Unit Heaters
There are a number of different gas furnace options available. You've
got "Unit heaters" that hang from the ceiling. They are typically
30x35x18 or so. I'm sure you've seen them. They are pretty good and
around 80% efficient. The Dayton units can be converted between
LP and natural gas. A 125,000 btu unit sells for around $650. A 75,000
unit sells for about $480. A little costly..
To meet OSHA requirements the bottom of the heater must be 8 feet off the
ground! Fortunately most residential installations don't need to meet
OSHA. My local inspector said he wanted 6 feet under the heater.
That's still a problem in some cases. I suspect he might have been
flexible given the right installation circumstances.
These furnaces vent outdoors. Ideally you'd want one that also draws
input air from outside, especially if your garage is well sealed. I
didn't find any units that did this. I probably wouldn't have been
willing to pay the price if I did.
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Big Radiant heaters
There are a variety of ceiling mounted radiant heaters available. Most
require a very high ceiling. Since I couldn't accomodate one I don't
have much info.
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Household furnaces
In some cases a regular household furnace can be the hot ticket. You
can buy new or used. A great many used furnaces end up on the market
as folks upgrade to higher efficiency models. These can be a great deal.
You can't sell a _new_ furnace that is less than 80% efficient.
A word about modern high efficiency furnaces.. I wouldn't put one in
a garage for various reasons (a furnace supplier convinced me it was a
bad idea) Most of them do NOT like to draw cold air. That is
important if you want to keep it cold during the day and flick it on
when you get home. I was surprised that they were so intolerant of
cold air. A contractor told me the Carrier brand units they sell
don't want less than 58 degree intake air! The manual for the brand
new (2 weeks old) Lennox furnace in my house specified similar
requirements. I believe it didn't want less than 55 or so degrees.
The >90% efficient furnaces condense the water out of the combustion
gases for more efficiency. You have to make certain that the drain
tube does not freeze.
Furnaces do not like fumes. The corrodes the heat exchangers. That's a
good reason to not invest in a mega dollar furnace.
Last weekend I purchased a used 80% efficient furnace for $70. It's
about 15 years old but in pretty good shape. It is a 135K btu unit.
The building inspector says it will need a safety inspection by a
certified furnace tech (the manual on my new Lennox even recommends a
full inspection each year) - check exchangers for cracks, etc.
A friend of mine did all of this with a $50 used furnace. A couple
of monthes later he had to spend $160 on a new motor. It was still
a good deal..
I don't intend to do any particularly elaborate duct work. It won't
be necessary for my space.
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There is a lot written about how to duct and vent furnaces and what is
acceptable. Fortunately it gets much simpler when you're only dealing
with one room.
In regard to venting you're going to have to seek out specifics
elsewhere. I can only give some general guidelines. Your local
building supply store should be able to provide you with a proper vent
setup but you must check local code.
You can't reduce a vent. If it is 5" at the furance it must be 5"
all the way out. You can't arbitrarily go larger either - it can
cause condensation in the vent. If you have to jog or "s" curve the
vent you'll have to consult the many tables that specify what angles
are acceptable, etc.
If you go into a crawl space, attic, etc you must use a double wall pipe.
I believe it is also mandatory when you go through the roof. Vent
assemblies sell for about $80-$100.
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Air Source
Your furnace can be setup to draw air from inside the garage or outside.
Again, it depends on the specifics of the install. If you're dealing
with fumes it may be much better to vent from outside.
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Car vs. Furnace
They tell me that a furnace installed in a car bay area (where a car could
hit it) must have cement posts sunk into the ground to protect it.
Fortunately that isn't the case in my installation!
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Gas Lines
You'll have to consult the charts to determine what is appropriate for
the Btu requirements of your furnace. The charts will tell you how
many cfm a given length of pipe will flow.
Line depth around here is supposed to be 18" or more.
Most suppliers will thead black or galvanized pipe for you. Flexible
copper tubing is generally considered the way to go underground. Note
that a given size copper does not flow the same amount as a given size
black pipe.
You'll probably have to pressure test it for the inspector.
Electrical
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I'm no expert here (or anywhere else on this stuff). Fortunately I have
a 2 week old furnace installation for reference.
Must be to code, of course. Furnace must be on its own breaker. The
furnace at my parent's house has a mini-breaker near the furnace. I
suspect the breaker at the box services something else, hence the
second breaker.
Flexible metal conduit to furnace and a switch in a metal enclosure.
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Tomorrow I meet with the inspector to review my plans and to obtain
permits. Saturday the 30 foot trench gets dug between the house and
the garage with a backhoe and most of the work gets done. Big fun!
Sorry for the length, but I thought others might benefit from my
recent experiences. This obviously is not a substitute for the many
books necessary to cover a complete and safe furnace installation, but
I hope it helps (probably not as much as my Dad will on Saturday ;->
Brian
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bkelley@ford.com
Not speaking for Ford.
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