20X8 supply equivalent to 26X6 supply? Heatloss guys says

[hvachelpneeded]

I cannot go 26X6 because the ratio is over 3 to 1... another heat loss guy says it's okay... can't find any answers on the internet... please help.

Thanks.

[pageyjim]

Short answere is yes

[Todd S 2]

I cannot go 26X6 because the ratio is over 3 to 1... another heat loss guy says it's okay... can't find any answers on the internet... please help.

Thanks.

What are you trying to do? 20x8 equals 30x6 as far as cfm goes.

[pageyjim]

I read it as he wanted 26x6 but couldn't do that because of "ratio" for some reason and they were going to use 20x8 which would be slightly more cfm

[hvachelpneeded]

I currently have a 20X8 supply air (2.5 to 1 ratio) and the equivalent cfm wise is indeed 30X6 but one heatloss guy says the ratios are over 3 to 1 on the 30X6 (5 to 1 ratio) and as such it is not a good idea from a design/flow perspective regardless of the cfm equivalence... the other heatloss guys says it's a bungalow so we can get away with it... confused as to why this ratio is an issue and I could really use the headroom in the basement but would never comprimise the performance of the duct work...

thanks.

[hvachelpneeded]

are important... I thought if the CFM is maintained there would be no problem... please clarify.

Thanks.

[hvachelpneeded]

I currently have a 20X8 supply air (2.5 to 1 ratio) and the equivalent cfm wise is 30X6 but one heatloss guy says the ratios are over 3 to 1 on the 30X6 (5 to 1 ratio) and as such it is not a good idea from a design/flow perspective regardless of the cfm equivalence... the other heatloss guys says it's a bungalow so we can get away with it... confused as to why this ratio is an issue and I could really use the headroom in the basement but would never comprimise the performance of the duct work...

Again, I'm really confuses as to why these ratios are important... I thought if the CFM is maintained there would be no problem... please clarify.

Can I go 30X6 or must I stay with 20X8?

Thanks Again.

[Todd S 2]

I read it as he wanted 26x6 but couldn't do that because of "ratio" for some reason and they were going to use 20x8 which would be slightly more cfm

Correct. The rule of thumb is not to go more than 3 times the width. But that said, we just go bigger. So, 30x6 will move the same air as 20x8.
26x6 is only good for 800cfm.

[hvachelpneeded]

I agree with you that the 30X6 will do cfm wise but what are the pitfalls/cons of breaking the 3 to 1 ratio rule and if it's not a big deal why have the rule in the first place? There must be a reason for it... perhaps the air does not move as easily/efficiently or the ducts are noisy?

Any other input would be greatly appreciated...

Thanks!

[lynn comstock]

Rules of thumb serve two pruposes.
1. The are useful as a quick double check in situations.
2. They are necessary for techs with little knowledge of the scientific fundamentals or little skill in applying such knowledge. They are better than a wild guess.

[hvachelpneeded]

your house and ignore the rule if you needed the headroom or would you stick with the 20X8.

Thanks again.

[hvachelpneeded]

with regards to cfm and the less than 3 to 1 ratio rule?

Would you do it in your house, why or why not, pros and cons for both?

Last post on this subject... just can't get a straight answer.

Thanks again for all or you help!

[Todd S 2]

your house and ignore the rule if you needed the headroom or would you stick with the 20X8.

Thanks again.

Lynn explained it well. It's a quick guide. If you need the headroom and 900cfm of air go with the 30x6.