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Thread: choosing an indirect fired water heater

  1. #1
    Residential Design and Consultant knowhow's Avatar
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    Default choosing an indirect fired water heater

    I am surprised to see that there is not a separate "indirect fired water heater" thread? forum?

    Terry, would you consider adding one under the "Water Heater Forum and Blog" category.

    I did a search on this site for "indirect water heater" and the few threads I found were all older.

    Am I missing something? Or is this just not a popular topic?

    This is my scenario :

    I am a general contractor in southern Ct. and am doing research for my customer.
    The plumber I use and my customer's plumber have given us a few options.
    We are all set on sizing. My customer is wary because he had a glass lined
    storage tank in the past, it failed and flooded his finished basement
    and he's been living with a poor quantity of hot water from the existing
    (probably scale fouled) tankless coil in his oil burner. I explained to him
    that it probably failed because he never maintained the anode. He wants
    my opinion, on stainless versus lined because I have more knowledge about
    his water quality. I recently had his well water tested before and after his water
    conditioner (softener). I wanted to find out if the softener was doing its
    job and if it was properly sized for his home and his water quality.
    Then I could consider what type of hot water storage I would advise he
    choose. Both plumbers gave us similar options of going with a storage
    tank or an indirect and options for stainless or lined with anode(s).


    The water tests showed 7 gpg hardness a pH of 6.7 and 16.5 mg/L of
    chloride before and after the .75 cubic foot (24,000 grain) conditioner.
    This proved my theory that the 16 year old conditioner was not doing
    its job. After analyzing their water consumption and usage patterns,
    checking the performance of the well and pressure tank, measuring the
    static pressure and gpm flow at the tank and various fixtures throughout
    the home we concluded that a new water softening system would be the
    first priority. We will be upgrading to either a 1.0 or 1.5 cubic foot system which I
    will purchase, install and program. I will retest for chloride once the system is online.


    Since the water will still contain chlorides ( I don't know how much
    yet), both plumbers agreed with my opinion that stainless may not be the
    best route.

    I researched different manufacturers warranties on their stainless
    versions and as I recall chlorides were acceptable up to a certain
    limit, but just because they would replace the unit doesn't mean I would
    jeopardize my customer having to experience another flooded basement.
    We may also add a Waggs valve (suggested by someone in another forum) and pan for protection.
    Does anyone know if a stainless tank can be protected by adding an
    anode? This is the only link I could find when doing a search for
    "stainless steel hot water tank with anode" I will be speaking with one
    of their reps today. http://alliedboilers.com/indirect_fi...ter_heater.php


    We will probably be choosing a lined steel tank with one (and I'll add
    another prior to install) or 2 anode(s). I will remind my customer to have the anode(s)
    checked annually when the boiler is serviced.
    We have decided to go with an indirect instead of just a storage tank because we will be
    bypassing and/or removing (any suggestions?) the boiler's tankless coil.
    We still have to decide on a tank in tank design or one with a coil.
    Would a stainless steel circulator be ok or is bronze preferred?


    Any suggestions or opinions will be greatly appreciated. Thank you!

  2. #2
    In the trades Dana's Avatar
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    Water softeners are notorious for shortening the lifespan of sacrificial anodes. The water conditioner may have contributed to the early demise by doing it's job a bit TOO well, rather than not well enough at some point in it's career. Annual inspection/swap-out is likely too aggressive a schedule, but every 3 years wouldn't be out of line for softened water. The glass liner is the first line of defense, the anode is only necessary after the integrity of the glass has been compromised.

    Tankless coils are fairly low performance even when new, and all will scale up eventually, and are the least efficient way to heat hot water. Oil boilers are also pretty low efficiency (~40%) in hot water heating only mode, even with indirects (though tankless coils are usually even worse), and at the current price of oil and electricity, even 18-20cent electricity, a (comparatively inexpensive) electric tank can be a cheaper way to heat the hot water. As long as the embedded coil can sustain flow (as opposed to not being able to deliver full temperature at high flow), it can be used to pre-heat water going to an electric tank during the heating season, and just turn off the boiler during the summer, letting the water flow through unheated, then in winter the heat purged from the boiler by hot water flows will improve it's average efficiency by lowering the average standby loss.

    If you install an indirect, (or even if you don't), install a heat purging control/economizer such as the Intellicon 3250 HW+ or the similar Taco unit, which will improve the boiler's overall performance by pre-cooling the boiler near the end of calls for heat, and heat-purging it at the beginning of new calls for heat (down to the programmed low-limit), which cuts significantly into standby loss.

    At 40% hot water heating efficiency a gallon delivers (0.4 x 138,000=) 55,200 BTU to the water.

    With 0.90 EF electric tank each kilowatt-hour delivers (3412 x 0.9=) 3071 BTU to the water.

    So a gallon of summertime oil is equivalent to (55,200/3071 =) 18 kwh

    At $4/gallon that's the same as heating hot water with ($4/18=) 22 cents/kwh electricity. If the client's electricity is much cheaper than that (which it is, for most of CT), an electric tank in series might be the better option. Even if oil prices drop, say it hits $3, that's still the same as 16.5 cent electricity, which is close to the current CT average.

    No matter how the water is heated, at these energy prices it pays to put 5/8" wall closed cell foam insulation (not the cheap 3/8" goods found at box stores) on all of the accessible hot water distribution plumbing, and all near-tank plumbing, including the nearest 6-10' of cold water feed and the T&P valve and outflow. If the client is a showering rather than tub-bathing family there's decent payback with drainwater heat recovery heat exchangers too.

    Last, (maybe least, since it's a stretch well beyond hydronic heating or hot water issues), anybody heating with $4 oil who can heat a large zone with a ductless minisplit heat pump would see a simple payback of 5 years or less on the heat pump, maybe even 3 years with the $1000 rebate subsidy being offered by the state, even with 18-20 cent electricity. In a CT climate a better-performance unit would deliver an average heating coefficient of performance (COP) of over 2.5, and in some instances even 3. At a COP of 2.5 each kwh delivers (2.5 x 3412=) 8530 BTU of heat into the room. A typical 3x oversized 85% boiler has an as-used AFUE of less than 80% (not counting distribution losses to unconditioned/semi-conditioned space), and at best would deliver 110,400BTU of heat into the space per gallon.

    So the worst-case mini-split against best-case boiler, the gallon of oil is equivalent to (110,400/8530=) 13kwh used in the mini-split, so $4 oil in the boiler would be equivalent to ($4/13=) 31 cent electricity in the mini-split which nearly 2x the actual cost of electricity. Even if oil hits $3 it's equivalent to heating with 23 cent electricity, which is still well above actual electric rates. Units with an HSPF of 9.5+ will average well above 2.5 for annualized COP. If sized large enough they can handle 100% of the shoulder season loads, with a COP greater than 3.5 when the average daily temps hit 40F and higher. (Seen any sub-$2 oil recently?) During the low-duty-cycle shoulder seasons an oil boiler at typical oversizing is at it's worst efficiency, and without a purge-control will be averaging 60%, maybe less (but maybe 70% w/purge-control.)

    In CT winter is warmer than in eastern Idaho, comparable in temp to Boise/Twin-Falls, but cooler than the Willamette Valley see the Metered COP averages in Table 33, p. 50, (p.63 in .pdf) in this document.

    Mini-splits and multi-splits cost about $2.7K/ton, installed, in our part of the world, and before swapping out a functional but low-efficiency oil boiler, for the same money even a 3-head 2.5-3 ton multi-split ought to be at least considered, using the aging oil-fired beast as the design-condition backup and for those relatively few mid-winter hours when the dew-point is too close to the air temp to get much capacity out of heat pump spending half it's time in defrost mode. (Defrost on a ductless cuts only modestly into efficiency, but does affect it's capacity under low-temp, high humidity conditions, say when it's both 15F and snowing.)

    When spending $3-4K/year in heating oil, or propane it pays to look at all options. A fuller-implementation multi-split can cut the oil bill by more than 80-90%, replacing that portion with $1-1.5K in electricity. It doesn't take a century to pay off the ~$8-9K installation cost for a 3 ton multi-split.

  3. #3
    Residential Design and Consultant knowhow's Avatar
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    Dana,

    Thank you for the suggestion of a 3 year anode swap-out, if needed. I'll make sure to replace the standard anode(s) with a segmented anode, or two, before the unit is installed. This way, when the maintenance is performed, the unit will not need to be drained but only depressurized when inspecting and or replacing the anode(s).

    I like the concept of using an electric hot water heater, but I think the existing tankless coil may be fouled.
    We can't shut the 2 year old boiler down in the summer because it supplies our heat pump? type AC unit.

    I see a few scenarios.

    One would be to replace the existing coil with a new one (aprox. $700 for parts and labor) and to use a standard electric DHW heater as a supplemental heater and storage device. I have read a few posts on various forums where they used a standard DHW heater ($500) as an indirect. The water would be preheated by the boiler and additional heating would be accomplished by the electric element. I understand the concept and like the cost effective virtue of a standard DHW heater over an indirect ($1500) plus the labor (plumbing labor would be about the same either way + an electrician). I just don't know if this can be accomplished to code and/or if it would be economical to run.

    The other would be to bypass the existing tankless coil and to use an indirect fired DHW heater off the boiler.
    I really like your suggestion about the "Intellicon 3250 HW+ or the similar Taco unit". Would this interfere with our AC needs?
    Any idea as to the ROI of the total cost (parts and labor) of the Intellicon or similar?

    I assume you may be an engineer. Would you have any links to a comprehensive answer on whether a magnesium anode would benefit a stainless steel tank's longevity, by balancing the effects of chlorides on the stainless steel. It would seem that even 316L stainless is susceptible at some point.

    Our average cost of supplied and distributed electricity is approximately 20 cents/kwh or more after all taxes and surcharges. It is approximately 10 cents/kwh for the supplied portion only.

    Thank you for reminding me about properly insulating the pipes most of the new pex is insulated with the 5/8" and the older copper has 3/8". Why do you suggest insulating the cold water feed and the T&P valve and outflow? For condensation?

    Thanks for the info on the ductless minisplit heat pumps. I'll print this out and give it to my customer. I"m certain it would have been an option if the boiler needed replacement.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    Even with heat traps, if present, the heat will migrate from the indirect, so insulating both the hot and the cold lines helps.

    FWIW, I have a SuperStor Ultra connected to my boiler that has served me well since installed. Hopefully, it will last a very long time. Assuming your water quality is within the accepted range, I would go for the SS tank verses any glass lined one...a glass lined one almost certainly will die before a ss one. I don't remember if this tank has a port for an anode, but there are some that can be hung from the inlet pipe, so that might be a possibility. Actually, I think the inlet on mine is in the side, so that may not work for you at all.

    An indirect is so much nicer than a tankless coil - full volume heated water when you need it, and if sized properly, unlimited shower time as well. My shower was on for an hour today, and was still producing a suitable shower temp. Then, I could have washed clothes, or filled the tub. One thing to consider is that the surface area of a round tank doesn't go up as fast as the volume it contains, so that helps with standby losses. A good indirect may only lose 1/4-degree/hour, and after say a lot of morning use and reheat, may never run the boiler again until the next day. IOW, there is NOT a lot of standby losses, and with a SS tank and an efficient boiler, is a much simpler thing with fewer restrictions than a tankless unit.
    Last edited by jadnashua; 05-24-2012 at 12:47 PM.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    Residential Design and Consultant knowhow's Avatar
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    Dana,

    I reread your post today, to better understand the mini-split suggestion/info.

    I think I now understand that an efficient way to use one would be to use the mini-split for the largest zone,
    thereby reducing the boiler's load (and thereby, its operating cost) and replacing that portion with a more efficient heating system. Would that zone be entirely separated from the boiler? Or would it be supplemented using the mini-split?

    How would one use "the aging oil-fired beast as the design-condition backup and for those relatively few mid-winter hours when the dew-point is too close to the air temp to get much capacity out of heat pump spending half it's time in defrost mode" ?

    Even if my customer was interested in making the additional investment in a mini-split we are restricted by space in the existing boiler room. Are these units small?

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    jadnashua,

    Thanks for your reply and for explaining the pipe insulation.

    Glad the SuperStor is working well for you. Like you said, it does not seem to have a port on the top to add an anode.

    I have found that the Weil-McLain Ultra Plus (made by Triangle Tube) and the Triangle Tube Smart Phase III, both have an auxiliary port to add a recirc unit. So if a recirc unit was not needed it could be used for a magnesium anode.

    I contacted Triangle Tube early last week, to ask if adding an anode to their SS tank would void the warranty, and asked if they thought an anode would extend protection to the stainless. I have not yet received a reply from them.

    The only stainless indirect I've found which incorporates an anode, is the one made by Allied Engineering Company (link above in my first post). I spoke with them last week and was given the phone contact info for their marketing rep., so that I could get the contact info for a local supplier. I called twice and left a message. No response. I wonder how they sell their products?

    What did you mean by the statement "One thing to consider is that the surface area of a round tank doesn't go up as fast as the volume it contains, so that helps with standby losses." ?

    Have a good day!

  7. #7
    In the trades Dana's Avatar
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    A typical installed cost to the client for a boiler economizer would be on the order of $500-700. Street price on the 3520 HW+ is $200-250 (multiple internet sources), and would take a DIYer a couple hours to install, but an experienced pro maybe an hour, sometimes less.

    Savings depend on the the oversizing factor of the boiler relative to the load, and it's condition, as well as what you program-in for min-temp. On an oil boiler holding the line at 140F at the low end would be important to limit both flue condensation and corrosive condensation on the heat exchanger plates in the boiler, which is significantly cooler than you'd need to keep it to get anything out of an embedded coil. (For gas cast-iron boilers you can usually set it to 130F without a problem.) Typical fuel savings would be on the order of 10-15% but 20% isn't unheard of. As a DIY project it pays for itself in the first heating season, installed by a pro it might take 1.5-2, depending on where fuel prices go and the actual fuel use reduction.

    The economizer controls the high-temp of the boiler (up to the safety-limit of the other controls), and the high-side aquastat that's on the boiler either needs to be bypassed or set to it's highest to allow the economizer to maximally utilize the thermal mass of the boiler. The economizers "learn" the system and turn off the burner (but not the circulation pump) in anticipation of the end of the call for heat based on the behavior of recent burn cycles. With the circulation pump still running heat its purged from the boiler into the zone-radiation (or indirect tank) until the room thermostat is satisfied, and the boiler won't re-fire until it hits it's programmed low point. The call for heat will typically ends before it hits the programmed low, but on a new call for heat the burner doesn't re-fire until the temperature drops to the programmed low. This leaves the boiler at a temp near the programmed low between calls for heat, minimizing standby losses. There's a big difference in standby loss between parking it at ~180F (or higher) and parking it at ~145F at the end of a burn. With embedded coils you typically need to maintain the boiler at 160F or higher to get adequate heat transfer rates through the coil at moderate hot water flows.

    Ductless mini-splits usually achieve highest average efficiently with a "set and forget" control strategy. Their highest efficiency mode at any outdoor temperature will be when both the compressor and the interior blower are running in the slowest to mid-range, and their lowest is when running flat-out, so any savings from setbacks get blown quickly in lower operating efficiency on the recovery ramp.

    To use the oil boiler as backup, set the boiler's thermostat in that zone to something a few degrees below where you set the mini-split. When the outdoor temps are too low (or temp-dewpoint delta too narrow) for the mini-split to keep up with the load, as the temp falls below the setpoint on the boiler zone's T-stat it calls for heat. But since the mini-split is still running (even if it can't keep up), it'll still be putting out a fraction of the heat for that zone, even during back-up calls to the boiler.

    The typical worst-case scenario is when it's 15F or cooler with dew points of 13F or higher. When that happens the COP efficiency will still be in the mid-2s, but it spends so much time in defrost mode it can't deliver very much heat to the space. (On for 12 minutes, defrost 10, on for 15 minutes, defrost 20, etc.) But the preponderance of the colder winter hours in CT have dew points well below the outdoor air temps, and the unit will normally be able to deliver something like 60-75% of it's 47F rated output at 17F (varies with make & model). Some models are capable of operating at 70% of rated output at temps as low as -13F (-25C). The Mitsubishi H2i Hyper Heating, Fujitsu Halcyon, and Daikin Quaternity series are probably your most likely bets, though Sanyo & LG both have decent offerings.

    Having multiple experienced installers/vendors in your area can be as important as the actual model, since getting it installed correctly and commissioned by competent techs can make a difference. Don't install it under the drip line of your eaves, or where it can get clobbered by a roof-avalanche- some overhead protection helps, but in most places isn't absolutely necessary. They can be bracket-mounted on walls if there isn't sufficient space for pad mounting at ground level.

    The interior head of a mini-split is typically 20-36" wide, 12-15" high, and projects into the room 9-12". A thing of beauty they're not, but there are flush-mount ceiling units that look like an oversized air-register (but for more money than a wall unit.) But even the wall units are no uglier than a window-mounted room air conditioners, and if you get a good one (SEER > 16, HSPF > 9) it'll be ~2x as efficient as a window mounted AC. The outdoor unit with the compressor looks like an oversized box fan, but they're much quieter than the traditional air-conditioning cube's we're all used to seeing.


    outdoor unit- probably a 2 or 2.5 ton


    typical indoor head


    Bracket mounted above peak wintertime snow depth, protected under eaves.

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    Because there's a squared factor to calculate area and area * height gives volume, a small increase in area (diameter) gives a big increase in volume. So a slightly larger exterior surface area from making it slightly bigger in diameter means a much bigger volume...check the specs, run the numbers for yourself. Standby losses are mostly because of surface radiation and piping - piping is the same regardless of the size tank, so it comes down to surface area (and, obviously, the insulation in the tank). So, a larger (volume) tank has a smaller increase in surface area than the increase in volume would suggest.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member BillTheEngineer's Avatar
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    I would not worry about an anode in a tank made of 316L SS. The concetraion of chloride in the water woould have to so high the water would be unhealthy. If you want to make 316L SS more corosion resistant, look for a tank that has been passivated. I don't know if anyone supplies succh a tank. Just about all 316L SS tanks have a lifetime warrenty. If i had to guess most 316L SS tank fail because of an installation issue of galvanic corrosion between the stainless steel and copper plumbing and/or stray electrical current from a bad ground orsome fault in the electrical wireing. Proper chemistry in the boiler water loop would be the first area keep an eye on. Like others have said the glass lined tank is going to fail, its just a matter of time.


    On just about any 316L ss tank the area that are going to fail first are usually the locations where welding has taken place. SS is not had to weld but it's not the most weld friendly. If the proper wweld filer material is not used or if the weld area is not properly shielded when welded the oxygen from the air basically ruins the weld and it is not as corrosion resistant as the base 316ll SS material.

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    BillTheEngineer,

    Thanks for the insight.

    I understand that stainless steel which has been passivated has a higher tolerance to corrosive elements. As I recall, a couple of the SS tank manufacturers I researched did state that the SS used for their tanks is passivated. If we choose to go with the SS I'll make sure to get a passivated one.

    Whether we go with a SS tank or glass lined, I'll make sure that high quality dielectric unions are used.
    Interesting discussions here: http://www.patchn.com/SMF/index.php?topic=753.0
    and here: http://www.thetankatwaterheaterrescu...rum3/2719.html

    I had not thought about the water properties in the boiler water loop. What would your concerns be? pH? TDS? chlorides? Would you suggest I replace the existing water with a fresh source once I have the new water softener installed and have tested the water and know its chemistry better?

    "On just about any 316L ss tank the area that are going to fail first are usually the locations where welding has taken place. SS is not had to weld but it's not the most weld friendly. If the proper wweld filer material is not used or if the weld area is not properly shielded when welded the oxygen from the air basically ruins the weld and it is not as corrosion resistant as the base 316ll SS material."

    As per the last paragraph in your post (quoted above), this is why I would like to add an anode to a SS tank. I just don't know whether an anode would have any negative effects on stainless steel. From my understanding, chlorides and pH are the major two factors which affect the corrosion resistance of stainless steel. I understand that an anode will not affect the pH, but wouldn't a sacrificial anode mitigate the corrosive effects chlorides would have on SS?

  11. #11
    Retired Defense Industry Engineer jadnashua's Avatar
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    As long as the material of the anode is higher on the scale than the material to be protected, the anode will be 'eaten' up before the other materials are attacked. Now, whether it's needed, is another story. It certainly helps on a glass-lined steel tank.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    update!

    Weil-McLain's New Aqua Plus line now incorporates a magnesium anode.

    Must be a reason why they've done this. I would assume it would be to assist in protecting the areas where the stainless steel may be more easily compromised such as at the weld joints and penetrations, and to better assist the passivated surfaces with corrosion protection.

    The Aqua Plus line uses the coil type heat exchanger. The anode is relatively small and is mounted horizontally.

    Their Plus, Gold Plus and Ultra Plus, which are stainless steel models also, do not incorporate an anode.
    I wonder if this is because they are manufactured for them by Triangle Tube. They are a tank-in-tank design and there is an auxiliary port on the top for adding a recirculating loop, I assume this port could be used for a standard magnesium anode.

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