I would not close the opening. It is needed to regulate the draft for the chimney. I assume this furnace was originally fired by coal and oil. If it was, then this would be a very dangerous set up. Even with gas you have the potential of carbon monoxide gas coming into the basement.

Here is a picture of gravity furnace with a damper. What I don't understand is if a damper was there originally how it could have worked with the opening pointing down.

I would bring in a expert to test the air flow and see what needs to be done to make it safe and minimize the cold air coming into the basement.

Thanks Mike,

Living in an old house, I just like to have a little information before I call in professionals, who usually have little to no experience working with the heritage equipment and materials found in older homes. In just a few years, I've gotten quite used to fixing or completely redoing the inappropriate work of "professionals." And in this case, I've already had this system serviced and the sellers did too, before selling.

Thanks to your lead, I believe this was at some point the location of a now-missing draft regulator, necessary when this system was fired on oil several decades ago. From what I've read, a draft regulator is either unnecessary or the wrong equipment for a gas furnace, and should have been removed or replaced. I'd still like a little more information about what its absense could indicate and what an advisable fix would be, before I call in a professional.

Here is a link that might be useful: Draft Regulators

I found the same web site you did. I was curious how these regulators worked.

I am not expert on chimney design. I wanted to caution you before you start closing off vents that seem unnecessary. Oil furnaces produce a lot more pollutants than gas so what was absolutely need for ventilation may no longer apply. But you still have to be careful. As people make their houses tighter a small problem becomes a big issue.

Even the professionals don't understand basic draft flow. Keep doing your research. I recommend installing a carbon monoxide detector near the furnace to monitor the levels in the event you make changes.

As Mike said, CO detection today. The old gas equipment had a draft diverter. Its purpose is to separate the appliance from the vent. Without it, in a high wind you could lift the flame off the burner causing outages etc. As well by drawing dilution air the dew point is altered giving a better chance for the saturated air flue gases to be vented to the outside a a vapor rather than condensing in the flue. That flue gas condensate is approx 3.2 ph and very aggressive to the vent material and masonry chimneys.

I would think that your old oil system had the barometric damper in that location, but the barometric must have been installed plumb and level to operate correctly, Some old timer may have figured out that by tipping it down it worked ok and thats all there was to it. As an old timer with a lot of this type experience, I don't know why however.

Get CO detection in the basement, and each floor level.

Thanks guys, will do on the CO detector near the furnace.

My current theory is that when they converted the unit from oil to gas, they removed the draft regulator from this spot for safety reasons, but didn't have enough vertical room to put in a draft hood, as would have been the correct thing to do at the time. So they twisted the flue pipe so that this open T would face downward, theoretically accomplishing something similar to the hood, but in a much less safe and wildly inefficient way.

So, now I'm thinking of finding someone who will install a "dual action" draft regulator designed for gas furnaces in this spot, which seems like it would greatly increase efficiency and be much safer. From what I've read it might be difficult to find someone these days who knows how to do that.

Thoughts?

How long do you plan to live in this house? How much are you spending on heating?

I don't think adding the damper is going to greatly increase the efficiency of this old monster. The basement might be a little warmer, but it is probably not going to have much of an impact on your annual heating costs.

I recommend replacing the furnace with a new one in the spring.

Mike, that's probably good advice, as it's the standard recommendation. But Gravity Heat does come with certain advantages, which from a certain perspective, outweigh the negatives:
1. Fuel Adaptability: in its lifetime, ours has run on wood, coal, oil and gas! If current trends towards economic instability and increasing resource scarcity continue this alone makes our "octopus" very attractive as a second furnace.
2. Dependable: few moving parts, strong long-lived materials, little to go wrong.
3. Ecological impact of embodied energy: As an annualized impact, this equipment's ecological footprint is tiny, even assuming large efficiency improvements, an honest evaluation including the embodied energy of new ductowrk, etc. makes it unlikely that furnace replacement would "pay off" in the lifetime of a furnace.
4. Financial viability: again, assuming huge efficiency increases only possible with home alterations that are likely to have a negative impact on the lifespan of our victorian home---and not including those costs in my calcualtions--a new furnace is unlikely to pay for itself over its lifetime! When I do the math, the payback time ends up being over 40 years, not including maintainance and parts replacement!
5. Depressurized heating efficiency in an old house: lab tests of efficiency assume the relative air-tightness of a modern home. In a leaky victorian home, there are efficiencies that come with not having heat "forced" out of the home by a blower.
6. Reasonably Upgradable: efficiency improvements can be made such as updating the ignition, adding thermal mass, and installing radiator coils in the flue pipe, which was popular in the 70s and can capture 40-50% of the "lost" heat, bringing efficiency into the range of modern heaters. Unlike a new furnace, these improvements would pay for themselves in a few short years.

So, it's not necessarily "cut and dried" and there are good reasons to "drive it into the ground" that you don't typically hear.