Going to tankless in older condo

Discussion in 'Tankless Water Heater Forum' started by Sam S., Feb 8, 2018.

  1. Sam S.

    Sam S. New Member

    Feb 8, 2018
    La Mesa, CA
    I am in a 35 year old condo on the ground floor of a 2-1/2 story building, and would love to replace my aging 40gal. with a natural gas tank-less. I'm working under some constraints, primarily that of the venting which can't realistically be altered.
    The current heater uses a 3" vent piped into the side of a 5" vent originating from the alongside furnace, leading up to the roof, and constructed of galvanized steel.
    Given the high vent output temperatures of conventional tank-less heaters, they would not work in this scenario, but I'm reasoning that a modest-sized (I'm looking at a 6.6gal/min. one) condensing one might, since they only produce temperatures under 150 degrees F.

    The problem I'm encountering is twofold:
    1. The manufacturer doesn't approve of a shared vent.
    2. The units seem to be designed for PVC venting, and although 3" diameter is okay, it's unclear whether it's allowed to be connected to metal!

    I don't really understand all this fussiness about the venting, given the relatively low temperatures involved. There is a separate condensate drain, so presumably the exhaust wouldn't be producing anything acidic/corrosive.

    Also, my (short) gas line is only 1/2".

    Does anybody have any insight and/or suggestions?

    I'm getting ready to just give up and go to a 30 gal. tank, but I'd really rather give this a try!
  2. Terry

    Terry Administrator Staff Member

    Aug 17, 2004
    Bothell, Washington
    The reason a tankless may use plastic for the venting? There is so much moisture in the exhaust, that it would rust out the metal chimney.
    Also, you can't combine the two. The old water heater didn't have a fan assist, the tankless does.
    Your solution is sticking with the tank similar to what you've had.
    Tankless can save $50 a year, but with the added maintenance, you can eat that up pretty fast. And they cost much more to install.
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    Unless you have a high pressure gas line, 1/2" gas supply would never be large enough. Your existing WH is maybe 30-40K BTU (or a little more)...a tankless capable of the volume you indicate is probably in the order of 199K BTU. It's also likely that you'd have to supply outside air for the thing to work properly whereas the existing tank can suck enough air through cracks, the tankless with its much larger burner may have trouble and thus, not work properly.
  5. Dana

    Dana In the trades

    Jan 14, 2009
    Even the gas meters on many condos don't have sufficient capacity for a tankless.

    There is always acidic condensation in the flue with any tankless water heater, and even more so with condensing tankless units. Plastic venting is only allowed for condensing units, since it can't tolerate the higher exhaust temps of non-condensing tankless burners (which require expensive stainless steel venting, aka "Z-vent").

    Navien's newer tankless water heaters have internal compensation that allows it to be hooked up to half-inch or 3/4" gas plumbing and still work OK, making it an easier retrofit replacement for a tank, since the gas plumbing doesn't need to be updated. But there still has to be sufficient capacity at the gas meter for the bigger burner.

    Any chance the condo assn. would allow drilling a hole and side-venting the tankless, with a suitably innocuous looking vent cap? Some can be low flush-mounted with the siding, others with just a hint of rain-hood.

    I assume the motivation here it that there only enough space for a 30 gallon tank, which is a bit less capacity than you'd like? Raising the storage temp to 160F or higher, tempered at the output to 115-120F prior to the distribution plumbing can help with the apparent capacity.

    If it's showering capacity (rather than tub-filling) that you're after, a 4" x 48"-60" tall drainwater heat recovery heat exchanger downstream of the shower can more than double the apparent capacity, turning a 30,000 BTU/hr burner 30 gallon tank into an "endless shower" experience at 2.5gpm at soCal type incoming water temperatures, if there is reasonable access to the drain plumbing, and not too hard to hook up to the tank & shower.

  6. Sam S.

    Sam S. New Member

    Feb 8, 2018
    La Mesa, CA
    Thank all of you for your helpful replies!
    Yes, I understand now why metal is disallowed.
    I had thought that the acidic moisture is what the condensation drain is for (I'm just talking about condensing units here), but apparently the exhaust is indeed still moist and acidic (plus it's indeed forced).
    Mine is just a 1 bedroom condo, by the way, on the ground floor with a neighbor's unit (with loft) above. The WH I'm looking at only requires up to 13K BTUs.
    There are no exterior walls nearby.

    You know, as I was awakening this a.m. I was thinking about the ceiling in the utility "closet" that contains the WH and furnace. There is a large (10-12" dia.) duct that leads up through the ceiling which is extended to a couple of inches off the floor. I guess that's an air intake for the water heater, since the furnace has its own intake via ducts below it leading into the kitchen ( this closet is about a foot above ground level). I imagine that, assuming it shoots straight through the neighbor's above and up through the roof, I could conceivably run a 25' or so length of 3" PVC through that to serve as the exhaust without obstructing much of its intake ability.

    But of course, this all requires:
    1. finding 3" PVC
    2. figuring out just how the heck to lower, say, a 10' length of tubing through the roof vent downwards, suspend it while I glue the next length, repeat until in place...
    3. getting the condo association to sign off on this

    But maybe this is all getting a little above my pay grade. Just getting a slightly smaller, new 30gal. tank might be WAY simpler (kind of what you were hinting at, Terry), even though I love the idea of tankless mostly for environmental reasons, secondly for economic. My old unit is not yet leaking--well made back then!
    Perhaps the newer tanks are more efficient than before. Plus, one bonus for us in SoCal is that a tank does in theory work as a sort of emergency backup water supply, no?

    Again, thanks, all, for your valuable input!
    - Sam
  7. Dana

    Dana In the trades

    Jan 14, 2009
    Even at a mere 40F rise (65F incoming water, 105F at the shower head) and a 2 gpm (1000lbs/hr) low flow shower head takes (40F x 1000lbs/hr =) 40,000 BTU/hr. A 13,000 BTU/hr water heater isn't even good for 1gpm. Are you sure you're not looking at 130,000 BTU/hr? That's still a big burner, probably 3x the burner size of the furnace (unless the furnace is completely ludicrously oversized for your space heating load, which it could be, often is...).

    Add up the input BTUs for the existing furnace and that of the tankless water heater, the kitchen stove, and any other gas appliances, then take a look at your gas meter. There is usually a CFH (cubic feet per hour) number on it. A cubic foot of natural gas is ~1050 BTU, so a CFH of 150 (a common size for condos & apartments) is only good for ~160,000 BTY/hr, and a 130,000 BTU/hr tankless and a 4 burner gas range with a gas oven would pretty much take up the remaining capacity, even without the furnace.

    If you were getting rid of the furnace too, replacing it with a hydro-air handler running off the water heater (which would probably make sense, and would be possible to right size for the space heating load) you'd have a bit more headroom on the meter, if it turns out yours is that small. In soCal it's pretty hard to get accurate heat load numbers from fuel use per heating degree-day, but if you ran this math on a couple of winter months' gas billing you could establish a firm upper bound, and be able to at least spec a hydro-air handler that would work.

    Alternatively, if the ducts are also used for air conditioning, a heat pump carefully sized for the cooling load (not ridiculously oversized, the way most are) would still be pretty efficient for heating the place, despite being oversized for your actual heating load, and would be a lot greener, given the increasingly low-carb grid mix in your area.

    But that's a lot of expense just to get you into a tankless, eh? :)

    A condensing 38 gallon tank type water heater like a Rheem RHE40S might work without blowing the gas capacity budget (it's only 40,000 BTU/hr), if it fits, and you can side-vent it or run the venting up your make-up air chase/vent. From a water-service point of view it's probably better than a 130K tankless in almost every respect except 24 hour continuous showers.
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