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Thread: Water heater sizing help

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    DIY Senior Member DavidTu's Avatar
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    Default Water heater sizing help

    3500 sqft house with finished basement (1 bath), main floor with kitchen (2 sinks) and powder, 2nd floor with 2 baths including master shower w/ 2 heads and separate tub.

    How big of a Tank water heater do I need? Model recommendations welcome. Or better to have 2 water heaters? main trunk is 1" how do I connect to tank? (ie can I get tank with 1" fittings?)

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The further away the WH is from the point of use, the larger the pipe run, whether there's any insulation on the piping, and whether it runs through unheated space all help to determine how long it takes to get hot water to the point of use...it could be quite a long time. You may be a candidate for a recirculation system.

    Then, you want to determine how many hot water users are likely to be running at once, and for how long. Use the manufactuer's specs for flow. If you like long, hot showers, and others may be doing this while you are, that would change things. If you have a large soaking tub, that would change things. Do you do clothes in warm or hot water at times? So, it's not one size fits all. You also didn't indicate whether you wanted either an electric or gas fired water heater.

    Then, if gas, what size flue? Would you be willing to vent out the side of the house, or must it go through the roof? Do you have room to put a WH on say each end of the house so one would be closer to the points of use? All of these things affect what kind, size, recovery rate, and applicability to your situation...no one size fits all. Then, it seems some codes have guidelines on what they expect, possibly based on the number of bathrooms.
    Jim DeBruycker
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    How many bedrooms? How many big tubs?

    If your main trunk line is 1" you better install a circ system or split the system up......it would take forever to purge the cold with a lav faucet.
    Last edited by Hackney plumbing; 02-03-2012 at 02:39 PM.

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    DIY Senior Member Runs with bison's Avatar
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    Natural gas? Electric? LPG?

    From what I've seen the limiting case is filling a big tub. A 50 gallon natural gas fired tank can fill a whirlpool tub to a very warm temp if the water heater temp is in the 130+ range (but comes up short at 120-125...speaking from experience.) However that will essentially empty the tank and then there is recovery time. Recovery time for electric is slow. For my 2800 square foot, 3 bath/shower home with 2 kids, 2 adults the only time we have run out of hot water with a 50 gallon tank is filling the tub. This was true even when all three showers were being used and we were using standard 2.5 gpm showerheads--although we are not long showerers. We now use 1.5-1.6 gpm showerheads.

    Assuming you have the physical space for it I would opt for a 70-80 gallon tank. The standby losses don't change much with a larger tank and it should suit all your needs. Not sure if you can get that with 2 inches of insulation foam for the tank, but that is what I would look for.

    If things aren't already enclosed, I suggest insulating all of the hot water lines all the way to the fixtures, particularly with the 1" main header. This will make the cool off time between draws much longer--very helpful on long runs to kitchen faucets and such. I also insulate my hot water line out of the tank--but with natural draft on a gas system you have to use non-flammable/non-melting insulation in the first foot or so adjacent to the exhaust vent. I also do pipe insulation on the T&P valve line and valve body (brass and copper projections like this waste a lot of heat because they have high conductance and a lot of surface area.)

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    Split the system up and use 3/4" pipe.........and two water heaters.

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    Moderator & Master Plumber hj's Avatar
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    Two water heaters will have more 'input" than a single one with the same total capacity, and probably cost less than the single heater. I would connect them in series, others would use parallel, but it is an individual preference.
    Licensed residential and commercial plumber

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    DIY Senior Member DavidTu's Avatar
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    We will be using electric. I have access to an 80 gal tank in good condition from another house that I could use so if that would suffice that's what we'll do. The main trunk is supposed to be 1" according to calcs. I've posted a whole thread on the distribution system in the Plumbing forum but I can repost here the diagram if it helps. The 1" trunk is a small section essentially tying together three 3/4" main branches in the basement. (According to our calcs we have 30 hwfsu at the water heater with 80 psi and 100 ft max developed length that required 1" branch to serve it.)

    The 80 gal tank has 3/4" connectors. I am still wondering about connecting that to 1" trunk... does it make any sense to 1" trunk w/ 3/4" water heater or can we change out the 3/4" connections on the heater or can we get a heater w/ 1" connectors??

    Attached is the diagram.
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    How many bedrooms? I dont care how many fixtures you have,its the expected use that matters along with your cold water inlet temp. I read that that average cold water inlet temp for seattle is 41 degrees???? Thats very cold if its true.

    You want to calculate your peak hour demand. Thats how much hot water will be used in one hour at the most active time in your household.

    The number of bedrooms is indicative of how many people would be showering and washing clothes...etc etc. in a normal house. Thats how septic systems are sized......that and how many big tubs you have. I understand your not building a septic system but the principle is the same,its based off expected use.

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    DIY Senior Member DavidTu's Avatar
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    4 bedrooms 3 up, 1 in bsmt

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    Retired Defense Industry Engineer jadnashua's Avatar
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    As I said earlier, figure out what your peak load is: how many showers running at once, how many back to back, is there a big tub that needs to get filled, do you want to be able to be using hot water with the DW or WM running at the same time? Then, and only then, can you figure out what you need. Say all four showers are running at the same time, that's about 10gpm. 7-minutes, and a 70-gallon tank is essentially cold, especially in the winter when you need more hot to cold ratio to keep things comfy. Fill a 100-gallon tub, and it won't be hot when you get in. We can't tell you what you need until you tell us your intended use. Some big WH may have slow recovery, some have MUCH bigger burners, and can help to maintain temp with a modest output. Raising the WH storage temp makes the tank 'look' bigger, but once it's cold, it's cold, regardless. Where I live, I've personally measured incoming winter water temps at 33-degrees. So, my wintertime hot water needs is more stringent than someone living in say Honolulu when the water temp may be 70-degrees all year long. If you have a deep well, regardless of where you live, your water is likely cold.
    Jim DeBruycker
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    Quote Originally Posted by DavidTu View Post
    4 bedrooms 3 up, 1 in bsmt
    What do you want to set your water heaters temp at? 120,130,140? The higher the better because with an incoming cold water temp average of 40 degrees your percentage of hot to cold is going to skyrocket.

    For instance with a thermostat setting of 140 degrees and cold water temp of 40 degrees and a shower temp at 105 degrees....64% of the water coming through the shower head will be hot water.

    if you do the same scenario with a thermostat setting of 120 degrees and 40 degree incoming cold water and shower temp of 105....81% would be hot water.

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    Quote Originally Posted by jadnashua View Post
    As I said earlier, figure out what your peak load is: how many showers running at once, how many back to back, is there a big tub that needs to get filled, do you want to be able to be using hot water with the DW or WM running at the same time? Then, and only then, can you figure out what you need. Say all four showers are running at the same time, that's about 10gpm. 7-minutes, and a 70-gallon tank is essentially cold, especially in the winter when you need more hot to cold ratio to keep things comfy. Fill a 100-gallon tub, and it won't be hot when you get in. We can't tell you what you need until you tell us your intended use. Some big WH may have slow recovery, some have MUCH bigger burners, and can help to maintain temp with a modest output. Raising the WH storage temp makes the tank 'look' bigger, but once it's cold, it's cold, regardless. Where I live, I've personally measured incoming winter water temps at 33-degrees. So, my wintertime hot water needs is more stringent than someone living in say Honolulu when the water temp may be 70-degrees all year long. If you have a deep well, regardless of where you live, your water is likely cold.
    If you figure 2.5gpm per showerhead that would be 10gpm. But not all of that 10gpm is hot water. lets say he has a thermostat setting of 120 degrees and a 70 gal water heater. The 70 gal water heater will deliver 70% of its capacity at the thermostat setting of 120 degrees.

    With a showering temp of 105 degrees and the incoming cold water at 40 degrees the percentage of hot to cold would be 81% hot water. That means the 70 gal water heater with a temp setting of 120 could only deliver 49 gallons before the 81% hot water mix ratio starts to go up to maintain a showering temp of 105 degrees

    81% of 10 gpm = 8.10 gpm of hot water flow at a temp of 120 degrees.
    The 70 gal tank can only deliver 49 gallons at the temp of 120 degrees.
    49 divided by 8.10 = 6 minutes and 3 seconds of showering before everyone of those 4 users start adjusting in more hot. At this point the 4 users would have 21 gallons of hot enough water to finish a shower at 105 degrees which would be equal to 2 minutes and 35 seconds. They would be constanly compensating from this point on.....unless they have a thermostatic valve and then the valve would compensate for them.

    So a 70 gal tank with a thermostat setting of 120 degrees with an incoming cold water temp of 40 degrees could supply four 2.5 gpm showers at 105 degrees for 8 minutes and 38 seconds.

    LOL I freakin love it.
    Last edited by Hackney plumbing; 02-04-2012 at 05:50 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Yeah...so without going too deep into it, my rough calc wasn't too far off. Also, this is one reason I like themostatically controlled valves...while you're diluting that WH tank with the cold(er) incoming water, the valve compensates and will maintain your desired temp until there's not enough hot water left.

    But, it still comes back to my main point...you can't size the WH until you know what you expect out of it. Someone that takes 'military' showers - maybe a minute or so in length could easily get by with a much smaller tank than someone who likes to turn into a prune before getting out.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Senior Member Runs with bison's Avatar
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    Being as this will be an electric water heater set up (per the response) it might be best to plan for two water heaters, even if one large one is used instead. This way the circuits are there and the floorspace for it is available if a two tank system is needed.

    Electric is a Catch 22 because of the competing problems of legionella and retrograde solubility of minerals in the water. Electric is more susceptible to fostering an environment that legionella can still grow in, so it really needs to run above 120 F. But at some point if the water is hard enough this will result in a lot of sediment because calcium and magnesium carbonate solubilies decrease as temperature rises.

    Series installation has been suggested. To maximize the system's recovery rate the 2nd tank could be set a few degrees hotter than the first (minimizing the lag time before the 2nd heater's elements pick up the load. I'm guessing on this but something like 130 F for the first tank an 135 F for the 2nd tank might make sense.

    One problem with series or parallel electric...how will the user know when one of the tanks has reached failure/near failure and therefore unsafe temperatures (below 120 F)?

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    Could you imagine having 3 teenage girls taking showers and a wife that loved to take baths in the jaccuzi tub every morning at the same time.......then its your turn to get in the shower???

    You could figure each girl would take ATLEAST a 10 minute shower. At a thermosetting of 120 incoming cold at 40.........a 70 gal wouldn't be big enough for just the girls showesr.
    Last edited by Hackney plumbing; 02-04-2012 at 06:41 PM. Reason: Corrected a mistake .

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