Any HVAC contractors here ? I've got a question...

[John Pollman]

Hi all,

I'm seriously considering a new furnace this fall. Right now the one I have may only be about 15 years old but it looks like it's about 30. I can't be sure but I think I remember the previous owner telling me that he did have it replaced when he bought the place in '90. But this thing has got to go. I'm considering a new 90+ unit. I've installed at least three furnaces myself over the past 15 years and I'm sure I can handle the work. But my question comes in the venting of the 90+. I know they use two or three inch PVC pipes that I can just run out the side of the house. I can handle cutting through the brick and installing the inlet and exhaust no problem but the question is their location. Because of the hearth of my firelplace I can't get them out through the end wall of the basement so they'll have to exit through the back wall. (It's only about 11-12' away from the furnace) My problem is that where they will exit will only be about 2-3' to the side and about 3-4' below the window for my family room. Is this going to be a problem ? I can call the building dept. on Monday and ask them but I just thought someone here might know off hand.

Thanks for any input !

John
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[Frank Hagan]

John, I work for a boiler manufacturer, but the rules found in the National Fuel Gas Code are the same for all combustion appliances. The caveat is that the manufacturer's instructions almost always trump the requirements, especially if they are more conservative. So you have to check with the manufacturer to be certain if the vent length is too long at 12', etc.

The basic answer is you need 4' clearance between the vent terminal and any opening into the building that is beside or above the terminal. If the terminal is ABOVE a door or window, it still has to be 4' away in the horizontal plane, but can be just 12" above it. That's because flue gasses generally rise.

The rules are different if you have an opening that sucks in air under power, but I understood your question to be in relation to passive openings like doors or windows.

If the appliance you are installing qualifies as a "Direct Vent Appliance" as stated by the manufacturer then you can install the vent termination within 12" of a mechanical opening such as a door or window. If the appliance does not qualify as a "Direct Vent Appliance" then you have to position the vent termination at least 4' from any door or window to the side or above it. You can terminate the vent terminal 12" above a door or window as long as you are at least 4' away from it in the horizontal plane.

A "Direct Vent Appliance" supplies all air for combustion from the outside, so you have a "combustion air duct" as well as the "flue vent" piping. Salesmen sometimes use the term to mean any higher effeciency unit. So just remember the official definition as found in the National Fuel Gas Code is that the combustion air is supplied directly from the outside in a combustion air duct.

Most furnaces use "room air" to supply the air for the combustion process, and send the air and flue gasses out a vent pipe. That means that the house needs to replenish the room air, so even without a breeze outside, there is air being sucked into your house whenever the furnace is burning. The house is put into a "negative pressure" condition. The 4' clearance in the code is to allow the flue gasses to mix with the outside air enough to dilute them so they are not pulled back into the house.

When you have a combustion air vent supplying the combustion air directly from the outdoors, the room air is not affected. Your house is not "negative". There are even sealed combustion, direct vent appliances that use a single hole and concentric pipe (a pipe within a pipe) to supply the combustion air and vent the flue gasses.

One more caveat, the higher effeciency units tend to condense in the vent pipe, so there are a couple of things you do differently. One, the horizontal portion of the pipe MUST be angled down toward the vent termination to allow the condensate to drip outside rather than back into the furnace (unless other arrangements are made for the condensation) and two, be aware that you may see steaming and condensation dripping from the vent termination. The condensation is a bit acidic, about like vinegar, so it may harm some plants under it. And you don't want to terminate the vent over a sidewalk or place where the resulting slime might cause someone to slip. If you have a sidewalk on that side of the house, there are some other rules that apply, but I won't get into that unless that's the case (this is already WAY too long).

[Ken Garlock]

John, I think Frank has given you more information than you will need. :cool: Let me add a couple comments.

When I lived in Ohio, I installed a 4" piece of PVC to the outside on one end, and up to the furnace combustion air intake on the other. This was an attempt to mitigate the negative pressure problem. It worked OK, but that was a long time ago (1977-79) and better solutions exist today.

A better solution is the concentric flue pipes. They are constructed so that the inter pipe exhausts the flue gas, and the outer pipe supplies combustion air and insulation around the hotter flue gas pipe. We have a sealed combustion fireplace by Majestic that uses the type S-8 concentric pipe.

Lastly, you still need a draft through the flue pipe. That may limit the length of your flue run. If the furnace uses an exhaust blower, no problem.

I may have missed it, but why not just use the flue system of the existing furnace? :confused:

[John Pollman]

Thanks for the info Frank.

I'll check with the instructions before I purchase the unit. I'll have to find out what the total allowable distance is for the intake/vent piping. I may even have to move the intake for my air makeup system. I just realized that is right next to where I was going to place the intake/exhaust pipes. But that's not a big deal.

I put all new windows in the house about 4 years ago. New roof and siding/soffit 3 years ago. And reinsulated 2 years ago. Now for the biggie, the furnace. The house is pretty "tight" and insulated but I think the high efficient furnace will help with the total energy bill in the long run.

Thanks again !

John

[John Pollman]

Hi Ken,


we were replying at the same time. :)

It was my understanding that with a 90+ unit, the old flue isn't useable. I guess because the new furnace will use outside air for combustion. There's a separate intake and exhaust pipe. I'll still need to install a flue liner for my water heater to help avoid destroying my flue from condensation. That's a pretty easy job. I've done three of them and it's not too bad.

[Jim Becker]

Our 92%+ efficient gas system, installed in 2003, uses two 2" PCV pipes for incoming air and outgoing flue exhaust. Given our basement has 2' thick stone walls, that made it easy to route them out one of the two "window wells"...something we could not have done with any system requiring metal flue other than using the existing chimney. (Which is in poor shape and now closed off, never to be used again) It's actually amazing how "cool" the exhaust air is coming out of the pipe outside the house!

[Ken Garlock]

John, you have a good handle on the problem, and as you said, how do you run the flue. OK on the old flue and the intake air.

The other solution is a geothermal system which is about 400% efficient. :eek:

[Frank Hagan]

Thanks for the info Frank.

I'll check with the instructions before I purchase the unit. I'll have to find out what the total allowable distance is for the intake/vent piping. I may even have to move the intake for my air makeup system. I just realized that is right next to where I was going to place the intake/exhaust pipes. But that's not a big deal.

I put all new windows in the house about 4 years ago. New roof and siding/soffit 3 years ago. And reinsulated 2 years ago. Now for the biggie, the furnace. The house is pretty "tight" and insulated but I think the high efficient furnace will help with the total energy bill in the long run.

Thanks again !

John

With your recent upgrades it sounds like you have what the code calls "unusually tight construction", so see if there are specific instructions in the instructions for that particular unit. If there is ducted combustion air, you are probably all set and you'll get the most out of the efficiency of the unit. If it draws air from the room, the appliance is probably relying on your house having a way to supply that air, and "tight" houses don't supply much air! It sounds funny, but sometimes you have to provide a combustion air vent allowing that cold winter air to enter the room with the furnace.

Most furnaces allow from 25 - 50' of piping before you have to reduce the restriction by increasing the size of the pipe. As a general rule of thumb, you count the linear feet of pipe and then add 10' to the count for each ell in the piping. So a straight shot up, over and out is what you are looking for.

You're right that you can't use the exisiting vent ... you need not only the right vent size but the right material. As the furnace gets the last few percentage points of efficiency out of the combustion process, it cools the flue enough that condensation forms. The condensation will rust through galvanized venting, so non-corrosive vent materials are used (PVC, polypropylene, stainless steel, and sometimes ABS).

I'm a critic of the energy efficiency claims, because I see people pulling out perfectly good equipment that is running at 82% efficient and replacing it with 95% efficient equipment, and spending thousands of dollars. The payback in those cases stretches out to beyond the REAL life of the equipment (15 - 20 years). But in a case where you have to replace the furnace anyway, the cost difference between a standard unit at 82 - 84% and one of the more efficient ones is much less. And if you have a unique installation situation like yours (where the house has gone from normal construction to unusually tight construction), then it probably makes sense.

Just be aware that the savings touted for heating appliances are like those MPG numbers on cars. They exist in the lab, but not so often in real life. There is probably about a 10% reduction in energy used between the old and the new furnace, so figure on saving about 10% of your monthly heating bill. That might make the new unit worth the extra cost.