# Drolet Wood Furnace

Registered Users Posts: 20 ✭✭
Hi, I was curious if anyone on the Forum has used the Drolet Heat Commander Wood Furnace on a off-grid system? I am trying to decide of my system would handle it through the night and also some cloudy days. The following info was in the Question & Answer section about this unit on the Northern Tool website. ( It is recommended to feed the furnace with its own electrical circuit of 15 amps at 120 volt with a breaker. The blower will use 5 amps at 120 volt 60 Hz. A standard HVAC unit would require 15-60 amps depending on how many tons.)

Where are you located (nearest city)? The unit seems to have a 330 Watt fan. Guess that it would run at 50% duty cycle (more at night, less during day?)? That would make the furnace use a bit more electrical energy than two Energy Star refrigerators.

https://www.drolet.ca/ca/en/wood-furnaces/df02003/

To give you an idea of how to do a generic design (you need to input your actual requirements):
• 330 Watts * 0.50 duty cycle * 1/0.85 AC inverter eff * 24 hours per day = 3,960 WH per day (fridge is around 1,000-2,000 WH per day)
• 3,960 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max planned battery discharge * 1/48 volt system = 388 AH @ 48 volt battery bank
Say you are in the Ithaca New York area... Fixed array facing south. Tilted to 41 Lat + 15 degrees for best winter harvest = 66 degrees. Assume 48% solar deratings/losses (i.e., our 52% system off grid solar AC lead acid efficiency). And a 1 kWatt (1,000 Watt) solar array (for easy math):

https://pvwatts.nrel.gov/pvwatts.php

# RESULTS

System output may range from 671 to 705 kWh per year near this location.

( kWh / m2 / day )
AC Energy
per 1 kWatt array in kWh per month )
January4.0662
February4.3561
March4.5968
April4.3260
May3.6751
June3.7749
July4.1055
August4.2457
September4.6061
October4.3061
November3.7454
December3.2249
Annual4.08688
For December (worst month), you would need an array of:
• 3,960 Watt array * 0.52 off grid system eff * 1/3.22 hours of sun per day (Dec Average) = 2,365 Watt array December "break even"
And for solar, you should oversize the array to allow for streaks of bad weather (less genset usage). Typically plan on using around 65% to 50% of planned/predicted average daily harvest:
• 2,365 Watt array (Dec break even) * 1/0.65 solar fudge factor = 3,638 suggested minimum array with "65% fudge factor"
If the fan uses more energy (as suggested by your vendor)--Larger solar power system. The above numbers are just the system needed to run the heater... Don't know the size of your existing system and loads, or where it is located. Lots of guesses. Always the trade off between larger solar+battery vs genset+fuel for winter system. That Ithaca NY has >3 hours of sun per day in Winter is pretty nice... Many areas in the North are less than 2 hours per day in winter.

Moving air/water tends to be energy inefficient (as is refrigeration). Running heating in winter with "winter sun" makes things more difficult.

Just some thoughts...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 20 ✭✭
I live at Westcliffe CO (South Central). We got lots of sun in our area all year long. I have been noticing some discussions on Mini Splits, would one of those take less energy than the Drolet furnace when heating? Of course I would than be able to cool with it in summer.
• Registered Users Posts: 20 ✭✭
Here is my current setup by the way. My inverter/charge controller is a Sol-Ark 16k. I have 2 strings of 72 cell Mission solar panels, 8 panels per string 340 watt panels in one and 385 watt panels in the other. I have16 Surrette, L16, 445ah lead acid batteries
edited September 2023 #5
The efficiency of a Mini-Split heat pump can be, down towards freezing and/or a bit below, is (at best?) around 3x input energy. If my guess at the wood burner's electric motor is 50% of 330 Watts, then the equivalent in electric heat would be:

330 Watts * 0.50 duty cycle * 3 heat pump factor = 495 Watt worth of "electric heat"

Is a 500 Watt Electric heater (heat pump equivalent) sufficient/interesting for your home?

Lots of research on units available in your area and their cold "efficiency" for heat pump operation. (this is not an exact or accurate reflection of what is out there today--Just some modeling of things I have read about in the past to give a rough starting point)

https://forum.solar-electric.com/discussion/23007/mini-split-update-for-offgrid/p1

for a 6,720 Watt array, Westcliffe Co, 38 degree tilt, a quick PV Watts estimate would be:
https://pvwatts.nrel.gov/pvwatts.php

System output may range from 6,021 to 6,349 kWh per year near this location.

( kWh / m2 / day )
AC Energy
( monthly harvest kWh )
January5.06481
February5.91502
March6.33582
April6.29547
May6.34556
June6.58534
July5.84485
August5.97499
September6.32527
October5.94530
November5.48496
December4.82457
Annual5.916,196

Lots of sun, large system, the estimated performance would be:

2 * 445 AH battery bank * 48 volts * 1/2 days storage * 0.50 planned max discharge * 0.85 AC inverter eff = 18,156 WH per day battery usage (bad weather, 2 days of "no sun")

The estimated harvest would be:

6,072 Watt array * 0.52 off grid system eff * 4.82 hours of December sun per day = 15,219 WH per average December day
15,219 WH per day (Dec Average) * 0.65 solar fudge factor = 9,892 WH per day suggested "planned" energy usage per day (December)

How much energy per day (December), on average, do you consume? The difference (estimated harvest - actual usage) would be the "extra" energy available for "electric heat"...

-Bill "just taking some wild guesses here" B.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 20 ✭✭
I think that gives me the information I need. It looks like the Mini Split option for heat would be far less efficient. The other part of the equation is that I do construction and produce a lot of wood scraps, so something wood burning makes a lot of sense. Thanks for the info!