Two Boilers One Flue?

I was wondering the same thing about my gas furnace that would not run at the same time, I have seen people do this code and insurance aside anyone know what the other problems would be??

Code itself is a big enough problem! That is, assuming your house is insured.......

There have been some previous discussions on this, but here is a basic summary.

While it COULD be safe to do this in some cases (in fact, it was 100% legal in our stare - Ma.), there are too many variables for it to be covered under the code. It is a catch-22 sort of situation:

How does anyone KNOW you are not going to use the other fuel?
Is the chimney up to temp. code for the higher temp output of wood as opposed to gas/oil?
Does the chimney have the BTU capacity for the two fuels or even for the new fuel?
Will the wood unit clog up the chimney with creosote, therefore making the oil/gas fumes back up into the house when and if it comes on?

Even when it was legal in Ma and perhaps other places, it excluded gas....was just for oil.
BUT
a listed (UL listed) Multi-fuel furnace or boiler can often be used in one flue - even with gas.

So that is the basic story. Too many possible scenarios.

Many codes refer to " manufacturers instructions". My take on this is since the man. tested & approved this installation, the code allows it.
In addition to the creosote issue, a trained chimney sweep will explain the mixing of products of combustion in the chimney, and corrosive nature of what is generated.
I'm sure someone here can better explain, but even oil, and wood flues in the same chase will destroy a metal chimney cap as teh gasses mix, galvanized, and SS both I've seen rotted out in short time. The installers in my area now use a heavy aluminum for this reason.

But back to you question, we've all seen oil flues with natural gas or LP water heater exhausts cut into them, sometime even done by an LP company.
NG, not safe- don't do it. Though i've been to many fires & even participated in the fire investigation at times, and most don't delve that deep, i would'nt want to be part of a denied claim because of non- compliant work. I fought a townhome fire in 10 degree weather for several hours, then assisted the FI with his investigation. He ruled the fire to be caused by a failure in the metal chimney system. Becuse of the magnitude of the claim ( over 2 million $), the Insurance Co sent out their own FI who took all of the chimney & the boiler with him. Don't know the final outcome, but you can be sure they tried to place that blame with The boiler man., the chimney man., the installer, the service co, the Architect etc.

Here is one older thread for reference:
https://www.hearth.com/econtent/index.php/forums/viewthread/11178/

Oh, goody, we get to argue code again!

FWIW, our gas company will shut down and tag any gas appliance that is connected into the same flue with a solid fuel appliance. The reason is that solid fuel can throw sparks and the gas appliance can dump unburned gas. This can create an explosive condition if they occur at the same time. Even if YOU never run both at the same time, it is impossible to assure that the condition won't happen.

That being said, I still think it is a bad idea for a couple of reasons. First, gas fumes temd to be a lot "wetter" than wood fumes and would probably contribute to more creosote production if they mixed. Secondly, there will be a lot of excess air coming from the gas furnace when it isn't in operation that will cool off the wood smoke and contribute to creosote formation and decreased draft. You also may have problems with draft on the wood boiler because most older gas appliances are basically an open hole into the basement at the draft hood. I don't even want to think of the effects of a downdraft in this situation.

It is possible to get a sidedraft kit for the gas furnace and keep the chimney for just the wood boiler. It's probably less money than installing a separate chimney, but price it out. Might be a consideration.

Chris

Stupid question... but why can't a gass boiler be plumbed into where an oil boiler was previously installed?

It usually can......the chimney may need a relatively low cost aluminum liner, etc.

but the water can be plumbed right in.

In short, there are no plumbing differences that I know of.

I had thought about a combo unit at one time but I don't think I'll do it for many reasons...but, can I attach a wood boiler to the existing chimney that my oil boiler now occupies? I plan on moving the oil boiler to another side of the basement and possibly power venting it.

That is surely possible. Have the chimney inspected to make certain it is up to snuff - my guess is that you may have to line it for a modern wood burner.