Flue sharing between wood and oil.

tuolumne said:

I am replacing an existing OWB from the 70's with an EKO gasifier. It shared a flue with a wood boiler serving the same house. These basic configuration is 8" round from each appliance going up 20", center to a tee and horizontal into the thimble. This condition has existed for 30 odd years without any issues. I know it would violate todays' codes (not applicable in our town) but what is the true danger? The gasifier will have much lower stack temps than the boiler it's replacing, as well as producing less creosote. What should I be concerned about? What can I do to mitigate these concerns? A new chimney/flue is not an option. Thanks, Chad.

Click to expand...

This sort of thing makes me nervous. Any chance of replacing the oil boiler with a direct-vent oil or propane water heater?

That code is national and applies everywhere. You can do it (I have, before I knew any better), but I don't recommend it for a number of reasons, including the very real possibility that it would void your insurance policy and could endanger your family.

If you decide to do it anyway, at the very least, disconnect and disable the oil boiler and dedicate the chimney to the wood boiler for the heating season. If you want to go back on oil again in the spring, CLEAN the chimney, then disconnect and disable the wood boiler and use the chimney only for oil.

Like nofo says, a power vent for the oil boiler would probably be the easiest and cheapest option.

Eric Johnson said:

Like nofo says, a power vent for the oil boiler would probably be the easiest and cheapest option.

Click to expand...

Actually, NoFo was thinking of a smaller wall-hung unit that could be vented via a 3" stainless low-temperature (Z-vent?) vent of its own through the wall. I ahve a propane model, and I think there are oil fired ones as well. These are condensing boilers - very efficient. My propane unit is replacing my oil for both DHW backup and space heating backup. It cost me less than $500 delivered, though there are other costs involved as always. It's something like 140,000 BTU/hr, but modulates to 20,000 BTU/hr.

Seems like a really good way to free up a flue if the oil is only going to be a backup / DHW heat source.

If it's national code to have separate chimneys, then how does TARM get approval for their dual fuel/ single flue unit ?

Or do they ?

I have just installed an econoburn boiler with an oil boiler in the same flue. It has passed the insurance . I had a stainless steel liner installed by a wett certified installer and that is all they needed done. The oil has to be above the wood pipe with separate openings.

Loki said:

If it's national code to have separate chimneys, then how does TARM get approval for their dual fuel/ single flue unit ?

Or do they ?

Click to expand...

I believe it is because it is ALL in ONE unit and only one fuel can burn at a time. There fear with connecting two appliances is having them both operate at the same time.

In NH You can use mutable appliances in the same flute as long as they are the same fuel only AND the chimney has to be sized properly.

Our house was built with a single flue and two connections, one for the oil boiler and one for the wood stove. Was like that for 15 years and then we had to change insurance and no one would insure it because it didnt meet code, even though it was fine when it was built. We installed a power vent for the oil boiler and all is legal now.

My understanding is for safety that if the homeowner doesnt use common sense and keep the chimney clean the flue could become blocked with creosote and the oil boiler would run and be unable to vent. With no draft sensing device it would run and blow fumes (not easily detectable im told) into the house potentially killing everyone inside (Say its at night and everyone is asleep) Where as if the woodstove is blocked it would blow smoke into the house but also snuff itself. The smoke is easier to smell and the thought is the people inside could smell it.

THis is what I was told by a fire chief that did a presentation at our business. I dont know how this means dual fuel devices are OK and allowable though...

~ Phil

nofossil said:

Actually, NoFo was thinking of a smaller wall-hung unit that could be vented via a 3" stainless low-temperature (Z-vent?) vent of its own through the wall. I ahve a propane model, and I think there are oil fired ones as well. These are condensing boilers - very efficient. My propane unit is replacing my oil for both DHW backup and space heating backup. It cost me less than $500 delivered, though there are other costs involved as always. It's something like 140,000 BTU/hr, but modulates to 20,000 BTU/hr.

Click to expand...

Which brand/model are you planning to use?

Loki said:

If it's national code to have separate chimneys, then how does TARM get approval for their dual fuel/ single flue unit ?

Click to expand...

You can't have multiple solid-fuel appliances on the flue. The Tarm Excel boiler is one appliance, so it does not violate that, even though it has two heat sources within it.

There is actually an accessory package to split it into two flues, if desired (or required by some local code).

RobCo said:

In NH You can use mutable appliances in the same flute as long as they are the same fuel only AND the chimney has to be sized properly.

Click to expand...

You can use multiple oil or gas appliances on one flue. You can never have multiple solid-fuel appliances on one flue in a residential application, or even most commercial applications.

Joe

You can never have multiple solid-fuel appliances on one flue in a residential application, or even most commercial applications.

Click to expand...

When I was visiting Tarm in New Hampshire they had multiple wood boilers installed on the same chimney...I am pretty sure it violated code, but the salesperson said because they were both wood is was OK. I didn't believe him, but found it funny anyway...

tuolumne said:

So...there has been a brief touch on what the dangers are. One seemed to be a blocked flue and the "automatic" oil boiler burning anyway and backing CO into the house and the other seems to be both appliances going at once and having an undersized chimney. In this case, only one appliance will ever be used at once, as the oil boiler can only fire when the wood boiler is cold. Also, the chimney is cleaned regularly. Even if both were running at once the flue size would likely be plenty. Is there any concern with exhaust backing down into one appliance from the other and then out into the room?

Click to expand...

Cinders/ashes from the wood boiler can fall into the oil boiler, as well. I've opened oil boilers to find that they were packed with wood embers...

And, even if you do everything perfect...

If something ever does happen, the insurance company can take one look at that and write your house off, without paying you a dime. It's really not worth the risk.

Joe

Although many of my learned qualified colleagues say no to your appeal ... I, as old man say yes. 2 burn units can be hooked up to the same flue as long as they both aren't solid fuel burners and at least one is of an oil, gas, propane type, but to further clarify my definition ...these 2 units of heat must be on different floors.... Keep good eye on burn residue and you will be OK. This used to be code 40 years ago and no insurance co will deny a claim if you, yourself, a tenant in the building in question has caught fire. Insurance co must honor accidental mistake by holder of policy...

Joey Chang

Up in these parts we have a weekend radio talk show on Saturday Mornings called "Hot and Cold" with 2 local heating brainiacs, Tom Gocze and Prof. Dick Hill from UMaine. This topic comes up at least once a month. The explanation of the creosote oozing down the chimney and covering the exhaust to the oil boiler is the only real hazard. It is the belief of these two men that there have been no documented house fires due to this condition. The oil boiler would probably blow soot all over the place before a fire ever happened. Please correct me if I am wrong and anyone knows of cases where an insurance company had to pay a claim on a fire due to wood and oil being in the same flue.

The only other scenario I can think of is if you had a chimney fire from the wood creosote you'd have a ton of air coming into the chimney from the barometric damper on the oil furnace. That would be a bad thing.

Right or wrong, I know of a few folks who do this, and some have 3 sources into one flue, and have never had a problem. But they keep their chimneys clean for good reason.

JoeyJ said:

Although many of my learned qualified colleagues say no to your appeal ... I, as old man say yes. 2 burn units can be hooked up to the same flue as long as they both aren't solid fuel burners and at least one is of an oil, gas, propane type, but to further clarify my definition ...these 2 units of heat must be on different floors.... Keep good eye on burn residue and you will be OK. This used to be code 40 years ago and no insurance co will deny a claim if you, yourself, a tenant in the building in question has caught fire. Insurance co must honor accidental mistake by holder of policy...

Joey Chang

Click to expand...

I've observed all sorts of situations that utterly defy not only code but common sense (in one case, an oil burner with auxiliary draft booster entering a single flue in a cellar, with two wood stoves hooking into the same flue a floor above) that did manage to function and at least to date, have not burned anything down or offed anyone with CO poisoning.

that said, I'd never do it, or leave such an arrangement in place in my dwelling for very long. why play Russian Roulette?

and as far as insurance companies, the claims handling practices of companies vary tremendously. some are very good at offering coverage when there are grey areas, others seem to think that their essential core mission is to find any and every technicality on which to deny or underpay even valid claims. again, why roll the dice on the reaction insurance will have.

power vents or other through-wall venting, as mentioned, can open up options for your fossil back up

in any event, with whatever system, install CO detectors, and maybe even a sprinkler above the boiler; residential sprinklering has become easier to DIY (look up Uponor's web site)

For the safety of your family and your largest asset, please follow the code and all professional recomendations.

JoeyJ said:

Although many of my learned qualified colleagues say no to your appeal ... I, as old man say yes. 2 burn units can be hooked up to the same flue as long as they both aren't solid fuel burners and at least one is of an oil, gas, propane type, but to further clarify my definition ...these 2 units of heat must be on different floors.... Keep good eye on burn residue and you will be OK. This used to be code 40 years ago and no insurance co will deny a claim if you, yourself, a tenant in the building in question has caught fire. Insurance co must honor accidental mistake by holder of policy...

Joey Chang

Click to expand...

In Canada, under no circumstance can a gas appliance share a vent with any other unlike fuel source. This is written in stone. Due to a possible burner malfuction and resulting raw gas release into the vent. As well a gravity vented gas appliance needs a specific sized vent to run properly as does a fan assisted appliance but in a lesser sense.

An oil furnace/boiler MAY share a vent with a wood appliance if they are interlocked so the wood appliance damper closes while the oil burner is in operation. Oil and wood commonly share the same vent as they both are able to create sufficient draft due to their high vent temperatures. In other words they are more forgiving when it comes to being able to creat draft.

The idea that a wood appliance can create enough creosote in the venting system to block the oil vent has merit but keep in mind most oil burning appliances do have a breaker system on their ignition system that sense if there is a blockage in the system. Anyone who has an oil burner knows when their burner gets dirty when they have to keep resetting the burner only to find out they need to clean the sensor in order for it to work properly. This sensor will also shut the system down when the burner flashes back due to vent blockages. Of course any safety system can malfunction or be bypassed. The barometric damper is not supposed to allow flue gases to escape back into the room but how many of you have discovered the damper to be stuck in a slightly open position? If this happens the oil burner could certainly support sufficient combustion and emit enough CO into the living space to cause health problems.

Two wood appliances may operate on the same vent with a larger vent size (eg 8" vent would handle roughly twice the products of combustion as a 6" vent) but you are only shooting yourself in the foot as the larger the vent, the less draft you will create. And as most of us know less draft is not productive to combustion.

I am confident that the codes will be similiar in other jurisdictions as our codes often chime with the US counterparts but some jurisdictions will have their own interpretations and changes.

At the end of the day it is always wise but not always practical to provide a separate vent for each and every fuel burning appliance in a building.

Be safe.

Very good point Paul. I was thinking an oil burner and a wood stove on a different level....The gas furnace or heater does change the equation....Thanks for that important piece of information.

I see what you are getting at but I would still not recommend using a shared vent 'especially' on two levels. The issue with that is there is a chance that the air pressure on the upper level could be lower than the air pressure on the outside of the building. (eg. strong winds, a reversed stack effect due to a cold vent, or depressurization due to air removed from the building via a clothes dryer, bathroom fan or range hood.) In that situation the products of combustion from the lower level would likely enter the upper level.

Anyhow, not trying to come across as a know-it-all or anything but I have considerable experience troubleshooting issues like this and I think it's important that we all burn as safely as we can. I'm also an on call firefighter so I get to see the often fatal results of unsafe installations.

Take care.