Designing and building a custom heater A to Z

By Alex ChernovWild Acres 2014

Estimating heat loss

• Always ask client for the heat loss statement first.

• Demand that the client must provide heat-loss statement if you are expected to meet certain heat loss.

• Doing own heat-loss statement calculations is not recommended. If interested, there are multiple online tools available.

Rule of thumb for estimating heat lossHeat loss estimated for the coldest day and for standard glazing conditions:

– Super-insulated houses (R30+ in walls): 10Btu hr/sq ft

– 2x6 walls with mineral or fibreglass insulation (R20): 20Btu hr/sq ft

– 2x4 walls with fibreglass insulation (R10-13): 25-30Btu hr/sq ft

– Uninsulated old houses: 50Btu hr/sq ft and higher

Calculating Energy Output

• Energy output of the heater depends on:– Weight of wood in one full load,– Number of loads per day,– Energy content of the particular type of wood

used,– Efficiency of the heater, which is determined by

size of the mass and heater design. – water heat-exchanger (if applicable) can take part

of the heat away.

Calculating Energy Output • One lb of wood on average has 8600Btu • One lbs of wood at 20% moisture content has 8600 x 0.8 =

6880BtuEnergy output of the heater in 24hrs equals:6880 x heater efficiency x daily load in lbs / 24hrsExample: 6880 x 0.75 x 100/24 = 21500 Btu hr If maximum load for a reasonably-sized firebox of a

residential heater is 75lbs, maximum possible energy output of a heater, fired twice a day with 75lbs load:

6880 x 0.75 x 100/24 = 32250 Btu hr

Fuel load and size of the mass must be balanced!

• Mass is too small for the load:– Low efficiency– High exhaust temperatures

• Mass is too large for the load:– Potentially poor chimney draft with all related

problems from slow start to back drafting and smoke spillage

– Condensation of the exhaust gases in the chimney or channels

– High emissions

Balancing heat output with mass

Rules of Thumb:• A heavy heater, fired two times a day, on average will

emit about 160Btu hr from one ft2 of its surface. – Depending on thickness of mass, this number can be

anywhere from 100 to 300Btu hr/sq ft– My opinion: for standard North-American double-wall

heaters we should have between 100-200Btu sq ft. • Maximum amount of mass in a heater: 300lbs per 1

lb of wood in a single load for heaters, fired twice a day and 150lbs/1lb for one fire a day.

Project 1: The Hortons

• Super-insulated house 1400 ft2 on the main floor, plus basement of the same size.

• Want to use masonry heater as the major heat source.

• Additional/back-up heat source is in-floor heating.

• Interested in a heater on the main floor.

Option1: heater with wood stove in the basement. Version 1

Option1: heater with wood stove in the basement. Version 2

Option1: heater with wood stove in the basement. Version 3

Calulations for Hortons

• Main floor: 1400 ft2 of super-insulated space at 10Btu hr/ft2 = 14000Btu hr

• Basement has lower heat loss of about 10000Btu hr and most time clients spend on the main floor. Total heat requirements: 24000Btu hr

• Two-story heater can deliver up to 60% of heat upstairs, leaving the rest in the basement.

Option 2: two-story heater with single firebox in the basement and a white bake

oven with direct-firing option

• Proposed heater has firebox, able to fit 60lbs of hardwood at once. Two fires a day schedule. Estimated efficiency 85%. Output claculation:

8600 x 0.8 x 0.85 = 5848Btu from one lbs of wood.

5848 x 120 / 24 = 24366 Btu hr average output

• is 17ft tall. Total 246 ft2 of surface area.24366 / 246 = 99Btu /sq ft. Within the range.

• Total mass: about 18500lbs Checking with load of 60lbs x 300lbs = 18000lbs. We are ok.

Project 2: Timoshenko

• Modern house. Good insulation. Two stories. About 1000sq ft on the main floor.

• Want a heater on the lower floor next to the area with sauna and hot tub.

• Want bake oven and heated benches: one on the sauna side and one on the second story in the bedroom.

• Want fireplace on the upper level on the opposite side of the heated bench.

Check list for other important things to verify for a heater job:

• Position in the house for proper performance, keeping in mind that 70% of heat is delivered in radiant form.

• Foundation– Proper size and reinforcement– Clearances to combustibles

• Venting – type of the chimney– Position and relationship to the structural elements– Height above the roof and roof system around

• Architect doesn’t plan your heater to be a load-bearing element

Last advice:

• Know your stuff.• Do not be afraid to admit you do not know things and

need expert advice. • Do not overpromise.• If client wants you to venture into an unknown

territory or to take a risk, make them take responsibility for something that is beyond your knowledge and experience or refuse.

• Trust your insights and do not be afraid to refuse a client if it feels fishy.

Thank you!