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Gas Tankless hot water needs

I found this site through a recommendation on a boating site.

I have a 2.5 bath, 4 bdrm home with a wife and 3 teen/pre-teen boys.....our 50 gal gas hot water tank runs out quick between showers and all the laundry/dishes we have to wash. The current hot water tank is in the garage with the exhaust vented with galvanized pipe up through the roof. I've wanted a gas tankless water heater and am quite handy, but I know nothing about these units.

What do I need to know? what type "rated" equipment should I look for? what brands to search for/stay away from?

For install, It seems I could remove the exisiting tank, mount a tankless unit to the wall, use flex tubing for a 2-3 ft run of gas and duplicate the exhaust venting so that it comes from the tankless unit on the wall at aprox the height the tank WAS at. I understand these units need 110V and the 200A main panel is in the garage so that won't be a big issue. (previous gas tank unit did not require electric). Do I have the install "figured out" or is there more to it than it appears?

Thanks for any help you can provide.


  • Jack
    Jack Member Posts: 1,046
    A couple things

    The biggest mistake in installation of tankless is inadequate gas line capacity. Your current gas line is likely undersized. You will be looking at a tankless in the range of max, 180-199kbtu vs yur current 36-40kbtu. With the increase in gas line, you will not be able to use the same vent system. You absolutely want a direct vent unit.
  • t500hps
    t500hps Member Posts: 4
    Gas Tankless hot water needs

    Thanks for the info, the main black iron pipe that feed gas to the house comes up right beside the current hot water heater and only has a 2-3 ft run from it to the tank. Replacing that line with a larger one is no big deal......no such thing as too large so something noticably larger than the fitting on the tankless unit should be fine especially for a 2-3 ft run. (right?) What is "direct vent". The current unit vents straight up through the roof, are you refering to the more effiecient type that vent out the side of the house? It would be in the garage and about 4 ft from the back of the house so installing that type wouldn't be difficult either, just curious what's required.

    also, any equipment recommendations? (I had the first shower of the AM this morning and it was a little cold, I need to get something else installed)
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,244
    I realize you are handy

    I also realize you feel plumbing is well within your skill set. But please gas piping is not a hobby. The gas line needs sized for the complete load of the building use. Being an informed consumer is critical in choosing a tankless heater. Confirm your water chemistry and the water usage you will have. Bosch is a heater I like as far as tankless units. Navien also makes a good unit. Like boilers the installation is as or more critical than the brand when you go with the major brands.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • t500hps
    t500hps Member Posts: 4
    might is the operative word....

    I'm asking cause it "seems" simple, but I realize there could be plenty I simply don't know. I have done minor plumbing, even installed the current hot water heater. I also installed a permanent gas grill on the deck and installed a "T" under the house in that iron feed line to supply fuel to the grill years ago (admit it made me nervous but it's been fine for the last 10-11 years).

    So what's the "going rate" for this type of installation? Unfinished garage. As your signature says, I want value and doing it myself may not be the best option......I'm just considering all the options right now.
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,777
    A different question

    Hello: Your focus is on increasing the available hot water, but reducing demand on the heater might work also.  One useful device is a "shower heat exchanger".  This can capture 60% of the heat going down the drain and put it back into the shower so you wind up using much less hot water overall. GFX, Power Pipe and ReTherm are three brands I know of.

    This exchanger needs room under the shower, so will only work in a two story home or one with a deep crawl-space or basement., but cost should be significantly less that a tankless heater.

    Yours,  Larry
  • rlaggren
    rlaggren Member Posts: 160
    If you have a detached garage

    Or an attached garage with a roof directly on the garage (no living space above), the flue can be modified fairly easily. If you have a garage under a living space, changing the flue is much more problematic, thus the mention of a "direct vent" unit which vents out a side wall. But it's usually easier and quicker (cheaper) to vent out the wall and most tankless heaters I've seen have kits to vent horizontally readily available. Many heaters now also incorporate a separate combustion air vent so there will likely be two (2) 4" ducts running to the exterior. This is a Good Thing because it makes it far more likely that the combustion appliance (heater) will function properly and not endanger you and your family.

    Provided, of course, that it gets installed properly. Charlie gave good advice - consider on it carefully. Besides the hundred little things a good tech does to make the job come out right and on time, you should get a warranty from the installer which will go well beyond any factory warranty. These are great little appliances when used in the right situations, but they do get tricky sometimes; just search through the forums here a bit and you'll see some of that. Besides helping to keep our medium of exchange flowing in your community, using a local pro will offer a chance to develope what might be a very good and important relationship to you down the road. And remember that one of those "little problems" that occasionally irk the pro's on this forum can take on a much more serious aspect to you as a DYI'er with money spent and a family hoping for hot water REAL SOON.

    You _may_ want to consider a larger standard water heater, especially if your family usage basically occurs at one time of day and that's it. Assuming there is a bit of space around your existing WH, this is likely a simple "bolt in" upgrade requiring only a couple of large muscular helpers for half an hour and a _strong_ two-wheel dolly. A 75gal WH will give you a good 50gal of hot water; if you set the HW temperature a bit higher you'll use less HW during your bathing.  I know a landlord who supplies nine units with a 100gal domestic heater, but I doubt that's appropriate in your case.

    Alternatively you can use your existing HWH and have a "tempering" valve installed on it's outlet - this will allow you to keep the the hot water in the heater at !50+ degrees F. while delivering HW to the faucet at, say 115F. This will greatly reduce your hot water usage because you'll be mixing lots of cold with it;  it will also greatly reduce the chances of bacteria and such growing in the tank. At least one member here had a brush with legionaire's disease from his own HWH and has firmly decided to keep his own heaters set at 140+ F. and use tempering valves - hospital time can lead to those decisions pretty quick. If you use high temperatures and a tempering valve you will need to ensure that your water pressure stays below about 60PSI and that your T&P relief valve is is good shape - ie. put in a new one - and that it's discharge is piped safely so there's no chance anybody getting hit with the discharge. Both precautions are easy and relatively cheap. If you use a standard WH, insulate it and insulate the first 3' of piping connected to the WH; this will save you a little money running it. When getting a new standard heater, insist on the most insulated version out there (it's easy to do some checking on the net) and then put a blanket on it anyway.

    Many hot showers to you and yours.

    disclaimer - I'm a plumber, not a heating pro.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Determing Size

    I think before you start looking at gas and venting you should first figure out what size unit your going to need. What's in those 2.5 Baths, body sprays, handhelds, rain head? How about dishwasher. You want to size the unit for expected demand from the occupants.

    After that then you can determine gas pipe and vent sizing. You may even have to change the gas regulator outside so that would have to be checked also. What else is gas? Boiler, Furnace, Dryer, Stove, Cook-Top, Fireplace etc.

    Do yourself another favor. DON'T SIZE IT OFF A PIECE OF LITERATURE...The ratings are generic. A particular unit can put out 7gpm in Florida but not in NY. To properly size it use this formula and then pick the unit based on the amount of btu's it can make.

    GPM x Temp Rise x 500 = Btu's Required

    Temp Rise is the difference between the incoming water temp in the winter (coldest water) and the water temperature you want the unit to make. Here in our market we use a 70 Degree Temp Rise. 50 incoming and make 120. Sometimes 75 or even 80 due to pressure balanced valves in the house.

    As an example

    7gpm x 70 x 500 = 245,000 Btu's

    5gpm x 70 x 500 = 175,000 Btu's

    2 Showers running is 5gpm with standard 2.5gpm heads. You can reduce by installing 1.6gpm heads. This is why sizing the unit is the first step. Then you can determine gas sizing and venting.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • t500hps
    t500hps Member Posts: 4
    edited January 2011
    2 tanks instead???

    I agree that the literature might suggest one model is acceptable, I figured if I did this I would get one of the largest units made for this size house.

    With 3 kids we have 3-5 showers in the AM, dishwasher runs nearly everyday, clothes washer does run everyday, the wife stays at home so she's using water sporadically throughout the day. AM showers are of primary concern, we manage fine the rest of the day.

    What I'm looking into now is adding a second tank unit, maybe even electric. A buddy built a river house and installed (2) 40 gal. units. He leaves the second one on all the time and turns on the first (lead) tank on for weekends when he has visitors. That lead tank can feed hot water into the second tank while isolating and heating the cold water that comes into the system. He feels it has the effective output of roughly 100 gallons of hot water....maybe more.

    My thought is adding an electric 20-30 gal tank on a morning only timer in front of this 50 gal gas unit. Installation space in the garage will be tight but do-able....would this give me an extra boost allowing 5 showers in under 1 hour in the AM?

    Thanks again guys!!!!!
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Your Missing the Point

    You stated you have 2.5 baths. I'm going to take it that means 2 showers running at a time. It your unit was giving you 5gpm it would make gpm all day long. You and a kid could both stay in the shower all day and it would never run out. Now if Mom runs the kitchen sink for hot water while your in there then that may be an issue. You have to size based on fixtures running at the same time.

    Are these just 2 showers with basic 2.5gpm heads? Do you have a whirlpool in the Master Bath? Is there a chance that there could be 2 showers and momma filling up the whirlpool? If so, that would be your biggest load at any given time. So you would size the unit to handle that peak gpm requirement.

    If your biggest load is 2 showers plus maybe a load of hot laundry, dishwasher or kitchen sink then you would size it according to that load. Remember as a previous poster said. You can change out the showers heads to 1.6gpm meaning now you only need to provide 3.2gpm when both showers are running.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • rlaggren
    rlaggren Member Posts: 160
    Designing for the worst case

    Is not always necessary. Consider the "100 year event" criteria used by many cities when planning their infrastructure - they do that because the consequences are extremely serious. But now consider _your_ worst consequences - your mother in law runs out of hot water because you didn't lay down the law to your kids about a quick shower that morning. Or something like that. Now, how much $$$ is appropriate to make sure that never happens, considering she visits once a year?

    OK, that's a little hoaky, but I think it gives the general idea. Running out of hot water once or twice a year probably won't matter too much; not doing laundry until _after_ shower time may not be that much of a problem. Or maybe it is, your call. But you don't _have_ to design for the absolute worst scenario; just try to be informed about all your options and keep it in perspective and safe and legal.

    disclaimer - I'm a plumber, not a heating pro.
  • bob eck
    bob eck Member Posts: 930
    need more hot water

    taking your 2.5 GPM shower heads and using Alsons 1.6 GPM shower head is a great start at making your hot water last longer. Look at using a Noritz tankless gas water heater NR98DVC delivers 4.8 GPM @ 70deg temp rise vents with double wall noritz ss vent pipe or NRC111 5.3 GPM @ 70 deg temp rise vents with PVC pipe or you can look at using A O Smith GPHE 50 ...Vertex 50 gallon gas water heater if nat gas this unit delivers 127 gallons first hour and 92 GPH @ 90* F temp rise (deliver double the GPH of regular 50 gallon pvc vented tank type water heaters) and is only 76,000 btu input might not have to change gas pipe size vents with pvc pipe. If LP look at their 100,000 btu Input Vertex GDHE 50 gallon unit. I beleve State water heater company has the same unit as the A O Smith Vertex. Here is to staying in hot water
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