Spacepak replacement with variable fan, what % fan speed is typical?

DCAC

Recently had an old Spacepak AH replaced and also went up from 2 to 3 ton. So far only two additional ducts have been added and I know I need to get at least another 4 added as well, so I suspect that is a reason why my fan speed on the AH is only around 35% typically.

It is set to auto adjust fan speed based on target pressure, which was at 1.2 and I revised it to 1.4 to see impact on fan speed and cooling. I understand 1.2 to 1.5 is ideal range.

System cools ok, not great - it can most of the time maintain temp in house when it is under 90 outside, but very hard to lower temp even when in mid 70's outside. LTA is good.

Main question is - if I had all the ducts I need to hit the ideal ducts per ton (and length of runs), what would the fan speed % usually be at? Am I currently WAY under utilizing the capacity of this thing or am I currently close to what the overall cooling capability would be even if airflow was maximized? Obviously it seems like I am using only a third of the fan speed possible, but don't know if running it closer to 100% would ever be possible or practical or recommended (which I assume I can't do now because of the lack of ducts anyway and would raise pressure way above ideal range)

If my understanding of all of this is totally off base please let me know! Many thanks

PC7060

Each outlet can output approximately 2KBTU, assuming line of 10’ -15”.  A 2 ton unit would typically have 12 outlets assuming no inline flow reducers.  A 3-ton will need 18. 

How many outlets does your system have?  I

Duct leakage outside the envelope (attic, crawl space, etc.) will hammer performance. Time spent checking/sealing leaks in ducts and the house envelope will pay big dividends. 

DCAC

Thanks for your reply. Had 13 before, now have 15. I have been planning to add 4 more, and trying to understand if this is the only problem that is preventing the system from performing like it should or if there could still be other factors. My guess was this is the main issue because the fan is limited to running at only a third of it's capability in order keep pressure in range. Is that accurate way to think about it? And that if it could move more air through more drops, then it could run faster and take better advantage of the cooling already being produced?

DCAC

And yes on the leakage, the new runs that were installed look very poorly done - definitely can feel air all around where the guy connected the new run to the trunk/plennum

EdTheHeaterMan

When I first learned to install ductwork, this is what we used in order to get our work clothes destroyed with stains that would not come out. https://www.supplyhouse.com/Polymer-Adhesives-CADS-1-Gray-CADS-Fiber-Reinforced-Water-Based-Duct-Sealant-1-Gallon-Pail ...but it was great at sealing up even large mistakes in the duct connections.

If the boss wanted to keep the uniforms clean and was ok with a little higher cost tape, this stuff has a rubbery cushion type of adhesive that can cover up large duct connection mistakes. https://www.supplyhouse.com/Venture-Tape-1580-2-Duct-Joint-Sealing-Mastik-Tape-2-x-100
It's a lot cleaner to work with and does not come off of what ever you get it stuck to.

PC7060

DCAC said:

And yes on the leakage, the new runs that were installed look very poorly done - definitely can feel air all around where the guy connected the new run to the trunk/plennum

I use yellow foam in a can for windows around all plenum to room line connection. The foam seal for the snap on connectors leaks over time even with perfect connection. 

Make an igloo of foam over the entire connection; ugly but great seal. I like the mastic Ed recommended but found it was a pain to apply around the odd shaped connectors. 

PC7060

Yes, you need to be able to dump all the BTU out of the AHU while maintaining  velocity at the outlet and pressure in the plenum.  I’d guess you don’t have sufficient airflow across the chiller frame. Did you check to see if it’s freezing up?

pecmsg

If you increased the capacity 50% then the air flow needs to increase as well as the delivery system.

PC7060

Regarding an accurate measurement, use an airflow meter to measure the velocity/CFM of the output and a manometer to measure the static pressure in the plenum about 18” from the air handler. 

Look the up the specifications for your unit.  IIRCmy system required Sp of 1.2-1.5 in. WC and  40CFM airflow per output assuming 10’ line and no restrictive plate (250 CFM per ton).  

Lance

I recommend 35 cfm per outlet or less to lower noise levels, it can be a silent system, and to improve air flow which saves money and performs better. 40 CFM per outlet was deemed too restrictive especially considering how people installed their systems. I have two systems in my home. I even put a service outlet in the attic when I need to cool down. stays shut till I need it. There is nothing that says you can't increase the duct to 4" either, just practice good duct sizing engineering.

DCAC

PC7060 said:

I use yellow foam in a can for windows around all plenum to room line connection. The foam seal for the snap on connectors leaks over time even with perfect connection. 

Make an igloo of foam over the entire connection; ugly but great seal. I like the mastic Ed recommended but found it was a pain to apply around the odd shaped connectors. 

Thanks, I will see if I can pick some up and seal up around the connectors, it seems like a substantial leak on the new takeoffs, not nearly as much on the 20+ year old ones. I dont think all the connector parts were used or installed right per instructions.

DCAC

PC7060 said:

Regarding an accurate measurement, use an airflow meter to measure the velocity/CFM of the output and a manometer to measure the static pressure in the plenum about 18” from the air handler. 

Look the up the specifications for your unit.  IIRCmy system required Sp of 1.2-1.5 in. WC and  40CFM airflow per output assuming 10’ line and no restrictive plate (250 CFM per ton).  

Thanks for the replies, very helpful. Yes system requires 1.2 to 1.5 SP range.

Question on that - the LCD control board on the AH reads out the current SP and it changes the fan speed to keep the SP at the target set, which is currently 1.4. Can I trust that is right or do I still need to manually measure it to confirm?

pecmsg

That measurement is a calculated measurement. It needs to be checked with an actual reading.

PC7060

What @pecmsg said. Needs to be measured a certain distance from AHU.