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Thread: Melted plastic and now popping noises

  1. #1
    DIY Junior Member FordBlueChevyRed's Avatar
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    Default Melted plastic and now popping noises

    Hello from a old newbie.
    This morning my wife uttered the feared "did you hear that?" I listened and figured out that the water heater was the culprit. It is now about 15 years old and probably needs draining/flushing or possibly replacement. However my main concern and reason for this post is... my mechanical room consists of a high efficiency furnace, a water softener and the now noisy water heater. The water heater is natural gas fed, natural draft vented with metal pipe out through the garage roof. This room also has a fresh air intake to supply combustion air to the room. Here is my problem. When the furnace and water heater are both running a the same time the hot gas from the water heater is not venting out but being pulled back into the room around the water heater vent hood and all the hot gas has overtime melted all the little plastic red and blue parts on the top of the water heater... I suspect that this is a "bad" thing. I'm of a mind to just replace this water heater with one that is power vented so this wont happen anymore.
    Questions ( if I replace the old with a new power vented type)
    1. can I use the existing metal vent to run the power vent into?
    2. Do I need to supply yet another combustion supply beyond what is already vented into the room.

    The house is small.. 1 bath in guest bathroom... not used unless guests visit ... My wife and I are only people in the house nowadays. We use masterbath shower. one dishwasher. I think it is a 40 gal heater but may be 50 currently.

    I am looking for recommendations, comments, directions, suggestions, jokes, good stories.... etc...

    Thanks in advance.

  2. #2
    In the trades Dana's Avatar
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    Power-vented hot water heaters are designed with stack temps low enough to run through PVC (or CPVC or ABS) venting rather than B-vent or single-wall vent piping to eliminate flue condensation concerns. Condensation in metal venting eventually corrodes through, but PVC et al are tolerant of the mildly acidic natural gas exhaust. I don't know of any power-vent units that specifies anything other than plastic venting materials. If the metal stack were all inside of conditioned space the risk of condensation would be low, but if it's passing through a cold garage or attic the condensation risk is pretty real in a MI climate. So, answer #1= "nope".

    The general combustion supply air requirements are the same whether it's an atmospheric drafted water heater or a power-vent when the power vent is taking it's combustion air from the space rather than ducted in (direct-vent). But if the current supply air is too small for the combined furnace + water heater combination (which it might be) it's a good idea to bring it up to spec. The installation manuals for each would tell you how much is needed, but add the grill areas together. If you install a sealed-combustion direct-vented water heater it's completely isolated from and doesn't interact with the combustion air draw from the furnace. There are a few direct-vented HW heaters out there (some are even sold through box-stores), and that would resolve the combustion supply air issut at least as far at the hot water heater is concerned, but it's still worth downloading and reading the installation manual for the furnace to make sure it's up to snuff.

    In some locations (not sure about MI) the subsidy for high-efficiency water heaters make them quite competitive with non-condensing power-drafted water heaters. The smallest condensing A.0.Smith Vertex has a burner ~2x the size of most atmospheric drafted versions, which may or may not require upgrading the gas plumbing, but it's standby losses are a fraction of the losses of an atmospheric drafted tank, and it's raw combustion efficiency is substantially higher too, and with subsidy it's likely to pay off eventually.

    Since it's just you and the missus, a heat pump electric water heater might cut it too. Few states/utilities will subsidize it unless it replaces an electric hot water heater, but in my area the subsidy is close to 100% of the list price of the heater, making it CHEAPER to install than a standard electric HW heater.

  3. #3
    DIY Junior Member FordBlueChevyRed's Avatar
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    Thank you Dana for pointing out the important and not so obvious particulars for not using the original metal vent pipe.
    I will look into a couple brands of water heaters based on some of the other posters recommendations. I appreciate your time and suggestions.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    A backdrafting WH means you are at a real risk of CO poisoning, and you should really resolve this before you potentially never wake up one day! A rough approximation of the required area for makeup air is in the order of 1sqin per 1K BTU. The size of the grate on the grill will affect the overall effective size. If the duct or grill is too long, that will affect its ability to supply air properly. Just where is this air coming from? IF it's in the house rather than outside, that means you must have a leak big enough to supply that air. Hopefully, that's not the case.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Plumber jimbo's Avatar
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    Please heed JAD's comments about CO, and makeup air. Lets say the furnace is 140,000 BTU and the WH is 40,000. SO you would need 180 square inches of make up air. ( less if it is a direct connection to outside). SO a 10" x 20" duct would be needed. Or about an 8" round duct.

    Check for blockage on the air duct, and please install a CO alarm immediately.

  6. #6
    DIY Junior Member FordBlueChevyRed's Avatar
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    Thanks JimD and jimbo for the meaningful information.
    The makeup air duct is indeed a direct connection to the outside.
    No blockage and the length is only about 4-5 feet.
    I will look into the sizes of the furnace BTUs and the Water Heater BTUs and use your suggestions to calculate the required make up air duct size.

    My mechanical room entry door is located in my garage. It also contains an entry into my crawl space. (square opening in the floor of the mechanical room).

    If more make up air is deemed necessary, can it come from the garage, say I install louvers in the mechanical room entry door? If not, I may be able to increase the size of the duct feeding the room from the outside.

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    Forum Admin, Expert Plumber Terry's Avatar
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    Make up air can come from the garage. Often the furnace and water heater is in the garage.

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    DIY Junior Member FordBlueChevyRed's Avatar
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    Terry,
    That certainly makes sense. So then I should be able to test the theory of not having enough make up air by just opening the mechanical room door when the furnace and water heater are running and possibly see the backdraft problem go away. I do understand about the hot gas/air creating the draft in the first place so maybe testing it this way may not produce the needed draft and i would still get the backdraft. I may have to open the door first, get the water heater going and then start the furnace to see if it still pulls back down the water heater vent. Of course then i should check it the other way too. i.e. open door, start furnace and then start water heater to be thorough. Does this sound like a good approach?
    This gives me a light at the end of the tunnel to work towards. Thank you kindly.
    Last edited by FordBlueChevyRed; 12-19-2012 at 10:06 AM.

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    Plumber jimbo's Avatar
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    I don't have the book in front of me, so hopefully someone will follow up on this for me: for a direct connection to outside, the rule is one square inch per 4K btu,so in my example that would be more like 50 sq. inch, 5 x 10. Or a 4 to 5 " round duct.


    IMPORTANT NOTE: in all the calculations about makeup air, the rule requires TWO ducts, each meeting the mentioned sq. area. One intake must within 14 inches from the ceiling, one within 14 inches from the floor.

    Since you ARE backdrafting the WH, you definitely have a makeup air issue. And on reflection, since you are in a mechanical room, I think the higher sq inch may be require. PLEASE someone with their CA plumbing code or National Fuel Gas code handy check my quotes on this! He has an issue, but we want to give him Straight Dope ( no plumbing pun intended!)

  10. #10
    In the trades Dana's Avatar
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    DO open the door to the mechanical room to see if that fixes the backdrafting problem. Test it with the clothes dryer, bath & kitchen fans etc. all running, not just the furnace & HW heater.

    DON'T cut a hole from the basement into the garage for makeup air if the mechanical room is in inside of conditioned space. In many locations that would be a code violation, but a bad idea even where it's allowed, especially if you're storing liquid fuels or cars in there. A small gasoline spill near the makeup air intake could become just a bit too exciting under optimally bad conditions.

    Terry: Furnaces and water heaters in garages are rare in climates as cold as MI, though they're pretty common in warmer climates, or the temperate coastal zones of the PNW where the plumbing freeze-up risk is rare. Furnaces & water heaters in garages are a bad idea in WA area from an energy-use and system-efficiency point of view (if not as bad as the ducts-in-attic-above-insulation practices common in slab on grade homes in the south.)

  11. #11
    Forum Admin, Expert Plumber Terry's Avatar
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    Correction.

    Before I get too carried away with make up air for the water heater from the garage, I do have to consider that it's firestopped between the living and the garage. However, the furnace has a cold air return and air ducts going out to the warm side of the home.

    I can't really picture an open grate between the garage and an inside room of a home. For that to happen, I would think a building code would require fire stopping for that room too. You need to establish a point of defense somewhere.

    But yes, pretty common in Washington State to have the furnace and the water heater side by side in the garage.
    The cold air return for the furnace is from the home, so you are heating air pulled from the home. Garages are pretty leaky, so it's no problem pulling new air for the fire.

    Dana,
    I have two CR-210 Naviens still in the box that I might run. If I do, they need $758.00 to change out my gas meter.
    The meter I have now is good for 325,000 btu's
    That would let me use one Navien. the furnace and perhaps a fireplace log set.
    The new gas meter would let me use a second tankless water heater.
    Last edited by Terry; 12-19-2012 at 02:56 PM.

  12. #12
    DIY Junior Member FordBlueChevyRed's Avatar
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    Another couple steps taken.
    1. Drained the water heater tank to see if it would help the popping noises.
    2. This helped because the WH now wants to run for quite a long period of time.
    3. Match / smoke test verified that the WH was venting properly and the exhaust flue was hot to the touch. (Door was open to mechanical room at that point as the hose was being used out that door)
    4. Waited until the furnace kicked on and re-ran the match/smoke test and still had good venting of the WH.
    5. Unhooked hose, closed the door and couldn't keep a match lit near the WH vent hood. Backdrafting was back.
    6. Cracked the door about an inch and proper venting was re-established.

    Terry,
    Yes the garage and mechanical room are firestopped from the living space.

    jimbo,
    Thanks for the heads up on the code requirements. I will make it a point to make sure that these are followed when I finalize the proper solution.

    Dana,
    The mechanical room is not all that "conditioned" as the room is fed with a direct 4" duct from the outside. The room is fully drywalled and insulated on all four walls and the ceiling. The make up air duct from outside connects from the outside wall of the house, through the crawl space and up into the floor of the mechanical room. It is a smallish room and keeps toasty when the furnace is running.

    My task now is to figure out the best method to supply the needed air to the room, through the door or a new vent in the wall to the garage. I am leaning towards the wall option vs cutting into my steel door.

    When this is solved ( very quickly now ) I can then look into a replacement WH to get this one updated.

    Many thanks to ALL for your concerns and helping me get to this point. I will live to fight another battle.

  13. #13
    In the trades Dana's Avatar
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    "Conditioned" isn't the same as "heated", but refers to it being within the thermal and (here particularly) pressure boundary of the house, and any ignition-barrier walls that are sometimes required by codes between garages and living space. (I don't know MI code, and don't want to look it up- but you probably should before drawing combustion air from the garage.

    Terry: A pair of CR210s is a LOT of water heater- are you filling 100 gallon tubs or something? (If it's a massive-flow shower configuration you're trying to keep up wih you'd be able to nearly the same performance with a single CR210 and a decent sized drainwater heat exchanger.)

    I'm a native of western WA and have some familiarity with how it's commonly done there. In colder climates houses (even garages) are usually built tighter than the western WA standard. I would expect (or at least hope) that WA code would require the garage to be vented to the outside with a cross section sufficient for combustion air for the furnaces & water heaters when they're in the garage(?).

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    Forum Admin, Expert Plumber Terry's Avatar
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    Dana,
    I have a new http://www.moen.com/iodigital in the box.
    Trying to decide whether I'm going to go ahead with the installation or bail on the idea.

    Of course if I go with the Tankless Navien, it's going to have it's own air intake.
    For some reason here in Washington, I'm not seeing anything for combustion air in garages.
    I have seen makeup air which brings in a mix of 10% outside air for the forced air.
    Last edited by Terry; 12-20-2012 at 12:47 PM.

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    Moderator & Master Plumber hj's Avatar
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    You may have another problem which has not been addressed. EVERY room with gas burning equipment NEEDS one "vent" low on the wall or floor, and another "VENT" high on the wall or in the ceiling. This is in addition to the "fresh air intake" and is for ventilation in the event fo gas leakage in the room. IF the room has inadequate fresh air, and that would definitely be the case if that 4" duct is the source of it, then using a power vent water heater will just make the situation worse.
    Licensed residential and commercial plumber

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