direct vent versus hoyme

[qualistat]

I have heard of using a Hoyme damper for combustion on a new sealed combustion furnace. The argument is that by venting the exhaust vertically through the roof and using this for combustion intake, you can reduce callbacks due to freeze up in winter.

Does this meet code? in other words, can you get back drafting if one pipe is open in the basement.
Also, would this affect the efficiency?

Thanks,
Darrell

[Admin]

Some furnaces like Lennox G71 are only approved for direct vent and cannot be one piped.  That damper is a good idea if you had a combustion air pipe dumping cold air into the basement.  The purpose of the combustion air is to prevent down drafting if there's something venting up a chimney.  Most new homes have high efficiency furnaces and power vented water heaters, so down drafting chimneys aren't a concern, but we still need combustion air by code.  (Section 8 of B149.1S1-07)

If your intake, on a two pipe system is freezing up, it's probably been installed incorrectly.
Remove any screen in the intake pipe, insulate the venting if it's installed in a non conditioned space, and ensure the termination is configured correctly.  Also sometimes the instructions require a tee and drain line be installed in the exhaust or intake when the vents are installed vertically.

If you look at the engineering data for a Lennox G61, which can be one or two piped, the efficiency is slightly higher for a two pipe, direct vent installation, but not by much.  In my area we can get a lot of snow, I prefer one pipe installations if possible, and use the separate combustion air pipe.  This prevents intake blockages and freezing cold air rusting out the heat exchanger.

What kind of furnaces have you been installing?

A sealed combustion furnace is a two pipe furnace and the damper could not be used.  The damper would fit 4" to 9" combustion air pipes, that don't connect to the furnace.  The combustion air pipe shall terminate within 1 ft above, and within 2 ft horizontally from, the burner level of the appliance having the largest input.

Usually those dampers are wired into the furnace, and open on a call for heat.  In some cases you might have a water heater that is above 50,000 BTU's, in which case combustion air is required and couldn't be closed after the furnace heat cycle ends.

So basicaaly if your furnace is approved for non-direct vent installation (one pipe), then you could vertically vent the furnace and install a sepreate 4" or 5" combustion air out the side of the house.  This would solve your intake icing up problem.  The combustion air pipe would not need a damper, that would only add cost to your installation.  However the house would lose some thermal efficiency having a 4" pipe dumping cold air into the basement.

If you do install a damper read the following Clauses;

Clause 8.1.3 Interference with the air supply for an appliance shall be prohibited.

Clause 8.3.4 Certified combustion-air-supply equipment may be used in lieu of a duct to provide the outside air supply to an appliance as required in Clauses 8.2.1 and 8.2.4.

Clause 8.5.4 An automatically operated damper or automatically adjustable louvre shall be interlocked so that the main burner cannot operate unless either the damper or louvre is in the fully open position.

Clause 8.5.5 An automatic combustion air damper installed in the air supply within a dwelling unit shall be certified.