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Old 02-16-2008, 08:04 AM   #1
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tankless WH questions


My NG tank WH is on its last legs and I am interested in going tankless. But I have a couple of possible issues.

First is that my 3 level townhome, with utility room on otherwise finished lower level presents limited venting options. The original WH is vented in the same type B stack as the NG furnace so that is not an option. My only option looks to be to take the Cat III stainless vent out the front side of my house on a piece of wall above my hose bib. It looks like a good place and meets all the required clearances except it I won't be able to maintain greater than 12" from an inside corner. It'll be more like 8", but it is brick. So irrespective of a possible code conflict is there any *practical* problem with this?

Second, the unit I am looking at is 199,000 btuh. My gas supply line is 1/2" diameter 2 psig to the manifold and regulator. The manifold is 1" iron pipe with 5 outlets to furnace, range, gas fireplace, gas grill, and tank water heater. Tank WH is 40,000, range/oven is 60,000, furnace is 80,000, gas grill is 40,000, gas fireplace is 30,000.

Is this gas supply likely sufficient to run the tankless WH and the furnace at the same time? I would run 10' of 3/4" supply line to the tankless.

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Old 02-28-2008, 10:36 AM   #2
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tankless WH questions


1) On low pressure systems you may also have problems with the meter sizing.

2) Watch the clearances on the CAT III vent.

3) Unless this is a direct vent unit pulling it's combustion air from the exterior adding 200K BTUs may:

A) Exceed the available interior combustion air.

B) De-pressurize the structure, causing other appliances to back-draft.

C) Draw in additional exterior air, causing the interior humidity in cold months to be lower than you might prefer.

I've put up a page on these and other tankless water heater installation issues I have encountered on my own projects and at Home Inspections here in Chicago / Evanston

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Last edited by Michael Thomas; 02-28-2008 at 10:46 AM.
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