not enough hot water

Discussion in 'Tankless Water Heater Forum' started by greentruck, Oct 6, 2009.

  1. greentruck

    greentruck New Member

    Messages:
    3
    Location:
    california
    I recently installed a new oversized bath tub. My current electric 40 gallon water tank cannot provide enough hot water to fill the tub to a comfortable level. After a so so bath, there is no hot water for quite a while in the house. I was thinking about switching to a tankless water heater, but am not quite sure what I need. What is the best way to remedy this situation?
    Here are the particulars of my house. It is a 1200 sq' 2 bed 2 bath in southern california. The house is 100 years old but has gas service and a 200 amp electrical panel. My current water heater is 40 gallons and 3 years old. I cannot put a larger one in due to size of the cabinet and don't want to relocate a new one due to outside appearance. The tub will hold almost 70 gallons before it hits the waste and overflow (according to the manufacture). It is a tub shower with a regular tub faucet and shower head. The other bathroom is a just a shower.
    Thanks for the help,
    Sam
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,485
    Location:
    Cave Creek, Arizona
    heater

    Unless the cabinet was made for that heater, a 50 gallon one might also fit. A tankless electric heater could be an investment in future problems and aggravation, not to mention energy bills.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,889
    Location:
    New England
    Personally, I feel tankless are overblown for our lifestyle. Now, you could comment on our lifestyle, but that's another issue altogether. They are in fairly common use elsewhere in the world - I've used them there, and have not been impressed.

    Living in SoCal, you probably have the advantage that your winter-time water isn't all that cold. Now, that isn't an absolute - it could be. You will probably need to upgrade your gas supply to use a tankless that can fill the tub at a good rate, and may actually need two in series. this isn't cheap. Probably much cheaper is to enlarge the cabinet. You have nowhwere near enough electrical to do a good sized tankless to get you good flow of say 5-6 gpm needed to fill the tub if it is a 1/2" valve. If it is a 3/4" valve, you'd need even more since it can flow up to about 13gpm. It's all about temperature rise and flow rates. Hand across the candle, or slowly through a blowtorch gives very different results. You need a blowtorch - a very big one to get good volume. especially if you want to take a shower in the other bath at the same time. Add that to the maintenance, installation, and (often) inexperience in the local servicing people to actually fix it when it does break (everything breaks eventually), mean you could be out a lot of money, and without hot water while someone orders parts that you may not need since they couldn't figure it out right.
  4. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Sam,

    If you've got natural gas hooked up to the house already, then why have electric water heating? That's a money losing proposition and about half as efficient. (Don't be fooled by the ~90% efficiency of the electric water heater itself, I'll explain in a minute.) Depending on typical electric and natural gas rates in your area, going to natural gas could pay for itself rapidly.

    The overall efficiency of electricity production is in the mid 30's, then you have transmission losses, and for an electric water heater a small storage loss for providing hot water. By comparison a traditional natural gas water heater will run about 60% overall efficiency. This makes more sense as electricity is a higher form of energy than simple combustion heat, so it is thermodynamically wasteful to produce electricity then convert it back to low level heat.

    Recovery time is longer for electric than gas as well.

    With a 70 gallon tub at the overflow you could subtract about 18 gallons of volume for a single occupant, so the net is close to 50 gallons. With that size tub a 50 gallon gas water heater would be close if not fully sufficient--not that I'm trying to discourage you from tankless.

    EDIT: This prompted me to check the fill volume on my whirlpool tub, appears to be about 70 gallons from a half fill, cold water test. With the 50 gallon gas water heater set at 120 F it comes up just a little short of doing a fill at the temp I want. I pause for ten minutes or so when the hot tap temp begins to tumble. This allows the burner to catch up and provide a comfortable fill. At 125 to 130 F water heater set point it might do the whole job without a pause. Fill rate is about 5-6 gpm (measured.)
    Last edited: Oct 7, 2009
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,889
    Location:
    New England
    As noted, if you did two things:
    1. Put a tempering valve on your existing WH.
    2. Raised the temperature of that WH to 140 or maybe a little higher.
    You'd potentially solve your problem. the tempering valve is to provide a level of safety...normally set to limit the output of the tank to around 120-degrees. But, since you are mixing it with cold to cool it off to 120, then you cool it off some more at the tub, the tank looks bigger, and you still maintain some safety since the temp can't be too high because of the tempering valve. BTW, where I live that tempering valve is required, not optional.

    A bigger tank does the same thing with probably lower standby losses.

    [​IMG]
    Last edited by a moderator: Aug 6, 2011
  6. Dana

    Dana In the trades

    Messages:
    2,805
    Location:
    01609
    What's actually possible depends a lot on the existing plumbing & proximity to gas lines etc.

    Electric tankless- fuggedaboudit, unless you're interested in spending a whole lot on upbgrading the power.

    Gas tankless- maybe, but it's not likely to live exactly where the 40 gallon tank is. If the cold water feed to the tank is easily separable from the other bathroom and there's space to hang a suit-case sized unit somewhere near both the gas meter and the (either inside or outside in CA) cold feed to the tank, you don't need much of a gas tankless to pre-heat the feed into 40 gallon tank (and it's temperature regulation doesn't need to be perfect either, since it's feeding only the tank.) The Bosch 1600H can be cheap to install as it's atmospheric-drafted (it can use cheap B-vent stack) and with now blower & flow-powered ignition it needs no power connection. It's heat output enough to run any single load at full flow, so as preheat to your tank you're very unlikely to ever run out of HW.

    If you're in a freeze free zone you may consider parking an outdoor version like the Noritz N-0531S-OD, Takagi Jr. Outdoors or Paloma PH-20R OFN, etc. near the gas meter. The the indoor versions of these would work too, but make for a more expensive installation (special venting, messier gas plumbing, etc.) (Even the outdoor versions do require a modest amount of electric power though.)

    By using it as pre-heat on the cold feed to the tank (which you should then insulate, where possible) you'll get pretty much continuous use flow and you'll avoid some (but not all) of the user-unfriendliness of gas fired tankless heaters. You'll still have the (fairly low) standby loss of the electric tank, but your electric bill should drop quite a bit, since the tankless is doing the heavy-lifting on the water heating, the tank's element just maintains the standby loss (more or less). The down side to the smaller output gas tankless units is the pressure drop through the units when 2+ taps are open at the same time, but it would be hard to rationalize a 200KBTU burner "whole house" version for your place. 80-100KBTU/h output is more than enough.
  7. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    To determine how much duty is needed, measure the flowrate when filling the tub in its present configuration (pretty easy to do if you have a city meter and a stopwatch.) Measure the desired bath temp, add about 5 F for heat loss during the fill, etc. He can estimate winter time water temps based on climate and knowledge of how the water arrives, the ground/water is probably not very cold in his location.

    From there one can calculate the duty. For example, pulling some numbers out of the air: Let's say he needs 5 gpm @ 115 F (110 in the actual bath by the time the fill is done) and that the incoming water in winter is 50 F. The duty would be Q = m*C*delta T = 5*60*8.33*1*(115-50) = 162,400 Btu/hr delivered (output.) This should be corrected with an efficiency factor to put it in terms of input rating.

    Lower duty would be okay if he was willing to accept longer fill times. Let's say the required fill volume is 50 gallons. That will take 10 mins at 5 gpm, 12.5 mins at 4 gpm, ~17 mins at 3 gpm.

    Dana, if you have a standard gas meter you can probably measure the burn rate directly by timing the spin of the smaller dials off in the left corner. (They are half cu. ft. and 2 cu. ft. indicators on mine.) I did this to check my water heater burn rate a month or so ago and it was about 15% off the 40,000 input nameplate on a ridiculously short test.
  8. Dana

    Dana In the trades

    Messages:
    2,805
    Location:
    01609

    I s'pose I could just read the gas meter with a stopwatch, but I have other curiosities about this system's performance as well.

    The incoming water temp in SoCal is likely to be well north of 60F, if the mean annual surface temp works as a model (which is mostly does, give or take a handful of degrees for local geology & water sources.):

    [​IMG]

    Mean annual earth temperature observations at individual stations, superimposed on well-water temperature contours.

    I'm roughly on the ~47F contour, but my late summer incoming water hit's the low to mid 50s, mid-winter it sometimes dips under 40F. (The main out on the street needs to be buried deeper, IMHO.)

    In SoCal it's 15-25F warmer that, making the min-modulation of a tankless more important than it's max. (Jakeru's contour line is 5F warmer than me, but I won't begrudge him the full flow of his TK3 even if it's overkill for his peak load. :) ) Odds are his winter incoming-water temps are above my late-summer temps.
  9. greentruck

    greentruck New Member

    Messages:
    3
    Location:
    california
    follow up

    Wow! I am impressed gentlemen. Thanks for all the information.
    Here is some more information that I should have included. My water heater is electric because the previous owner wasn't willing to do to extra work (ie vent pipe and gas line). The heater is inside of the bathroom closet in the house. A bigger size tank will not physically fit. It is really tight. Enlarging the cabinet isn't very feasible due to lack of space. The house is a raised foundation so running gas and water pipes underneath isn't a big deal. My tub has about 5' of 1/2" copper plumbing going to it off of the 3/4 feed.
    According to the water temp chart, I am on municipal water in the 62 degree range. I am an hour northwest of los angeles along the coast. Never say never but in the 30 years that I have lived in the area, I have never had a frozen water line.
    I wasn't sure about tankless gas versus electric water heaters because the advertising seems one sided pending on what the manufacture is. Energy efficency is nice to know but in so cal gas and electric costs are both expensive pricing seems to fluctuate. After reading the post, it sounds like my new 200 amp panel isn't big enough anyway to run a decent sized tankless. I get the idea that no one here likes electric anyways.
    If I went with a gas tankless and mounted it outside, how far away from a window does it have to be? ( My area uses the California plumbing code)
    I do not have a tempering valve as mentioned but it sounded like a solution worth trying.
    Here is my idea to start: Crank my water heater up to 120 degrees, wait a few hours, and fill my tub. I will then add cold water to the tub ( to simulate a tempering valve before spending $100 to sweat one in) and see if it gives me enough hot water in the tub to make the little lady happy. If this works, I assume that it is worth installing a tempering valve and running the water heater hot. Any problems or ideas with this? If it doesn't work, I will go with another idea.
    Thanks again for all the help,
    Sam
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,889
    Location:
    New England
    Crank it to 140. If you set it to 120, as you use water, it will be cooled off by the incoming water, at least later on. You're simulating what would happen if you had the full tank at 140, so start that way. just be careful, as at this temp, you can get burned. If it works out, then keep the tank at that temp, but temper it to a safer output. You could fill it as normal and see when you run out. In this case, instead of mixing cold at both the tank's tempering valve and at the tub fixture, you'd only be doing it at the end, but you'll get more accurate results.

    Adding a tempering valve is likely to cost more than you indicated, as you need to include some t's and other piping to get everything hooked up in addition to the valve itself.
  11. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    In such a warm climate and with the high electric/gas rates in CA, a solar water heating arrangement could be helpful. This could be especially true for electric water heating.

    120 F is the lowest set point you really want for a water tank. Below that there is some legitimate concern about the potential for growth of legionella. With an electric water heater it is even more of an issue based on some study results (gas heaters fared better), so it probably is wise to set the tank above 120 for electric anyway.

    Assuming it is already at 120, you might try 130 F set point first, then 135, then 140. This will give you an idea as to how much scald potential the different settings have. Remember to allow a major use/recovery cycle before trying to quantify the result of a set point change.
  12. Dana

    Dana In the trades

    Messages:
    2,805
    Location:
    01609
    What the bovine-companion said, although I'd work backwards- start at 140F- if that doesn't cut it you'll know right away. If it works then back it off 5F at a time to where it becomes problematic. If 130F or higher is where it wants to live, a tempering valve is in order. (In many places tempering valves are required no matter what temp you set your water heater to.)

    The general rules for USA are that the exhaust vent of a gas fired heater must be at least...

    4 feet laterally or below an operable door or window, or ventilation air intake...

    1 foot away from the gas meter or gas line in any direction...

    1 foot away from the vertical centerline at any point on the wall directly ABOVE the gas meter...

    1' above an operable basement window...

    3' away from any air intakes for other combustion equipment

    As an example of what it takes, flip through the installation & operation manuals for any of 'em, such as:

    http://www.takagi.com/download/product_manuals/T-KJr.pdf
  13. greentruck

    greentruck New Member

    Messages:
    3
    Location:
    california
    Update

    I finally got around to raising the water temp on the water heater up to 145, filling up the tub and adding cold water so that it was a decent bath temp for the little lady. By doing this I was able to fill the tub, but the water was only luke warm. If I install a tempering valve, it seems like I have maxed out the current configuration and the results are not as good as I had hoped for.
    I think that I am going to bit the bullet and get gas a tankless that mounts outside. Thank you for the general code requirements about mounting locations. I have a good location on the outside that is 4 ' away from a window and out of public view.
    What size tankless would you go for with for a 1200sq' house with two baths? Thanks for all the help again,
    Sam
  14. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,889
    Location:
    New England
    Figure the max flow you want to support at any one time (take a bucket and a watch to see what flow you get), then, you need to know how cold the worst winter incoming temps of your cold water are. A tankless needs to be bigger the more water you want hot at one time and how many degrees it needs to be raised. Unless you are willing to make compromises, they can require some pretty hefty fuel requirements if the water gets cold in the winter. Where I live, I've measured the incoming water temp at 33-degrees, and a typical tankless would need to be quite large.

    The things are usually rated at a certain temperature rise at a specific flow rate (think hand through a flame). To raise the temperature a lot and a lot of water in the short distance in the heater requires a LOT of energy. The warmer your incoming water is, the larger the flow you might get, and the hotter you can make it. Some reduce the flow to ensure you get the desired temperature, some just let the water cool off as you use more, and warm up as the flow rate drops. If you need a lot at once, many of them can be run in series (i.e., two or more). It can get really expensive for large flow rates with big rises required. you might need to upgrade your gas lines. It is much easier to put in a larger tank. don't even think about doing this with electric.
  15. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Agreed. Size it for the tub fill rate you want. Set the fill flow like you normally would, then do the bucket or meter timing test. Ask plumbers/local utility what the coldest winter supply design temp is likely to be. Figure out what sort of tub water temperature you want, then add about 5 F for tub cooling/pipe losses/evaporative losses during the fill. Flowrate converted to pounds/hr times this total temp delta (desired +5 F - coldest winter supply) will give you the output duty required. Note that this differs from input by the thermal efficiency factor.

    Another thread here mentioned insulating the tub. That is something that would really be nice. If I were putting in a new tub, I would do it. This will better retain the fill heat and bleed it away more slowly. There would still be substantial evaporation losses, but no more of the "cold tub wall in winter against-the-butt-cheek effect." And it won't substantially reduce the larger "miss" you are experiencing in heating capacity.
  16. nofears67

    nofears67 New Member

    Messages:
    186
    Have you looked into the Bradford White GX155S6BN high recovery model?

    - 200 gal 1st hr delivery
  17. Thatguy

    Thatguy Homeowner

    Messages:
    1,460
    Location:
    MD
    Proportion of hot and cold water
    HG = gals of hot water
    HWT = temp of hot water
    CWT = temp of cold water
    ST = shower/bath water temp

    The gallons of cold water taken to be 1.
    {HG(HWT) + 1(CWT)}/(HG + 1) = ST
    and
    HG = (ST-CWT)/(HWT-ST)

    With 2 gpm showers for 10 min. ea. you need 15 gals. of 120F and 5 gal of 40F water to give you 20 gals. of 100F water.
    Last edited: Dec 4, 2009
  18. MechGuy

    MechGuy Plumber @ Mechanical Contractor

    Messages:
    9
    Location:
    Iliinois
    Production rate

    Have you ever measured the GPM of that tub faucet? @ 48F inlet and 122F setpoint (74F Rise) that T-K3 should produce 4.3 GPM. And for you exact freaks the T-K3 is 199,000 Inside and the O/S model is 190,000.
    Curious if you have any water towers in your area? We've started seeing increased inlet temps due to solar gain by end of Summer and also chilling of inlet water in Winter that none of these towns anticipated. Luckily this is only in a 1-1.5 Mile radius of the towers.
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