Questions on airflow performance specs of two dif register designs.

[Overland46]

Hello all,

First off, sorry this is a bit long for such a mim subject, but I just figured I would give you all the info I could upfront.

So as an ex HVAC contractor I am a little embarrassed to post this question, but for my own house for what sidewall registers I have, I have been mostly using the Lima 2 way registers that are of a 1/2” fin spacing design here: https://limaregister.com/products/sw/stamped-lanced-blade-register-with-multi-louver-damper

To which for that unit its fins are at a 40-degree angle.

Which to be honest, the reason for that was at least in part as since my house has a heat pump and is in the Northeast I am always looking for anyway to make it more efficient, so I guess at the time in just looking at them I felt the larger spacing or openings, and less angle would have the least restriction so would make the system work less to get the same amount of air around.

Recently though I have had a situation where I have to replace one and so was studying the design data for these as opposed to their 1/3” fin spaced, 45 degree angled units here: https://limaregister.com/products/632/stamped-ceiling-wall-register

Note on that, while on their main page it says 20 degree angle, on their spec sheets it says 45, which I think is actually correct.

So in comparing the two via their performance/engineering data sheets and at least for the 12 x 6 units I would be using and per the same face velocity & pressure lost columns, it looks to me that actually the 1/3” finned space units outperform the 1/2” finned spaced units, such as in at 500fpm face velocity & .016 pressure loss, the 1/2” unit produces 115cfm & 7.5 throw. Whereas the 1/3” finned spaced unit at the same spec produces 150cfm & 11 throw.

Interestingly by reviewing what are essentially the same spec-ed versions of the Hart & Cooley units, they produce about the same results.


So I guess what I wanted to ask first is...am I reading this right as do the 1/3” finned spaced units actually move more air?

An if so why does that happen? As I would think with the lesser fin angle and more space between them the 1/2” spaced finned units would all allow more air to pass, over the 1/3” spaced unit that has more of an angle.

But if not, does anyone know why this is?

For one thought I had was and would note I can’t seem to find any free area specs on any of these so am not even sure if the 1/3” units have less free space, but is this the typical case due to that or some other thing of a restriction they somehow are creating more static pressure and thus that is really what moves the more air?

All in all my thoughts are to possibly switch to the 1/3” units if they are going to more air for whatever reason, but if that is doing that by a restriction that is increasing my SP, my thoughts are my variable speed air handler may have to work a little harder, and also my air noise may go up a bit, so in the end it could be that I don't actually end up with a more efficient system and all with a noisier house to boot.

Thanks for any help.

 

[MintJulep]

The damper design seems different.

 

[LittleInch]

In terms of what the data is telling you I agree, the 1/3" units seem to have a greater airflow for the same size and same velocity / differential pressure.

Without comparing one in each hand it's not easy to see from the photos, but I think they will both have a very simialr open area. The difference I think is in the length of the "blades" or louvers with the 1/2" version creating a higher flow disturbance and very slightly more friction.

The issue will be how many of your units are fully open all the time?

The Ak rating is different though which is something to do with area factors... https://www.krueger-hvac.com/files/white papers/white_paper_area_factors.pdf

And define "efficiency" here. Or do you mean effectiveness?
The impact o being able to shift more air is up to you and your system and its control function as to how noisy it might be. If you suffer from lack of heating or cooling in any rooms then increase air flow. If not why change it? I can't see the fan motor being the cause of that much power usage.

 

[Overland46]

The damper design seems different.

Hello MintJulep & thanks for the reply,

Is interesting that you mention that as actually if I recall one of the other reasons I chose the Lima and in particular the SW 600 version over say a same Hart & Cooley unit or something else was as it was the only model that had a front damper handle that stubbed out a bit like it does, which I think is at least in part why the rest of the damper design is also so different.

Which primarily I did as the plastic handled slide types were one’s I felt could be fussier to deal with and get dirty faster and be harder to clean over time. Which I am all talking about in small proportions of course, but seeing as it had then had the larger openings too is likely why I chose it over their other unit.

Also I probably thought it’s easier to see at a glance if any of the units had somehow closed either fully or in part by just looking at the position of the handle.

What is interesting though per our discussion points is if you review and compare the data for the similar Hart & Cooley models here:

661/A661 — Steel/Aluminum 2-way Register, MS damper, 1/3in. Fin Spacing

Hart & Cooley is an industry leader in the HVAC supply industry, providing unmatched customer care and high-quality commercial and residential grilles, registers, diffusers, vent systems, and chimney systems for more than a century.

hartandcooley.com

682/A682 — Steel/Aluminum 2-way Register, MS damper, 1/2in. Fin Spacing

Hart & Cooley is an industry leader in the HVAC supply industry, providing unmatched customer care and high-quality commercial and residential grilles, registers, diffusers, vent systems, and chimney systems for more than a century.

hartandcooley.com

They are virtually the same for all the specs I have listed for the corresponding Lima units, and yet at least by their drawings at their site the two Hart & Cooley units look to have the same damper setup.

 

[Overland46]

In terms of what the data is telling you I agree, the 1/3" units seem to have a greater airflow for the same size and same velocity / differential pressure.

Without comparing one in each hand it's not easy to see from the photos, but I think they will both have a very simialr open area. The difference I think is in the length of the "blades" or louvers with the 1/2" version creating a higher flow disturbance and very slightly more friction.

The issue will be how many of your units are fully open all the time?

The Ak rating is different though which is something to do with area factors... https://www.krueger-hvac.com/files/white papers/white_paper_area_factors.pdf

And define "efficiency" here. Or do you mean effectiveness?
The impact o being able to shift more air is up to you and your system and its control function as to how noisy it might be. If you suffer from lack of heating or cooling in any rooms then increase air flow. If not why change it? I can't see the fan motor being the cause of that much power usage.

Hello LittleInch & thanks for the reply,

Yeah, in per the dif of the blades, is a good thought, but if I am reading you right as I posted above, the similar Hart & Cooley units have pretty much the exact same specs though they look to have the same damper setup.

So if I had to guess and as both brands are so close for their data, this is more about the fixed blade design than anything to do with the movable dampers.

To which then to also answer this part of your reply, I just leave all my supply air register dampers open all the time.

I do think you are more onto something with that Ak number though and thanks greatly for the link to that article as is the one thing I didn’t look up nor thought it had anything to do with the free area, but it looks like you are probably right on that so the whole thing could be that while the 1/2" spaced units visually look to have more space between them, the fact is the 1/3” units have more free area and thus are actually less restrictive.

I think this could be even more put to the point as after posting my OP I was looking at the returns in much the same way as in the same I need to replace one and am trying to make everything the same throughout the whole house.

So in looking at this in the same way for just the Hart & Cooley units and at least for the 12 x 6 units here:

650/A650 — Steel/Aluminum Return Air Grille, 1/3in. Fin Spacing

Hart & Cooley is an industry leader in the HVAC supply industry, providing unmatched customer care and high-quality commercial and residential grilles, registers, diffusers, vent systems, and chimney systems for more than a century.

hartandcooley.com

672/A672 — Steel/Aluminum Return Air Grille, 1/2in. Fin Spacing

Hart & Cooley is an industry leader in the HVAC supply industry, providing unmatched customer care and high-quality commercial and residential grilles, registers, diffusers, vent systems, and chimney systems for more than a century.

hartandcooley.com

Again while it's actually less than the registers, the 1/3" beats the 1/2" unit just slightly, but what is even more interesting is the Ak number is only slightly higher for the 1/3 unit in this case, whereas for the registers is a bit more to which then we see more of performance increase.

So overall I would lean to the idea that the Ak number reflects something about how much free area the units have.

All in all its pretty strange then to think about why anyone would want to go with the 1/2" returns as while I could see the design difference desires of the angle of the fins of a supply registers as they diffuse the air differently , why would anyone want a more open showing return grill over one that shows less, when the more open one restricts more air?

At any rate, just for the info I also wrote something quick to Hart and Cooley about this later in the afternoon yesterday, (which had to be quick as the form on their site did not allow too much room), but hopefully they understood what I was asking about and if I here back from them I will update this here.

But unless anyone else has something else to post to the contrary, to me it seems like while probably very small, if you are looking to get the best performance you can out of things the 1/3 units over a ½ look to do better.