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Double Combo vs Double Tee
Asked Aug 25, 2005, 08:01 AM
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24 Answers Hi All, The drains of my two storey home drain to one point at my basement ceiling, and then vertically to the basement floor via a 3" DWV pipe to a 4" sanitary tee that horizontally provides 1) a clean-out and 2) drain to the septic out the foundation wall. The center-line of the 4" sanitary is 9" off the floor. I wish to add a basement toilet and sink within 3 feet of the drain, and tap into the drain. I've found a wall-mount rear-drain toilet with a 3" drain whose center-line can be 10" to 14" above the floor. (Geberit pressure assist toilet with in wall tank and carrier). So, I reckon I should be safe with the toilets drain slope to the main drain line mentioned above; eighth inch of slope per horizontal foot. However, this would require replacement of the drain's sanitary tee with a double connection. To make this work, I'll need to replace the existing 4" sanitary tee with a horizontally mounted double connection; using either a double sanitary tee, double combo wye with eighth bend or double wye. I'd run the existing house 3" drain to one side and the basement's new toilet/sink drain to the other side. I'd run the 3" drains against the wall and come away from the wall to connect to the double connection -- I hope you can visualize this ugly looking thing from a top view; basically, both 3" lines run along the wall and then come way from it far enough to connect into each side of the double connection that comes away from the wall. Therefore, a double wye or combo wye would require more "looping" away from the wall to make the connection than a double sanitary would require. Two questions: 1) Should I be concerned with going with a double sanitary tee, rather than a double wye or double combo wye? I'd prefer the smaller foot-print of the double sanitary tee, but I'm worried that the flow of the 1st & 2nd floor drainage coming into the tee on one side will affect the new toilet/sink drainage (e.g., traps) on the tee's opposite side. 2) I'm certain I'll need to vent this configuration. Yes? I'm thinking my best option would be an AAR. Thank you for you thoughts, David |
24 Answers
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I realize that you can see this lay out in your minds eye but you confuse me when you say you wish to replace a single vertical sanitary tee,( sounds more like a clean out tee to me) with a double horizontal combo or sanitary tee laying on its side. Let's get some terms straight before I start asking questions. Looking at a upright combo or tee the side opening is called a branch while from top to bottom is called the run. As I understand it you're going to connect the existing 3" house main to one side of a branch of a double combo or tee on its side and pick up the new toilet and lavatory from the other side. That leaves both ends of the run open. What am I missing here? Can you make a drawing that you could E mail me? My addy is; speedball1@hotmail.com Regards, Tom | ||||
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| Hi Tom, You are kind for replying so quickly. I figured any description might be troublesome. I have a digital photo I'm uploading, and a sketch of what I'm trying to do. I'll send them to you, and attach them here. You do have it right when you say "... Connect the existing 3" house main to one side of a branch of a double combo or tee on its side and pick up the new toilet and lavatory from the other side. That leaves both ends of the run open. " One end of the run is the clean-out, and the other immediately passes through the basement wall to the septic tank. Any clearer? David | ||||
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Thanks David I'll look for the pictures. In the mean time how do you plan to convert the 3" vertical house drain to a branch of a horizontal double combo? Cheers, Tom | ||||
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| Hi Tom, To move the existing vertical branch pipe to the side of the horz. Double combo, I would extend/reduce the branch's higher horizontal portion to bring the vertical portion down to the side of the existing location. I've sent you a drawing... Sorry for the mind bender. David | ||||
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Good morning David, I can find no fault with your basic layout,( nice drawings) however a few suggestions. Use a short sweep at the base of the 3" house drain. Increase the pipe size of the lavatory drain to 2". If you can not vent to the roof a spring loaded mechanical vent off the lavatory's acceptable. I can't see any advantage to using a combo as against a DWV sanitary tee to exit the house. Go with the tee. Good luck, Tom | ||||
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| Good morning Tom, Thanks for the encouragement and advice. I've attach the diagram here, but the resolution is not good because of having to reduce the file size to fit within this site's limits. If you have a moment, can you clarify a few things? I've not heard of a "spring loaded mechanical vent". I'll look into it. Would a separate vent for the toilet be needed? If not, why? Why use a short sweep at the base of the 3" house drain? Are you assuming 90 deg? Is a "short sweep" less than a standard? (I'm thinking you make this suggestion because the short sweep has a smaller "foot print" than a longer sweep, and a longer sweep would not further improve drainage because the head of the vertical drop in the drainage has plenty of power.) Am I correct in understanding... In using a horizontal double sanitary tee, you think the 3" house drainage from one branch will 1) not negatively affect the house's drainage passing from the branch into the run of the tee, and 2) not negatively affect the trap of the opposite branch's new toilet. Cheers, David | ||||
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When you said in your first post you were going to vent with a AAR I thought you meant AAV,(air admittance vent) which is a mechanical vent used in installations where a vent through the roof isn't practical. To check these out click on; http://www.studor.com/homeowners.htm The toilet will be wet vented through the lavatory vent and should not require a separate vent. You asked," in using a horizontal double sanitary tee, do you think the 3" house drainage from one branch will 1) not negatively affect the house's drainage passing from the branch into the run of the tee, and 2) not negatively affect the trap of the opposite branch's new toilet.?" I wondered about that but your layout doesn't allow for any other setup. However may I make a suggestion? Since you have a pressure assist toilet I don't think the back pressure caused by a gravity swing check valve would hinder the flush action, ( check this out with Gerberit and let me know what they say). I don't see a problem with any discharge backing up in the lavatory since the tee is installed above the level of the toilet branch. Is your layout iffy? Perhaps. Do I wish you would go back and configure it differently. Yes I do. But with what you wish to accomplish you have configured it the simplest way it could be done. Good luck, Tom | ||||
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| Thanks Tom. I respect your thoughts and time, and I don't mean to be too much trouble. We are finishing our basement to make space for our 5 young kids, and I'm hoping to include this toilet/sink. I'm in the planning stages, so suggestion of a better arrangement is possible. I have 1) the existing drain, as shown in the diagram, 2) limited floor/wall space that will allow for a toilet on the one side and a sink to the other and 3) concrete wall and floor. I'm planning on adding a 6" stud wall to the foundation wall to support a wall-mounted rear-drain toilet. Concerning the vent... 1) I mistyped, as I meant an AAV (air admitance valve) as you mention. I've found that the Studor AAVs seem to be the most accepted by municipalities, so I'm comforted by your suggestion. 2) I'm familiar with a "wet vent", but I was not sure a wet vent could be considered through a DWV double sanitary tee and the house drain between the toilet and and lavatory with AAV. Regarding the house drain branch affecting the new toilet branch... I'm not sure I have the space for the 9" length a 3" check-valve would require. I'll have to see. If you have a suggestion for a different configuration, please feel free to share it. In the mean time, I may get a measurement of the distances I have to play with on either side of the existing drain. Cheers, David P.S.: Thanks for your "closet bend" response. | ||||
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That way you wouldn't get the discharge from the house drain shooting directly into the toilet. Sound like a plan? Regards, Tom | ||||
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