• Tag Archives Integrating
  • INTERIOR VS. OUTDOOR WOOD BOILER SYSTEMS — SOME OBSERVATIONS

    Having now maintained our own Interior, Integrated Wood & Oil Heating System since 1975 and more recently integrating Outdoor Wood Boilers to our Weil-McLain FHW Heating Systems,  we are motivated to pass on some of our observations.

    (Note hyperlinks within the document.)

    Disclaimer: We are exclusive Weil-McLain Heating Designers & Installers here in NH only (thus far). We do not nor have not specified, sold or initially installed any interior or outdoor “Solid Fuel” (Wood, Pellets, Coal, etc.) Boilers. Our participation has been to occasionally help out a customer integrate these products with our Weil-McLain Hydronic Heating Systems per our personal experience. Included has been converting Steam Boiler Systems to FHW and conjoining these systems.

    Note: We offer as an option full-size fittings (plugged) at our boiler supply and return points to integrate other boilers. Specify at ordering.

    The recent popularity of Outdoor Wood Boilers in particular has prompted many inquiries and discussions on our part. They seem however to neglect the interior-located solid fuel boiler in the scope of options. Each of these have distinctive attributes that both complement and complicate their integration and operational effectiveness.

    Inside or outside? That is the (first) question and may likely be answered by The Regulators (Zoning, Fire and Insurers). Specifically to wood/coal boilers:

    1. The Local Zoning Code will define permitted use and location parameters. (Outdoor Boilers are particularly vulnerable to NIMBY (Not In My Back Yard) and more specifically regulated.)
    2. The Fire Codes usually reference both, but are tough on Interior Boilers.
    3. Insurers (risk takers) then determine how much it is going to cost you to save money. Again, interior boilers take the major hit, especially if it is the primary energy source.

    Note: The delineation is between Gas/Oil-fired Power Burner Heating Systems and Solid-Fuel Fire (Coal/Wood, etc.) Systems. Insurers will usually bend for Automatic Stoker-fed Coal and Automatic Wood Pellet Systems with a two-week continuous-feed fuel storage supply. Check your local codes and insurers, however.

    At the risk of oversimplification and generalization we offer the following:

    TypeFlueLoop LengthLoop LossesStandby LossesEfficiency (Overall)
    InteriorReq'dShortLowLowModerate to High
    OutdoorN/ALongHighHighLow to Moderate

    Explanations:

    Flue:
    Any Interior Wood/Coal Heater REQUIRES A DEDICATED CHIMNEY FLUE! Flue quality must also be considered to support a continuous fuel fire. A “sometimes option” is to convert the primary powered burner (gas/oil) appliance to a “direct vent” device vs. the additional flue construction cost. Check it out.
    The Outdoor Wood/Coal Boiler has its own integral exhaust stack. However it is not unusual to see vertical stack added, sometimes up to 20 ft. to improve draft or smoke dissipation conditions.

    Loop Length:
    An Interior Wood/Coal Boiler can be readily be coupled to a FHW System, given flue location options. Interior Boilers are typically like-pressurized and therefore operationally compatible.

    An Outdoor Wood/Coal Boiler is typically “Zero Pressure” to simplify construction, ease of operation and complexity. This comes at a price with a continuously-powered, “high head” (flow resistance) system loop to the FHW System using a tough circulator. An in-line Heat Exchanger is required to mate the pair. Significant heat losses occur as well.

    Loop Losses:
    Interior Wood/Coal Boiler Loop Losses are typically dissipated to the ambient surrounding area and usually indirectly contribute to the heating of the structure.
    Outdoor Wood/Coal Boiler Interior Loop Losses are as prior, but Exterior Losses are very significant! (We have a system with over 400 feet of high quality, insulated two-pipe, buried down several feet. You can readily observe the reduced/absence-of-snow line path.) You must further isolate the “Zero-Pressure” Outdoor water from the interior “Pressurized” water using a “Plate-To-Plate” Heat Exchanger System with controls & circulators. This system contributes some heat to the ambient area, however.

    Standby Losses: (Defined as energy losses between heat demand cycles.)
    All Interior Boilers dissipate energy between cycles to the ambient and incur flue losses (heat up the chimney). The chassis and piping energy is generally accepted to be contributing to the heating environment positively excepting during the non-heating season.

    All Outdoor Boilers lose significant energy both from their chassis (never any snow on them) and their exterior service loop if circulation is not inhibited with demand. All this is lost energy by definition.

    Efficiency (Overall):
    Albeit being an empirical determination, the relative efficiencies of interior vs. outdoor boiler systems will be effectively higher due to their placements (in heat value areas) and reduced loop and standby losses. Thus the terminologies assigned and relationship to each other.

    Note: We are not addressing the Heat Generation Efficiencies of any boiler products in our discussion, these varying by configuration(s), but only their overall performance by type. Open the NH Climate Audit Calculator (Excel Doc). It’s a useful tool to both qualify and quantify your options by “playing the numbers”. Our “trick” is to calculate the “Cost per Million BTU” for a selected fuel, then adjust other fuel costs to equal your selection MBTU value. Remember to adjust your fuel appliance efficiencies if known or use the defaults. The resultant is a great “apples to apples” economic comparator!

    Operational Observations:
    Further observations of field operation leads us to believe that many outdoor boiler users fire them virtually continuously, incurring very poor fuel utilization, during low heat demand periods. Perhaps there is a continuing need for DHW (Domestic Hot Water) Generation without a storage means provided, or an unwillingness to build a new fire.

    We have evidenced outdoor wood systems consuming 11 cords of wood that in our humble opinion should be consuming no more than 5 cords with an interior boiler, 45% of the outdoor unit. Granted this is a very heuristic and unqualified determination, but yet an indicative one.

    Whatever the case, if one presumes to maximize his fuel utilization the household must adapt to the characteristics of the solid fuel fired system. “Showers-On-Demand” will likely require a generously sized Indirect Water Heater (Super-insulated DHW Storage Tank) to accommodate this lifestyle. Otherwise an incremental or even seasonal fallback to the gas/oil-fired system may be necessitated, and desirable.

    There is a tendency of auxiliary wood/coal users to “over-engineer” their systems. A prolific use of circulators, relays and controls result. We would respectfully suggest that exploring the use of Taco® Delta-T ECM Circulator Technology would benefit energy management. Further, employing Taco® Zone Sentry Valves with these permit some emergency (no power) gravity heating on interior boiler systems.

    Conclusions:

    1. Outdoor Wood Boilers appeal to users who have larger fuel demands, an abundant, low-cost supply of particularly cord wood, time and resources available to process it! (As we say: “Wood warms you twice!”) Higher equipment installation costs and poorer total fuel efficiency is a definite factor in their consideration.
    2. Outdoor Wood Boilers in particular require continuous electrical energy for combustion and distribution. No Power — No Heat! (Or it’s own a generator, too.)
    3. Interior Coal/Wood Boilers are operationally more desirable, but regulatory influences must be qualified and satisfied.
    4. Installed Costs of any dual-fuel system are substantial. To minimize this and achieve predictable performance, use of a qualified technical resource is recommended. Utilize a “system guy” and not just a “tech-sketch” provided by the boiler salesman.
    5. Not maintaining ideal and consistent heated water delivery in any system takes its toll on equipment, in particular circulators, through very long duty cycling.

    Our Personal System and Performance Data:

    Referencing our Blog on our personal system, we offer the following:

    1. Operating as an oil-only system between 1971-1975
      • Annual Fuel Oil Consumption: 475 Gallons x 140KBTU = 66.5MBTU/Yr @ 85% Efficiency = 56.5MBTU/Yr. Net Usable Energy
    2. Operated as a wood-only system between 1986-1995 (9 yrs.)
      • Annual Wood Fuel Consumption:  4-1/2 Cords average x 20MBTU/Cord = 90MBTU/Yr. @ 60% Efficiency = 54MBTU/Yr. Net Usable Energy
    3. Operating as a dual fuel system otherwise between 1975-1985 and 1995 to date. We have been using only a modest amount of wood in recent years, restricted by wood fuel costs and time constraints.

    2016/2017 Full Heating Season: 460 Gallons of Fuel and 1+ Cord of Wood + combustible paper & other waste, as available.  (Admittedly approximations.)
    10 Room Single Family Home, Built 1970, Heated Area: 2,016 Sq Ft
    3-1/2 Zone, Programmed T-Stats, Coupled FHW Oil and Wood Boilers. DHW Immersion Coil on Oil Boiler. Planned Upgrade: Large Indirect Water Heater (HTP SSU-80) for DHW to permit less low/no heating season boiler cycling. TBD.
    Annual Degree Days: 7,500 Average (SW NH)

    Comments:

    Dual-fuel and in particular interconnected FHW boiler systems offer a good heating option, given they be properly configured and operated. We have a long ways to go however to inform and guide customers in their economics, application and execution. It is our hope that this document is of some help toward this end.

    Author’s Note: Updated 10/13/2017


  • INTEGRATING A WOOD OR COAL BOILER WITH FHW (HYDRONIC) SYSTEMS

    Making that wood or coal fired boiler perform well when coupled to your gas or oil fired FHW heating system can be a challenge. They just operate differently. Only by knowing their characteristics and utilizing them to advantage can we affect positive system behavior.

    An automatic, powered FHW system necessarily attains a high water temperature (just below boiling) during heat demand for heat transfer efficiency. The heating system is pressurized (like your automobile) to enhance circulation and increase the boiling point. Controls adjust burner operation to compliment demand levels (instant on-off). Your radiation is sized to compliment its heat generation capability.

    A coal or wood boiler has a far different firing (heating) profile. Their fuels have “burning stages”, typically ignition, charcoaling (wood) and gasification. Each stage increases combustion temperature (and efficiency). Maintaining a fire necessitates replenishment (mixing) of new fuel, continually changing the heating profile. This can be moderated and controlled in degree by sizing, i.e. wood pellets vs. split firewood or rice coal vs. chunk. In either case, the fire is modulated by air and draft control.

    Note: Exterior Coal and Wood Boilers are typically “Zero Pressure”, necessitating a water-to-water heat exchanger to couple to the typical 12-15psi FHW System. This will require an additional controlled, properly sized and powered circulation loop.

    The major control element of an integrated system is the FHW Boiler Master Aquastat, controlling powered system temperatures and burner operation. It is typically designed to operate in a narrow and high temperature range for efficiency. This must be widened to accommodate the fluctuation in solid fuel delivery or your burner will be cycling often and shortened cycles. (Tough on equipment as well.)

    You will have to check the specifications of your Master Aquastat as they vary widely with your system type. Briefly,

    1. If you have an Immersion Coil to generate DHW in your boiler, your Aquastat is a “Triple Action” or “Ranging” Type that maintains temperature with a modest, adjustable temperature differential.
    2. If not, your boiler provides only heated water for area heating of your home or structure. It has (or should have) a “Cold Start” Aquastat that only fires the boiler when heating is required. Otherwise the boiler cools down between cycles, approaching ambient temperature. Additionally,
      • Older systems will likely have narrower, fixed differential controls.
      • The newer, high efficiency boilers will have adjustable or programmable digital controls. Check specifications again.

    Your Master Aquastat should have a fully adjustable temperature range, wide, adjustable differential(s) and mode switching to compliment coal or wood boiler interaction. We employ the Hydrolevel Model 3150 Universal Aquastat exclusively:

    Specifications: http://www.hydrolevel.com/pages/new.html

    Installation: http://www.hydrolevel.com/pages/pdf_files/HydroStat.pdf

    It does everything, and well. You can “range” up to 30 deg F on the high and low ends, inhibit burner within high range and cut off at low limit (mode). Nice, visible display!

    Check one of our site photos for interfacing and general piping details (powered boiler).
    Link: http://www.merchantcircle.com/business/Mercier.Engineering.603-588-2333/picture/view/2875310

    This provides your basic interface, but more is required, particularly hydronically.

    Particular attention must be paid to the operational habits of your coal or wood boiler, particularly in ultimate temperature control. Can you fully fuel it when the boiler is red hot and walk away unconcerned? If over-firing is undesirable, you must dissipate this excess energy somehow. Options:

    1. A “Dump Zone” – Actuate heating zone(s) automatically through an in-line high limit “make-on-rise” Aquastat. They can be normally or optionally heated zones. Result is an overheated house, basement or garage, or all. Wiring this option can be tricky, however. Watch your control wiring. Add isolation relays if in doubt.
    2. A “Dump Tank” – Configure a storage tank in parallel with the feed lines to the FHW Boiler. Depending upon your physical layout and attributes it could also be behind and near the solid fuel boiler to act as a direct “Tempering Tank” to it. Between the boilers it can function as extra heating capacity, in degree. Again, take care with your circulator circuit and temperature sensors.

    Note: Reading our blog entitled OUR UNPOWERED FORCED HOT WATER (FHW) GRAVITY HEATING SYSTEM may be of some help in this regard, where applicable. Employing gravity elements used in our personal system takes some expertise and effort. BE CAREFUL!

    To further control inhibiting the burner at a lower temperature than the 3150 ranging allows may require placing an immersion style (where boiler provision is available) or a “strap-on” style Aquastat on the inter-boiler loop. This would be a “make-on-rise” Aquastat configuration.

    Another option is a manual one – placing a switch in series to the burner wire and located conveniently. Just don’t forget to turn it back on before you go on vacation!
    Operation requires “tweaking” of controls and in particular of temperature and differential settings. Heating demands vary widely with external temperature, lifestyle patterns and solid fuel firing schedule. If you still find that your FHW Boiler is cycling too often despite your best efforts, look at your total boiler water capacities.

    EXAMPLE: We have a client who is attempting to supplement in a larger home using a Chunk Stove with an internal water coil. The system has such low water capacity that if he delivers water at a reasonable heating temperature it is short cycled. Water temperature is either too cool, correct for a short period and tripping the “Dump Zone”. The quick patch is to install a “Dump Tank” with circulator-loop controls to accumulate energy along with aggressive firing of his chunk stove. Even then it will only be modestly effective – unless he wants to fully occupy himself in the basement.

    We alluded early on to radiation capacity as a concern. Radiation is designed to operate at a high, normal FHW heating temperature of 180-190 deg F. Lowering this temperature necessarily lowers heat delivery significantly. If your FHW System was slow to warm your home, it isn’t going to get better with a solid fuel boiler’s varying, if not lower delivery temperatures!

    Newer homes we find are radiated so closely to peak cold expectation as a cost constraint that a solid fuel boiler supplement may be disappointing. Older home FHW Systems on the other hand tend to be better radiated by design – and – home energy improvements have reduced their heating demands, effectively increasing radiation capacity! Their original boilers if still in use also exhibit this over sizing effect, coupled with the earlier tendency to oversize them more aggressively by design.

    So, if you are having difficulty maintaining warmth you have a few options:

    1. Increase the water capacity of your system with a “Dump Tank” to hopefully provide a damping effect against deep day/night fluctuations.
    2. Qualify pipe sizing and routing compliments boiler capacities.
    3. Verify that capacity of your solid fuel system is adequate.
    4. Increase the radiation capacity of your home, proportionately for maximum comfort. You can always close registers to cool selective areas, but not to increase others.

    Note: Use a Heat Loss Calculator to qualify items 3 & 4 above! (See our Blog.)

    We have purposely omitted piping and electrical diagrams from our discussion. There are just too many factors and system configuration details to present generic solutions. This is where a qualified heating technician or engineer should assist you to obtain a successful outcome.

    An integrated system while offering a likely significant benefit in heating cost reduction does present its own operating characteristics:

    1. Circulator run time is significantly increased due to lower average system temperatures. Expect a modest increase in electric cost with more wear and tear on circulators and controls over the longer term. Keeping a spare circulator on hand is probably a good move for a DIY.
    2. If you use Setbacks with a Programmable Thermostat, be prepared to adjust not only the setback temperature but the times to “lead and lag” for comfort. Experimentation here.
    3. If you don’t utilize the wide differential range of the recommended Master Aquastat and install a second Aquastat to inhibit your FHW Burner at a lower temperature, you may exhibit significant heating lag. Adjust your temperature set point upward to moderate this condition (or listen to the “cold blooded” complain).
    4. Exterior Boilers in particular are vulnerable to power outages. A small backup generator in any case might be a judicious move.

    Hopefully this introductory discussion will help toward this end. You may also wish to read our other blogs to fill in more detail and explanation.

    Last Edit: 10/10/2012 pdm