Detached Wood/Coal Boiler Projects

We offer IPS tees on our Delta-T ECM Hydronic Appliance Supply & Return Ports to accommodate any wood/coal (or other) separately fired appliance. This is the “E” (Extension) Option noted in our Configuration Charting. It may also be inclusive with an Indirect Water Heater as the “C” (Combined) Option.

It must be noted that any extended fuel system boiler must be similarly pressurized (typically 12-15 PSI) or adapted as necessary.

  1. An ASTME Rated Pressurized Boiler (Wood/Coal) can interface directly. We do recommend that the piping sizes be same as our supply/return lines (1-1/4″ or 1-1/2″ to match).
  2. Un-Pressurized (Zero Pressure) Outdoor Wood/Coal Boilers require an appropriately sized Plate-To-Plate Heat Exchanger with distribution to interface with our or any pressurized system.
  3. In-line temperature switching is required to manage operation.

Consult a Heating Engineer to assure that your dual boiler system controls are appropriate and effective.

A Blog in our Library entitled “OUR UNPOWERED FORCED HOT WATER (FHW) GRAVITY HEATING SYSTEM” may be of some initial assistance.

It must be said that all of the Integrated DIY Wood/Coal Projects that we have seen are woefully installed. Beginning with the very generic piping diagram supplied, the installation goes rapidly downhill thereafter. There is seemingly a total misunderstanding of Hydronic (Hot Water) Distribution Methods & Efficiency, evidenced by a preponderance of circulators, relays, controls and wiring that defy reason. If you have a current central heating boiler, it must become the hydronic “distribution box” with the auxiliary wood/coal boiler augmenting it. Our Blog entitled “THE DELTA-T ECM CIRCULATOR — The “Automatic Transmission” for Boilers” is a MUST READ to acquaint you with modern hydronic distribution basics.

Several projects in the works, as detailed below. We are assisting customers interfacing their indoor/outdoor boilers with our or their FHW Heating Systems as a service only. The resulting documentation is presented as “public domain” to anyone interested without guarantee or liability.

Project Updates:


Click HERE to view the overall installation mentioned below.

Click HERE to view the Delta-T Distribution Head designed by


We currently have a Taco VT2218, originally a “BumbleBee”, prototype distributor configured on an outdoor wood-fired boiler system to distribute from the plate exchanger to multiple heating zones.  The distribution head is built over an 80-gallon buffer storage tank to further manage water delivery temperature variations. There is no internal boiler available on this particular system.

To date this system has exhibited very stable Delta-T performance with very low electrical requirements.  Now with five (5) of the six (6) zones operating it still flows on average 6.8 gpm using only 12 watts of power with an outside ambient temperature of 25 deg F!

These results continue to be encouraging ….. stay tuned.

The customer’s Outdoor Boiler System is shut down seasonally for the summer (no heat) cycle. He continues to provide DHW via his dedicated Electric Water Heater. This is the shortfall of these three season systems.

Several options are available to provide low-demand domestic hot water heating, namely:

1. Activating the 80-Gal. (electric) Hot water Tank to provide low-temp heating water. This can be automatically deployed by low-temp supply line switching.

2. A variety of fueled hot water heating devices similarly coupled into the system.

Note: Zone Six (6) on the current Distribution Header (Manifold) is intended to supplement the current DHW Heater during the heating season and will be configured prior to fall system start-up. We have recently configured a DHW Heater “Booster” on one of our Delta-T ECM Systems at our client’s request. It will default to electric heating during a high heating season demand or fully electric on a shutdown.

Update 02/20/2020: A Taco VT2218-HY2 Circulator has replaced a contamination-failed “BumbleBee”. It is displaying 8-11 Watts & averaging 4 GPM during heating demand. The owner is installing a precipitate filter to avoid a future occurrence.

Update 01/12/2023: Our customer’s Outdoor Boiler System Circulator failed on Christmas Eve, and we substituted a Taco 007e Indoor Circulator as an expedient. His complaint of an increased heating season electric bill was obvious in that his Taco 0011F5 was consuming 200 Watts of power 24/7 during operation ….. about $30/per month @ our $0.20+/KWH local rate. The Taco 007e is consuming 20/25 Watts or $8 or less/month. This is prompting us to install a Circulator Aquastat in his second 3/4″ “Bung” Port and use it to cut in at a lowered water supply service temperature ….. sufficient for peak demand only! A classic over-pumping condition …..


We have committed to aiding a friend to integrate his newly acquired (used) outdoor boiler with his home (FHW) heating system, winterizing his outdoor hot tub (in NH?) and heating a detached workshop. The condition is that we re-pipe his dated Oil-fired Ultimate Boiler FHW Home System with Delta-T ECM VT2218 Distribution Technology to attain more “hands-on” conversion and interfacing experience. This has been done, including utilizing a Taco “BumbleBee” Delta-T ECM Circulator on his inter-boiler loop via a Plate Heat Exchanger.  The “BumbleBee” is using 8 Watts of Circulation Energy @ 6.8 GPM Flow. Completion data and pictures will be posted soon. His Ultimate Boiler is about half the weight of our Weil-McLain UO Boiler, and it shows. Combined with a marginally sized Plate-to-Plate Heat Exchanger, the Ultimate drops temperature forcing the reduction of Delta-T on the “BumbleBee” Circulator from 20ºF to 15ºF (so far) to increase delivery volume. Stay tuned …..


In late 2010 we were approached to convert a classic Two-Pipe, Reverse-Return All-Copper Steam Heating System ….. an absolute beauty! It was in an equally stunning New England Colonial Reproduction Home of the Great Depression Era. The objective was to convert from steam to hot water heat and interface with a remotely located, very large Outdoor Wood Boiler that would eventually service the Home, a detached Large Garage & Apartment Structure and a Multi-Function “Barn”. The boiler and the nineteen (19) Wall Convector Cabinets were reconfigured using a Delta-P (Differential Pressure) Circulation System. The Outdoor Wood Boiler was then interfaced with a Plate Exchanger and Push-Pull Supply Circulators to overcome the 400+ foot supply distance. A rewarding but challenging pre-Delta-T Project indeed ….. !

Last Fall (2017) we were again approached by our customer to provide an Anti-Freeze Alternative to protect his very large Outdoor Wood Boiler during prolonged absences. He had during the interim extended Wood Boiler Service into his “Barn” as well, comprising a Utility Room, Workshop, Tack Shop and an acclimated Horse Watering Trough via a large SuperStor Indirect Water Heater.

Our solution was installing a Delta-T ECM (Oil) Hydronic Appliance in the Utility Room, zoned for all functions, including tempering the Outdoor Wood Boiler just above freezing through the Plate Exchanger Circuit. Utilizing Delta-T Circulation along with WiFi Interfaced Temperature Sensors on both boilers permit remote, real time monitoring.

NOTE: All future systems will utilize the improved Taco VT2218 00e Series Circulator, replacing the “BumbleBee” mentioned above.