Sounds like the 30,000 BTU unit is the one for me. The garage is quite
well insulated, but as you say I will have to weatherstrip the door a
bit better. With the two electric heaters in there I can maintain
decent heat, but it takes quite a while to heat it up. I never thought
about air conditioning. That's another possibility. Maybe even just a
window type unit.
I just picked up a 35,000 btu "Reddy Heater". My shop is 20x20 and the
walls are insulated but it has no celing (want storage) and the trusses are
not insulated yet. It was 8 degrees in Iowa the other morning and I fired
it up - in twenty minutes, I could take my coat off.
I had a window unit before I ripped it all out and went this way. It helps
that I do HVAC work for a living! The condensing unit cost me nothing, it is
actually parts from two differant units that were heading for the trash. The
furnace was a scratch and dent from a distributor we deal with.
First, the required BTU's are *NOT* a function of volume, it is a function
of heat loss. Heat loss is a function of area, insulating value, and
temperature differential with the outside.That said... It's complicated.
When I tried to figure this out for my shop ( 500 sf over a 2-car attached
garage, 5.5" fiberglass in the walls/floor and a foot over the ceiliing,
similar climate, northern NY). I ended up guessing at 2 10K BTU toe-kick
heaters. In retrospect, could have gotten by with one.
You could approach this problem imperically. That is, borrow some electric
heaters (if you have enough circuits) and see how they perform. If you could
get four of the little 5K BTU jobbies going and see how they perform. I
would think that 20K BTUs on a chilly day would be a goos starting guess
give you a sense of how much oomph you need.
I would be concerned with dumping all that water vapor (the result of
natural gas combustion) into a space with cold cast iron. Unless you always
keep the space heated, that water vapor will condense on the cold cast iron
and rust it.
The procedure to measure heating cooling needs etc is I believe called
a Manuel J....Not sure but one of my sons is in the heating business
and he wipped out his clipboard and figured out what I needed in about
10 minutes....Bet a local heating/ air contractor could tell the OP
just what he needs... BUT yea you know damn well he would want to sell
you a heater...
After researching, I got the impression that what the best method would be
to have someone with really good experience to "adjust" the formulas. In
theory the formulas work. They are a simulation, and with all simulations,
they simplify/round/estimate/assume. It really depends on the sophistication
of the algorithm. Does it account for the size/quality/tightness of windows
and doors? It probably works reasonably well with tight new construction,
goes downhill with 100-year-old houses where there is alot of leakage and
you don't know what's in the walls.
My gut says that a formula in the hands of the inexperienced could yeild
results that were off by a factor of two. In this case, a poorly fitting
garage door could double your heating needs. The trick is telling the
difference between an experienced heating pro and a hack with a formula that
only takes into account wall size and assumed insulation.
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