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  1. #1
    Join Date
    Nov 2012
    Posts
    18

    Venting requirements for Tankless Water Heater?

    Hi,

    I live in a condo and I'm thinking about a tankless water heater, or a system like Rheem's "integrated heating and water heating system." http://www.rheem.com/products/integrated_systems/ I also am in need of a furnace, so this would make enormous sense.

    The existing vent is a combined vent for the furnace and hot water heater. It follows a somewhat convolute path, and the mechanical closet does not allow a simple through-wall direct vent to the outside. What are the venting requirements (generally) for a system like this or a tankless heater? (I'm guessing that my 24 year old vents can't be used for this type of system.) Thank you.

  2. #2
    Join Date
    Feb 2010
    Location
    In a boiler room
    Posts
    7,162
    It depends on which model heater you get. Some can be vented with PVC and some must be vented with stainless steel.

    Any of them can vent vertically through the roof, but none of the will be able to use your existing vent.

  3. #3
    Join Date
    Nov 2012
    Posts
    18
    A contractor who bid my furnace told me that a high efficiency furnace would be impractical because the vents would all need to be a different type. Is this the same problem with the tankless system? There doesn't appear to be a simple way of replacing the vents. There is a room above the mechanical closet, the vent is tucked behind another closet and probably follows some kind of chase up through the roof--there is no easy access to them.


    Every picture posts upside down, my apologies. That pic shows the 2nd floor mechanical (three story building).

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  4. #4
    Join Date
    Feb 2009
    Location
    Portland OR
    Posts
    2,001
    Any new tankless is going to require a special type of venting, whether it be PVC or a proprietary brand. The high efficiency(90% or higher furnace) will also require PVC venting.
    The biggest issue I see with tankless water heaters is contractors using the existing 3/4" gas line. A Typical 3/4" gas line can supply about 140,000 BTU's based on running 40' from the meter to the waterheater/furnace. Most tankless water heaters are 180,000 BTU's plus a furnace that is 40-80K BTU's and you can quickly see the issue so make sure a new gas line is specified or have them ensure the existing on is adequate(typically 1" or larger inside diameter)
    Check out my YouTube channel - http://www.youtube.com/user/skyheating1 We have customer testimonials, product reviews and more!
    Like us on FACEBOOK if you like our advice here!

  5. #5
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,580
    As Sky pointed out, the problem is usually size of gas supply and access for new flue and intake pipes. The older tank water heaters used 35-45k burners, and the new "instant" water heaters use burners using at least 3 to 4 times the fuel due to the instant rise in water temperature needed. Putting a new instant unit on an old undersized gas line will create horrible problems, with performance, reliability, and warranty.

  6. #6
    Join Date
    Nov 2012
    Posts
    18
    Thanks for your help. I think I've about given up on the idea of using a tankless + furnace system, and now I have one more reason to do so. Someone I spoke to today gave those hybrid systems a bad review--not to mention the hurdles to overcome to do it. I'm leaning towards a conventional 2-stage furnace/2-stage AC, just have to find the right installer, price and product.

  7. #7
    Join Date
    Jun 2007
    Location
    Minneapolis, MN
    Posts
    742
    Right idea, wrong water heater. We install and upgrade combi-water heating systems every day here in Minneapolis and maintain hundreds of condominium heating systems in which a tank-type water heater provide both domestic hot water and space heating using a variety of terminals including radiant floors and as in your case, forced air.

    In your case we would give you an option to use a conventional water heater coupled with a variable speed fan coil (like a furnace, minus the burner). You may also use a high efficiency condensing tank-type water heater such as American's Polaris or an A.O.Smith Vertex. This type of closed-coupled water heater, fan-coil combination has been used all over the US and Canada for 30 years or so. The systems you have is primitive.

  8. #8
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    I've gone back and forth on the benefits vs/ costs myself. I think in a retrofit, by the time you've installed a pump, piping valves and controls, you're probaby have spent more than the incremental cost of going form a air handler to a furnace in the first place.

    I see the greater benefit when heat loads are <30k BTU and/or you are looking at zoning.

    One other thing to consider, is that with domestic water with recirculation, a water temp of 110-115F is adequate, if not ideal for both effciency, capacity, safety and equipment service life (less scaling of the heat exchanger). When you go to a combi system, you're having to run at a minimum of about 120F and depending on the HW coil and load requirements, it could be 130-160F then use a mixing valve to drop the domestic water temp. I think once you're much above 120F on a tankless, you definitely should be looking at a combi boiler with a closed loop heating system and an integrated heat exchanger for DHW.

  9. #9
    Join Date
    Aug 2001
    Location
    Pavilion, NY
    Posts
    2,173
    And running your space heating temp at 160 degrees turns your 98% combi boiler to around 85% efficient thus a traditional 95% efficient forced air furnace would be considerably more energy efficient. Combi boilers have there place in society but not when you are using forced air if you are looking for top efficiency.
    ...

  10. #10
    Join Date
    Nov 2012
    Posts
    18
    Quote Originally Posted by BadgerBoiler MN View Post
    Right idea, wrong water heater. We install and upgrade combi-water heating systems every day here in Minneapolis and maintain hundreds of condominium heating systems in which a tank-type water heater provide both domestic hot water and space heating using a variety of terminals including radiant floors and as in your case, forced air.

    In your case we would give you an option to use a conventional water heater coupled with a variable speed fan coil (like a furnace, minus the burner). You may also use a high efficiency condensing tank-type water heater such as American's Polaris or an A.O.Smith Vertex. This type of closed-coupled water heater, fan-coil combination has been used all over the US and Canada for 30 years or so. The systems you have is primitive.

    Thank you, that's informative. We are probably going the more conventional route, but I'll keep your suggestions in mind.

  11. #11
    Join Date
    Nov 2012
    Posts
    18
    Quote Originally Posted by kangaroogod View Post
    And running your space heating temp at 160 degrees turns your 98% combi boiler to around 85% efficient thus a traditional 95% efficient forced air furnace would be considerably more energy efficient. Combi boilers have there place in society but not when you are using forced air if you are looking for top efficiency.

    80% is probably as efficient as we can get due to venting limitations and the difficulty of installing drains, etc., in a condo retrofit. Still, everything you said makes sense. 80% will be a significant improvement over what we are running, and we won't be here long enough to see the return on investment in either case. We are only doing this because the valve is gone on the old furnace and we don't like the idea of replacing a valve on a 24 year old system that is cobbled together to begin with.

  12. #12
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,580
    Stupid question, but did you get a second opinion on the bad valve? We actually had a customer call us with the explanation that another "mega" company young tech had told her that her gas valve was bad because it was "cycling too often", and therefore was "going bad" and she simply needed an new furnace (which suprisingly they could do right away). Pure BS...

  13. #13
    Join Date
    Nov 2012
    Posts
    18
    Quote Originally Posted by wahoo View Post
    Stupid question, but did you get a second opinion on the bad valve? We actually had a customer call us with the explanation that another "mega" company young tech had told her that her gas valve was bad because it was "cycling too often", and therefore was "going bad" and she simply needed an new furnace (which suprisingly they could do right away). Pure BS...
    Yeah, I had two contractors last winter in to look at it, and each told me that it needed replacement. It has since gotten worse. The wiring harness is really finicky, I have to play with it to get the furnace to fire. It's pretty scary, the valve shuts and opens in rapid succession now before it finally ignites. I am mostly using space heaters, and only use the furnace first thing in the morning, if at all. It's not that the valve is so expensive to replace, but the system has had some other issues and we don't want to pump money into it only to have something else fail such as the heat exchanger or the condenser. There is also a problem with the limit switch sticking on the furnace. The condenser can be really noisy, there was some evidence of freezing back a couple years ago.

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