A/C sizing help

[CMcAllister]

We are to the point of being ready to get some estimates for HVAC for our new shop. We have information from a design professional and some HVAC guys with unit size recommendations that are all over the place. Some real world experience would be appreciated.

This is a stick built building, 52x40, steel walls inside and out, R19 in the walls, R30 in the ceiling, ceiling height is 12'. 2 insulated garage doors, 4 energy star windows and 2 man doors. It will be used for an auto restoration/hobby shop, some metal fabrication and other metal work. Doors will stay closed. We are in the Mid-Atlantic area where it has been 90 and humid for 5 weeks.

I intend to go with a heat pump with an auxiliary electric heat unit for when it is real cold out. 65 to 75 would be the target temps and humidity control is important.

I've been told anything from a 2 ton unit to a 4 or 5 ton. Is there a resource on line that is useful? What are the recommendations or experiences from those here? Thanks in advance.

 

[rsa]

We are to the point of being ready to get some estimates for HVAC for our new shop. We have information from a design professional and some HVAC guys with unit size recommendations that are all over the place.

Did any of them perform Load Calculations, e.g. Manual N?

IIWM, I'd ask your same questions over at hvac-talk.com, probably in the commercial "Ask Our Pros" section, maybe in the AOP-Residential section. If you do, please read their stickies. Basic rules for the AOP forums are no DIY questions and no price quotes.

 

[danski0224]

You will have to do a load calculation.

This load calculation will tell you whether or not a heat pump will work in your area. You need the BTU's at 47* for your load and the equipment.

Is this a true commercial building in a commercial zoning area subject to an inspection, or is this a commercial-style building for private use and no inspections? I'm guessing the latter because if it was a plan and spec job, equipment sizes and ductwork would have been part of the design.

Commercial spaces typically have minimum ventilation standards (fresh air), and this radically changes building loads. Fire codes may dictate duct alarms once the air handlers exceed XXXX CFM.

You also need to account for interior heat generation, such as lighting.

The "stick built with steel walls inside and out" sounds a lot like a Morton building to me. I envision 1000's of screw holes and potential air leaks. If the building is up and finished, a blower door test will establish actual infiltration levels. Your "energy star" labeled windows mean nothing.

A typical commercial rule of thumb for commercial retail space is 400 sf per ton, which will vary with climate zone, ceiling height, building orientation, glass area and orientation and occupancy/activity levels.

Your space is 2080 sf, which translates to 5 tons of cooling... which may or may not be accurate.

I assume that you will have some type of mechanical exhaust for "dirty" work, which means you need some type of makeup air.

I will tell you this: trying to stuff 5 tons of cooling into 4 tons of ductwork will give you a system that doesn't work.

You really should pay a professional to design a system and create a set of drawings with specifications (equipment sizes, registers, diffusers, ductwork, duct smoke detectors, ventilation/fresh air...) . This is the ONLY way you will be able to compare submitted bids.

If you have ever looked at split system evaporator coil charts, it is possible to get 42k btu out of one "5 ton" coil and 60k btu out of another. This is why specifications are important. Obviously, one is cheaper than the other.

If you do not know what it takes to condition your space, and if you are comparing bids based on pricing, you are setting yourself up for failure. If this is how you are approaching this, then don't blame the HVAC contractor when it doesn't work.

 

[Full Throttle]

Contractor must perform a manual N load calculation

my concern with using a heatpump for this type of space is the A/c load is going to be much less than the heating load. for example only needs 2 tons to cool it but needs 5 tons with electric back up to heat it with a heat pump contractors use te size needed for heat, and this leads to a grossly oversizeda/c. My reccomendation as a HVAC contractor to you is gas heat and straight a/c. Make sure your contractor has performed a manual N load calculation. Manual J residential design will not work for this application due to building material, ventilation and interior equipment and lighting loads.

 

[pop pop]

Dan and FT are right. Without the load calculations, you're just taking a hit or miss at sizing (I'm a retired PE, Mechanical). I would suggest your speak with them about a fossile fuel heat system, even if as a back up for the heat pump. For humidity control, you can get staged/and or variable speed AC with humidistat control. I'm in central Va and I do understand humidity control necessity.

 

[mayday0017]

not to hijack but it Manual N for commercial and Manual J for homes?

 

[mayday0017]

FYI if you got say a 2 stage 4-5 ton unit, you should have enough HVAC to handle the hottest of days. You would be able to cool the place down quickly if you didn't want to keep it at your target temp 24x7. And it would drop down to 2ish tons when it doesn't need full capacity to keep up.