Residential (IRC) Mechanical Ventilation Question

[jar546]

I am doing some IECC CEUs, and the training and code book say one thing, but the practice quiz says another. Section M1505 is directly related to the IECC training. Here is the situation.

There are two options under M1505.4.3 for Mechanical Ventilation Rate. You can use Table M1505.4.3(1), or you can use Equation 15-1. That is clear, cut and dry. Here is the problem. The exception allows the ventilation fan to run as low as 25% of the time. However, you have to use Table M1505.4.3(2) to adjust the required CFM. Again, easy no problem. Here is where the confusion comes in.

If you are not going to use a continuous fan, and want to go intermittent, can you use Table M1505.4.3(2) for the number based on Table M1505.4.3(1) only or could you use the same factor table if you choose to use Equation 15-1?

The way it was taught was that the factor is always multiplied by the table, not the equation, but one of the questions could only be answered correctly if you used the equation to multiply by the factor.

 

[wwhitney]

Are you referring to Exception 2 of 2024 IRC M1505.4.3? The text from iccsafe.org (excerpted below) seems unambiguous to me, you can apply the Table M1505.4.3(2) factor to a ventilation rate determined by any of the methods allowed for continuous ventilation.

BTW, Table M1505.4.3(2) is stupid. It's just a look up table for the function x -> 1/x. The text of the exception could just say "multiply the required continuous ventilation rate by the reciprocal of the duty cycle of the intermittent ventilation" or the equivalent.

Cheers, Wayne


"2. Programmed intermittent operation. The whole-house mechanical ventilation system is permitted to operate intermittently where the system has controls that enable operation for not less than 25 percent of each 4-hour segment and the ventilation rate prescribed in Table M1505.4.3(1), by Equation 15-1 or by Exception 1 is multiplied by the factor determined in accordance with Table M1505.4.3(2)."

 

[ICE]

Is there a formula to use after the occupant disabled the fan?

 

[jar546]

Are you referring to Exception 2 of 2024 IRC M1505.4.3?

2018 IECC

 

[wwhitney]

2018 IECC

OK, that has no M1505 section, so this answer is not computing for me. The only M1505 I can find is in the IRC, in which case my previous post would apply.

Cheers, Wayne

 

[jar546]

OK, that has no M1505 section, so this answer is not computing for me. The only M1505 I can find is in the IRC, in which case my previous post would apply.

Cheers, Wayne

I should have typed 2018 IRC. It was the IECC that sent me there, so I still have energy on the brain.

 

[wwhitney]

I should have typed 2018 IRC.

OK, the wording in that code year is a bit different:

M1505.4.3 The whole-house 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.

Exception: The whole-house mechanical ventilation system is permitted to operate intermittently where the system has controls that enable operation for not less than 25 percent of each 4-hour segment and the ventilation rate prescribed in Table M1505.4.3(1) is multiplied by the factor determined in accordance with Table M1505.4.3(2).

I agree the 2018 wording requires you to use Table M1505.4.3(1) if you want to provide only intermittent operation.

However, I would say there's no valid performance reason to impose such a limit, and this oversight was fixed in the 2024 IRC as per my first post. Perhaps the question you referenced in the OP was based on the 2024 IRC?

Cheers, Wayne

 

[jar546]

OK, the wording in that code year is a bit different:



I agree the 2018 wording requires you to use Table M1505.4.3(1) if you want to provide only intermittent operation.

However, I would say there's no valid performance reason to impose such a limit, and this oversight was fixed in the 2024 IRC as per my first post. Perhaps the question you referenced in the OP was based on the 2024 IRC?

Cheers, Wayne

I believe we agree with the interpretation of the 2018 IRC, even though using the equation should allow it to be factored in my opinion. For some reason, the powers that were for the 2018 version wanted to 'penalize' you if you did not use the Table, and instead used the equation. My question was specific to the 2018 IRC.

 

[wwhitney]

For some reason, the powers that were for the 2018 version wanted to 'penalize' you if you did not use the Table, and instead used the equation.

Or it was just sloppily written, and not fixed until the 2024 version. Hanlon's razor.

Cheers, Wayne