I have a boiler that both heats my house and provides the hot water.  It runs
on fuel oil.  Could I substitute vegetable oil for the fuel oil?  I
know that vegetable oil will work in Diesel cars, but am not sure if 1.
Diesel fuel is the same thing as fuel oil, and, 2. If it is (or isn't), 
would it work for my boiler?

pdq2...

The vegetable oil used in diesel engines is not pure vegetable oil.
It's undergone a process known as transesterification, which makes
it usable as oil or diesel fuel. This page on BioDiesel.org explains:

"Is Biodiesel the same thing as raw vegetable oil?"

"No! Biodiesel is produced from any fat or oil such as soybean
 oil, through a refinery process called transesterification.
 This process is a reaction of the oil with an alcohol to remove
 the glycerin, which is a by-product of biodiesel production.
 Fuel-grade biodiesel must be produced to strict industry
 specifications (ASTM D6751) in order to insure proper
 performance. Biodiesel is the only alternative fuel to have
 fully completed the health effects testing requirements of
 the 1990 Clean Air Act Amendments. Biodiesel that meets ASTM
 D6751 and is legally registered with the Environmental
 Protection Agency is a legal motor fuel for sale and
 distribution. Raw vegetable oil cannot meet biodiesel fuel
 specifications, it is not registered with the EPA, and it
 is not a legal motor fuel."
http://www.biodiesel.org/resources/faqs/

This entire page, on the same site, discusses the potential
of its use in replacing heating oil:

"All throughout New England numerous micro-managed pilot
 projects are underway many not being managed as
 professionally as we would like. However, all those we
 spoke with have provided encouraging results that
 improved operational performance was realized with
 blends of 10 - 20%."

"One fuel oil dealer in Maine claims to be providing a
 homeowner with 100% biodiesel at the request of his
 customer and has enjoyed problem free operation. It is
 not recommend at this time for blends higher than 20%
 to be used until industry leadership completes the well
 thought out testing protocols which have been designed
 to validate all parameters of biodiesel blends
 performance from emissions to overall operational
 impacts."
Much more on the page:
http://www.biodiesel.org/markets/hom/


One caution, which also applies to its use in vehicles:

"The materials in the burner were compatible with soy
 diesel with the exception of some gaskets and label
 adhesives which runs parallel with what we know of
 material compatibility issues with biodiesel in the
 diesel fuel marketplace which is clearly outlined on
 our website listed under the Fuel Facts."
[same page as above]


You didn't say where you live, but here's an interactive
map which lists retail outlets in the United States:
http://www.biodiesel.org/buyingbiodiesel/retailfuelingsites/default.shtm


Here's another site, the biodiesel supplier for Texas,
whose site is user friendly. Their page discussing 
biodiesel vs vegetable oil is here:
http://www.distributiondrive.com/technology.html

They are the suppliers for Willie Nelson's retail outlets
in Texas. Naturally, he uses both biodiesel fuel and oil
in his tour bus. Here's Willie's biodiesel site:
http://www.wnbiodiesel.com/


Please do not rate this answer until you are satisfied that  
the answer cannot be improved upon by way of a dialog  
established through the "Request for Clarification" process. 
 
A user's guide on this topic is on skermit-ga's site, here: 
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sublime1-ga


Additional information may be found from an exploration of
the links resulting from the Google searches outlined below.

Searches done, via Google:

biodiesel
://www.google.com/search?q=biodiesel

willie biodiesel
://www.google.com/search?q=willie+biodiesel

HI,
this site that is/was a sponsored link to your question reports on a
book that says that you can treat used cooking oil to burn in your
diesel furnace.
It would seem to make sense that if "bio-diesel" works in cars it
would work in a furnace, which is much simpler and less demanding than
a motor.

http://www.electricitybook.com/

There's some info at
http://www.findarticles.com/p/articles/mi_m1279/is_201/ai_111269259

Ian G.

Contrary to what sublime1-ga stated. Straight Vegetable Oil (SVO) or
Waste Vegetable Oil (WVO) can and is used in diesel vehicles without
going thru the transesterification process (Turn to bio-diesel) The
only thing is that oil must be heated to reach the same viscosity as
diesel before it is pumped into the injection pump so the vehicle must
be modified (In the sense of pre-heating the oil, no motor
modifications) to run SVO or WVO. The same can and is being done with
waste oil burners. Although your boiler will burn SVO or WVO it would
have to be modified to heat the oil to a lower viscosity before being
pumped into the burner head. This way it will atomize and not just
spray in a straight stream(I've been there and done that!) You can buy
modified waste oil burners to fit your furnace. They will also still
burn #2 diesel.