Not able to calculate any decrease in savings?

Discussion in 'Tankless Water Heater Forum' started by clock, Dec 7, 2009.

  1. clock

    clock New Member

    Messages:
    3
    Location:
    USA
    Hello,

    We're currently on a highly efficient Buderus 30 gallon (86.6%) tank. My best guess for its operating cost is $261 per year or so based on my calculations.

    Based on the same calculations, increasing to an 80 gallon tank (which would cover my main interest in tankless - the showers) would only increase the price to $275.

    At the same time, an 116 AMP electric (cheaper install) at 5 GPM with 70 degree rise (to run the tub) appears to cost $50 extra a year to run (if I have it cover four showers a day). That doesn't include installation, and the propane units didn't seem any better but had higher installation costs.

    That appears to be more expensive - both to install and run - than just increasing the tank. My question is - do those numbers sound right? Am I missing something?

    Thanks guys!
  2. Dana

    Dana In the trades

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    In most markets electricity is more expensive than even 60% efficient fossil-fueled hot water heating, but when that fuel is propane it's often flipped on it's head.

    But without knowing what fuels & prices and assumptions you're making about hot water volumes used it's all just a WAG.

    Since your primary reason for upsizing was showering compacity, DO consider keeping the tank small and going with drainwater heat recovery, which turns just about any tank heater into an endless-shower at very high net efficiency. Installed downstream of the shower with it's output plumbed to feed both the input to the HW heater and the cold feed to at least the shower (if not the whole house.) It cuts the energy used in showering literally in half, and boosts the first-hour rating of system in showering mode immensely. It's like adding a 25-35KBTU/h burner to the system (a burner that doesn't use any fuel!) The hardware is a large fraction of a grand, (and in some instances difficult to install) but it's a bigger efficiency boost than going from a standard-efficiency tank to a condensing tank/tankless, and the apparent capacity boost in showering mode alone can be worth it. Anticipated lifetime is 30-50 years, maintenance free.

    It basically pre-heats the incoming cold water up from 40-60F to 70-80F by extracting heat from the ~100F water going down the drain. You end up mixing in less hot water from the water heater, and the water heater is being fed higher temp water, so it's load is less. The effect on capacity is pretty dramatic for showering, but ZERO for tub fills. The tank can be sized for tub filling, and you can be fairly assured that with any reasonable burner on the HW heater, it'll mostly keep up with 2.5gpm shower flows indefinitely.

    [​IMG]

    For a cost effectiveness based on showers per day see:

    http://www.renewability.com/uploads/documents/en/analysis_dwhr_minnesota.pdf


    The take-away from the cost effectiveness document is, if you're going there go BIG - as big & fat a heat exchanger that still fits. Taller & bigger diameter=higher performance, and the performance increase is in excess of the higher up front cost.
  3. Thatguy

    Thatguy Homeowner

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    Location:
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    Assuming 100% efficiency, 1 Therm of energy can be had from approx. 0.71 gal of fuel oil, 100 cubic feet of natural gas, 1.1 gal of propane, 29 kwh of elec heat, 8.4 kwh of heat pump heat (COP = 3.5), 4.2 kwh of heat pump heat (COP = 6.9).
  4. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Your usage might make sense for drainwater heat recovery if you have good access to the drain collection stack below all of the showers. For us a retrofit would only allow 50-75% of the daily showers to make use of such a system. Unlike the report we take ~7 min showers @ 1.5-1.6 gpm and are therefore using less than half as much as the report's "worst case."

    DWHR won't do anything for filling a tub. A larger tank will. Our 50 gallon water heater at 120-125 F won't quite fill the tub at our desired temperature, but stopping the draw for 15 minutes awaiting recovery does the trick.

    It sounds like a bigger tank is in order. The efficiency factors don't seem to change appreciably from a 30 to a 50 gallon tank. So unless there is a space issue I would go bigger.
  5. clock

    clock New Member

    Messages:
    3
    Location:
    USA
    Thank you for the input so far! These ideas are really interesting, and I’ll have to research them further.

    Demand: There are three bathrooms. Once is the 80 gallon tub and a base flow rate of 6 GPM. Its shower is 2 GPM or so. The other two bathrooms are 2.5 and 2.5. We're fine except when we have 3 or more showers in the same period of time, and we can't use the tub at all. It looks nice, though...

    Assumptions: The rise we have to accommodate in this area is probably 77 degrees as we’re in the northeast US. I made an assumption that while we have a tub at 6 GPM, we could cut in cold water, and perhaps run it at 3.43 GPM (showers at 1.33) when using tankless. (Perhaps this is in error, but I wouldn’t be able to get electric tankless anyway otherwise as I don’t see models running much above that.)

    Tankless: At a 70 degree rise with at least 3.43 GPM, I assume we can get away with a unit like the Stiebel Eltron Tempra 36 - 240v, around $900 + install, for example. (http://www.tanklesswaterheatersdire...rheaters/stiebeleltron/stiebeleltronspecs.htm) . 150 max amp load.

    Comparison: I used another technique at first, but then I just used Rheem's energy calculator. (http://www.rheem.com/products/tankless_water_heaters/energy_calculator/)
    I believe that their estimates of cost/therm are based on national averages (although I could change them). Put in my boiler, .87 efficiency, my rise (77), and 100 gallons – which is about what I think we use for showers each day.

    This is quite interesting. Apparently, because the cost of electricity can, for comparative purposes, be put at $2.46/BTU, even at 100% efficiency, electrical heating could not compare with my 0.87 efficiency boiler. My boiler definitely beats out tankless propane, especially when installation is considered. Pretty incredible, if this is right. If your natural gas tank is any better than 0.82 efficiency, it beats out the tankless. However, there are standby losses to account for, and with these Buderus models, I wouldn’t know where to start to find that.

    For calculating the differences in tank size, I used: (http://www.doa.state.wi.us/docs_view2.asp?docid=2249). 7 people 14-64; all trues except the last one; 43 cold inlet temperature; .87 energy factor; 98 MBTU/hr input. What I’d love to find is a calculator that does standby loss increases, but I think this accounts for that, as the gallons used increases each time.

    To cut to the chase – even if I upgrade to 80 gallons, it appears the impact will be around $20 more a year. Am I missing anything here? Any system I install to achieve the same end would need a GPM of 6 and run on $20 a month. It would have to cost less than a new tank (the company’s efficient tank is $1,777. I don’t doubt that installation would be cheap. So, say, $1,900.), yet run on so little that I would recoup its cost in a reasonable amount of time. If I stayed in the house for 10 years (doubtful) that means it would have to cost, with installation, and run on less than $200 over the course of those years.
  6. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Clock,

    With 7 people in the house you really need more than a 30 gallon water tank. As you observed that'll never feed an 80 gallon tub either...my 50 gallon tank is challenged by that at 120 F setpoint...it can do it after a 15 minute pause for recovery and my jacuzzi tub is approximately the same capacity as yours by my fill measurement. The 50 gallon was fine with two 2.5 gpm showers running simultaneously, but adding a dishwasher cycle would deplete it near the end of their showers. (I used this as a way of timing the kids and providing incentive to get out. ;) I would start the dishwasher when they entered the shower and tell them that they had only enough hot water for a reasonable shower...worked like a charm! :cool:)

    The tank efficiency is important only for calculating the energy use, not for calculating the volume it will put out. A standard gas water heater is about 40,000 Btu/hr input at about 75-80% burner efficiency. That will supply about 1 gpm at 120 F with 55 F feed. Be careful not to confuse this with the Efficiency Factor which includes storage losses for a given usage profile. Storage losses are essentially a fixed annual quantity and that makes the mathematics confusing.

    There is another high dollar tank type unit that gets you the instantaneous capacity of a tankless, with the burner efficiency of a tankless, without the drawbacks of a tankless: a condensing storage tank. These typically have about 100,000 Btu/hr burner rates with about 96% thermal effiency (tank storage losses not included.) I should be able to run two showers simultaneously endlessly at this burner rate because I use very low flow showerheads (1.5-1.6 gpm). I've not run three very low flow showers simultaneously although I want to try it--my calcs suggest it will be just fine. Filling an 80 gallon tub at 3-4 gpm and 105-110 F should not be a problem either. The negative is that these are not yet fully rated and they are very expensive! I'm hoping they become mainstream and less expensive in a few years so that I can go that route when I have to replace my water heater.

    Electric resistance heating is expensive because the average electric power generation facility operates at less than 35% efficiency. Newer units are above 50%. Then you have 7% transmission losses and about 8% storage losses for a net of about 30% overall for the present 35% efficient unit. By comparison a gas heater runs at about 75-80% efficency minus storage losses that drop it to around 60-63% overall. With natural gas this is a no-brainer. Propane is much trickier.

    It makes thermodynamic sense (see entropy): electricity is a higher form of energy than combustion heat. Therefore if your desire is heat, combustion is more efficient because it is more direct with fewer transfer steps.

    The caveat is with heat pump water heaters. These can operate with about a 2.0 factor for electrical efficiency (resistance heaters being ~1). However, they need to be positioned properly where the absorbed heat can be handled (heat removed from the room and put in the water), they appear to have very low recovery rates, and even the manufacturer projects typical installations as being about 1.5 electrical efficiency wise since they use resistance heaters when the heat pump can't keep up.
    Last edited: Dec 8, 2009
  7. Dana

    Dana In the trades

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    It may be difficult to fill an 80 gallon 80 gallon tank all at full flow, but with 98 MBH into the boiler it'll be quicker than than with a 3.4gpm tankless. When looking at tub filling with hot water heaters, look at the first-hour gallons ratings, and figure on doubling what you'd need for a tub fill. (Don't go under 160 first-hour gallons if you need to fill an 80 gallon tub.) With 98MBH input & 87% efficiency gives you 85KBTU/hr, which delivers ~2.2gpm at your 77F temperature rise, tank volume. That 2.2gpm is 132 gallons/hr, but you'll get at least 50 out of the tank before you might have to slow down the flow to keep the temperature up. If you're filling at 6gpm that first 50 will take ~8minutes, in which time the burner makes up ~18 gallons more worst case you'll have to throttle back the flow to keep the temp up, but figure on something like 15 minutes on the coldest-water day in winter. With the 3.4gpm tankless it's gonna take you 20 minutes to fill that sucker.

    Heat pump water heaters only really make sense in cooling dominated climates, or if you have VERY cheap heating fuel. It's no more efficient than your heating system, since drawing heat from the room adds directly to the heating load in heating dominated climates. But in cooling dominated climates it lowers the air conditioning load. Most tank-top heat pump water heaters have fairly slow recovery too (meaning limited first-hour capacity.)

    Energy factor (EF) tests are based on 62 gallons/day of use, independent of tank size. ACTUAL use will vary quite a bit, and with tank heaters, the more volume you use, the closer you get to the steady-state efficiency of the appliance. With a tankless the EF test is pretty close to the steady-state efficiency of the appliance, but doesn't account for short-cycling losses that occur in the real world. (A 0.82EF fossil fired tankless really only runs about 75% average efficiency in real-world residential use. A condensing 0.98 EF tankless never breaks 90% in a residential app, but will in pool heating or car washes, etc.) Very high volume users get 75%+ out of standard tank heaters, and you're likely to break 80% with a Buderus. I'm not convinced you'd get much more than 85% out of a condensing tank, but you might.

    But whatever the tank efficiency, that's only part of the equation. Near-tank plumbing on tanks contribute a large fraction of the standby loss, and can be mitigated substantially by insulating them to a minimum of R4 for the first 6 feet (5/8" wall foam pipe insulation), including the cold feed and the pressure/temperature safety outlet plumbing. Insulating the entire hot water distribution plumbing is also cost-effective.
  8. Thatguy

    Thatguy Homeowner

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    Location:
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    98' of 1/2" ID pipe holds one gallon.
  9. Dana

    Dana In the trades

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    Who cares about the volume? It's the thermal conductivity of the plumbing continuously sapping heat in standby from the tank, radiating/convecting it into the surrounding space that's the more significant loss. It's quite unrelated to it's volume, since it stays well above room temp by the continuous sapping heat from the tank.

    But on the distribution plumbing the volume of abandoned BTUs that have to be replaced by draws also adds up. Say you live in a fairly short tight distribution system, with only 25' of 120F water, and half inch, not 3/4". If it has to be fully replaced 10-25 times/day vs. 3 times/day it becomes a measurable difference in loss:

    Call it 10x/day times 25' is 250', or ~2.5 gallons.

    @ 8.34lbs/ gallon & a 70F delta-T on the incoming water, that's 1500 BTUs/day x 365 days/year it's ~550,000BTUs

    Assuming a ~70% efficient hot water heating system that's, ~ 8 therms of source fuel. In a busy household where it's 25x/day that's ~20 therms/year.

    If instead it's 3/4" pipe and actually 50' long...

    It multiplies considerably, and most houses have much more distribution volume than 25' of half-inch.

    If you insulate the pipes, subsequent draws that occur within a half-hour of the initial draw are usually "hot enough" for the intended purpose, but that's reduced to well under 10 minutes if uninsulated. Residential use tends to be bunched up in the mornings & evenings, with occasional mid-day draws so the amount of abandoned BTUs drops dramatically with insulation, only fully abandoning the heat a few times/day.

    Now 8-20 therms/year is not gonna break the bank, but the payback on 25' of pipe insulation is way better than your 401K. But that's a best-case scenario. Real world measurements average around 15%+ of the total heat drawn from the heater is lost to the distribution plumbing.

    According to real-world measurements by PG & E, in "typical" 200-250therm/year residential water heating use, that's 30-40 therms burned to no good end, and insulating the distribution plumbing will cut that loss to under 10 therms/year for way less than $50 in materials. (You can probably find the raw data online, if you really care.)

    Or better yet, ask Runs with bison- he's probably measured it at his place! ;-) But seriously- he's cut his tank standby losses down to the nubbins, and no matter what his utility rates are, the after-tax return on pipe insulation is way better than Wall Street, and at much lower risk- practically guaranteed.
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,990
    Location:
    New England
    Humm, unless you are using recirculation, the heated water that eventually fills the pipes, regardless of how it is heated, is lost to radiation in between uses - doesn't matter if it is from a tank or a tankless system. If your tank or tankless system is fitted with recirculation, the losses from the distribution systems are identical, and they're almost the same otherwise.

    If you use the generally available heat traps on the input/output of a tank, you don't get much convection heating of the water above the tank...you do get some from conduction. If the tank is in a heated space, it's a wash as it's less heat you need from your heating system to keep that area warm.
  11. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
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    Location:
    Penticton, BC
    Crank the temperature of the tank up to 160* and put a mixing valve in above the tank to deliver 120* water to the faucets.
  12. Dana

    Dana In the trades

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    I think we agree here- distribution insulation is important for the efficiency of any HW heating system, but near-heater insulation cuts standby losses on tanks (heat traps or not- it's a matter of degree), but of minor to no importance with tankless systems. Standby losses being typically greater than distribution losses raises the relative importance of near-tank plumbing where tanks are concerned. Anything beyond 5-6' from the tank isn't much of a standby issue unless you're storing 160F water or something.

    On distribution plumbing, the higher the insulation value, the more time you get between draws with the distribution water at still useful temperatures. But anything left for an hour is in fact abandoned & lost, unless one uses non-cost-effect levels of insulation. Here's some data from one amateur engineers DIY at-home experiment:

    [​IMG]
  13. clock

    clock New Member

    Messages:
    3
    Location:
    USA
    Wow, you guys are incredible. I have to admit, upfront some of this is quite over my head.

    It seems that insulating the pipes sounds like a pretty good idea (although it seems not for too long) if I know what to use and how to do it properly. I think?


    In terms of cranking up the heat on the tank - I just looked it up. Pardon my naivete! It seems to be kind of like "on-demand" for a furnace based on what I did find (maybe). So this would allow me to fill the tub, and solve the shower issue, correct? However, it would probably cost more - is there any way to tell whether this would work or whether it is cost effective?

    This is the article I found on the topic:
    http://www.naturalhandyman.com/iip/infwaterheater/infwhadjust.html
    Last edited: Dec 10, 2009
  14. Dana

    Dana In the trades

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    It's not really an "on-demand" (which is another term for tankless), but an "indirect fired tank", which gets it's heat from the heating system's boiler. If I read your post right, the boiler runs at ~87% efficiency, with 98KBTU/hr input? In that case you needn't raise the storage temp on an 80 gallon indirect to meet your peak loads- 125F-130F should pretty much do it. If you went the smaller indirect fill times on the tub would be slow because you'd have to limit the flow to keep the temp up after the first 20 gallons, but you could still blast multiple shower flows for 10-15 minutes at a time without running cold (and the recovery time to get the water back up to temp would be very short.)

    Running the smaller indirect at higher temp would help but not enough to fill the tub quickly and it's standby loss would increase dramatically. The 80 gallon indirect maintained at more moderate temps will be more efficient and reliably deliver the goods.
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