water heater sizing

chroniekonJune 12, 2013

I am doing a complete house remodel and I want to put in the proper size hot water heater. There are two of us living in the house. The energy source is natural gas. The largest use will be a 78 gallon whirlpool tub. The plumber has suggested a 65 gallon heater for filling the tub. This makes sense on the surface, but in all actuality, the tub will be used infrequently after the newness wears off. My concern is the standby energy cost of such a large water heater. Can I use a 50 gallon heater with the temperature set way high and add a tempering valve set at 120?
Will the standby energy use be less with this set up, equal or more?

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My wife and I briefly considered getting a larger tub. We came to the conclusion they were monstrosties with a myrid of problems: Including having to regularlly fill and tub and clean it - because the water remains in the internal piping - and you get mold.

An on-demand water heater seems like a good fit for this application.

    Bookmark   June 12, 2013 at 1:59PM
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The tub we are putting in has something called antimicrobial EverCleanî system. It's 'supposed' to reduce the problem. I'm not sure about the monstrosity issue as it has the same footprint as a regular bathtub.

    Bookmark   June 13, 2013 at 3:41PM
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You will use more standby energy with a 50 gallon set hotter than with a 65 gallon set less hot. Even at the same temperature, I doubt that the loss will be much different. If the same exact construction except diameter, I would expect about 14% more standby heat loss from the larger vessel. You would also need to know your input cold water temperature, to calculate what the net in-tub would be. My plumber put in a 40 gallon for my 63 gallon tub -- I made him replace it with the 50 gallon I had specified.

    Bookmark   June 14, 2013 at 3:30PM
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I'm on a well and the cold water is 52 degrees.

    Bookmark   June 15, 2013 at 12:05PM
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Go tankless. I've never regretted mine.

    Bookmark   June 15, 2013 at 4:26PM
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I'd go with a 60 or 65 gallon model, you never hear anyone complaining that their water heater is too big. A 50 gallon heater could be too small anyway, unless your house is just a two person house. Meaning, only large enough for two, I understand there are just two in it now.

Tankless water heaters can be expensive to buy and own, especially if your well water is hard or not passed through a water softener.

    Bookmark   June 18, 2013 at 12:22AM
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With 52F cold, a 50 gallon tank will probably not be satisfactory if you want to fill it, and have hot water, and be able to "top it off" after a few minutes as it cools. Maybe doable if your tank is quite close to your tub, and you set it for 135F, but you will not really know until you try it.

If you want to plan your use ahead, you could fill it 1/3, then wait for your tank to recover, and top it off. That way you would be mixing at least room temperature "cold" rather than 52F "cold".

    Bookmark   June 18, 2013 at 6:05PM
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Thanks guys for all the suggestions. I'm leaning towards the 65 gallon size. Does anyone know of a formula for sizing a water heater. One where you enter the gallons of hot water you want to produce (in this case 78) at a certain temperature (say 105) with a cold water input of 52 and the water heater set at 125.

    Bookmark   June 18, 2013 at 10:25PM
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For a 50 gallon tank, assuming that it is all at that 125 (and no temperature drop while travelling along the pipe to the tub), and assuming 78 gallons fill.

Ttub = ( 50*125 + 28*52 ) / 78 = 98.8 degrees

where 50 is gallons of hot, 78-50=28 is gallons of cold

For 65 gallons, same assumptions,

Ttub = ( 65*125 +13*52 ) / 78 = 112.8 degrees

    Bookmark   June 19, 2013 at 1:16AM
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Thanks attofarad!!! Exactly what I was looking for. Yup a 50 is too small. A 60 would work but with little extra if the dishwasher, washing machine or whatever else was in use. Anybody have recommendations for a good 65 gallon unit?

    Bookmark   June 19, 2013 at 1:56AM
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Your approach is right, atto, that's the algebra for calculating the result of a mixture.

Your answer would be right if the water heater were turned off. But it's not. Within a minute or two of the hot water starting to run, the burner will ignite. That does two things - it raises the temperature of the existing hot water to above 125 (the assumed setting in this example), and it also will produce more "heated" water that's warmer than the cold.

Every heater has a stated recovery rate - how many more hot gallons that can be produced per hour, starting with cold. For most gas tanks (other than ones made with a larger than standard burner to increase the recovery rate), it's usually 90-100 percent of the tank's size more hot water per hour.

At 5 gph filling and 2/3 - 1/3 hot cold split in your first case, you get to 75 gallons in 12 minutes. Also, in 12 minutes, you have at least another 10 gallons of hot water, and the hot that went in would have been warmed above 125.

So, the math isn't so simple. But as before, a 50 gallon tank is probably too small. Alternatively, one could run just hot water for 5 minutes, wait 10 minutes for the heater to recover, and then resume filling to get more and hotter hot water to finish filling the tub.

    Bookmark   June 19, 2013 at 2:19AM
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Two people and a 65 gallon tank? I understand the 78 gallons for the tub, but a tankless will fill that. It will take 13 minutes to do so, but that isn't a problem. You plan for it. Also, pay attention to how much hot water you are each using per day. How many minutes per day is your hot water tap actually open. Maybe 25-30 minutes a day, and that is pretty heavy use for two adults? Why do you want to store hot water all day for such low usage? I'd at least give tankless a strong look.

    Bookmark   June 22, 2013 at 2:16PM
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I don't understand the resistance to tankless. It does cost more to install up front, but it pays you back in the medium to long term. The only people who don't care about that would be those who will be moving in the next couple of years or those who don't care about energy saving options.

I've had my tankless for almost 20 years. A tanked heater would have had to be replaced 2-3 times during the same time period. My energy bills each month are much lower than when I had a tank, and my tankless can probably operate another 20 years without any issues. And it can handle a monster tub just fine. Plus do the dishes and 2 loads of laundry and even a dozen showers for a visiting football team after filling that tub if I needed that to happen.

    Bookmark   June 22, 2013 at 3:41PM
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I'm not opposed to the tank-less option, but I do have a few concerns. The first is the size of the gas line that feeds it. My gas meter is on the far end of the house 70 feet away. A 3/4 inch pipe comes off of that and feeds the dryer, furnace and hot water heater. Is that size of pipe capable of supplying sufficient fuel to all three devices if they happen to be running at the same time? Another issue I've read about is the burst of cold water one can get from a shower if the shower was used previously by someone. I think they called it a 'hot water sandwich.' Another issue is my cold water supply is 52 degrees. So tank-less fans, what size of unit do I need to meet my requirements. I do have the water treatment system already in place.

    Bookmark   June 24, 2013 at 11:53AM
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My tank heaters last more than 15 years, with no maintenance. If I were to replace the anode at 7-8 years in, and maybe drain/flush occasionally, they would probably go much longer. Like the OP, my meter is pretty far from where my far tankless heater would be; 130 feet, and I doubt that I could run two tankless water heaters and my 115kBTU/hr furnace simultaneously without re-piping the gas.

My total annual gas bill for water heating was about $225 (two water heaters), for mostly just me. My total bill when my wife moves in post-remodel will likely be ~$280 (got rid of 5 gpm shower heads, got high efficiency washer and dishwasher, but have two people). Approximately $108 of that is the cost of keeping two tanks (40 gallon and 50 gallons) heated at standby, all of which could be saved with tankless. Of the $172 for heating water used, I could save another $43 or so in better efficiency with tankless. So, for two heaters, I could save about $151 for the year, and put up with the other issues chroniekon mentioned, such as not being able to sustain a low flow rate of hot water. If I don't need to re-pipe the gas to enlarge (some local pipe work still be needed for different hookup), I can break even in maybe 11-12 years, unless you take into account the cost of money, in which case it will take more like 16 years. If I have to re-pipe the major gas run, it would never pay off.

    Bookmark   June 24, 2013 at 10:11PM
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Your gas line is definitely an issue. A tankless for your home will be wither a 180k or 199kbtu unit. 3/4" will not be enough to run all of your appliances. Cold water sandwich is spoken of as the great boogie man of tankless when you listen to people who don't want to work with them. Here's how it works. Every time a tankless is turned off it is in fact...off. Once water begins flowing again the unit has to re-start. It takes a few seconds for it to re-establish the set- point temp. For instance, if my wife showers before me, turns off the shower, steps out and I step right in (telling her of course how we could save water by showering together, but alas, there's not much of that going on any more) I turn the shower on and have hot water immediately. I know that once the water that is in the pipe from the water heater has travelled to the shower that there will be that "start-up" cooler water that has to travel through the shower head. I simply step aside and in about 5 seconds it has passed and I have hot water all day if I like. It is not a big deal.

With 52* water a 180k tankless will deliver 4.3 gpm at a 70* temp rise. A 199k unit will make about 4.8 gpm. With a 2.5 gpm shower head at the temps I shower at it took 2.1 gpm (you can read the flow rate on the touch pad display) of hot mixing with .4 gpm of cold to give me the 2.5 gpm shower. 120 is plenty hot for domestic use. If you fill your tub with 105* water (if you sit in water over 105, you are no longer bathing, you are par-boiling) the supply will be in the 5 gpm range. What I have found to be very nice with tankless and bathing is that in my claw foot tub (no jets) within several minutes of getting in the tub I need to add hot water. The tankless will ppeak that tub as long as I choose.

I would look to see if there is a better location for the water heater than where it is now. These are veery compact units and can fit in closets, etc. Look at your house. Is the water heater in the best location or only in the location that was convenient for the original low cost bidder?

One other thing about my unit. I have the Bath Fill Control on it. I set the control to deliver 40 gal of 102* water, turn on hot only and go about my business. When the tub is filled with 40 gallons of 102* water it beeps to tell me it is full and it shuts the water off. Pretty cool actually and very convenient.

    Bookmark   June 25, 2013 at 10:27AM
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The location for the water heater is going to be in the garage, unfortunately a fair distance away from the kitchen and one bathroom. Which brings up another question. Can a circulating pump be added to a tankless to have instant hot water at the farthest faucets? Also, is it feasible to combine a smaller tankless unit (one that my existing gas line would work with) with a conventional 40 gallon tank heater. With the tankless output feeding the tank type input, it seems like maybe the reservoir of the tank would allow some mixing to eliminate the 'cold water sandwich' talked about earlier and also accommodate the circulating system. Also should mention my furnace is 60K BTU.

    Bookmark   June 25, 2013 at 7:06PM
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If you have a tank inside the conditioned envelope of your house don't forget that for 6-8 months of the year (here in the north anyway) the heat lost from the tank is not "wasted", it simply displaces the need to heat the house with your furnace. Same is true for "wasted" heat from incandescent light bulbs. I would cut those energy savings numbers for standby water heating vs tankless about in half in heating climates. A hot water tank also provide a backup source of 40-60 gallons of fresh water in the event of an emergency that disrupts supply.

    Bookmark   June 26, 2013 at 2:42PM
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Many plumbers in my hard-ish water area discourage using tankless heaters. I got a quote, in my case it would have taken units with double the BTUs as my forced air heaters and about $200 a year for maintenance. Having them would be considerably more expensive than my current setup. I've replaced each of two tanks once in >25 years, and they're still doing fine.

Where I live (in the temperate West) water heaters are rarely inside the building envelope. Lost heat is lost but compared to many other "utilities", it doesn't cost a lot of money to have hot water.

    Bookmark   June 27, 2013 at 8:49PM
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