Commercial HVAC question

[wanderer]

I just bought a commercial office building. I previously rented the space from the owner so I was very aware of these problems but one of the first things that I am going to do to the property is some energy efficiency and HVAC work.

The property is a 6000 ft.² office building. Single-story with concrete block walls and a web truss ceiling, built in the 1970s. The building is divided into seven individual office spaces. Two are quite large, maybe 1500 square footage, three are more modest, five or 600 ft.² each, and the last two are very small. Two or 300 ft.² each. The building has three rooftop furnace and AC units with ductwork in the ceiling. Originally the insulation was put on top of the drop ceiling but the majority of it is missing now. Because there are seven units and three thermostats, poorly sized ductwork, and insulation problems, there are some pretty extreme comfort and energy efficiency issues with the building.

I’ve had a few local guys come out and look and they tell me that the current system can’t be zoned because the old age of the equipment insufficient ductwork sizing would not tolerate running that way. Ok. So far I’ve had three companies look at this and all are giving me slightly different information. One was a commercial company that is recommending removing all three rooftop units and completely reinstalling with new ductwork throughout, with proper zoning and thermostats in each space. Another was a small company and the last was a medium sized company, these two I have a good relationship with and trust but I think this is a bit outside their experience ad comfort zone.

My opinion is that I would like to move away from the rooftop units and towards some thing that is easier to service and more efficient. I think the layout of the building is perfect for mini splits. The outside unit could easily be put on the backside of the building so they are accessible with the ladder without getting on the roof, and there are cartridge units that will easily install in the ceiling tiles and look attractive. Another benefit of this is that I can spread the expenditure out over time, fixing units units immediately with the really unhappy tenants but leaving for the existing commercial unit installed in the large spaces that are not really having too many issues right now.

Another big advantage of these mini split Is the ability to install occupancy sensors because I pay for electricity at the building. I have noticed in many of the spaces are unoccupied during the day or for long periods. One is leased to the school district so anytime there is no school the space is completely empty. Basically, they occupy the space 180 days per year. I’d like to only be minimally heating or cooling spaces on these other days.

So I’m thinking I need to do some more looking for installation companies who have experience with the right equipment. I’ll probably get on the equipment companies website and do a search for local dealers. So far I’ve reached out to a Mitsubishi mini split installer. Are there other brands or equipment I should strongly consider? Any tips or advice is appreciated. I Have never owned a mini split before so I’m relatively unfamiliar.

 

[RossABQ]

Depending where you live, and what energy codes they've adopted, you are likely going to be forced to have the building modeled for heating/cooling loads, by a registered engineer. This is the case where retrofits are more than a certain % of the system or over a threshold $ amount. In the end it is the right way to do it.

Does the building have a flat roof? What climate are you in?

 

[Wrench97]

If the 7 units have 7 different tenants I would want each unit to have it's own system run off that unts meter.

 

[wanderer]

Depending where you live, and what energy codes they've adopted, you are likely going to be forced to have the building modeled for heating/cooling loads, by a registered engineer. This is the case where retrofits are more than a certain % of the system or over a threshold $ amount. In the end it is the right way to do it.

Does the building have a flat roof? What climate are you in?

I’m in the Midwest. I think design heating loads vary from -4°F to 104°. Eating capacity in the winter is a concern of mine.

I was planning on having the contractor perform a manual j calculation on the space. No engineer required. Yes it has a flat roof.

 

[wanderer]

If the 7 units have 7 different tenants I would want each unit to have it's own system run off that unts meter.

There is only one meter. I’m not happy with it but changing it is not feasible.

 

[Wrench97]

I would stay with the roof tops, at least around here condenser theft is a issue, and you need heat that a mini-split can't provide.

 

[bzinsky]

I just bought a commercial office building. I previously rented the space from the owner so I was very aware of these problems but one of the first things that I am going to do to the property is some energy efficiency and HVAC work.

The property is a 6000 ft.² office building. Single-story with concrete block walls and a web truss ceiling, built in the 1970s. The building is divided into seven individual office spaces. Two are quite large, maybe 1500 square footage, three are more modest, five or 600 ft.² each, and the last two are very small. Two or 300 ft.² each. The building has three rooftop furnace and AC units with ductwork in the ceiling. Originally the insulation was put on top of the drop ceiling but the majority of it is missing now. Because there are seven units and three thermostats, poorly sized ductwork, and insulation problems, there are some pretty extreme comfort and energy efficiency issues with the building.

I’ve had a few local guys come out and look and they tell me that the current system can’t be zoned because the old age of the equipment insufficient ductwork sizing would not tolerate running that way. Ok. So far I’ve had three companies look at this and all are giving me slightly different information. One was a commercial company that is recommending removing all three rooftop units and completely reinstalling with new ductwork throughout, with proper zoning and thermostats in each space. Another was a small company and the last was a medium sized company, these two I have a good relationship with and trust but I think this is a bit outside their experience ad comfort zone.

My opinion is that I would like to move away from the rooftop units and towards some thing that is easier to service and more efficient. I think the layout of the building is perfect for mini splits. The outside unit could easily be put on the backside of the building so they are accessible with the ladder without getting on the roof, and there are cartridge units that will easily install in the ceiling tiles and look attractive. Another benefit of this is that I can spread the expenditure out over time, fixing units units immediately with the really unhappy tenants but leaving for the existing commercial unit installed in the large spaces that are not really having too many issues right now.

Another big advantage of these mini split Is the ability to install occupancy sensors because I pay for electricity at the building. I have noticed in many of the spaces are unoccupied during the day or for long periods. One is leased to the school district so anytime there is no school the space is completely empty. Basically, they occupy the space 180 days per year. I’d like to only be minimally heating or cooling spaces on these other days.

So I’m thinking I need to do some more looking for installation companies who have experience with the right equipment. I’ll probably get on the equipment companies website and do a search for local dealers. So far I’ve reached out to a Mitsubishi mini split installer. Are there other brands or equipment I should strongly consider? Any tips or advice is appreciated. I Have never owned a mini split before so I’m relatively unfamiliar.

<I have several buildings....retail, office, and multifamily. I also have personally installed 20 mini-splits, and I used to service my own equipment. My largest being a 20 ton, and smallest being 9k btu mini-splits.

The downside to the mini splits is that they are not as reliable and long lasting. They also require frequent filter cleaning. Something that you basically can’t rely on the tenant for if you’re paying your own electric. So you will be inside each office space once every 6 months cleaning filters. A bit easier when you have 1/2 big units for the building. It’s just ends up being a pain over time.

They are indeed efficient though. I’ve also found offices create a lot more heat than apartments due to computers and people.

I’d go with fujitsu first, mitsu second, just because it seems like there are more techs familiar with fujitsu.

Btw, single zone 9-12k btu mini splits are by far the most efficient if you can swing it.

The big downside to mini splits.....
You have office space right, well the tenant can fit out the office space as they see fit. They often want to put up walls. That’s going to be difficult with mini splits as if you create an office and you only have 2 mini splits per unit, then what are you going to do? That office needs heating and cooling too. Much easier with traditional duct work.

My vote......if you’re going to use this long term, traditional. They just always work.

Keep in mind as time goes on you will get tired of managing it and you will want easy easy easy. You want systems in place so you don’t need to do anything. I’ve found traditional units do a better job with that.

 

[RossABQ]

...
The big downside to mini splits.....
You have office space right, well the tenant can fit out the office space as they see fit. They often want to put up walls. That’s going to be difficult with mini splits as if you create an office and you only have 2 mini splits per unit, then what are you going to do? That office needs heating and cooling too. Much easier with traditional duct work.

My vote......if you’re going to use this long term, traditional. They just always work....

Exactly right, IMO. Zoned VAV with rooftop unit(s) are the most common for commercial leased space, and for exactly the reasons stated -- they are easily changed to suit new tenants or new arrangements. Much easier to provide required ventilation and "free cooling" (economizer) too.

 

[nostromo7]

Yikes. I'm a professional engineer who does building systems (and I'm expressly disclaiming any liability for the generalized advice I'm about to give ), and this:

IÂ’ve had a few local guys come out and look and they tell me that the current system canÂ’t be zoned because the old age of the equipment insufficient ductwork sizing would not tolerate running that way. Ok. So far IÂ’ve had three companies look at this and all are giving me slightly different information. One was a commercial company that is recommending removing all three rooftop units and completely reinstalling with new ductwork throughout, with proper zoning and thermostats in each space.

... is what you really ought to do. Packaged roof-top units can still be very efficient, as long the ductwork is well-built and insulated to prevent losses.

You have several factors that are complicating your issues:

Originally the insulation was put on top of the drop ceiling but the majority of it is missing now. Because there are seven units and three thermostats, poorly sized ductwork, and insulation problems, there are some pretty extreme comfort and energy efficiency issues with the building.

Red flag #1. You should have insulation at the roof, not on top of the ceiling. Be sure that the roof and walls above the ceiling are properly insulated. Don't band-aid a **** building envelope with an oversized HVAC system, it's a waste of money.

My opinion is that I would like to move away from the rooftop units and towards some thing that is easier to service and more efficient. I think the layout of the building is perfect for mini splits.

Okay, but you have two problems doing that.

1) You say you live in the midwest, and "design heating loads vary from -4°F to 104°". I hope what you're meaning to say is that your winter design heating condition is -4 °F. That's a big problem for ductless mini-splits: most manufacturers' outdoor units are not effective at that cold a temperature. Some will state that they are capable of running down to -4ish, but in reality that's with one or more caveats applied: for example it may be possible only with one outdoor unit matched to one indoor unit, or the unit will still run at -4 °F but the heating capacity is de-rated (which is a colossal problem because the colder it is the more heat you need!). It is very, very likely that a split system will not be sufficient for your heating needs.

2) You have to meet the minimum requirements of ASHRAE Standard 62.1. In an office space that's 5 cfm/person and 0.06 cfm/sq ft floor area. This needs to be calculated in detail to account for the actual occupancy of your building. Ultimately this means you'll still need to have an air handler that will distribute ventilation air to your various office suites; your proposed ductless mini-splits don't ventilate.

Depending where you live, and what energy codes they've adopted, you are likely going to be forced to have the building modeled for heating/cooling loads, by a registered engineer. This is the case where retrofits are more than a certain % of the system or over a threshold $ amount. In the end it is the right way to do it.

I was planning on having the contractor perform a manual j calculation on the space. No engineer required.

Manual J applies only to low-rise residential; it does not apply to commercial buildings. Any reputable contractor being asked to do this should refuse you.


You didn't mention whether the existing RTUs have electric heat or are gas-fired. If the latter, chances are you would need to upgrade your electrical service if you went with a ductless split system. I would also undertake an analysis of energy prices in your area. In most areas gas is much, much cheaper than electricity, and obviously if you're the one footing the bill you ought to pick the one that minimizes your op costs.

 

[wanderer]

The existing units are gas fired.

Regarding the insulation, I have a contractor coming in to add R30 insulation in between the trusses.

Guess I need to reconsider things, I had really thought the mini splits would be better. You guys make some pretty good points though.

 

[Captain Spaulding]

Talk to your utility providers. They may have programs that can reduce your usage, or have connections with vendors who can reduce your costs in exchange for a portion of the savings. Costs nothing to check.

 

[Denwood]

Just sold my commercial building (about 8600 sq/ft) with six tenants. We also did an extensive retrofit, complete gut with furred air space assembly, radiant heat (insulate and overpour) with six air handlers. Office and light commercial tenants. 1 large venmar HRV for fresh air was ducted into each system.

One triangle tube 99% efficient on demand wall unit provided all heat (hot water to air handlers), and we had external AC units for each air handler. Nine ecobee Wi-Fi thermostats with multiple external sensors to monitor occupancy. Hydro costs were $450 a month, and highest heat bill ever (during a month with -20C temp average) was $223 for NG.

The system was awesome, and I could program and keep every tenant quite happy. The ecobee stats would reliably alert me of any issues with low temps etc if someone left a door open or otherwise.

 

[danski0224]

The existing units are gas fired.

Regarding the insulation, I have a contractor coming in to add R30 insulation in between the trusses.

Guess I need to reconsider things, I had really thought the mini splits would be better. You guys make some pretty good points though.

Fiberglass insulation provides its rated R-value only if it's touching (but not compressed by) solid (air tight) building materials on all 6 sides... plus all of the detail work around stuff like penetrations and conduit.

Simply slapping up some insulation will be almost worthless.

The insulation should be rigid foam on the roof deck, under the roofing.

 

[danski0224]

Zoning a traditional ducted system *****.

The only way that it really works is with a modulating medium pressure air handler (equipment) and VAV boxes.

Air sealing at the roof curb/ductwork/equipment interface needs to be paid attention to. So many ****** installs out there.

Too many installers have this differentiation between supply, return and exhaust ductwork with respect to air sealing and fitting construction. Again, so many ****** installs.

You will only get efficiency with a properly engineered system with coordinated drawings that are actually followed by the contractor and crew. And someone needs to be held accountable if the drawings are ****.

And then when the system is commissioned and tested, you need to be sure that it isn't pencil whipped from the street. The job I'm currently on might have 10% of the ductwork tested and the rest is not installed to the same standards as the cherry picked test sections. And believe me, the person doing the testing knows how to cheat.

Otherwise, just slap a concentric diffuser on the bottom of the RTU and call it good.

 

[wanderer]

Fiberglass insulation provides its rated R-value only if it's touching (but not compressed by) solid (air tight) building materials on all 6 sides... plus all of the detail work around stuff like penetrations and conduit.

Simply slapping up some insulation will be almost worthless.

The insulation should be rigid foam on the roof deck, under the roofing.

I originally was looking for spray foam but was talked out of it due to a variety of reasons that made it impractical. The roof is new, adding insulation up top is not practical.

 

[Denwood]

The aforementioned system was spec’d completely by a mechanical engineering firm, but even that is no guarantee. I fired the first firm we contracted as they made a real cluster of the drawings and specs.

 

[Steve W.]

. . . . ..

 

[karoc]

Glad you decided to stay with package units,those mini splits are really made as kinda a fill in. Where I work the mini splits are there for storage rms, or where band equipment may be stored. They just can't handle renters where a package unit can if stats has lock boxes on them which maybe the programmable type say to shut off at 6pm and come on say at 6am and maintain say 73* settings. To mount several VAV's with elec reheat and controls for that will cost you a penny. This is something that you need to call a commercial outfit who also does control work.