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    Hydronic (FHW) heating system installations can be likened to creating something with a giant Erector Set™. As a kid, mine came with an electric motor enabling powered cars, airplanes, ferris wheels, etc. Days, weeks and months of childhood imagination and expression ensued. Great memories!

    Now as “big kids” hydronic heating system installations offer us a similar fulfillment of free expression. Within the limits of ultimate system functionality hydronic “artists” can seemingly paint their own canvas while getting paid for it! The question logically arises however as to where are the cost benefits of our efforts? We argue that heating guys are indeed losing direction, both technically and functionally. The adage “Old Dogs need to learn New Tricks” comes immediately to mind, regretfully also being one. It’s not only our innate trade resistance to change but not to think forward in applying what has preceded us ….. beginning with Physics 101!

    Gravity convection was the means of distributing heating water from the Roman Age onward. From the mid-1800’s to about 1930 it was “Queen” Gravity Hot Water along with “King” Steam. However gravity hot water systems required large boilers, piping and radiation sizes. They were slow to react, single-zoned and fuel hungry, whether burning wool, coal, oil or whatever.

    Introduction of the electric circulating pump changed hot water (hydronic) heating forever. Now multiple zones, rapid response through less intrusive radiation and powered burners do it all very conveniently. Gravity is still there, but we have necessarily learned to control it with flow check valves. Apart from the brief and unsuccessful introduction by American Standard of their “Ash Can” Gravity Boiler System in the late 1950’s (our nickname – we were Am-Std Installers back then) FHW has predominated. Circulators began by being placed on the (cooler) returns with flow checks on the (warmer) supplies to control convection. Then circulators moved to the supplies along with the valves. Boilers are now migrating from two-pass to three-pass or condensing types for greater efficiency. That’s arguably been it ….. until now!

    Enter the Delta-T (differential temperature) sensing and managing, ECM (electronic commutator motor) driven Variable Speed Circulator, specifically the Taco® VT2218-HY1-FC1A01 (Current Model). This latest iteration in Taco® Hydronic Distribution Technology is making its inevitable mark on radiant and other hydronic circuits. Its “smart” capabilities are many however, having five (5) selectable operating modes, but we are focused only upon the “DELTA-T MODE” within its appetizing menu.

    We have been experimenting for some years now with Taco®’s prior Delta-T’s and the current VT2218 ECM as a dedicated system circulator, beyond that denoted inTaco®’s Product Brochure. During this process we discovered and integrated differential temperature management with natural hydronic convection (gravity) for an optimized design with superlative results, again well beyond those claimed by Taco®. Multiple “Beta Site” installations have provided us with sufficient data and results to seek “Provisional” and now subsequent “Non-Provisional Intelligent Property Protection” on our:


    Quite a mouthful indeed, but place the emphasis on APPLIANCE. You see, delta-t hydronic technology is not merely a “circulator swap” for radiation and energy efficiency gains. It offers additional opportunities in hydronic process integration and management as yet either unrecognized or underutilized. Our efforts fall on the latter, particularly when cooperating with “Mother Nature” by employing natural hydronic (gravity) convection to an operational advantage.

    Falling back upon and employing the principles of “non-powered” gravity heating we optimized a “contemporary” Delta-T ECM FHW System. Utilizing supply and return iron-piping “stacks” and compacted manifolding minimized distribution head losses. Combine these with the thermal storage capacity of mating to a cast-iron high-mass boiler enables a true, free-standing APPLIANCE. Its profile is contained within the boiler footprint plus about a foot of exhausting & piping space to the rear. Modular construction is a natural for hydronic elements as well as system wiring and controls. Module variations to suit particular site and application requirements logically follow.

    Taco® promotes energy reductions of up to 85% and 15% for electricity and fuel respectively by using their Delta-T ECM Circulator Product. We emphatically agree ….. and even more! Measurements on our multiple “Beta Sites” indicated a deviation that was not anticipated. Namely, upon the dedicated VT2218 system circulator achieving preset “delta-t” and stabilizing, the wattage displayed further deteriorated to approximately half this value along with some further speed reduction! A subsequent “Beta Site” circulator failure confirmed our suspicions. First, that failure event was not noticed for an estimated 2-3 days, and then only with a gradual decay in house temperature. The event was camouflaged by an integral Indirect Water Heater (IWH) based domestic hot water (DHW) delivery that was never affected!

    The circulator failure not only quantified the now measurable convection (gravity) heating effect witnessed, but qualified our overall near-boiler piping configuration, purposely designed with a de facto priority circuit for the IWH Option. The latter eliminates the need for that particular optional control feature. Further witnessed on our “Beta” circulator failure was boiler temperature operating near maximum. TheHydrolevel® 3250-Plus Aquastat on our Weil-McLain® UO Boilerlogically incremented system temperature upward, attempting to satisfy prolonged zone demand(s). The higher supply/return temperatures and differentials further enhanced convection. The effective 700 pound plus thermal storage mass of our “appliance” also served to dampen the close-coupled IWH zone demands! It is reasonable to presume that the IWH thermal storage would also be “pumped down” during a burner failure, passively prolonging thermal decay.

    Radiation configuration and piping integrity are paramount to convection (gravity) heating effectiveness however. Drawing from over 40 years operation of our personal dual-fuel, fully no-power emergency capable hydronic heating system, we can attest to radiation attribute importance. Simple, well designed serial and split baseboard loops work best. De-aeration is absolutely necessary and system water pressure must be present.Taco® Zone Sentry Valves utilized in our design have a manual open-close feature that can be utilized to regain a heating level upon an actuator failure, as do current flow check valves. (NOTE: You can replace a failed Taco® actuator head on our appliance in 20 seconds – if ever a need!)

    Complimenting hydronic performance is our “Compact Steel Hydronic Header” (Patents Pending) that positions, orients and provides flexibility in both zone and return valve mounting with expansion capability. Placing Taco® Zone Sentries at their minimum (2-3/4″) spacing with wired harnessing eliminates the need of a significant valve or circulator relay expense ….. and with greater diagnostic and system functionality!

    Currently applying our manufacturing process expertise we are projecting the “economies of scale” provided in our construction. The physical modules of boiler, supply and return are configured for capacity/fuel and zone complement and inventoried. A natural and complimentary option of an Indirect Water Heater (IWH) for domestic hot water (DHW) generation falls into this methodology as well. Adding assembly fixtures, applied tools, wiring assy’s, etc. further contribute to appliance value.

    A Hydronic Heating “Appliance” changes everything. The historical loose-piece “process” of hydronic (FHW) installation now becomes a “product” installation of lesser content with higher, predictable system performance and costs. Not just having a stock product that readily configures to the specific requirement, but being available on demand to fulfill both routine and seasonal emergency failures is a valuable resource indeed. Appliance costs and performance are also guaranteed, with all components warranted by their U.S. Manufacturer(s) as with any contemporary installation.

    Our motto:“Simple, Durable and Efficient FHW Heating ….. Period!”sums it all up. Technology has finally caught up with hydronic heating, and it’s about time! The end game after all is providing real value to the consumer, is it not?

    Author’s Note: Updated 01/20/2018


    For the past year Mercier Engineering has been immersed in developing and preparing for market it’s Packaged Delta-T Hydronic (FHW) Heating System™, based on our past heating experience projected into the new world of “Delta-T Circulation”. You may have noted our preoccupation with this technology in “The Heating Blog” on our www.boilersondemand.com  website. Time to “put our money where our mouth is”, so to speak. The results of our efforts we deem noteworthy and are initially reflected in this writing.

    As the titling of this blog purposely implies, we must get beyond weighing hydronic heating system efficiencies solely upon the boiler’s Annual Fuel Utilization Efficiency (AFUE) Rating.  It is only one of multiple elements in an operational formulation that is seldom if ever approached, even more poorly understood, and we allege almost universally misapplied. Strong words which must be tempered by the reality that there has been little market incentive to change our approach to serving the residential FHW heating market in particular; but we ultimately must adapt and change it for the consumer’s benefit.

    AFUE is a regulatory, laboratory testing procedure intended to establish an efficiency value for a hydronic (hot water generating) boiler under a defined operating sequence and conditions. It can be presumed that it executes this comparison very effectively, under its terms. However, what it does not measure from our observations is in practice very significant. Specifically these Non-AFUE Test Attributes are:

    1. There are no provisions for qualifying or measuring between-cycle “stand-by” or “idle-time” losses. This is the time between burner firing cycles when the boiler is prone to radiated energy and convective exhaust (flue) losses, presumed to be non-productive.
    2. Similarly, the testing is “steady-state” in execution, providing no qualification or quantification of individual boiler attributes that may contribute to site application efficiency.

    These test attribute observations have been borne out in field applications, where system performances have not correlated well, boiler-to-boiler or system-to-system. To further complicate this is the variability of physicals to each application, however subtle. The forums and blog sites are rife with these seemingly “apples-to-oranges” commentaries. Our developmental efforts may be able to provide some explanations.

    From our observations there are necessarily five (5) elements contributing to total system energy efficiency:

    1. The boiler (heat engine) energy conversion efficiency or AFUE.
    2. The physical attributes of the specific boiler complimentary to system operation.
    3. The energy required to move heated water through the distribution system (radiation).
    4. The effective matching of radiation elements to heating demand.
    5. The control algorithm(s) to match energy creation with varying system demands.

    Our initial efforts have been with oil-fired hydronic systems and is the focus of this document, with gas-fired and solid-fuel applications to follow as resources permit. However, much of this effort is applicable as the basis of other heating systems.

    Varying the output (energy creation rate) of any heating resource is paramount. This has been readily achieved in gas-fired boilers by “modulating” combustion with sophisticated valving and controls. Typically they adjust from 20 to 100% of capacity, from “idle” to “full speed” to use the automotive analogy. However, direct modulation of oil-fired systems is not feasible using current technologies. A fixed (capacity) firing rate via pressurized, nozzle induced fuel atomization is the norm. Therefore, the only option is to adjust the operating temperature of an oil-fired hydronic boiler via controls to compliment heating demand. This is reasonably well-managed with modern “cold-start” aquastats, external temperature sensors, etc.

    The prior unaddressed penalty to particularly residential hydronic systems has been the toll on equipment and electrical energy requirements of circulating heating water with fixed-speed circulators. They are notoriously and arguably universally misapplied and inefficient in practice. Reducing water temperatures merely aggravates the situation by prolonging circulator cycling.

    Fortunately technology has come to the rescue in the form of the “Delta-T” Circulator, now becoming very applicable and affordable to the residential/light commercial markets. The undisputed pioneer and flag-bearer in this market is the Taco Viridian VT2218 found at this link: http://flopro.taco-hvac.com/media/Viridian_VT2218_100-114.pdf  To use the quote “This changes everything” is not an exaggeration! The Viridian is in fact the second generation, replacing the entry product Taco “BumbleBee” found at this link: http://www.taco-hvac.com/uploads/FileLibrary/100-101.pdf We mention the “BumbleBee” only because it has rapidly become a “cult product” in the HVAC Community, somewhat akin to the “Trekkies”. It was our initial “new tool” in developing and thence refining our product(s). Like our brothers, we hate to see it go as we move to the refined and more sophisticated “Viridian”.

    Referring back to our five (5) elements to total system efficiency, Delta-T Circulation is number three (3) on the list but is deservedly and necessarily the foundation of any hydronic system improvement. Taco reports system Delta-T Circulator-only swaps yielding 15% fuel usage reductions. It is the keystone of our Packaged Delta-T Hydronic (FHW) Heating System™, and should be the first improvement to any system! We caution however that this will require substantial near-boiler system re-piping and your installer must be knowledgeable. It is discouraging to note how few of our fellow tradesmen are cognizant of Delta-T or have used it beyond a radiant heat loop. We “Old Dogs must learn new tricks”, and we have!

    The second element of import is the necessity to employ “Cold-Start” Boiler/Aquastat Hydronic Technology, which overlaps Nos. 2 and 5 in our list. We are unabashed in our praise of the Hydrolevel 3250-Plus “Fuel Smart” Aquastat, found at this link: http://www.hydrolevel.com/new/images/literature/sales_sheets/fuel_smart_hydrostat_sales_sheet.pdf   It is now standard equipment on all our Weil-McLain Ultra Oil Boilers, and none too soon! The inter-action of the 3250-Plus with the VT2218 Circulator’s operational software is paramount to total system performance, as we have learned.

    Note: “Cold-Start” Technology applies to “heat-only” boilers. DHW (Domestic Hot Water) must be effected by an external Indirect Water Heater or another dedicated appliance. We combine the Indirect Water Heater in our design for optimized Heat and DHW Generation.

    Element 3: Our development indicates individual boiler attributes are significant. Specifically,

    1. Boiler supply and return tap placements are crucial to system “packaging”, i.e. the ability to compactly (efficiently) structure near-boiler piping. (We can pipe into a space as close as 11″ from the chimney, with all piping and controls behind the boiler, yet readily accessible.)
    2. A very high boiler mass (weight) for its capacity, i.e. for both thermal damping and storage.
    3. Favorable exchanger flue passage routing and exhausting.
    4. Burner type to compliment its attributes.

    The noted attributes lead us to our “Boiler-of-Choice”, the Weil-McLain Ultra Oil Series with the Beckett NX Burner. Refer to this link for detail: http://www.weil-mclain.com/en/assets/pdf/Ultra%20Oil%20Brochure_8%20Pg_web1.pdf   We have had “conventional” system design and installation experience with this boiler for over ten years now, with only one “no heat” service call, a failed aquastat. Weil-McLain has since upgraded it to the Hydrolevel 3250-Plus, thank God!

    The Beckett NX Burner has been likewise flawless in operation. Literally a “plug and play”. Its dual vent typing capability (direct & chimney) has proven beneficial to problematic venting applications, especially when encountering “cold chimneys” in our northern climate. Fully exposed exterior chimneys are sure to give a rough startup without utilizing its pre-purging and pressure firing features.

    The key attribute to system performance outside of Delta-T Distribution has proven to be Thermal Mass (Storage) provided by the sheer robustness (weight) of the Weil-McLain UO Series High-Mass, Triple-Pass Boilers. They are “The Heavyweight Champions” by far and as a result exhibit lower mean boiler operating temperatures and very less frequent burner cycling.

    As a matter of policy we do not cite or criticize our competitors, but we must make a single attribute comparison to emphasize our point. The approximate block weights of the top hydronic (approx. 100KBTUH, 87% AFUE) oil boilers are:

    Manufacturer/ModelApprox. Ship Wt.
    less Tare (lbs.)
    % of HighestComments
    Buderus G115/G21537560%Adjusted for 100KBTUH
    Burnham MPO-IQ11545072%
    Weil-McLain UO-3625100%

    AUTHOR NOTE: Very noteworthy, the Weil-McLain UO is also disproportionately the lowest cost per pound (by nearly half) of the three. Just what is the consumer paying for, we wonder? In our development experience increased boiler mass equates to improved system longevity and hydronic performance!

    Radiation (Element 4) efficiency is the remaining, but least controllable variable in a heating system. It is substantially outside the scope of our system application, yet there are some performance elements we can address.

    Existing hydronic radiation:

    1. Removal of unnecessary valving in zone supplies and returns. All zone supply functions are integrated into our system package.
    2. Zone interconnection and functionality can be optimized by correct pipe sizing and routing. It confounds us as to why some plumbers use virtually no 45° fittings! You can use 3-4 of them vs. a 90° elbow for the same flow resistance, and only 70% of the pipe required for a 90° elbow routing.

    New hydronic radiation:

    The contemporary approach to radiation varies widely, from simple radiation loop(s) for zoned heated areas to individually heated rooms throughout. The more finite the control, the more piping, fittings and control valving, the more hydronic distribution energy is required.

    Ironically, the same Delta-T Circulator Technology we employ to maximize our system performance has preceded us and become the darling in particularly radiant system applications. We have also employed them in these and they perform admirably. They reduce the energy requirements significantly but yet still camouflage that basic issue.

    If your concern is total energy consumption of a system, we would invite you to consider using less sophisticated radiation distribution schemes. A properly designed, installed and balanced series or split piping loop exudes simplicity and will likely be a lower installed cost. The KISS Principle applies — keep it simple ….. (Refer to our Heating Blog Library for additional detail.)

    To Summarize:

    1. Additional Boiler Attributes are important, beyond the AFUE Rating. In particular heat exchanger thermal mass (weight) will lengthen service life while minimizing repair costs. Burner attributes related to exhausting and tuning must also be considered.
    2. Delta-T ECM Hydronic Distribution Technology is key to improving any system’s energy performance, both heating fuel and electrical power consumption.
    3. Inter-related “intelligent” controls determine system operational performance. They are currently the Hydrolevel 3250-Plus Boiler Aquastat and the Taco VT2218 Delta-T Circulator Logic.
    4. Near-boiler plumbing in particular affects system performance. This is optimized in our system piping configuration to include fail-safe “natural gravity convection”.
    5. Interconnections between our system zone access points to existing radiation must be executed with the goal of minimizing flow anomalies.
    6. Existing and/or new radiation installations must likewise be executed by idealizing flow conditions inasmuch as possible.

    We strongly recommend referring to Taco’s website link http://flopro.taco-hvac.com/deltat_resources.html and refer to the various Delta-T resources therein. There’s a volume of resources here that will properly inform you of this new technology and its place in your Hydronic (FHW) Heating System.

    Author’s Note: Hyperlinks updated 08/27/2019


    Well, we’re deep into the heating season and listening to that boiler run and run is not going unnoticed. Every time it starts we feel our wallets getting a bit lighter and our blood pressure going up. On top of this things are pretty tough economically. Believe us, we notice it too! Let’s go over some things we can do to lighten the load at least.

    Our remarks must be predicated by noting that the older your system, the more likely you are to benefit from this posting. Additionally refer to our other postings to gain insight to other improvement opportunities.

    Please note the green highlighted text links for referencing ease.

    Firstly, if your system has any age to it, most likely the boiler is over-sized for your application. This happened two ways. Your boiler was over-sized when originally installed, the “standard procedure” until recently, or you have made energy improvements over time to make it become that way, or both. Listening to your boiler cycling rate on that deep cold January morning will confirm this situation.

    Now we are assuming that your boiler has been THOROUGHLY CLEANED when serviced and properly tuned. If your serviceman shows up without heavy cleaning brushes and vacuum cleaner, trade him in!

    Opinion varies on this, but if your unit is running less than 50% of the time on your deepest outside cold point(s) you are likely over-sized.So much for peak demand assessment and tuning. The aggregate of the more subtle energy-stealer items is likely more significant overall.

    Applicable to your situation or not, the most common and most aggravating situation we find on older systems is incorrectly wired controls, particularly of circulators and the occasional zone valve. This winter again we have had several calls related to poor installation of controls and relays. (Refer to our blog ‘Plumbing Guys Plumb, Heating Guys Heat’ that vents our frustrations in this regard.) Additionally is having the wrong Master Aquastat for your system operating parameters.

    The Master Aquastat is the primary control on your heating system and defines its sequencing, correctly or not. To determine if yours is correctly defined we must go down a checklist.

    1. Do you generate your Domestic Hot Water (DHW) directly off your boiler by means of an Immersion Coil, internal to the boiler? If so your options are very limited. The boiler must be at temperature to produce DHW. Train yourself and users to switch the boiler on and off (seasonally) to produce DHW only when you need it. Don’t let your boiler run all summer to have on-demand supply! A waste of energy. (See other blogs.)
    2. If you generate DHW via a separate, detached water heater of any type and your boiler stays hot when no heat is required (as all summer long?), UPGRADE YOUR AQUASTAT from your current “Ranging Type” (keeping boiler water hot all the time) to a “Cold Start Type” that only warms the boiler when heat is demanded. Our favorite is the Hydrolevel 3250-Plus for oil and the 3200-Plus for gas fueled systems. Note that the additional use of their “Electro-Well” provides Low Water Cutoff (LWCO), a nice boiler safety feature. Nice display, easy to install & adjust; and reasonably priced to boot. Pays itself back in a hurry! Also check out their great videos!
    3. Now, are your aquastat and circulator or zone valves and controls wired properly? They must talk to each other electrically or your boiler runs erratically, zones don’t heat properly and circulators can circulate cold water. The quick way to tell is by removing the Master Aquastat Cover and looking at the “T-T” terminals, usually located near the upper right of the control board. What do you find?
      • If you have only one zone (thermostat) and one circulator, there should just be a pair of smaller wires from your thermostat to the “T-T” terminals. You should be good-to-go, otherwise it just won’t run.
      • If you have two or more zones (thermostats) with a circulator for each there should be a two-wire on the “T-T” terminals from a Multiple-Circulator Control Relay or to EACH of several individual Relays.
        • Case 1: If the wires on the “T-T” terminals are from one of your thermostats, IT IS WRONG! This is the most common condition we find. Someone has added additional zone(s) to your system using a SPST Switching Relay like a Honeywell RA89A, Argo, etc. The quick way to tell is to open the Relay(s) and see if it has terminals numbered 1, 2, 3&4 ONLY. You must use a DPDT Switching Relay like a Honeywell RA845A, Argo, etc. that has terminals numbered 1, 2, 3, 4, 5&6. Terminals 5&6 are not powered and are wired to the “T-T” terminals on the Master Aquastat to call for burner operation when any thermostat calls for heat. Changing them will allow the boiler to run ONLY when calling for heat. Energy and equipment saver.
        • Case 2: If the wires on the “T-T” terminals have a “jumper wire” across them, IT IS WRONG! Your system is running on a “Ranging Aquastat” that just keeps the water temperature in its setting range and from which the circulators pull heating water on-demand. This method is tough on equipment and effectively reduces both the capacity and the recovery of your system. Referring to preceding Case 1 go through the steps with Circulator Relay qualification and inter-wiring to bring the system up to peak, efficient operation.
        • NOTE: If in either Case 1 or 2 you are using a Circulator Switching Relay, or a Zone Valve Control the interior “End Switch” terminals are wired to the “T-T” terminals on the Master Aquastat only. Finished.
    4. If you are using Zone Valves in your system, are your aquastat and zone valves and controls wired properly? The first step is to verify that all of your zone valves have wiring to all THREE terminals (1, 2&3). Next is to trace and verify that there are leads from terminals 2&3 on all valves to the “T-T” terminals on the Master Aquastat. Refer to this Taco Zone Valve Link to verify wiring: http://www.taco-hvac.com/uploads/FileLibrary/ZoneValveBroch.pdf NOTE: If you are using a Multiple Zone Valve Control Relay, the interior “End Switch” terminals are wired to the “T-T” terminals on the Master Aquastat only. The 3 (R-W-G) wires from each zone valve and the 2 (R-W) Thermostat wires from each zone are wired to their respective interior relay locations. Refer to this Taco ZVC Relay Link to verify wiring: http://www.taco-hvac.com/uploads/FileLibrary/CAT100-5.1ZoneValveControl.pdf
    5. If you are using older Taco Zone Valves in your system consider upgrading them, particularly if they have Green-colored Heads (Tops). They were troublesome and Taco upgraded them with Gold-colored Heads. These Zone Valves are typically slow to actuate on & off, use measurable electricity to energize and maintain position and restrict hot water flow in the system. (Consequently we have preferred circulators.) Recently however Taco introduced a new, low power use, full-ported Ball Valve to replace these. Check this link: http://www.taco-hvac.com/uploads/FileLibrary/100-82.pdf The “Zone Sentry™” does it all well, very well, and with a useful indicator LED. By the way they are also less expensive than the older valves! Why repair it when you can swap it? Standard Ball-Porting will also definitely help out with those slow warm-ups, and lazy zones. Saves the circulator(s) a bit too. A winner.
    6. As you replace Circulators, necessarily or not, specify and utilize new generation products only! Recent advances in hydronic circulator and distribution technology are astounding! Despite a higher initial cost, they save lots of energy (electrical & some fuel) and extend service life. Two (2) basic examples. a. Single Circulator System with Zone Valves: Install a Delta-T ECM Circulator ASAP. It’s “thrice the price” but dramatically smooths out energy delivery and radiation pulsation. Swaps out directly with your little green Taco 007 (or other brand). Just attach the sensors to your boiler supply & return piping and go. b. Multiple Circulator Systems: Substitute the Taco 007e Circulator for existing, based on system usage. Again, about “twice the price”, but what a value! Your constant running “system circulator” where used is first, with your following based on zone sizes and usage. Note that radiant heating zones will benefit as well. (Weil-McLain now supplies Taco 007e’s only as their residential system circulators.)
    7. Perhaps the best investment a FHW System owner can make is installing an Indirect Water Heater for thermal efficiency and system value/performance. (Refer earlier in this blog and others to “Cold-Start Aquastats” and their benefits.) Don’t care what you are using for a heating fuel, the Indirect Water Heater tops them all in DHW generation. Problem is they are not cheap. You have to be serious about your energy management. There are two methods of getting there, however. a. The DIY or “Home Brew” Method: Your current boiler MUST HAVE AN IMMERSION COIL FOR DHW. Search out a very good, quality 80 Gallon Electric Water Heater that can be had cheaply, to be used as your storage tank. Strip out all the electrics. The only electrical function to be used is the (upper) thermostat terminals. Procure a Bronze or Stainless Steel bodied Taco 007 Circulator (or equivalent) ONLY! No iron-bodied ones — you are drinking the water going through it. Obtain a DPDT RA845A or equiv. Switching Relay (see prior above in this blog). Plumb a circulation loop from the boiler immersion coil taps to tees at the input and outputs of the 80 gal. water heater. Circulation direction is to the top tee. Wire in the relay as any zone addition, with a two-wire between both “T-T”‘s (relay and water heater). It isn’t a perfect solution, but a cost-effective one. b. The Professional Method: Applicable to any FHW Boiler configuration. Purchase a quality Indirect Water Heater Unit (we swear by, and not at, the HT Products “SuperStor Ultra”). Link: http://www.htproducts.com/literature/lp-81.pdf They use boiler water to heat DHW via an internal coil and therefore use iron-bodied system circulators. Other components are similar to the prior above. We would also suggest professional installation of these units. NOTE: Always put a small, correct “Thermal Expansion Tank” on the cold water feed side of any DHW Heater to prevent over-pressuring resulting from heating a cold tank of water.
    8. The final budget item must necessarily be a boiler upgrade. It’s also the point where you should review your heating fuel options. Refer to our extensive Boilers On Demand Heating Blog Library to assist you in this regard. Regardless of your selection, employ top-of-the-line appliances! Do your homework carefully but don’t select solely on a prior service relationship basis. The Weil-McLain Replacement Guide is invaluable in this regard! These new, high efficiency appliances are virtually self-diagnosing and trouble-free. Not only the Maytag Serviceman is lonesome these days.

    Last Edit: 12/31/2019 PDM