11 Comments

1. 
   saltarch | Jan 16, 2017 01:37pm | #1

   Combustion Air
   Am I seeing the image correctly that the combustion air is being pulled from the interior of the mechanical room? Is that legal? Seems like that could impact comfort in the house unless it is somehow balanced elsewhere?

2. Expert Member
   Dana Dorsett | Jan 16, 2017 03:03pm | #2

   Neither a comfort or legal problem @ Chris Harris
   Unless explicitly barred in the manufacturer's installation instructions it's legal to draw combustion air from the mechanical room so long as there is the code specified minimum makeup air path for the size of the burner. Codes vary on the makeup air opening requirement- not sure what it is in CA. A typical makeup air requirement for an atmospheric drafted burner is 1 square inch per 1000 BTU of make-up air opening into the mechanical room from the conditioned space is sufficient even for supplying the draft-hood dilution air of an atmospheric drafted burner. But stack dilution air isn't even used for condensing burners. It only needs enough for the combustion air.

   Then, a 60K burner doesn't draw enough combustion air to affect indoor comfort:

   Even a low-efficiency gas-burner draws no more than one cubic foot of combustion air for every 1000 BTU of source fuel, and condensing burners about 10% less. So 60,000 BTU/hr condensing burner draws about (60,.000/1000= ) 60 cubic feet per hour, or (60minutes/60 cfh = ) ONE cubic foot per minute. That's only ~1% of the air of a decent bathroom fan, and "in the statistical noise" of the amount of air leaking into & out of a typical home. A 2 mph change in wind speed would make a bigger difference in the amount of infiltration air.

3. 
   AlanB4 | Jan 16, 2017 05:35pm | #3

   @ Dana
   I used to have a 75K in/60K out indoor air sourced furnace, replaced with a dual pipe unit and i noticed the indoor humidity went up 10-20% the winter after replacement. So its enough to make a difference, and one can probably consider it an exhaust only ventilation system.
   I am much happier with the dual pipe and i hope my water tank replacement can also be dual piped (then comes the dryer).
   Though i will then need an HRV once i do some air sealing :)

4. 
   charlie_sullivan | Jan 16, 2017 10:03pm | #4

   Climate Consultant
   Thanks for prompting me to try Climate Consultant. What a great free tool! It seems like a good way to answer questions like whether a whole-house fan is useful in a particular climate, for example.

5. Expert Member
   Dana Dorsett | Jan 17, 2017 02:04pm | #5

   It's the dilution air, not the combustion air @ Alan B
   75K-in / 60K-out= 80% efficiency, and whether it was atmospheric drafted or forced draft, the largest fraction of the air needed to operate it without flue condensation is the dilution air going up the draft hood.

   The dilution air draft runs 24/365, not just when the burner is operating. The flue damper (if the old unit even had one) that opens only during burns is on the appliance side, before the dilution hood. Even if the furnace or boiler is right-sized for the load, the daily volume of passive dilution air flow is quite large.

   With condensing appliances there is NO dilution of the combustion exhaust, and the exhaust venting materials are selected to be condensate tolerant, and flue condensate management details are built into the equipment & venting design.

   If your water heater isn't a forced draft type (with plastic venting), and it vented into a masonry chimney that used to be shared with the old furnace, there is high risk of even the diluted exhaust condensing/adsorbing into the masonry, along with higher risk of backdrafting. Natural gas exhaust condensate is mildly acidic- it degrades mortar and destroys the chimney from the inside out. Look up "orphaned water heater".

   http://www.ashireporter.org/HomeInspection/Articles/Orphaned-Water-Heaters-It-s-Adopt-a-Chimney-Time/14813

6. 
   AlanB4 | Jan 17, 2017 02:33pm | #6

   @ Dana
   That is very interesting about the dilution air. It was forced draft with no damper i am aware of. The orphan water tank is going to a non masonry chimney, its actually a metal chimney that runs up the center of the house. I am planning on replacing it with a power vent water tank so the chimney will be capped, and there is not much in the way of water dripping out of the chimney, i have removed the bottom cap a few times and its dry, even after rain.

7. 
   user882465 | Jan 18, 2017 03:00pm | #7

   Code Requirements for Furnace Replacement
   California's Title 24, Part 6 Building Energy Standards require an ACCA Manual J, S and D for a furnace replacement. This leads me to ask whether this project was permitted. Was it? If not, this points out a benefit of getting a permit. It it was permitted, and the contractor can't show that they did these calculations, it would strengthen the homeowner's case that the contractor did not do the minimum requirements called for in this job.

8. Expert Member
   Dana Dorsett | Jan 18, 2017 03:13pm | #8

   Dilution air on hot air furnaces @ Alan B
   Hot air furnaces need an induced draft to overcome the impedance of the heat exchanger, but the dilution air is introduced post- heat exchanger in a draft hood (typically on one side of the box, sometimes two and is not dampered-off in many/most older 80% AFUE models. Retrofit automatic flue dampers were sometimes added post-draft hood with interlocks to inhibit ignition until the open flue was "proved" by an electromechanical switch, but it's unlikely that yours had one of those, given the over-drying symptom.

   The exhaust condensate will eat holes through B-vent over time. It won't show up as liquid water when you break it open for inspection at some random time- it DOES dry, due to the 24/365 parasitic drafting through the draft hood of the water heater. If it's a stainless liner it will tolerate it, but dual or single walled galvanized won't.

9. Expert Member
   Dana Dorsett | Jan 18, 2017 03:32pm | #9

   CA Title 24 @ Bill Burke
   It always amazes me, but that part of the code seems to be regularly overlooked in older homes, even though that's where it is most likely to do some good! Newer up-to-code houses are much tighter, but new construction seems to be where inspectors focus on the HVAC details.

10. 
    LukeInClimateZone7 | Jan 21, 2017 12:20am | #10

    CA Title 24
    @ Bill Burke, @ Dana
    The requirement for Manual J, (or ASHRAE HoF) calcs has been on the books for years. As someone who works on the Peninsula, I've experienced this bit of code (Section 150.0 (h) of Title 24 part 6) quite a bit since I advocate for it in my own work. My personal experience is: plancheckers and inspectors don't know about this section of code, nor have they received training as to how to review such calculations, and the vast majority of HVAC subcontractors have never heard of them either.

    On the good side-- Bill and his colleagues are actively addressing this with ongoing education at the PG&E Energy Centers in SF and Stockton. (keep up the good work!)

11. 
    AlanB4 | Jan 21, 2017 11:49am | #11

    @ Dana
    Thats very good to know and i will keep it in mind, i am already planning on replacing it, hopefully this year, and i'm not sure what kind of metal it is or how to find out. Its at least 30 years old that about all i know.