# Time for a new HVAC Unit



## Sar-Con (Jun 23, 2010)

Looks like our current equipment is shot. We think the old unit is oversized (very noisy, short cycled, etc) and that may have caused it's early demise. 

I'm trying to do some research before talking to the HVAC guys, and all the sizing guides that I've found are geared to residential applications.

Our office is one floor, about 3000sf with 8ft ceilings. Regular occupancy is about 10-12 people. We're about 1 hr drive north of Detroit.

So, what I'm looking for is a guide to tell me the rough tonnage needed for cooling - 5 ton, 8 ton? Does such a thing exist? I'd like to educate myself a little before talking to the HVAC guys...

Thanks!


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## CompleteW&D (May 28, 2011)

You should get "educated" by a professional HVAC contractor. They will know how to do a proper load calculation on your space. It's based on a LOT more than square footage and occupancy. Things like insulation, duct sizing, what kind of duct work and is it insulated, supply and return air duct placements, the square footage of the glass you have - including the type of glass and what direction it faces, the number of degree days your location has and on and on and on. All of these factors and many more go into the load on your particular space.

Do yourself a favor and have a professional commercial HVAC contractor do a thorough and proper evaluation of your space and give you detailed recommendations and a quote.


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## beenthere (Mar 5, 2009)

Commercial, you'll need to have fresh air brought in, that adds to the cooling load. 

Then there is the lights, office equipment, what kind of traffic in and out of the building. All these things are taken into consideration, to determine size. Above and beyond just wall area, window area, orientation, infiltration.


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## Sar-Con (Jun 23, 2010)

CompleteW&D said:


> You should get "educated" by a professional HVAC contractor. They will know how to do a proper load calculation on your space. It's based on a LOT more than square footage and occupancy. Things like insulation, duct sizing, what kind of duct work and is it insulated, supply and return air duct placements, the square footage of the glass you have - including the type of glass and what direction it faces, the number of degree days your location has and on and on and on. All of these factors and many more go into the load on your particular space.
> 
> Do yourself a favor and have a professional commercial HVAC contractor do a thorough and proper evaluation of your space and give you detailed recommendations and a quote.


Yep, fully intend to call the HVAC guys and get their opinion, just thought I'd poke around on the web first to get some background.


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## Dr Heat (Dec 25, 2008)

If you are concerned you will need a certified Manual J. Then the ducts need to be checked to verify they can handle the load of the new RTU.


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## DuMass (Feb 6, 2008)

Probably Manual-N for light commercial office space applications like this one.


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## rdvd (Dec 12, 2012)

I agree you may want to get the opinions of the experts on this. I know that some office buildings who employ building maintenance services will often offer HVAC maintenance or will have a person knowledgeable in HVAC, so you may want to check with them first.


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## flashheatingand (May 3, 2008)

Dr Heat said:


> If you are concerned you will need a certified Manual J. Then the ducts need to be checked to verify they can handle the load of the new RTU.


As mentioned before, If the ducts are sized for a 5 ton (just a number) then put in a 5 ton unit. Unless one is ready to rebuild the ductwork and all, what good does a heat/cool load serve?


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## flashheatingand (May 3, 2008)

I take it the building is primarily for office stuff? Maybe a coffee machine, a copier, conference room...etc.? Lots of open cubicles or closed in offices? The biggest variable I would think is how much window space. Have you considered mini splits? I know Michigan does get cold, so you may want to keep the old behemoth, and invest in mini splits. Just food for thought.


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## beenthere (Mar 5, 2009)

flashheatingand said:


> As mentioned before, If the ducts are sized for a 5 ton (just a number) then put in a 5 ton unit. Unless one is ready to rebuild the ductwork and all, what good does a heat/cool load serve?


Many duct systems claimed to be size for X tons. Really aren't. Even if it is. If a 4 ton is the size the building needs. it will do better on that 5 ton duct system then the 5 ton would.


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## flashheatingand (May 3, 2008)

I differ on these grounds: A major component we discuss is the efficiency thing. If you are moving the air in a less than efficient manner, you are heating or cooling in an inefficient manner as well. Take care of the air flow thing, and the BTU's will follow.


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## Dr Heat (Dec 25, 2008)

close is close as long as the duct is big enough. But undersized duct is death to efficiency. Flash and been I know you know this but to the op it is important to check the existing. 

If the duct is over sized by too much the velocity (FPM) will be to slow.

In office spaces where remodeling is common you often end up with one space with a grossly over sized system just changing the unit will not fix the problem. (this is not an opinion this is a lesson that cost me several thousand dollars in the 80s) your welcome.:whistling


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## beenthere (Mar 5, 2009)

flashheatingand said:


> I differ on these grounds: A major component we discuss is the efficiency thing. If you are moving the air in a less than efficient manner, you are heating or cooling in an inefficient manner as well. Take care of the air flow thing, and the BTU's will follow.


Under sized duct is very inefficient. 

What many term as over sized duct, is not over sized, and is very efficient.

There are of course considerations as to where the duct is. You don't want to size your duct for 300FPM when its exterior to the building, or in an vented unconditioned attic. Heat gain and loss would be too much in summer and winter. Unless you insulated it to R30.

I have a little 5 ton heat pump on duct work that starts out at 14X34. In linear length the supply duct is 135 foot long(maybe a tad more). Can't remember its TEL anymore. Its been working fine for 12 years. 

The TESP on it was .7", only reason I can remember that is because I was surprised it was that high. And an inspector was telling me it was too big, and the air wouldn't know to come out the registers. ROFL.


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## beenthere (Mar 5, 2009)

I seldom get office buildings that can use a smaller unit that won't have atleast 600FPM velocity. Once in a wile, but not often.

Did get burnt on one building whee they wanted a larger unit. They gave me the model number(12 ton), and asked for a price to change it out to a 15 ton. Told them the duct work would be too small. They said they gave the same info to 3 other contractors, and the one that came out said no problem. 

Being busy at that time, I conceded and gave them a price without ever going on the roof, or checking the duct(had done a fair amount of work for them in the past, always good pay).

Day of the change out, I find out the old unit is a 10 ton, and the model number they gave me was from the unit beside it. Ended up having to eat a high static drive kit for the unit.

I was a bit younger then, and a bit hungrier then. Now I make sure I know whats going on before giving a price and signing a contract.

Lesson learned. 

Tore out a 5 ton heat pump, installed a 3 ton in its place. Zoned it and of course had to transition the plenums. Even with all zones calling, good air flow to all zones. All duct work was in the basement. 

If main trunk line FPM is going to drop to 500 or less. Then you have to be concerned. And you need to re-think what type of supply registers you are going to use, and if you can install ones that will work for the application your in. If not, then duct alterations will need to be done.

The duct system/distribution system needs to be looked at as a whole, which includes duct size, register placement, as in position in room/area, and height from floor, along with the throw the supply register will have.

Its a long learning curve. I'm still learning.

When changing out a system. if the blower still works on the old unit. its always best to check the ESP the old blower is working against. This tells you what the new system will work against. Weather your installing the same size, larger or smaller.

A duct system moving 3000CFM at a static pressure of 1". When the air flow is reduced to 2600 CFM, the static pressure will drop to .75"


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## accessheating (Mar 22, 2012)

http://www.accessheating.comGreat use of this forum here, Sar-Con! Glad to see you are getting some advice...
It is difficult to provide much advice, other than the basic/stock info (have a heat load calculation, size the ductwork/manual J) because we don't reside in your town...

Keep looking, have 3-5 companies come out and you will most likely see a vast difference in quality and affordability; and don't forget- your "gut-feeling" of a company is most often right on the money. 

Thanks and good luck!
<><><><><><><>


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## Dr Heat (Dec 25, 2008)

3-5 @#$%#@$ 3 reputable companies is all anyone should call estaments are not free they cost the contractor plenty.


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## flashheatingand (May 3, 2008)

They shouldn't be that expensive. Every merchant should know their costs and be able to give a number they can live with, on the fly. Granted, there are exceptions, but...


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## bellemech (May 27, 2009)

*Hello*

Give me a call.
I am in Grand Blanc, Michigan. 
I would be happy to get you a quote.
Thanks
Brian with Belle Mechanical
810-931-3762


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## slowsol (Aug 27, 2005)

airtrackhvac said:


> To make it clearer let us say an expample
> 
> 
> 
> ...






Sent from my iPhone using Tapatalk


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## TimberlineMD (Jan 15, 2008)

This thread is 3 years old. :laughing:


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