looking at your house, but it would be a very common thing to put it in the attic space above the garage.
01-26-2005, 06:37 PM
I've seen upflow furnaces installed in a garage, and horizontals in the attic over a garage. Garage odors shouldn't enter the house if the return air plenum to the furnace from the house is properly sealed.
By the way, welcome to Big D. I've lived in this area most of my life...I hope you're ready for hot, muggy summers and winters where the outdoor temperatures swing all over the place.
What is ICF?
01-26-2005, 08:10 PM
Insulated Concrete Form
01-26-2005, 08:15 PM
Shophound- ICF is Insulating Concrete Forms. You've probably seen them since they've really gained popularity but maybe hadn't heard them called that. Basically, expanded polystyrene a.k.a. styrofoam enclosing concrete and attached to each other with nylon, plastic or metal ties. Lots of brands, types and styles out there. As with anything else, some better, some easier to use, some friendlier to other trades, etc. than others. Greg
01-26-2005, 08:16 PM
You don't have anything to worry about, attic installations in the Dallas-Fort Worth area are very common.
But, I would suggest that you insulate the garage ceiling if your unit is installed there. Some G.C. do not insulate the garage. Make be a good idea to have ventaltion in the attic in that area also, ask the G.C. to have fresh air brought into air space in the attic..
01-26-2005, 09:12 PM
Originally posted by dallasbill
It will be in an enclosure suspended from the 8 foot ceiling there, against the corner of the back wall next to the interior wall of the attached garage.
This doesn't sound like an attic to me :(
Bill do you know if this is a condensing furnace or an 80%?
01-26-2005, 09:17 PM
Thanks all! The welcome is appreciated, but I have been here 10 yrs -- but it's the first time we have built a house from scratch and the 2nd furnace positioning was puzzling to me!
I see that someone answered the ICF question. For those wanting more, the block we used is Amvic -- the 6" concrete core variety...http://www.amvicsystem.com/index.html
The space above the garage is a future media room and is part of the 2nd floor at the back of the house... no attic space there. That's why the 2nd zone furnace is in the garage -- space efficiency. I have been assured the return air plenum will be sealed with extra care.
BTW, the garage space is uninsulated in the traditional stick house sense. BUT 2 walls of it are 11" ICF walls (2.5"EPS foam, 6"concrete, 2.5"EPS foam) and 1 interior insulated wall and 1 garage door wall. So, there is a certain amount of thermal mass energy retained and a lot less air leakage than a full stick garage.
The HVAC sub says he has done this before and there will be more than enough combustion air available via the garage space without a vent to the outside. Is there any way to more accurately determine this?
And, I am checking tomorrow to see if it is an 80% -- I don't think it's a sealed super-efficient kind.
01-27-2005, 11:46 AM
Probably not. Especialy if there's no fresh air intake. How will the unit be vented? Sidewall out the back of the garage?
01-27-2005, 11:50 AM
It is vented via a stack passing up through the sidewall and up through the roof.
RE: "Probably not. Especialy if there's no fresh air intake."
Sorry... what is this referring to?
01-27-2005, 05:06 PM
50 CF of free air for every 1000 btu's input of your furnace.
A 50,000 input gas furnace would require an area of 2500 cubic foot.
A 16' x 20' x 8' garage would have 2560 cubic foot.
01-27-2005, 05:14 PM
beenthere... thanks so much! ;-)
I shall get out my tape measure this weekend!
01-27-2005, 05:36 PM
One thing to keep in mind with this location.
In the winter you are guaranteed a frozen
garage in that area.All that combustion air
is coming from some where you know.Most of
the 80% furnaces have a inducer blower motor
that move some where 65 to 80 cfm.
So your 2500sqf garage whould have a
complete air exchange with the cold out
side air every 32.0 min of furnace
operation.16'x20'x8'=2560 cubic feet.
2560 cubic feet/80 cubic feet a min.
(inducer blower)= 32.0 min.
Can you say cold!!!
01-27-2005, 08:44 PM
Thanks 41gasman... and please help me here, being an air exchange & heating details newbie.
When you say "can you say cold" what are you getting at? We get max 10 days a year where the daytime temp is at 32 or below continuosly -- and 10 degrees max below that at night.
And 25/15 day/night more than two days in a row here has not been seen in the 10 yrs I have been here. Longterm Jan/Feb avgs are low 50s day and high 30s to low 40s night.
Are you saying that the furnace/ducts themselves will lose heat due to those temps they are sitting in the garage with? Well, so does the furnace sitting in the attic of my 1926 Tudor cottage now, and I stay toasty at 71.
Please help me understand where you are going and how this relates to your equations -- I am confused.
01-27-2005, 08:49 PM
He's trying to say that your garage itself will become the temp of the air it is pulling in to replace the combustion air that the burner is using.
Uhnless you build in the furnace, and install a combustion intake.
01-27-2005, 10:25 PM
beenthere... thanks again... makes perfect sense when netted out that way.
As stated in my OP, it *is* being built into an enclosure. I will now ensure with the GC that it has an external combustion air source... it will be very easy to run through an outside wall, or even a parallel pipe up -- next to the combustion vent pipe going up ~27 ft ultimately through the roof.
Is there any recommended "size of pipe vs. distance of draw to outside" calc I should talk to him about, esp. if we run it up the side wall & out the roof?
01-27-2005, 10:59 PM
What I was trying to bring to your attention
is what is called "combustion air".This is
the air that get burned in the fire in your
furnace.It goes through heat exchangers
and then out the vent pipe though the garage
roof.The 80% efficiency furnaces have
what is called a inducer motor.This motor
inturn has a small fan mounted on its shalf.
What all this does is suck the combustion
air(the air required for fire) and the flame
its self into the heat exchangers.It than
blows (pushes)the exhaust(all the nasty
byproducts from the fire) up and out your
vent.I had gotten the impression from
one of the posts after your initial post
that in the winters there it can get pretty
damm cold. So I was try to bring to your
attention.That for all practical purposes
the air in the garage will be the same
temperature as the ambient air outside.
If it doesn't get all that cold outside
than no big deal.However if it's ten or
twenty degrees outside than that can
make for some very cold cars in the
One other problem that I have been seeing
more of lately. With the advent
of much tighter(meaning air)homes and
garages.Is a lack of combustion air
in ghe garage! They are building
the lids with double 5/8"x sheetrock
and some really good garage door seals.
So dispite the 50 cubic feet of
air per thousand btu rule.There is
not enough combustion air.
I have been called to "JUST FIX IT"
By two differant contractor.
The problem was the flame would drop
out.What took forever to figure out
was it would happen right after the
garage door was closed.The pocket
of air it would create would pull
the flame off the flame rectifier
"JUST" long enough and dead furnace.
Also would die on longer run times.
Cure,more outside air.
Hope all this helps/not confused
you and good luck with your new
01-28-2005, 01:25 AM
That's why you might want to look at a 90% efficent unit. It will use PVC for fresh outside air intake and exhaust. Just two sidewall pipes or a single concentric flue. Only issue, some people don't like to see water vapor cloud and frozen condensate on the side wall outside when it's cold.
01-28-2005, 10:11 AM
41gasman and johnsp... thanks both so much!
I am meeting with my GC today to discuss options. On one hand, we do not have a lot of *really* cold days here.
Avgs: Nov 66/45, Dec 58/37, Jan 54/34, Feb 60/38, Mar 68/45
I can fully understand now the combustion gas and the air volume and draw required and what can happen. I can see how the temp inside the garage would drop and also can see what could happen if it's sealed too tight.
With those temps, even when it does drop into the twenties here for the short periods it can, I think we could live with that.
My actual concern is "sealed too tight, because 2 of the 4 walls are 11" of ICF and nothing leaks through that. The other is an interior wall that is well sealed. I need to check on the garage door sealing if we go with garage air. And on a vent pipe to the outside for air for combustion gas if we go that way!
I am not sure that we need to go to the expense of a 90% furnace in there and we do not want such pipes out the side wall.
Thanks again... :-)
01-28-2005, 11:01 AM
The pipes out side are not too bad we have lots and lots in Nebraska. My first question is 3000 SF, 11 inch ICF walls, Warm climent , How big are these unit going to be? I wonder if you could get by with much smaller ones or heat pumps and electric strip in the furnace cabinet..... Heat pumps work very nice to 15 degrees above for us.
You may not need any thing but heat pumps... very cheap to operate here. My house is 1500 SF we have a well and a hot tub, all electric home $1100 a year total 6800 heating Degree days, design temp -4. cooling load @95 degree design. 2-1/2 ton 13 seer 10K back up
You seem to be on top of things, check the load sizing cal. It is possible that they forgot the CIF. or wrong out side design temp. You can reduce the load by adding more attic insulation.
The author of Manual J has writen us that there is 15% oversizing built into his prorgam. Most ever other sizing program out there is using Manual J as its bases. Then the person doing the calulation adds some fluff and the furnace contractor adds some more by using the next step size unit and you are way over sized.
01-28-2005, 12:37 PM
You make a very good point on sizing.
The example used by other's for illustration here has been an 80% efficient 50,000 BTU furnace. I still do not know what the BTU rating is on this garage unit. So, I may have more than enough make-up air and I may not see a full air exchange in the garage every 32 minutes on a rare 20F night.
My GC says it's "a 4 ton furnace" and I am getting him to get this info and brand/modle for our meeting later today. I have him convinced that the HVAC person needs to provide us with some more concrete calcs before I authorize anything further!
01-28-2005, 04:14 PM
TO COOL!!! Got'um om the run.
In a good way.Information and
knowledge can be so much fun.
I think I saw something about
enclosing the furnace and bring
in combustion air from outside.
Probably not a bad idea.
BUT make shure that the poor tech
that has to do the maintenance has
plenty of room to work.If you have seen
many of those guys.They have a real
propensity to be rather JOLLY
fellows.If you know what I mean:}
A few other things to thik about.
Light in there with a switch at the
accsess door.Power outlet next to
furnace.But most important!!!
A really good filter and eazy
access to it.Can you say filter
cabnet!!! I almost always use at
least a 4"x16"x25" If were only
using a media filter.
Please come back here and let us
know how your meeting with the
GC and hvac guys went. we are living
viacariously though you!!!
Good luck,and may the comfort
be with you.
01-28-2005, 04:18 PM
Thanks Jedi Master... I shall report back by Monday -- I am now off to Tatooine!
02-02-2005, 04:54 PM
I'm baaaaaaack! It's been quite a week and this is where we are at, some good, some up in the air and that's where I'd like some further advice on how to proceed.
1) The HVAC guys is now adding venting to the outside, through the roof -- in addition to the garage air.
2) Ran new numbers and we are going w/ 2 44,000 BTU units. We are looking at Carrier Performance 80 vs Infinity 80. Variable speed blower on Infinity is an adavatage because one zone is rated at 32,000 BTU, but there are no furnaces that small.
3) A/C is two 3 ton units, one for each zone, w/ the second one having a balancing damper put in as it supplies a lofted space too. Both are Carrier Performance 12.
What was my delay in getting back, you ask??
Well... HVAC guy is using QuietFlex ducting and whoever his installer was did not exactly do it right. I went in there over the weekend and there was 120 degree corner bends in some places, 2" sags in others, and up to 20 degree twists on axis upon entry throught some cut-outs through the second floor joists passing across the garage for the room above. There was lots of duct tape used to seal the ducts to the registers and get this -- no mastic used for the inner air bearing tube sealing, no banding to the register on that and no banding to the register with the outer insulation wrap! Just duct tape to attach and seal! They were also supporting 5 foot spans with 3/4" metal strapping when the spec calls for 1.5" min strap!
I walked through with my GC yesterday to point all that out and he has them ripping and replacing today! Thank goodness for Internet research -- 5 years ago I would have been scr#wed!
More as we go.
[Edited by dallasbill on 02-03-2005 at 12:30 PM]
02-02-2005, 07:35 PM
Just for your info.I don't
know what code is in dallas
but all the manufacter specs
I have seen state that inside
radius is to be 3x duct dia.
So if you have a six"flex
duct the min. inside radius
is to be 18".6"x3"=18" If this won't
fit structure than you have to
use metal elbow.Building it
this way makes a huge differance
in air flow!!! Also make sure
all metal joints Have masic
sealent if joint have a kinda
big gap(air wise)than platsic
webing is required.And always
three screws min. per joint.
120 degrees apart.or as close
as possible.Insulation on
metal ducting here is 3"min.
My statementon insulation
in general is cost you ones.
And it will pay back you
back forever.With no more
work for you!!! Alway remember
A good job is only a pain
in the A$$ once.This is alot
02-02-2005, 09:09 PM
Thanks 41gasman... we have that covered!
Hopefully some of the Carrier experts (Dash?) can comment on the combo question I also have.
DallasBill, best of luck with getting a good installation. Sounds like you are well on your way.
I would like to ask you, what did your heat load calculation say for BTU/Hr at your winter design temp? And what design temp do you use in the Dallas area? If you happen to have the corresponding cooling load figures, that would be very satisfying to a numbers junkie too.
I could offer my figures in return, but mine is not an ICF home and it's very speculative that you are as curious about a 1989 stick built house.
My parents have a 1963 house in North Dallas which is uncannily energy efficient and I have not figured out all the reasons why. It happens to have a full concrete basement which must add a large amount of thermal mass, for one thing. The way Dallas has grown over 40 years, your house might be quite a ways farther out.
Best wishes -- P.Student
P.S. Hope we can set aside our not seeing eye to eye on that insulation subject. I am genuinely interested in learning what I can about your project.
02-03-2005, 09:58 AM
p-student... no problem on the insulation stuff!
I do not have those numbers w/ me... GC has the Energywise engineering study at the site. Will post when I get a chance. I do know that the furnace at the front end of the house (master suite, master bath, walkin closet and 2 feeds to the open-vaulted living room) will be oversized because there are not a lot of units around less than 44k BTU.
That's why I am seeking comments on an Infinity 80 instead of a Performance 80 furnace in one area. I have read that a variable-speed blower is a good idea when a unit is somewhat oversized.
(BTW, we are a block from Flagpole Hill at White Rock Lake).
[Edited by dallasbill on 02-03-2005 at 12:32 PM]
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