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Ignition / Operation & Chimney Venting Recommendations For High Efficient Heating Appliances

Most of the heating appliances we inspect are higher efficiency. Yes there are some dinosaurs out there, but most have been updated. The efficiency of some units are over 90%. I want to go over the sequence of operation and important recommendations a home inspector should be looking for when inspecting higher efficiency chimney vented appliances. We will define “high” efficiency heating units as whole house furnaces / boilers that have an induced fan. You can see past newsletters discussing direct vent units. First the sequence of operation:

1 – Thermostat calls for heat

2 – Combustion air pressure switch proves induced blower operation. Pressure switches are factory set and require no adjustment

3 – After a 15 second pre-purge, the hot surface igniter or electronic ignition energizes

4 – Within 20 seconds, the gas valve solenoid opens

5 – The gas should be ignited within approximately 4 seconds

6 – Flame sensor proves the flame and the combustion process continues uninterrupted

7 – If a flame is not detected after the first ignition trial, the ignition sequence will repeat 3-4 more times before locking out the gas valve.

8 – If the unit does not respond, it will automatically repeat the entire heating sequence. Some units will have to be manually reset

9 – Depending on the unit, a reset button can be depressed or the thermostat can be turned to “off”, then back to “heat”

 Many times we see these units go through the starting sequence a couple of times before they activate. I would make a note on my report that “The heating unit did respond to the thermostat, however it cycled twice before properly activating.” It would be a good idea to advise your client to have it evaluated by a heating contractor.

Many high efficient units are not vented properly. Sometimes there seems to be some confusion related to this. Often times we see a new high efficient unit installed in place of an older low efficiency unit. What about the flue pipe lengths? What about lining the chimney? What about the size of vent pipes? How many other appliances can be connected with a high efficiency unit?  Here is what a home inspector should be looking for:

  • Metal flue pipes should be secured to the furnace collar with 3 self drilling sheet metal screws or a mechanical fastener
  • The first metal vent connector should not have an elbow within 6 inches from the unit
  • The vent pipe should be properly supported without any dips / sags and shall slope a minimum of ¼” per foot
  • High efficiency (direct vent) appliances should never be vented into a masonry chimney without a liner
  • Many manufacturers recommend a masonry chimney that is tile lined must be lined with B1 vent or a listed insulated flexible metal vent (check with manufacturer recommendations)
  • An exterior tile lined chimney that is sealed and capped should be lined with a listed flexible metal vent
  • Insulation for the flexible pipe should be an encapsulated fiberglass sleeve recommended by the flexible vent pipe manufacturer
  • Do not insulate the space between the liner and the chimney wall with puffed mica or any other loose granular insulation
  • For multiple appliances venting into one chimney refer to the chart above
  • A type B1 vent liner shall terminate above the roof surface with a listed cap
  • A manual damper, barometric draft damper / regulator, or flue restrictor should not be installed between the unit and the chimney. (This may exclude oil fuel burning units – refer to the manufacture)
  •  A flue pipe should never share a chimney with a solid fuel appliance or wood burning fireplace
  • When the flue pipe for a high efficiency appliance must be located or pass through a crawl space, attic, or other areas that may be cold, that portion of pipe shall be constructed of double wall type B vent material or material having equivalent insulation qualities
  • No portion of the venting system can extend into, or pass through any circulation air duct or plenum
  • The vent connector should not be sized more than 2 size diameters over the size of the draft hood outlet or flue collar outlet
  • The maximum length of a flue pipe for a high efficient chimney vented appliances:

      3” connector diameter = 4.5 feet

      4” connector diameter = 6 feet

      5” connector diameter = 7.5 feet

      6” connector diameter = 9 feet

      7” connector diameter = 10.5 feet

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