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Thread: Adding 2nd holding tank as pre-heat for incoming water

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    DIY Junior Member cfoster's Avatar
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    Default Adding 2nd holding tank as pre-heat for incoming water

    We're considering buying a cheap electric hot water heater and NOT turning it on. It would just to act as a 60 gallon holding tank for incoming water.

    In winter, the water temperature of our incoming line is about 3'C/37'F. We thought if that water could sit overnight (even in an unheated tank), it would gradually come up to room temperature (or thereabouts). The other active hot water tank would draw from this holding tank so it would only need to raise the temperature about half what it usually would.

    I know the house will ultimately be heating this unit, but it seems like the house would be an order of magnitude more efficient at raising the temperature than a hot water heater would.

    Is there a flaw in this logic?

    Thanks,
    Colin.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    An electric tank is better insulated than a gas one (since it needs a flue up the middle). A good one will keep the cold in nearly as well as keeping the heat in. You may not see that much of a temperature rise.
    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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    Geologist sjsmithjr's Avatar
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    What you want is a tempering tank which, installed ahead of the water heater, will do what you want assuming you install it in a space where the ambient temps are high enough.

    You want an uninsulated tank, not an insulated tank like a water heater. Installing the tempering tank and the water heater in a conditioned space will yield the best results.
    Last edited by sjsmithjr; 06-03-2009 at 12:53 PM. Reason: Double Signature
    -Sam Smith
    Licensed Professional Geologist - AL, TN, KY

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    DIY Junior Member cfoster's Avatar
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    Hi Guys,

    Thanks for that feedback. (Very glad it's of the "it would be better if" variety and not "you can't because".)

    I was going with an electric only because it seemed like the cheapest way to get a tank. I was hoping by getting a cheap model I'd also get poor insulation.

    I figured it probably wouldn't be too hard to remove the insulation from the tank if I needed to, though I've been warned I'll then have more problems with rust and have to collect/dispose of the condensation on the tank.

    A PVC tank would probably work well as it wouldn't be affected by the condensation. If anyone can recommend an inexpensive tempering tank that would be great! (The 60 Gal. Electric I'm looking at is $CD 300.)

    Thanks,
    -Colin.

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    Plunger/TurdPuncher kingsotall's Avatar
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    Quote Originally Posted by cfoster View Post
    I was going with an electric only because it seemed like the cheapest way to get a tank. I was hoping by getting a cheap model I'd also get poor insulation.
    I love that logic! Get one made in China!
    I just post cuz I like to see my avatar.

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    Geologist sjsmithjr's Avatar
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    Try calling it a water storage tank instead of a tempering tank and you'll probably find something that'll work for you, possibly for less money.
    -Sam Smith
    Licensed Professional Geologist - AL, TN, KY

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    Extreme DIY Homeowner Scuba_Dave's Avatar
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    I picked up a tank for free off Craigslist
    They changed from electric to gas
    I just need to strip off the outer shell
    DIY Handyman (not 4 hire)
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    DIY Senior Member chris8796's Avatar
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    Quote Originally Posted by cfoster View Post
    I know the house will ultimately be heating this unit, but it seems like the house would be an order of magnitude more efficient at raising the temperature than a hot water heater would.

    Is there a flaw in this logic?
    You don't mention your furnace type or your water heater type, but its not going to be much more efficient. You don't mention what your goal is, save money, add capacity, etc.

    If you use a decent amount of hot water, especially for showering, I'd consider a drain heat recovery unit. I have one and it makes a noticeable difference.

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    Master Plumber nhmaster's Avatar
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    These days the tank insulation is urea foam and getting it all off the tank is a major undertaking, even for the cheap ones.

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    In the trades Dana's Avatar
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    Quote Originally Posted by cfoster View Post

    I know the house will ultimately be heating this unit, but it seems like the house would be an order of magnitude more efficient at raising the temperature than a hot water heater would.

    Is there a flaw in this logic?

    Thanks,
    Colin.
    There's the rub!

    More efficient? A very squishy "maybe", but don't count on it.

    An order of magnitude more efficient? NEVER! (At least not on an annual basis in heating-dominated Toronto!)

    An electric HW heater & electric-resistance heating- no gain except in summer (but quicker recovery on the tank.)

    A gas hot water heater v.s. 80% AFUE hot air heating system (assuming the furnace was properly sized), could be same story, but maybe not:

    Gas hot water heaters have very high standby losses (primarily from flue-convection, but also from the insulation gap at the burner. As a result, the net efficiency is largely dependent on the volume of hot water used. The US Efficiency Factor (EF) is based on ~60gallons/day of use. If you use more than that you'll beat the EF number, if you use less than that you'll fall short- it's as simple as that. At 25 gallons/day a gas-fired tank with an EF of 0.65 will be under 50% efficent.

    But AFUE is also a very squishy test, based on a ~30% duty cycle, and doesn't measure serious system-issues like duct leakage & insulation, duct balancing, etc. Most "typical" hot air systems run fully 15% or more below their AFUE as a system, and it's the system that will be heating your tempering tank. If you're a average or high-volume user with an 80% AFUE (properly sized, so that the ~30% duty cycle test even has some validity) with you'll roughly break even on the tempering tank.

    If it's a well balanced properly sized 95% condensing furnace, you'll beat the standard gas-fired tank efficiency tempering tank somewhat, but you haven't changed the standby losses of the tank-heater, only lowered the duty cycle of the burner a bit.

    If it's a forced hot water heating system you have some of the same issues, but the distribution losses tend to be much lower than forced hot air, so only take 5% off the AFUE number assuming it's correctly sized, which means you'll beat it. But it probably isn't- 100-300% oversizing is frightfully common, and if it's 300% oversized you'll just break even with the tank efficiency.

    But if you have forced hot water with an AFUE greater than 80% (even if oversized) the greenest thing you can do is install an indirect-fired HW tank heater running off the boiler- preferably a reverse-indirect plumbed as a buffer tank for the heating system (eg. ThermoMax, ErgoMax), which will raise the net efficiency of the heating system as a whole (with very SIGNIFICANT gains on an oversized boiler system, pulling it back up the efficiency cliff it had fallen over) and heat your water at the same system-efficiency, beating a self-standing tank every time. During the summer season it's efficiency will fall to about what a 60gallon/day tank heater would be, but not much lower than that. But the gains in system efficiency to the heating system more than make up for any slightly lower summertime HW heating performance.

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    DIY Junior Member cfoster's Avatar
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    Thanks for the "water storage tank" tip, Sam. That seems to be much more on target. I haven't seen any with the correct in/out lines, but I'll keep looking.

    Does anyone know if that would cause a problem for the water pressure out of the water storage tank? (I've heard that a pipe that, say, goes from 1/2" to 3/4" and back to 1/2" doesn't have the same pressure. If that's true, what effect does a tank have?)

    Just to clarify: the goal is to find a way to get incoming water to *slowly* come up to room temperature over time (8 hours or so) so the hot water heater (combo unit for radiant floors & domestic hot water) doesn't have to work as hard.

    My original plan was just to run a 200' coil of 1/2" copper line (~25 gal.) before the hot water heater. That would sweat a lot, of course but it shouldn't be too hard to drain it. The "unheated hot water heater" option just seemed like a cleaner solution, but perhaps not.

    -Colin.

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    Extreme DIY Homeowner Scuba_Dave's Avatar
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    Problem here is the incoming water can be down close to 32 in the winter. So the water heater is trying to heat water from that temp to close to 140. That's a 100 degree change. If the tempering tanks sits overnite then the water should be around 57-63 - cellar temp. And actually the utility room stays warmer then the rest of the basement
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    In the trades Dana's Avatar
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    Quote Originally Posted by cfoster View Post
    Thanks for the "water storage tank" tip, Sam. That seems to be much more on target. I haven't seen any with the correct in/out lines, but I'll keep looking.

    Does anyone know if that would cause a problem for the water pressure out of the water storage tank? (I've heard that a pipe that, say, goes from 1/2" to 3/4" and back to 1/2" doesn't have the same pressure. If that's true, what effect does a tank have?)

    Just to clarify: the goal is to find a way to get incoming water to *slowly* come up to room temperature over time (8 hours or so) so the hot water heater (combo unit for radiant floors & domestic hot water) doesn't have to work as hard.

    My original plan was just to run a 200' coil of 1/2" copper line (~25 gal.) before the hot water heater. That would sweat a lot, of course but it shouldn't be too hard to drain it. The "unheated hot water heater" option just seemed like a cleaner solution, but perhaps not.

    -Colin.

    Not work as hard? Are you saying that the burner just can't keep up with both a hot water load and a heating load at the same time, at the incoming water temps you experience?

    Is the combi unit a tank, or set up using a tankless?

    If tankless:

    It'll take a bit of system design to get there, and it won't be super cheap, but the "right" solution is to use a reverse-indirect as the heat exchanger for the DHW, turning the indirect into the hydraulic separator for the primary/secondary on the heating system:

    http://www.ergomax.com/New-Tanks.htm

    http://www.thermo2000.com/pdf/en-US/manu/turbomax.pdf

    Set the aquastat to whatever temp the radiant needs (or if it's a low-temp slab, set it to 135-150F and use mixing valves on the heating end.) Set the flow & delta-T on the tankless/boiler such that it can deliver design-day heat at normal radiation returns. (Go with the lowest flow on the primary that will reliably get the tankless to fire, and program the output temp to whatever it needs to be to deliver the BTUs. Unless the place is huge you won't need more than 2-3gpm or output temps more than 180F.

    Then, when the cold water comes into the heat exchanger coils at the bottom of the tank, the return water in the primary loop will drop and the tankless will modulate the fire up to keep the output at the programmed temp. With 25-50gallons of buffered heating system water to draw from, even if the burner can't keep up with continuous load it'll go awhile before the DHW output temps drop to the uncomfortable range, and the recovery period will be very quick. With the indirect acting as a buffer in the system it'll lengthen the burns to reduce cycling losses as well.

    It's a lot more money than 200' of copper pipe or a surplus tank, I know, but it works, and will improve the efficiency of the heating system. And if you're not comfortable with re-designing the heating system at this level, don't go there without professional assistance.

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