I have the same question. I found this reference:

http://hes.lbl.gov/hes/makingithappe...rcombined.html


In summary:

" If you have a boiler or a heat pump for home heating, you can use it to
provide hot water in what is called a combined, or indirect, system. These
systems are more efficient than separate systems because they eliminate the
extra standby losses of another tank or unit."

--
wrote in message
oups.com...
I have hydronic heat in my house and use a oil fired Weil-McClain
tankless boiler that is about 6 years old (WGO Gold or WGTO Gold I
think). It has one output for the hydronic heat and another for the
house hot water...so I don't have a hot water heater. The whole thing
works fine.

My question is whether this is an efficient setup as far as oil usage
goes. The boiler runs year round cause of the house hot water. I know
the heat circuits aren't being used, but it does heat both at the same
time.

I have been considering getting a Boderus direct vent boiler cause I
hate the power vent I have right now and hear Boderus are great
boilers. Would my oil usage be less with a combo boiler/oil hot water
heater year round? I have also heard about indirect fired water
heaters (superstore?). Any experience and suggestions would be
welcomed. I figure I could sell my Weil-McClain cause it isn't that old
and still get a decent $ for it.