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Todays inspection had a Johnson Furnace in it. The serial number on the unit was #3 stamped into the plate. Mad# looks like BTS100. 100,000/80,000 btu. No blower door switch. A single huge burner (like a hot water heater. The unit ran flawlessly and was very quiet.

There is a strange vent configuration.

I need help with two things:

Age and what do you think of this flue set up? The flue runs through the return air into a box on the outside of the furnace and then up the flue to the chimney. But the box on the side has no bottom.

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flue runs through the return air into a box on the outside of the furnace and then up the flue to the chimney. But the box on the side has no bottom.

The flue/ combustion air should not run up through the return air. The flue gases/ combustion air is running up through the interior part of the heat exchanger. The opening that you can see in the 2nd picture is where the flue gases come out of the exchanger and enter the exhaust vent. That is a typical set up for an old system.

The open bottom just makes it easier to evaluate the heat exchanger for damage.... If you think about it, heat rises. If heat does not rise, what's gonna happen? As long as the return air plenum is sealed, the combustion air will be looped back through the heat exchanger.

There isn't supposed to be a bottom in this case.

I'll probably re- read this post in the morning and need to change it. I'm kind of groggy right now.........

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I agree with Brandon. It was configured this way by the manufacturer, probably to recover some of the heat loss in exhaust to pre-heat return air - high tech for it's day.

Seeing the burner setup would be a great help in guessing its age, but simply based upon the cabinet color and the appearance of the information plate (dating myself) I'd guess that furnace is pushing 45 to 50 years old.

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I'm guessing that furnace could be somewhere between 1959-1964? or so?

I see brand-new oil-fired furnaces that have the exhaust-vent running through the return just like that today. Brand-new. It has always struck me as not a great idea, because you're robbing the exhaust of heat which could possibly adversely affect the quality of the draft. Then what happens if the vent gets rusted-through or compromised with a bad weld? (I always check for that with a mirror, but I've yet to see a problem.. yet..).

Some of these lowboys were used because of the location of the fuel source and the chimney (I guess..) and the duct work in general.

I've never seen a gas-fired unit with the 'draft hood' outside nearby the blower-end like that (yet).

I'd say that the blower door should be kept tightly-closed (maybe even taped) to prevent negative pressure from affecting that draft. How was the 'draft' when the unit was running (with the blower-door closed?).

I'd have put my mirror around the base of that draft hood during burner+blower operation..

If you had given us the age of the house and a stand-back shot...it may have helped... :)

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I see these in apartments or apartments gone condo. Is it part of a multi-unit hi-rise?

Are you sure it passed through r/a for the dwelling and not fresh air for the a/c condenser? Been a few years since I've seen one.

BTW, Trane makes a replacement for these.

Rick, I was going to Google to see what I could scare up in the way of a manual however I can't make out the model & serial number on the plate - what is it?

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My furnace is a Richmond of the same era. I'm guessing mid to late fifties.

The flue runs through the return air for efficiency, the draft hood is open by design to temper exhaust and to prevent scavenging. The micromillivolt gas valve operates off voltage generated by the thermocouple. The input/ output data on mine indicates 82% efficiency... not bad for the times and I have no plan to replace it any time soon... it's bullet proof and simple.

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This raises a thought in my mind regarding these old reliable Sherman Tank heating systems. Why hasn't anyone ever developed an aftermarket add-on untit to take the exhaust and wring out a lot of the remaining heat - or have they?

It should be a pretty affordable retro-fit producing a pretty handsome return.

Anyone ever seen anything like that?

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That furnace is pushing 60, and it's 82% efficient. How much more are you gonna wring out of it? If it ain't broke, don't fix it.

My boiler is at least 40 years newer than Chad's furnace and is 80% efficient. For about $4k I could replace it and reach 84%, or go to $6k for a munchkin and see 86%. I realize there are different ceilings for air and hydronic efficiency, but there is such a thing as diminishing returns. Complicating such a beautifully simple machine would be heresey in my book.


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The furnace pictured is more likely from the early to mid '60s. The external "box" is no different than draft hoods installed inside the furnace cabinet.

It's not a flue connector passing through the return compartment. It's a heavy gauge, welded-seam steel tube. I've never found one leaking byproducts of combustion or found anything but minor surface rust from humidifiers. That's a very common style of furnace here. They're still manufactured in that style for oil-fired systems.

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You guys are a wealth of knowledge. This unit is in a town home. It was built about 1950. A newer Lennox AC has been added and a Kenmore humidifier.

The unit is Mod# BTS100 the serial number is 3.Except for the bottom of the cabinet there is no rust. It runs great and may for many more years to come. I could not find much on the internet so I turned to the best knowledge base I know of. All You guys.

Thanks Again

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Well, there ya go! It's a pretty good bet that's the original furnace based upon our insights and the house age, unless you see evidence of previous electric baseboard heating.

Gee, now that I think about it, I honestly have no earthly clue when electric baseboard heating came on the scene. It may not be an applicable factor here.

Any insights from the brain trust?

Enjoy your weekend!

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