VS motors on poor ducting

RoBoTeq

Any of you guys install vs motor bearing units to help with issues of poor ducting? For what circumstances do you use vs motors and how do they work for you/

JohnH1

We have used them for some under sized systems that we could not get to the duct system for repairs. If you run the static .95 or higher there is a possability the motor will start hunting up and down. It is defently better to try and do some duct modifacations if possable. The VS is only tring to cover up the underling problem.

RoBoTeq

Agreed; 100%.

However, like you stated, you may have a duct system that is just not conducive for being fixed.

I don't like taking a vs motor over .8 SP in a system. At .8 SP a vs motor is pretty much costing the HO as much to operate as a psc motor is, so why bother?

If you cannot increase the size of the ducting, does it make sense to reduce the amount of air you are pushing through that ducting?

JohnH1

It's not about the cost of operation it is more about improving the comfort for the home owner.
As far as redusing the amount of air through the duct. If you need the btus to heat or cool the space then run the static up untill you can keep the people comfortable. Hopfully the velosity does not get so high it becomes noisy also.
I would assume someone did a heat load to confirm the equipment is not oversized before investing in a vs piece of equipment. As we all know many systems are oversized.

RoBoTeq

You hit on what I consider the most important factor which is the proper sizing of the equipment. Many systems with high static issues can be helped greatly just by reducing the capacity of the equipment to the proper size.

By reducing the capacity of the equipment, you are reducing the amount of air needing to be pushed through that system. This increases comfort by increasing equipment run time and lessening the air sound.

So, say we have the proper amount of cooling/heating. In most systems, heat is not a factor as much as cooling is. Heat pump systems of course are a different story because we need as much air if not more in the heating mode.

For argument sake, let's take a situation in an old row home with all 5" branch ducts in the walls and a duct system designed for gas heat only imbedded in the ceilings of all finished rooms, including the basement.

Let's assume that the total system SP is .8 with a 2.5 ton cooling capacity with a psc blower set to the highest tap setting because any speed lower allowed the evap coil to freeze up on very humid days. Could a vs blower help with this scenario and how?

Yes, I am leading toward an answer which has been working for my contractors. I am just trying to get a feel for a better way to describe why.

JohnH1

Yes and No. I you increase the static pressure weather VS or a higher HP motor you should move more air to be able to reach your target cfm.
Now once you go beyond the max tested SP for that equipment you need to actually check the delivered cfm. Blowers can only go so far.

Also a note re: higher cfm for cooling. This is not accurate any more. especially for the higher efficency equipment. The heat many times requires more cfm that the cooling does nowadays

RoBoTeq

Ah! Your last point can be used to negate your first.

A vs blower can effectively perform in a system with a reduced amount of air without creating the issues that a psc motor is prone to create. By reducing the usually required 400cfm per ton (1,000 cfm in this case) to around 350 cfm per ton, a very easy thing to do with vs dip switch selections, you now reduce the SP, decrease coil temperature, increase humidity removal (at a slight efficiency cost) and reduce air noise.

hvaclover

Boy did I take in the neck on my first VS install. Changed wholesalers cause my guy wouldn't put me in a training session. Then expected me to buy his VS stuff any way.

When I switched suppliers and my rep said h'de put me in his first available class. Like an idiot I fell into the efficiency trap my self after listening to the new rep.

I thought the VS was a remedy for under sized ducts. I sold a job with VS and new 3 1/2 ton system. Ducts weren't up to snuff. Set the dip switches for 1400 cfm and the motor was drawing ten amps!

Oye, was I mad!! I had to rip out a finished basement ceiling to correct the supply and return trunks on my dime. Good thing I made a mint on the job.

Still need a good seminar on VS stuff but I'm afraid to ask my Goodman guy cause I haven't moved squat for equip this year.

__________________
Not Stupid, Just slow

beenthere

They having this trouble with Luxaire blowers/air handlers?

Their PSC single piece 2.5 ton air handlers, at 230 volt, and .8" static don't move 1000CFM to begin with. Some of the older ones would drop below 800 CFM at that static.

The 3 ton ones will move over 1200 CFM on high tap with .8" static.

Have you been out to these sites, and tested the static yourself. Or are you just going on what that customer is telling you.

__________________
How many times must one fix something before it is fixed?

hvaclover

Been we are talking about VS blowers. How does the psc motors fit into thid topic

__________________
Not Stupid, Just slow

beenthere

From post #5:

__________________
How many times must one fix something before it is fixed?

RoBoTeq

Had you taken a measurement of the SP, I'll bet it was at least .8

So, had you trimmed that motor -15% using the trip feature, you would have been putting out 1190 cfm of air, or 340 cfm per ton which, based on a nominal 100' of static (may as well make it work with the ductulators) that same system would have a SP of .55 which would have reduced the amp draw quite a bit.

More importantly, it would have reduced air noise, increased humidity control and protected the motor for a longer life running at lower amps.

Had you installed a psc blower motor and simply reduced the air speed, you would have taken a chance on coil freeze up when an extremely wet coil or dirty filter occured.

RoBoTeq

Going mostly from past site inspections. I rarely hit residential systems with .8 or higher SP if there were no restrictive filters installed. Commercial systems? Yes. But most residential systems that I inspected with air flow issues were between .5 and .7 SP.

hvaclover

Whole point was that it was a lesson that nothing replaces a formal factory training session.

__________________
Not Stupid, Just slow

RoBoTeq

This may be true, but that is now in the past and we must live for the future because the present is always the past as soon as it happens.

JohnH1

Can you say Deep Thoughts

hvaclover

No, but I can BUY hip boots

__________________
Not Stupid, Just slow

JohnH1

Then you would be like a plumber standing in Deep S * * t

RoBoTeq

Too late for boots, save the dumbbells;

JohnH1

Thanks for the thought but Im not a dumbell Ha Ha Ha Ha