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Thread: Tankless water heater install question

  1. #1
    DIY Junior Member David212's Avatar
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    Default Tankless water heater install question

    I am about to install a new HVAC system. In doing so, we want to change out our traditional water heater for a tankless unit. All of the units say I need 3/4 inch gas and water lines. I only have 1/2 inch gas and water lines. How hard would it be to use reducing fittings and what problems would I run into?

  2. #2
    DIY Junior Member jimdarcy's Avatar
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    call Nicor and get the fittings and meter size increased, if you play your cards right they may upsize your meter for free

  3. #3
    In the trades Dana's Avatar
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    Don't worry about REDUCERs, worry about replacing the entire run with gas plumbing of adequate capacity, and the max-rating of the regulator at the meter!

    The gas connection on a tankless heater may be 3/4" but you need WAY more than 3/4" plumbing on the gas runs to meet the BTU-rate requirements of a tankless! Most houses would need a dedicated inch or 1-1/4" line back to the regulator, and not teed off from a run to the furnace/boiler/whatever to get reliable results. In fact the high-fire burn rate of most tankless water heaters far exceeds the peak space heating loads of a typical house in IL by 4-5x or more.

    A smaller tankless runs 140MBH, but many residential units are between 180-199MBH. The size of the plumbing that can deliver that depends on the pressure drop that can be tolerated at max flow, which is a function of diameter, length, and the number (and severity) of the turns at tees & ells.

    For most gas fired appliances, you can use BTU/equivalent length charts such as these:

    http://www.engineeringtoolbox.com/na...ing-d_826.html

    http://www.aprsupply.com/support/tec...-for-iron-pipe

    (There are "equivalent length" charts for the fittings if you think you're cutting it close on BTUs and raw length.)

    On the water plumbing using reducing fittings is fine. If the narrow plumbing hasn't been a problem with flow rate with your tank, it won't be an issue with a tankless.

    May I ask what you're installing for a new HVAC unit, and what it's replacing?

    Indirect fired tanks operated as a zone on a modulating-condensing boiler do about as well as a tankless on water heating efficiency, and by avoiding the extra burner you are less likely to go over the top of your regulator's capacity, and won't have multiple runs of high-capacity gas line to install.

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    DIY Junior Member David212's Avatar
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    Right now I have 20 year old Tempstar that was rated at 80% but is actually running at 60%. I have two rooms that are a good 10 degrees cooler than the ret of the house. Three different contractors told me that my ducts are collapsed (flex aka fuzz duct) and that the blower was plugged up. I'm replacing all of the duct work, getting a Trane 96% furnace and a matching ac unit. We are looking forward to lower utility bills and a more comfortable house.

    As for the water heater, I'm actually leaning towards leaving it alone. My house is on a slab so changing the gas line is not a realistic project. Too bad they don't make a programmable thermostat for gas water heaters. That would be a good compromise.

    Thank you for your input.

  5. #5
    In the trades Dana's Avatar
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    What are the BTU/hour heating & cooling specs for both the old and new systems? (Or the model numbers, so's I can look 'em up.)

    Almost all heating systems in the US are more than 2x oversized for the actual loads, in part to make up for duct inefficiencies. It's worth using designing the ducts to Manual-D spec, using hard-piped ductwork and mastic-sealing every joint & seam, and even FSK-taping the seams on the air handler cabinets, then "right sizing" the mechanical systems to the actual loads. With ducted-air systems this is more of a comfort issue than an efficiency issue, but at 5x oversizing even 2-stage gas furnaces start slipping down the steep part of the efficiency regression curves.

    If you haven't already, start buy doing a room by room heat loss calculation based on realistic indoor temp and the 99% outdoor design temp for your area, the window type/U-factors, wall & attic types & R-values, etc. I can show you how to do a quick & dirty I=B=R method estimate using spreadsheet tools if you like.

    I can't tell you how many time's I've seen 150KBTU/hr furnaces & boilers in houses with 30KBTU/hr heat loads- it's rampant! With 5x oversizing they are theoretically good down to about absolute zero as an outdoor air temp- what's with that? Hopefully yours isn't that gross, but even 2x oversizing is a comfort-mistake. AFUE is tested at 1.7x oversizing, and beyond that furnaces start slipping a bit on efficiency (except for 2-stage or modulating condensing furnaces.)

    BTW: Since this is slab-on-grade, are the ducts & air handler in an attic, above the insulation? If yes, controlling air leakage on the ducts is CRITICAL, since leakage would then drive air-infiltration losses for the house (quite literally) through the roof! Controlling the air leakage at the attic floor/conditioned space plane is also huge. On houses like that when it's time to re-roof, consider putting 2.5-3" of rigid foam above the roof deck and 5-6" of open-cell foam on the underside of the roof deck, sealing off all attic venting (and blower-door test it before the foam guys break down). This puts the ducts & air handler fully inside the thermal & pressure boundary of the house, typically gaining more than 25% on system-efficiency, and even more on comfort levels (particularly on muggy mid-summer days.) It's not a cheap solution, but it's worth it on comfort, even if the payback on utility savings is long.

    Timers/ programmable thermostats on water heaters save next to nothing, since to save anything on standby loss the temperature in the tank has to actually drop. The only way to save substantially that way is if you turn the water heater off THEN take a shower or fill a tub, leaving the tank at a much lower temperature until you're ready to heat it back up again.

    Even at cheap gas rates it's worth insulating all of the hot water distribution plumbing, and then near-tank cold-feed & and T&P overflow plumbing to R4 or so with closed cell foam pipe insulation. This reduces standby loss measurably, and extends the amount of time the already-drawn water in the distribution plumbing remains warm enough to be useful. Most of the foam pipe insulation at the box stores is 3/8" wall R2. Some TrueValue stores will carry 5/8" wall R4 suitable for half-inch plumbing, and Grainger has a wide variety available. Otherwise you may have to go through a local plumbing supply house or internet source and buy a box of it. But the 5/8"-3/4" wall goods is worth seeking out.

    See: http://www.leaningpinesoftware.com/h..._cooling.shtml

    Last edited by Dana; 03-14-2013 at 09:06 AM.

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