Water Heater(s) vent question

I currently have one (NG)water heater in the basement venting into an unlined chimney. I am adding a second one on the second floor for the hot water needs for an upstairs apartment.
As I understand it the chimney is much to large for the single water heater to be venting there.
I was planning on lining the chimney by running down sections of galvanized pipe through the chimney from the roof, then capping off the top of the chimney arouond the new vent. In my mind I'm using the old chimney as a sort of pipe chase.
For the section with just one water heater, I'm assuming I just need 3", what size should I increase it to after I tee in the second heater vent? 4" isn't quite double the area as 3", so would 5" be the correct size?
Is galvanized ok, or do I need stainless? Class B? Besides powered vent water heaters, is there another option I should consider?
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This is important stuff!
The best source of information is the water heater manufacturer's installation instructions.
In years past, there was a lot of waste heat going up the chimney and "draft" was easy to obtain. Draft is heating of the walls of the chimney, this causes heat to rise, and an upward draft in the chimney which then carries the dangerous waste gases up and out of your home.
BUT these days they are making things like this energy efficient. This means less heat goes out the chimney. Draft is harder to obtain. A smaller chimney may be needed to obtain draft.
And of course an air vent to the outside so fresh air can enter the area of the water heater. In order for air to flow out of the house, fresh air needs to be able to flow in somewhere.
If the installation manual does not say you can install two water heaters on the same chimney, I would call the manufacturer of both water heaters and ask if this can safely be done and what size chimney.
The manufacturer's instructions may call for different size chimney pipe depending on the specific model! (In the case of some new super efficient furnaces, small PVC pipe is actually used these days! Very little heat.)
Then another concern I would have is that cold air could come down the old "large chimney" and cool the new metal chimney pipe inside. This could kill any draft you are trying to obtain. You want the metal sides of the chimney pipe to get warm/hot. So perhaps double walled insulated chimney pipe, or a cap on the top to keep cold air out, or some sort of filling inside the chimney?
Again the manufacturers of these products are the best source of proper installation based on the specific model/heat output through the chimney, height of chimney, etc.
"michaelcherr" wrote in message
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michaelcherr wrote:
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you should have a qualified HVAC man put in a proper liner. They make such products for just such an application. They are made of aluminum or stainless.

michaelcherr had written this in response to http://www.thestuccocompany.com/construction/Re-Water-Heater-s-vent-question-17979-.htm : The General Handyman unlicensed types are the only ones I could afford right now, and I know more than most of them, so I'm going to do it myself. I'm unemployed and have plenty of time to do research and do a professional quality/safe job. I am worried about the safetey of the existing setup, which is why I'm asking questions and wanting to do it up right. (I'm also probably loosing the grandfather clause because of me adding the second)
I was planning on capping the chimney with masonry around the new vent to stop drafts. Will that be enough. Going with double wall the whole way seems overkill, but if it's needed, then I can do it. opinions?
The owner's manual shows illustrations where more than one appliance share a vent, so I know at least teh one i'm adding allows it. I will consult the other manufacture.
I am wondering how a professional HVAC person determines the correct size for a vent? I am familiar with/and can find charts and tables for every other trade it seems.
My intuition tells me that since both appliances need a 3" flue which is approx 28.27 sq in then I need a 5" flue after they join which is approx 78.53 sq in.
The rule I found was that for venting into a chimney the area can't exceed seven times the flue size feeding it. Since 78 is less than seven times greater than 28, then I should be fine?
I was thinking about going with stainless class B for the last section exposed to the elements. I've seen way too many rusty galvanized ones.
Normally I see galvanized used for inside the house, but Steve suggested aluminum or stainless. Just guessing, but I don't think I will like the price of stainless. Which would be better for inside the chimney: Aluminum or Galvanized?
Thanks for the time and advise Mike

On Sep 6, 3:13 pm, michaelcherr_at_gmail_dot snipped-for-privacy@foo.com (michaelcherr) wrote:
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I think you should redo you math, a 3" diameter circle has less area than a 3" square and a 3" square has on 9 sq in of area.
Your building department or a good venting book should tell you the size needed. Try NFPA-54 2006 Edition (National Fuel Gas Code). there may be a new addition out there. There is a standard on sq in cross section of vent per 1,000 inbtu btu of the unit. Remember the manufactures instructions or building department may over rule the manufactures instructions.
As for hiring a qualified technician, good luck. I do whole hose performance and test for Worst Case Depressurization and combustion analysis (BPI) and about 40% of the contractor installed units fail worst case, spillige or draft. Unless you can or who you hire can do these tests do nto touch it.
My advice is to install a sealed combustion unit in both the upper and lower or one to service both and forget the natural draft unit.

michaelcherr had written this in response to http://www.thestuccocompany.com/construction/Re-Water-Heater-s-vent-question-17980-.htm : michaelcherr wrote:
Thanks for catching my math error. I used diameter instead of radius. (And I just tested into calculus at the locall community college)
I havent heard of totally sealed combustion for water heaters. I knew that was available for furnaces. When mine dies, I will probably go with that. I had thought about the direct vent ones, but decided otherwise because of price.
I will return the one I already bought and buy a direct vent one, or sealed combustion if available.
Thanks for your helping me to get this done safe.
The direct vent ones are a bit more expensive, but thats far cheaper than the cost of worrying about poisoning the occupants.
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*I've noticed that in several towns in my area the building inspectors are requiring that the original 3" vent installed by the builders be changed to 4" when the water heater gets replaced. It might have something to do with the increase in BTU's, but I am not sure. You should check with your town regarding your installation. As Bill mentioned fresh air is important for combustion and draft. I am wondering if the top of the chimney became clogged with a birds nest or other critter, would the exhaust from the lower water heater come out into the upper apartment? I guess there is probably some sort of back draft damper that can be installed.

michaelcherr had written this in response to http://www.thestuccocompany.com/construction/Re-Water-Heater-s-vent-question-17982-.htm : In case anybody has a similar question, I found the answer to my original question.
http://books.google.com/books?id=W3p0btt2GXEC&pg=PT122&lpg=PT122&dq=sizing+vent+for+multiple+appliances&source=bl&ots=oH5cddS5Uf&sig=S_gJOfKwLne6lUCo54o9SaVnsYs&hl=en&ei=-nakSpu_GJCLnQeL073SBA&sa=X&oi=book_result&ct=result&resnum=5#v=onepage&q=sizing&f úlse
In section 5.11 of the 2006 International Plumbing codes handbook, it says that two appliances sharing a common ventalation system must be on the same story, with an exception for engineered type systems.
I guess I will be going with electric, power vent and run PVC, or running two flues inside the chimney.