Installing a NG Tankless where an electric tank was before

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Brandon Belew, Dec 19, 2018.

  1. Brandon Belew

    Brandon Belew New Member

    Joined:
    Dec 19, 2018
    Location:
    Kansas
    Hello,

    I've taken on a DIY task of installing a 9.5GPM tankless water heater to replace a 50 gallon electric tank I have currently. This is being installed in a closet under a staircase, so space is limited.

    I had a NG heater installed last year and when the plumbers were here I had them put 3/4" stubbed up to where I eventually planned on installing a tankless. So that part should be straightforward, I just need to stub the pipe through the wall and put my shut off and trap in.

    The issue i'm running into which I might just be overthinking is mainly venting. This is a direct vent using 3" PVC condensing Rheem unit. The wall i'm mounting it on is an exterior wall, so I had planned on just coming straight out the wall with minimal piping. But reading the instructions it looks like it calls for building a condensate trap on the intake, but none of those are examples are 1' or less piping. So my question is, can I get away with not doing the condensate trap on such a small run? If not i'll have to run the pipe a couple feet over and then go out.

    The other question I have is mounting -- we have 18" on center studs, and one stud lines up perfect with the center mounting holes on the unit. but the outer edges are all hollow wall, but it appears to be a decently thick sheet of sheetrock. Could I get away with using the middle stud, and putting snap toggle hollow wall anchors on the outside edges? If I were to mount a piece of plywood to the wall over the sheetrock between the two studs and connect the tankless to it. It would bring the tankless out a little too far in the space I have. I'm trying to install it alongside my existing tank so I can take my time, do it right, and not lose the little hot water I still have until the new one is plumbed and ready to go. With that extra space taken up I wouldn't be able to remove the water tank when done.

    Any suggestions?

    Attaching a picture of where i'm adding it and space i'm working with.



    Thanks!
     

    Attached Files:

  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
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    Retired Systems engineer for defense industry.
    Location:
    New England
    As to the studs, personally, I'd put up a piece of thick ply behind it that was attached to the studs, and then screw into the ply. You could use cleats instead.

    It's best to follow the manufacturer's instructions regarding venting. Now, how much condensation you'd get in such a short run, don't know, but you do not want any that may form to pool inside of the burner assembly! This is best discussed with their technical service group to get an answer that will not void your warranty. Get any decision in writing in case you have a problem down the road with warranty when you deviate from the printed installation instructions.
     
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  4. Dana

    Dana In the trades

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    Unless you house gas lines are high pressure and regulated down at the water heater you're not going to be able to run a 199KBTU/hr Rheem burner off a run of 3/4" gas line. Most houses require at least a 1-1/4" dedicated run (no branches or tees) between the tankless and the regulator/meter to operate properly. The lengths of this chart is included "equivalent feet" of all the ells & tees on the run, and it's in 1000s of BTU:

    [​IMG]

    Unless that 3/4" run is VERY close to the meter it's going to be an issue.
     
  5. Brandon Belew

    Brandon Belew New Member

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    Dec 19, 2018
    Location:
    Kansas
    It comes off the meter at 3/4" and runs roughly 10-12' before it T's up to go inside to the tankless, it then reduces to a smaller size and runs to the furnace. The plumber that installed the line for me was informed it was for a tankless water heater which is he put the T there for me. Assuming he knew what he was doing. I do have a call in to Kansas Gas to make sure my meter is up to the task.
     
  6. Dana

    Dana In the trades

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    If it's going to tee off to the furnace the tee should be at the meter end of the run, not the tankless end. In addition to the additional BTU load on the 3/4" pipe, the pressure wave passing the branch to the tankless when the furnace kicks on/off is going to interfere with the tankless. If the tee is at the meter that doesn't happen.

    See this chart for the equivalent lengths:

    [​IMG]

    The 3/4" branch off the tee is another 5.3 equivalent feet, so you have at least 15' assuming no other twists & turns.

    What is the input BTU rating of the furnace?
     
  7. Brandon Belew

    Brandon Belew New Member

    Joined:
    Dec 19, 2018
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    Kansas
    I'm not sure on the BTU rating of the furnace. It's a high efficiency condensing furnace multi stage, thats all I can remember. I am seeing if I can get the plumber that ran the original line to come out and check and/or upgrade it if needed. It's not that far of a run it's just a few short sections of pipe with elbows around two corners and the T before the pipe reduces down.
     
  8. Dana

    Dana In the trades

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    Hi efficiency furnaces could be 30,000 BTU/hr or 300,000 BTU/hr or anything in-between, and if it's a big 'un it might even require upgrading the meter to handle a 199,000 BTU/hr tankless.

    The model name & number is on it somewhere and would be a way to look it up. The BTU in/out numbers are also on a label the furnace, sometimes on or behind an accessible interior panel.
     
  9. Brandon Belew

    Brandon Belew New Member

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    It's a 100,000 BTU furnace. I searched back through my text messages to find it. It's under my house it's fun to get to.

    I have the gas company coming out on Friday to give me some information and have a look. The pressure test crew I guess are off until after Jan 1. If I have to get a bigger meter i'll have to wait until then.
     
  10. Dana

    Dana In the trades

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    At 95% efficiency that furnace could heat my 1920s vintage ~2400' (+ 1600' of insulated basement) house to 70F down to about -100F outdoors. Expecting another ice age? :)

    To find out your actual 99% design heat load, run these numbers on the wintertime gas bills. ASHRAE recommends 1.4x oversizing capacity beyond the load at the 99% outside design temperature as the ideal compromise for hot air furnaces. So if your heat load came in at 65-70,000 BTU/hr in the fuel use load calc you have the right furnace. If like most house it's half that, it's 2x oversized, and could be more than 1.4x oversized even at it's lowest stage. If it's possible to lock it out of going into high fire that might buy you some margin on the gas meter capacity.

    A typical residential gas meter is good for about 275 cubic feet per hour, which is 275,000 BTU/hr. So, with 299K of gas appliance hooked up to it don't be surprised if they insist on upgrading the meter. If the gas pressure can be boosted to something more than 0.5 psi you have some leeway, but you still want to make the run to the tankless a dedicated line, not teed off in the middle of a run to the furnace, where the on/off cycling of the furnace burner can interfere with the operation of the tankless.

    If the gasfitter/plumber reduced it to half inch for the run the 100K furnace it would only be good for about 11 equivalent feet, interpolating from the 0.5 psi lengths chart, and that's only if the run up to that point has excess capacity with all teed off loads running. A home-run for each of those big-burner appliances is in order, and possibly a bigger line to the furnace than what's currently there.
     
  11. Brandon Belew

    Brandon Belew New Member

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    That unit is just half of my house ( which is just our master suite ). The other side of the house has a package unit that, not sure how big it is on BTU's. It has trouble keeping up when it's -10 outside, that side of the house was built in 1920 and isn't very efficient. The two units cover the house which is a little over 3,000 sq ft. That unit has a dedicated run from the meter. The new addition didn't have gas lines ran to it, which is why I had the original one ran last year.

    Kansas Gas will be out Friday to take a looksy at the meter and see if we can upgrade it. The meter we have now is labeled as 250 C.F.H @ 1/2" Diff, with 5PSI as it's MAOP.
     
  12. Dana

    Dana In the trades

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    I live in a 1.5 story 2400' house built in 1923 over 1600' of poured concrete foundation basement (with a very inefficient shape- 14 corners!) I have retrofitted with 3" of reclaimed roofing foam on the foundation, and shot some cellulose into most the framed cavities where it didn't involve cutting into antique horsehair plaster walls. Maybe 5-7% of the wall area remains uninsulated. Most of the windows are original, with clear glass storm windows (1980s vintage). Suffice to say, this is not a superinsulated dream house, and well below current code minimums.

    Buy the heat load of my house at -10F is still under 50,000 BTU/hr. Before retrofit air sealing & insulation it might have been 70,000 BTU/hr @ -10F, but not more. At my 99% outside design temp of +5F (a typical KS design temp) it's under 40K. At the water temps I'm running in the heating system I'm radiation limited to less than 50,000 BTU/hr, and it keeps up at -10F.

    If your ducts and furnace are under a pier-foundation or an uninsulated & well vented crawlspace it's adding quite a bit of parasitic load from duct losses, but nowhere 50K. If your losses are that high it's worth taking a serious look at retrofit air sealing & insulation, as well as duct sealing/balance issues.

    The description "... has trouble keeping up..." is subjective. It either keeps up or it doesn't. But the perception is related to what the observer's expectations are on how fast the thermostats are satisfied, or how long it takes to recover from setbacks.

    A right sized (=1.4x oversize factor on the 99% heat load) furnace for a location with a +5F design temp would only be about 15% oversized for the load at -10F, and running an 87% duty cycle when it's -10F outside, which is nearly constantly. A 3x oversized furnace at design temp might normally satisfy the thermostat in 15 minutes or less at the average wintertime temp of +25F, but stretch it out to a half-hour or more when it's -10F outside, and may take over an hour to recover from a 10F overnight setback on nights when it hits negative double digits, even though it's still more than 2x oversized for the actual load at -10F. For some people that "feels" like it's struggling, but if you measure the duty cycle just maintaining temperature it's possible to get better handle on it.

    Most furnaces out there are about 3x oversized for their design loads. While that level of oversizing is not an issue for efficiency with hot air heating, it's noisier and less comfortable. The furnace is most comfortable when it's running at only moderate cfm, but running nearly constantly when it's cold outside.
     
  13. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Retired Systems engineer for defense industry.
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    New England
    IDeally, the heat source would need to run continuously all of the time. That's not possible with today's equipment, but you can get closer if the heat source has some modulation, the more, the better. All you 'need' is enough to hold a certain temp at a certain external load. Having a bit more than that will help it to recover, which may not be possible if it is only able to maintain. What may take a bit of thought is that if the unit is capable of holding a temp at say -10F, if it got to -11F, the world doesn't end...your inside temp would drop a degree over time, depending on the insulation, and recover as the outside temp does. There are losses each time the thing turns on and off, so running constantly has advantages. Now, if the walls don't have great insulation, they will 'feel' colder, but the room temp may be the same.

    Radiational heating helps, like being in the sunshine on a cold day, you 'feel' warmer than you would if just the air temp were warmer. A draft can mess that up quickly, though. From the Romans to Koreans, warm floors have been around for millennia. They did it mostly with fires and invited CO poisoning if there was a leak, but still, it was comfortable. If you've ever had the chance to tour an old Roman bathhouse, for example, you'd see how they used fires to heat the floors. There's a good example in Bath, England.
     
  14. Brandon Belew

    Brandon Belew New Member

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    Location:
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    The unit on that side of the house has an input of 80,000 BTU, it's an outside package unit. When I said it had trouble keeping up, I meant at a setpoint of 68 when it was -10F outside, it ran all day and never made it to 68. It was lucky to get over 60.

    The way I understand it is the unit is sized correctly for the size of house served, but not so much for an old drafty house. The ductwork is also undersized if I recall since they were limited on space to install it. That unit was installed before I bought the house, Dec of 2016. The unit on the south side of my house, the 100k BTU unit I had installed late 17 early 18. Old side of the house is two story, 3 bedroom. New side of the house is one room ( master living room ), with a master bathroom, and a loft master bedroom.
     
  15. Brandon Belew

    Brandon Belew New Member

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    Dec 19, 2018
    Location:
    Kansas
    I'm pending confirmation from the gas company on my meter -- but I think i've decided to change up my project a little. My house currently has 2 - 50 gallon hot water tanks, one purchased in 2006 and the other in 2007. The 2007 vintage tank is failing, the 2006 seems to be OK for the moment.
    So what I think i'm going to do is remove both tanks and install the tankless on the old side of my house where there is a little more space in the closet, and it's a few feet from the meter so I can easily add a home run 3/4", connect the hot water to the other side of the house so the tankless will run both bathrooms and the kitchen.

    While my kids are still small I figure the 199,500BTU tankless can handle the whole house, eventually, I figured maybe I could add a small 20 gallon tank after the tankless to give me 20 gallons of instantly available hot water even if the power is out and to handle the possibility of everything needing hot water turning on at the same time.
     
  16. Dana

    Dana In the trades

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    The plan to move the tankless to the other side of the house with an ultra-short home run of 3/4" or 1" is sound.

    Outdoors package units are a pretty lousy solution in areas that can get down to negative double digits. The duct losses are terrible! A typical non-condensing package unit has an 80% efficiency gas burner, so your 80K system should be delivering ~64,000 BTU/hr into the ducts, but how much of that is getting into the house? 50K? 40K? 30K? A lot of system design factors will drive that, and some of it is correctable. It's probably worth doing some DIY "recommissioning" of that system if it isn't keeping up with the load- 64,000 BTU/hr is quite a bit of heat, but not enough to heat the whole outdoors.

    In the professional HVAC world furnaces are sized to the size of the heat load, not the size of the house, and include factors such as duct losses and the air tightness of the house. The "gold standard" is the ACCA Manual-J load calculation methodology, but some other approaches can still be pretty good (or even better, with a lot of data input.) If the ducts are undersized, leaking and not well insulated, and not well balanced for supply/return you could be losing easily half the heat to duct design/implementation issues alone when it's -10F outside. Undersized ducts results in higher static pressures and higher duct leakage. Unbalanced ducts creates room-to-room pressure differences, which results in "the great outdoors" being part of the return path.

    The duct leakage can be minimized with buckets of duct mastic, aluminum tape and caulk, if you have access to all of the duct work. Duct balance issues can be mitigated by verifying return paths- do all doored off rooms with supply registers have return registers? If not, partition walls and transoms over doors can be used to create lower impedance return paths to keep more of that return path indoors.

    An Energy Star duct system would have no more than 3 pascals (0.012" w.c.) room-to-room pressure difference under all operating conditions, doors open/closed all air handler speeds, and less than 4cfm of duct leakage per 100 square feet of conditioned space at 25 pascals pressure on the ducts. Typical systems will be multiples of those numbers, some by an order of magnitude or more. Sealing every seam & joint on the duct system, and sealing the duct boots to the subfloor or gypsum board can do a lot for putting the heat where it was designed to go rather than under the house in a leaky crawlspace.

    Air sealing the house reduces both air-handler air infiltration losses as well as parasitic 24/7 heating/cooling losses. The current IRC calls out less then 3 air exchanges per hour at 50 pascals pressure for new construction. That's pretty easy to hit for houses sheathed in 4x8 sheets of plywood, and 4x8 sheets of drywall with any sort of attention to detail during construction, but it's amazing how many houses fail that test. With plank sheathing and plank subfloors it may or may not be able to hit that mark without a lot of expense, but it can usually be brought to less than twice that cost effectively. With vented crawlspaces it's usually easier & cheaper to insulate and air seal the crawlspace walls than the subfloor, especially if there are ducts & plumbing in the crawl space. With pier foundations and a plank subfloor it may take 3-6" of open cell foam on the subfloor in the joist bays to tame that beast, but every home is a bit different. I've never personally had to assess or fix air leakage in a house with an outdoor package unit. Air sealing the attic floor is the other primary focus for air sealing. Leaks of warm air out of the top of the house from stack-effect buoyancy depressurizes the house, drawing cold air into the rooms below. If you stop the leaks at the top and bottom of the stack, the leaks in the middle matter a whole lot less (except when the air handler is running, and the ducts aren't balanced.)

    Just for yuks, run a fuel use heat load calculation using some of last winter's gas bills. December through February fuel use usually has the least error from other factors such as hot water use or solar gain through windows. The fuel use load will also include all of the parasitic duct losses too, since it can't be separated out, so it's really a true upper bound. Calculate the load only at the 99% outside design temp. Multiply by 1.4x to come up with how much furnace output it would take to cover the whole load, including duct losses even at record low outdoor temperatures. The load at -10F is only about 1.25x what the load is at +5F, 1.15x the load at 0F. Whatever your 99% outside temperature happens to be, 1.4x oversizing really is enough to get it through the lowest temperature one would expect to see over the lifecycle of the equipment.
     
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