combustion air source for high eff. gas heater

greg0380

Hi, I am preparing for an oil to gas heating conversion; Warm forced air furnace and 50 gallon separate hot water heater. I am using high efficiency gas furnace and hot water heater and will vent them outside via PVC pipe. Can I use my abandoned chimney flu as a combustion air intake for both devices? The flu is located 3 feet behind the furnace and hot water heater. The square inch area of the flu is larger that would be required if I used PVC piping. I want to do this instead of running PVC pipe and cutting more holes in the side of my house and don't want to worry about snow drifts potentially blocking the combustion intake pipes.

HVACNUT

Well the short answer is you dont do any venting unless it absolutely meets the manufacturers specs and meets code in your area.
I&O manuals are very specific how they want their equipment vented. And you most likely cannot combine the two.
Not really a DIY thing if that's where you're going.

greg0380

I am not talking about venting. I want to draw combustion intake air from the abandoned flu.

STEVEusaPA

Usually they are of a balanced flue type and will want vent and combustion air together at the same location.

Leon82

There's a good chance it would inhale old soot and debris plus possibly a bird or squirrel if you don't have a screen

Zman

There are 3 ways to provide combustion air to an appliance:

1.Using a sealed combustion appliance, duct directly from the appliance to the outdoors using manufacture approved ducting (PVC in your case). You absolutely cannot combine multiple appliances to one pipe without a manufacture approved design. The appliances will fight for the air and it will not end well.

2. Provide CA ducts to the mechanical room and have the appliances pull air through the space. The sizing of these ducts is spelled out in the mechanical code and sometimes by the manufacture. The duct size is based on BTU rating of equipment. The sizing of the ducts using this method will be much larger than the size in the first option.

3. Use the air in the space as well as the adjacent spaces for combustion air. The mechanical code spells out how much space is required for this method based on BTU rating of equipment. This is not the preferred method of most knowledgeable contractors as it is counting on a leaky building envelope to provide combustion air. The available air is in this scenerio is also influenced by other factors like kitchen and bath exhaust fans.

That being said, How are you providing combustion air?

EBEBRATT-Ed

You cannot use the chimney for combustion air. In most cases the outlet and intake must be run to the same location so that they are subjected to the same draft/pressure.

A chimney drawing warm air up from you basement will not work

rick in Alaska

I have been running my 95% furnaces using the existing chimney as a chase, which it sounds like you are also trying to do. What I do is put a tee in to the bottom of the chimney and put plugs in both outlets. Then, I drill holes in both of them large enough to put my pipe in. I run the exhaust up through the bottom of the tee and run it up through the chimney cap. Then, I just put the combustion air pipe into the side of the tee and just terminate it there. You just have to make sure the space that is left after you install the exhaust pipe has enough square inch capacity for the needed combustion air requirement.
All that being said, you need to find out if it is ok in your neck of the woods to use the chimney in that way. We only have metal chimneys here, and the codes allow it, but you might have problems where you are at, especially with masonry chimneys.
Rick

HVACNUT

@rick in Alaska
I don't know if something like that would pass for code in my area but that's a pretty cool design. Patent pending? Kind of a self made concentric vent.
When you say metal, do you mean B vent and triple wall, or 26 ga. gal.?
If I understand correctly, the vent penetrates through the center and approx. 12" or more above the chimney cap and combustion air is drawn through what was previously the cap vent.
Like you said, as long as the cap and chase can supply sufficient free air to the appliance.
Do chimneys by you need to run a minimum 3 ft above the roof peak if its within 10 ft of the peak? Any chance of snow drifts blocking the intake?
Again A++ if safe and code approved.
If you're accepting revision options, instead of PVC venting, polypropylene can be sleeved with attached feelers to keep the vent centered within the outer diameter. Might even be a labor saver.

Zman

Running the vent through and existing B vent, making an on site concentric is pretty common. I believe that Lochinvar has an approved detail for that.
What the OP is trying to do is run the exhaust out one way and use the old B vent as CA for 2 appliances.

rick in Alaska

HVACNUT said:

@rick in Alaska

I don't know if something like that would pass for code in my area but that's a pretty cool design. Patent pending? Kind of a self made concentric vent.

When you say metal, do you mean B vent and triple wall, or 26 ga. gal.?

If I understand correctly, the vent penetrates through the center and approx. 12" or more above the chimney cap and combustion air is drawn through what was previously the cap vent.

Like you said, as long as the cap and chase can supply sufficient free air to the appliance.

Do chimneys by you need to run a minimum 3 ft above the roof peak if its within 10 ft of the peak? Any chance of snow drifts blocking the intake?

Again A++ if safe and code approved.

If you're accepting revision options, instead of PVC venting, polypropylene can be sleeved with attached feelers to keep the vent centered within the outer diameter. Might even be a labor saver.

That is correct. The exhaust pipe goes through the existing cap about a foot, and then the combustion air is just drawn in around the underneath part of the cap that the exhaust used to come out of. The existing pipe is Type L vent for oil. I also do a quick brush if there is any soot buildup so it doesn't get in to the combustion air pipe.
I use extra heavy combination cap/plugs I get from my supplier, and screw them down with at least five screws, and do the same with the Tee. I drop my pvc pipe in from the top with a coupler and a 3" piece of pipe sticking out, and run it through the plug in the Tee. Then, I usually will have a 90 fitting coming off the stub pipe that is sticking down, that will turn and go to the appliance outlet. I use the coupler as a way to hold the pipe from falling down.
I basically do the same thing with the intake. The pipe going through the side plug has a short nipple going inside through a coupling, or possibly a 90, depending on where it will go from there, and then a coupler on the inside. If you keep the nipple short enough, it keeps the two fittings close together and keeps the pipe tight to the plug.
I know code says you can not use a chimney as a chase, but a type L vent is considered a vent, and not a chimney, so they allow it here. As usual, check with your code people. We don't really have any here, so it was just up to the gas guys to accept it, and they do.
Rick