Gas gun in oil burner and Chimney Liner

wickable

Can someone explain the rationale for a chimney liner for an existing boiler, not currently having venting issues, being replaced by a gas gun? Assuming the same heat rise, due to the size of the boiler, why are some people saying we need a liner for this application?

-There is no water heater on this, so that's not a factor.
- We had the Chimney swept for safety.
- The chimney is not outside wall.

I'd like to avoid the expense as we are going direct vent in a few years.

jesmed1

I am not a pro, just a homeowner/mechanical engineer in the MA area. We recently looked at the same option you are, with the same setup, ie an existing oil boiler using a terra-cotta lined chimney which is mostly internal to the house, looking to convert to a natural gas burner.

Here in my MA town the building inspector told us we needed to have a reputable chimney sweep inspect the chimney and make a determination as to whether a metal liner would be required. We found a good local sweep, and he said that, despite the chimney's good condition, converting to gas would require a liner.

The apparent reason for the liner requirement is that natural gas combustion produces more water vapor than oil combusion, with the result that the vapor is more prone to condense inside the chimney and mix with combustion gases to form a slightly acidic condensate that will then eat away at the insides of the chimney.

The liner will prevent that from happening, and by virtue of its lower internal surface area (think of the circumference of, say, an 8" circular duct vs the internal perimeter of a 12" x 12" chimney) will keep the flue gases hotter as they move up the chimney, again reducing the potential for condensing.

Again, I'm not a pro, but that's how it has been explained to me. We too decided a liner was too expensive and decided to stick with oil for now.

wickable

Thanks for the reply. Makes sense for a long-term application. Seems the chimney guys are somewhat predatory as I got a quote for 5K for a single chimney, which caused doubt in my mind. It seems pretty simple to add a liner, but I honestly have no idea how difficult they are. For now, we plan to have an annual inspection until we go direct vent.

EBEBRATT-Ed

@wickable

The gas code determines weather you need a chimney liner. Newer boilers and burnere are more efficient. More efficiency =lower flue gas temperatures. Lower flue gas temp=condensation in the chimney and flue. A liner heats quicker than a masonry chimney so less condensation. Gas contains more water vapor (much more than oil ) and generally runs with a lower flue temperature.

This is decided by the heat output of the appliance, fuel burned, size of chimney, location of chimney (inside or outside) and climate location.

Heating contractors don't like liners. They add to cost of the job but if code says you need one you have no choice.

wickable

The contractor says we're ok and is doing chimney cert, but we are getting conflicting opinions. Seems a bit arbitrary, so was looking for more insights. Thank you!

Jamie Hall

It's not arbitrary. The problem is that the combustion gasses from a gas fired appliance contain, as noted, a lot more water than those from oil. That wouldn't be a problem if it was just water, but they also contain various nitrogen oxides and, almost always, some sulphur oxides. When that gas condenses, these dissolve in the water and make an astonishingly aggressive condensate -- much worse than even old oil, and far far worse than ultra low sulphur fuel oil, which is all you can get these days.

Now if the chimney is newer with a terra-cotta liner in good condition, and you are using a conventional -- not mod/con -- gas fired appliance, you may be able to get away without a liner. That would depend on your building inspector or fire marshal. However, if it's older, or has no liner, or you are dealing with a condensing boiler, you really have no choice: you have to have a stainless steel liner or the chimney will be quickly compromised.

realliveplumber

The contractor is incorrect. The chimney must be lined.

I can tell you how many times I have heard "we just want to patch it up, we're moving in 2 years", or "we're going to do this and that in 6 months."

Then life gets in the way.

The chimney is life safety.

If your contractor is not going to be truthful when signing a legal affidavit, I'd reconsider my choice of contractor.

realliveplumber

@Bob Harper is the chimney guy. He writes the chimney books. Maybe he will chime in.

wickable

EBEBRATT-Ed said:

@wickable

The gas code determines weather you need a chimney liner. Newer boilers and burnere are more efficient. More efficiency =lower flue gas temperatures. Lower flue gas temp=condensation in the chimney and flue. A liner heats quicker than a masonry chimney so less condensation. Gas contains more water vapor (much more than oil ) and generally runs with a lower flue temperature.

This is decided by the heat output of the appliance, fuel burned, size of chimney, location of chimney (inside or outside) and climate location.

Heating contractors don't like liners. They add to cost of the job but if code says you need one you have no choice.

It's the same boiler, using a gas gun, so isn't that less efficient?

wickable

realliveplumber said:

The contractor is incorrect. The chimney must be lined.

I can tell you how many times I have heard "we just want to patch it up, we're moving in 2 years", or "we're going to do this and that in 6 months."

Then life gets in the way.

The chimney is life safety.

If your contractor is not going to be truthful when signing a legal affidavit, I'd reconsider my choice of contractor.

Without knowing the details how can you say he's incorrect? From what I'm reading the conditions determine the need ie. the size of the chimney, terra cotta liner, and inside vs outside wall, btu's etc.

Understanding your point of out of sight, out of mind, which is a good flag for sure.

Not trying to argue the point, just want to make sure I have the correct information. Seems like the risk comes down to chimney erosion over time.

EBEBRATT-Ed

@wickable

Burning gas in an oil boiler is fine but you will probably run a lower flue temperature due to the fact that gas contains water vapor mor so than oil. Liners are usually always needed in an outside chimney. With an inside chimney you may not. Depends on the parameters I posted above.

Bob Harper

All chimneys must be suitable for the class of service. If you search this site, you will find me repeating this several dozen times. The code requires a liner. The liner must be properly sized and intact. Terra cotta liners are never installed properly, never hold up, run cold and sweat, and the mortar is incorrect so it erodes even if properly placed, which is rare.
The industry std. is NFPA 211. In Ch 15, you will find levels of inspection. You need a Level II internal inspection by a qualified agency. That will 100% reveal you need a liner if they're doing their jobs.
Many great points above by the guys on sizing, efficiency, condensation, etc. The liner not only contains corrosion and erosion, it sizes the flue to the appliance and, being low mass, heats up quickly to minimize backdrafting at startup. Your chimney sweep should have recommended a level II- call him.
If you check into your state and utilities, you may find incentives to do the CAT IV condensing unit now rather than wait. That money for the liner would go a long way to defraying that cost plus you begin saving on energy costs.

realliveplumber

wickable said:

realliveplumber said:

The contractor is incorrect. The chimney must be lined.

I can tell you how many times I have heard "we just want to patch it up, we're moving in 2 years", or "we're going to do this and that in 6 months."

Then life gets in the way.

The chimney is life safety.

If your contractor is not going to be truthful when signing a legal affidavit, I'd reconsider my choice of contractor.

Without knowing the details how can you say he's incorrect? From what I'm reading the conditions determine the need ie. the size of the chimney, terra cotta liner, and inside vs outside wall, btu's etc.

Understanding your point of out of sight, out of mind, which is a good flag for sure.

Not trying to argue the point, just want to make sure I have the correct information. Seems like the risk comes down to chimney erosion over time.

Its more than erosion over time. As Mr. Harper stated, its also got to do with drafting and introducing it in a timely fashion. And condensation. He is the authority. He literally writes the code books on chimneys. You would be wise to take his advice.

What will you do if you do install the boiler and you start getting a stream of oily water running down the chimney, raining over the heat exchanger, and pooling on the floor?

pecmsg

Just install a liner and be safe.