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Thread: Winter problem with gas water heater: gas line too small?

  1. #1
    DIY Junior Member Bowsher1973's Avatar
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    Default Winter problem with gas water heater: gas line too small?

    My water heater doesn't produce enough hot water in the winter months, yet it's perfectly fine during summer months. My online research suggests the problem may be that my gas line is too small to run both the water heater (40,000 BTU) and the gas boiler (130,000 BTU) connected to this line. Both gas appliances run off of a 3/4" natural gas line, with 40 feet of pipe between the meter and the appliances (located next to each other), and maybe a half-dozen 90-degree elbows along the way. If I'm reading the charts correctly (and that's a big "IF"!!) it looks like 40 feet of 3/4" pipe only supports 136,000 BTU, and I assume the elbows decrease this capacity even further. So, okay, fine, I get it, technically my pipe is too small. [Insert joke here.] But here's what I want to understand, and I'm hoping someone on this forum can explain to me: since my boiler is not running constantly -- let's assume it's running for maybe 10 minutes out of every hour -- shouldn't the water heater be able to function just fine most of the time, struggling with a smaller flame for just the limited period of time when the boiler is on simultaneously? In other words, regardless of the pipe size, when I wake up in the morning and nobody's used any hot water for at least 8 hours, shouldn't there still be a full tank of hot water in the winter, just like there is in the summer, because the water heater has had all night to maintain the temperature? I mean, doesn't the internal thermostat in the water heater tell the flame to keep running, small or big, until the tank is full of hot water? I don't understand why would it matter that the water heater doesn't get its full capacity of gas for a fraction of each hour. Thanks in advance for any help you can offer!!

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    Forum Admin, Expert Plumber Terry's Avatar
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    The farther North you live, the colder the incoming water is. The temperature differential makes makes the heater work harder. Also, if the heater is in the garage, you will be losing some temperature overnight. Granted, the thermostat is supposed to keep heated water in storage, but unless the water dips to the reset point, you will be losing some heat.

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    Plumber jimbo's Avatar
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    One symptom of an appliance "starved" for gas is the pilot going out frequently. More likely, you just lose a lot of heat in the hot pipe between the heater and the garage. If there is any gas at all, the heaterwould run until the thermostat is satisfied.

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    DIY Junior Member Bowsher1973's Avatar
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    Terry -- Thanks for your suggestions. But I don't live very far north (Maryland), and my water heater is not in my garage.

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    DIY Junior Member Bowsher1973's Avatar
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    Jimbo -- Thanks. I agree, it seems like so long as the pilot isn't going out (which it's not), then obviously the appliance is getting gas, and if there's any gas at all (which obviously there is), then it seems like the heater would run until the thermostat is satisfied, even if it takes longer to satisfy it because it's not getting enough gas due to the smaller-than-optimal gas line. Seems like if nobody's used the hot water overnight, there should be plenty of water in the morning to take showers. I don't want to go through all the hassle of increasing my pipe diameter and then discover it doesn't solve the problem.

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    DIY Senior Member SteveW's Avatar
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    Have you measured the temperature of the hot water at the bathroom? I would do that next. If it's 100 degrees F or lower I would be tempted to increase the temperature on the WH. You do have to watch out for the possibility of scalding, however, if the water temp gets too high (like 120 F or more).

    If your hot water isn't hot enough, you'll use more of it (i.e. deplete the water in the tank more) compared to the situation where the water temp is higher, and you thus use less water from the tank (because you are mixing it with a higher proportion of cold water at the shower).

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    It could also be a bad or defective thermostat gas valve !

    It may have lost it's calibration

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The incoming water in the winter could easily be 30-degrees or more colder than in the summer. This cools off the hot water in the tank, and your total available volume is less. The first draw, before this happens, is determined by the thermostat and how long the run to the point of use is. The longer that run, and the colder it is, depending on how well it is insulated, would determine how much it cools off on the way. You will NOT get the same amount of hot water out of a tank in most places in the winter verses the summer when the 'cold' water might approach 70-80 degrees verses 33-50 in the winter.
    Jim DeBruycker
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    In the trades Dana's Avatar
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    With a 130KBTU boiler in there, the "right" solution would be to replace the ineffcient & small-burner standalone tank with an "indirect" hot water heater running as a separate zone off the boiler. The boiler has enough burner to give you endless-showers if you operate it as a "priority zone" that turns off the space heating loop whenever the tank is calling for heat, but that's enough boiler that priority zoning probably isn't necessary unless you need a LOT of hot water (filling a spa, perhaps?). It's output is probably over 100KBTU, and even the 1% coldest hours of the season heat loads for typicial MD homes are less than half that, and many tigher homse are only 1/4 of that, meaing you have PLENTY of burner to spare for hot water (even at 5AM on the coldest night in January.)

    An indirect will provide more hot water, and improve the AFUE of the heating system by increasing the duty cycle for less standby time on the boiler. And it'll typically outlast a standalone tank heater by a decade ormore. It's more money to install, but there are incentives & rebates available in most places due to the efficiency enhancement aspects.

    Much cheaper, but still relevant- insulating all of the near-tank plumbing with 3/4" walled closed cell foam (available at Graingers, plumbing supply houses, or online stores but not big box stores) reduces the standby losses of the tank, and insulating all of the accessible distribution plumbing abandons less heat in the pipes, stretching tank capacity a few percent. (It's cost effective as a DIY project, no matter what sort of water heater you use.)

  10. #10
    DIY Junior Member Bowsher1973's Avatar
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    Steve W: Yes, I measured the temp. Itís 132 degrees at the faucet. I increased the heat setting in order to help compensate for the lower volume of hot water weíre experiencing in our morning showers.

    MacPlumb: Thatís what I was thinking too, except that a faulty thermostat gas valve wouldnít explain the huge seasonal differences Iím seeing. It works just great in the warmer months.

    Jadnashua: Thanks. Yeah, I totally understand that the colder incoming water in winter will reduce the amount of hot water available in my tank. But my incoming cold water in the summer is never warmer than 55 degrees, and my incoming cold water in the winter is never less than 35, and the bottom line is that in the morning when the tank has had all night to heat up, thereís no reason I shouldnít be getting better performance out of this two-year-old 50-gallon water heater. In my prior home, just a block away from where I live now, with the same water supply and same seasonal temperature changes, this was never an issue. The length of pipe from my water heater to my shower is not longer than average, and it doesnít go through a garage or any unheated space. I could obviously help the situation by insulating my exposed pipe, sure, but this will only give me a few more minutes of warm water, itís not going to solve the problem.

    Dana: Wow, thanks, thatís good to know. Great info! When we replace our water heater (eventually), Iíll definitely have to consider getting an indirect water heater. No, Iím not filling a spa, just normal hot water usage. And yes, youíre right, the boiler (with a 130K BTU input) has an output of just over 100K BTU (103,000). Thereís not more than 20 feet of exposed hot water pipe for me to insulate, but it seems to be a common suggestion, and itís cheap enough, and certainly couldnít hurt (maybe add a couple minutes of hot water) so Iíll do that as a band-aid approach to the broader problem.

    Thanks again, everyone, for all the input.

  11. #11
    Moderator & Master Plumber hj's Avatar
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    1. IF the gas line were too small, the burner AND pilot would go out, and then you would not have enough hot water until your relighted it.
    2. An indirect heater does NOT get "free heat" and if the boiler is not adequate for the heating AND hot water demands the performance will be less than desired.
    3. How large is the water heater?
    4, Once the water heater is full of hot water, such as overnight, it will ALWAYS provide the same amount of hot water until the tank starts to cool down.

    THerefore, we have to know what you mean by "doesn't produce enough hot water", since that could have more than one interpretation. Every heater I have encountered with an undersized gas line, the main burner and pilot both go out, because the burner depletes the supply of gas pressure.
    Last edited by hj; 12-03-2010 at 01:14 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Generally, all water heaters have a dip tube that directs the cold water to replenish the hot you use towards the bottom of the tank. You draw the hot water off the top of the tank to allow you to draw the maximum hot water volume before it gets tempered by the incoming cold water. On an electric WH, the upper element helps to maintain that water closest to the discharge point hot as long as possible. But, you do get some mixing between the hotter at the top and the fresh colder water at the bottom. The length of the dip tube could be different, allowing the water to mix higher, or it could be non-existent (left it out when they built it), or broken. If your shower has multiple heads, or they are older, or someone modified them to produce a 'stronger' spray, it is easy to run out of hot water sooner than your old house. For an experiment, take a bucket and collect the water from the shower(s) and measure how much you get per minute. You should be able to use about 75-80% of the hot water tank (in your case maybe about 40-gallons) before you notice significant lowering of the output temperature. Since you are mixing cold for your shower, you should get more than that value, but it would depend on how cold the incoming water is as to how much is hot verses cold (more in the summer will be cold and more hot in the winter).

    BTW, with supply temperatures greater than 120-degrees, it is a good idea to install a tempering valve to keep it below 120 on the outlet. You'll still get the advantage of the hotter water, but be safer in the process. Where I live, they are mandatory on any new or replaced tank to pass inspection on a residential installation.
    Jim DeBruycker
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    In the trades Dana's Avatar
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    Quote Originally Posted by hj View Post
    1. IF the gas line were too small, the burner AND pilot would go out, and then you would not have enough hot water until your relighted it.
    2. An indirect heater does NOT get "free heat" and if the boiler is not adequate for the heating AND hot water demands the performance will be less than desired.
    3. How large is the water heater?
    4, Once the water heater is full of hot water, such as overnight, it will ALWAYS provide the same amount of hot water until the tank starts to cool down.

    THerefore, we have to know what you mean by "doesn't produce enough hot water", since that could have more than one interpretation.
    Nobody said it was "free heat", only that it would raise the average operating efficiency of the (very likely oversized) boiler and provide beaucoup more hot water than a standalone tank while remove any question as to whether the gas plumbing was undersized for the combined load.


    The 99% outdoor design temps for VA are quite modest (17-20F), and unless this is the leakiest, least-insulated house in the state, or some 6000' McMansion I'd be surprised if the design day heat load were over 50K, which would leave more "spare" burner capacity for hot water than the typical 40-50 gallon standalone tank.

    If the indirect is zoned "priority", it's more than enough boiler for the task(s), period.

    To find out if it's enough boiler for non-priority zoning on the indirect, what's the zip code, and the annual fuel use at this place? (Assuming you don't also heat with wood on a regular basis.)
    Last edited by Dana; 12-01-2010 at 02:21 PM. Reason: danged grammar & spelln' #$%*

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    Master Plumber Dunbar Plumbing's Avatar
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    I answered on another site already. What is the brand name/mfg. of the water heater.
    Read what the end of this sentence means.

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    DIY Junior Member Bowsher1973's Avatar
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    hj: Water heater is 50 gallons. And no, I disagree, I think if the gas line were too small, the pilot would not necessary go out. The there could be enough gas for the pilot to remain lit and the main flame to be low, bilowy, and orange. My flame is fine, fortunately, even when the water heater is running concurrently with the boiler, but my point is that the theory has merit under other circumstances. To better quantify my comment that it “doesn’t produce enough hot water”, I’ll can tell you I’m getting about 15 minutes of hot water in the shower in the morning, as opposed to virtually limitless hot water in the warmer months. I haven’t yet counted the actual number of gallons of hot water just yet, as a few folks have suggested, but as soon as my wife stops running the dishwasher and clothes washer during the limited hours that I’m actually home and awake, I’ll do that. It’s just such a massive drop-off in hot water production from summer to winter, in terms of how long we can take showers in the morning before the hot water runs out… gallons shmallons, I’ll count the gallons just for giggles and for the amusement of this board, but regardless, there shouldn’t be such a dropoff, something’s not right.

    jadnashua: thanks for the primer, but I’m fairly up to speed on the basics, including the dip tube. It’s a two-yr old water heater, and it works beautifully in the summer, so I’m pretty sure we can rule out the dip tube theory. No, my shower doesn’t have multiple heads, nor has the head been modified… it’s a standard 2.5 gpm. Yup, I understand the risk of supply temps exceeding 120 degrees, and the availability of tempering valves; thanks, though.

    Dana: Nope, definitely not a 6000’ McMansion! Ha!! 2800 ft, 1947 brick colonial. And no, not leaky/drafty. I’ve got new double-pane low-E argon-filled high-end replacement vinyl windows throughout the house. Zip code is 20815, but I’ll have to get back to you on the annual fuel consumption. Regardless, it’s obvious by standing next to my boiler for an hour that I have capacity to run an indirect heater, because the boiler just simply isn’t on for more than maybe 10 minutes out of every hour. Okay, maybe 15 minutes when it gets “really” cold for this area’s standards. (I’m originally from Michigan so living here in southern Maryland feels like the subtropics to me.)

    Dunbar: We meet again! The brand of the water heater is GE, manufactured by Rheem.

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