Make up air vs air for combustion....

MikeL_2

I'm hoping some of you can help with a discussion I'm having on another forum. Is make up air the same as air for combustion?

STEVEusaPA

Which other forum? Probably many of the same people.

MikeL_2

I hope so, Steve. I see an opportunity for learning..........Plumbing Masterminds

jumper

No, no, no! People require make up air even when there's no combustion.

Tim McElwain

Make up air is air required to replace air removed by mechanical means such as an exhaust system in a commercial kitchen. Or in the case of gas dryer in a home which by the way is not vented but is exhaust it must have make up air. Air for combustion is the air that needs to be supplied for proper burning it includes primary air, secondary air, excess air air. Many times folks call all air needed "make up air" but that is not a correct use. i hope that helps.

Gsmith

Building codes for residences require outside air for ventilation, (people breathing, etc) at typically 5 air changes per hour (whole house volume changed over 5 times per hour), see International Residential Code 2018, PLUS, combustion air for any fired appliances.

Tim McElwain

@Gary Smith you are correct but that is not defined as make up air by definition. Without getting into pages of definitions air for combustion has a lot to do with the size of the fuel burning equipment zone, how the air is supplied such as fan in a can, or two openings communicating directly with out doors one 12 inches from the ceiling the other 12" from the floor and it goes on as to just how we are getting that air. So you are correct after all of that required air we still need to make sure there is air for humans, plants, pets etc.

Gordy

5 air changes per hour?

STEVEusaPA

I want to selling heating oil to the houses getting 5 complete air changes per hour.

Gordy

That number is not a code required whole house ACH.

Gsmith

Ah, sorry for the slight simplification, IRC2018 requires window or other openings Capable of 5 per hour but doesn’t require operating at that rate, otherwise mechanical ventilation for people must be supplied. I can provide the text a little later. But the point was codes require people air in addition to combustion air.

Gsmith

For completeness here are the IRC 2018 code sections:
"R303.4 Mechanical ventilation. Where the air infiltration
rate of a dwelling unit is 5 air changes per hour or less where
tested with a blower door at a pressure of 0.2 inch w.c (50 Pa)
in accordance with Section N1102.4.1.2, the dwelling unit
shall be provided with whole-house mechanical ventilation in
accordance with Section M1505.4."
"M1505.4.3 Mechanical ventilation rate. The wholehouse
mechanical ventilation system shall provide outdoor
air at a continuous rate as determined in accordance with
Table M1505.4.3(1) or Equation 15-1.
Ventilation rate in cubic feet per minute = (0.01 × total square
foot area of house) + [7.5 × (number of bedrooms + 1)]
Equation 15-1"
"TABLE M1505.4.3(1) has listed rates for various house sizes and number of bedrooms, both table and equation for a 1,500 sq. ft. house with 3 bedrooms works out to 45 cfm continuous ventilation rate.

GroundUp

We've got a local contractor here that has never, ever installed a combustion air intake on any furnace or boiler and always passes inspection. He runs a 6" flex to the mechanical room with outdoor air and just lets the furnace/boiler intake port hang open to the mech rm. I often wonder how he can get away with it, but apparently it works out

Henry

Indirect Make Up Air or IMUA are use to replace the air that is exhausted by WC exhaust fans and kitchen fans in apartment blocks/condos or restaurants and industry. Direct Fired Make Up Air are for industrial use.

Rich_49

Gary Smith said:

For completeness here are the IRC 2018 code sections:
"R303.4 Mechanical ventilation. Where the air infiltration
rate of a dwelling unit is 5 air changes per hour or less where
tested with a blower door at a pressure of 0.2 inch w.c (50 Pa)
in accordance with Section N1102.4.1.2, the dwelling unit
shall be provided with whole-house mechanical ventilation in
accordance with Section M1505.4."
"M1505.4.3 Mechanical ventilation rate. The wholehouse
mechanical ventilation system shall provide outdoor
air at a continuous rate as determined in accordance with
Table M1505.4.3(1) or Equation 15-1.
Ventilation rate in cubic feet per minute = (0.01 × total square
foot area of house) + [7.5 × (number of bedrooms + 1)]
Equation 15-1"
"TABLE M1505.4.3(1) has listed rates for various house sizes and number of bedrooms, both table and equation for a 1,500 sq. ft. house with 3 bedrooms works out to 45 cfm continuous ventilation rate.

That's a far cry from 5 ACH Nat my friend . Where exactly do you work or reside ?

mikeg2015

GroundUp said:

We've got a local contractor here that has never, ever installed a combustion air intake on any furnace or boiler and always passes inspection. He runs a 6" flex to the mechanical room with outdoor air and just lets the furnace/boiler intake port hang open to the mech rm. I often wonder how he can get away with it, but apparently it works out

My last home in michigan had this. A 6” passive vent near the ground for the water heater and 80% furnace.

When I replaced my furnace (with one 1/2 the size) I took it and connected it directly to the furnace and used a electronic damper, then left it about 20% open all the time.

At my current home I have a 6” duct with a powered damper powered off the heating call to the boiler. Might change it to go off the main gas valve call instead. We have some radon where we are at, so keeping the basement form going negative is beneficial on a 3 story home counting the tall attic.