Two separate HVAC systems for one space in a 3-story office building

[earshavewalls]

Is it allowable under California Codes (Energy, Mechanical, Building) for a 5,000 square foot office occupancy (no special uses) to have two separately controlled, separate sources of outside air, separate registers and ducting systems that serve exactly the same spaces. One system is the existing 3-story building system, which consists of a constant velocity supply air system with VAV boxes for each zone and hydronic heating and cooling coils supplied by the building boiler and chiller system. The other system is a split system with fan coils and condensing units that takes outside air from a louver cut into the existing building exterior wall.

Two sets of thermostatic controls operate each separate zone. Zones are identical with each system for this space. There are double grills for supply (one from each system) in each room and in the open areas. They have added a note to one of the mechanical plan sheets stating, "Split systems to run during off hours and weekends, to be controlled by programmable thermostats".

This is basically a 5,000 sq. ft. sales office for a tract home builder that is located on the first floor of a multiple use, 3-story office building.

What are the issues with this type of configuration? This seems extremely wasteful as to energy and construction costs and is the first such design that I have seen submitted in 8 years of performing commercial plan checks.

The tenant apparently wishes to stay open longer than the building HVAC system currently remains in operation during each day, and also on weekends. Has anyone seen this type of double system being used in California?

This just seems like it may be a way for the design/build HVAC contractor to pad their pocketbooks, but I have made incorrect assumptions before.........any input???

 

[mtlogcabin]

This seems extremely wasteful as to energy

If they both run at the same time but the thermostats should shut them down when not needed

This seems extremely wasteful as to.......construction costs

So are GREEN and LEAD requirements but not your call

This just seems like it may be a way for the design/build HVAC contractor to pad their pocketbooks

Again not your callI saw a few redundant systems in Florida in the 70's and 80's but most served computor rooms or churches where the cooling loads increased with the amount of people and the use within the building.

 

[Paul Sweet]

We did something similar recently in a classroom building with a large multipurpose room that could be divided with folding partitions. We used a 5 ton or so rooftop unit to handle the space loads when it was unoccupied (which was most of the time) and a 10-15 ton rooftop unit for each section for high occupant loads. The control programming was interesting, but it seems to be working well.

 

[RJJ]

I have some buildings that have a many as 10 units on one floor. With geothermal I have 3 20K plus buildings with even more. They are all zoned!

 

[Frank]

Controls need to be such that they are sequenced by load and cannot be one in heating while other is in cooling mode. IECC 503.2, 503.2.4

 

[RJJ]

Frank: I agree.

 

[earshavewalls]

The issue has been taken care of. The tenant intends to occupy the space during hours when the CV system is shut down for the building (normally unoccupied at that time). To be able to provide continuous heating or cooling, they wish to use the backup system during those off hours. The only thing needed to comply with energy useage is to set up the systems so that they do not operate concurrently.

If they submitted the two systems as one, it would not comply because it would use twice the energy necessary, on paper. This was a simple solution that the tenant, architect, engineers, and building & safety were all in agreement on.

This is the first time I have seen such a system. It is sort of an afterbirth of the energy regulations. Years back this would never even have been considered because of the waste of money involved.......they would have just left the system on, as usual. Now, with energy such a concern, I feel we will see more of these "secondary" HVAC systems. It was just odd seeing two sets of supply registers in each area, fed from different systems, controlled by different controls, that are meant to do the same thing in the same area....

 

[earshavewalls]

Just for kicks, here is an excerpt from the 2008 Nonresidential Compliance Manual from the California Energy Commission:

Mechanical Systems – HVAC System Requirements Page 4-73

2008 Nonresidential Compliance Manual August 2009

4.6.1 Sizing and Equipment Selection

§144(a)

The Standards require that mechanical heating and cooling equipment (including electric heaters and boilers) be the smallest size available, within the available options of the desired equipment line, that meets the design heating and cooling loads of the building or spaces being served. Depending on the equipment, oversizing can be either a penalty or benefit to energy usage. For vapor compression equipment, gross oversizing can drastically increase the energy usage and in some cases cause premature failure from short cycling of compressors. Boilers and water-heaters generally suffer lower efficiencies and higher standby losses if they are oversized. On the other hand, cooling towers, cooling coils, and variable speed driven cooling tower fans can actually improve in efficiency if oversized. Oversized distribution ductwork and piping can reduce system pressure losses and reduce fan and pump energy.

When equipment is offered in size increments, such that one size is too small and the next is too large, the larger size may be selected.

Packaged HVAC equipment may serve a space having substantially different heating and cooling loads. The unit size should be selected on the larger of the loads, based on either capacity or airflow. The capacity for the other load should be selected as required to meet the load, or if very small, should be the smallest capacity available in the selected unit. For example, packaged air-conditioning units with gas heat are usually sized on the basis of cooling loads. The furnace is sized on the basis of airflow, and is almost always larger than the design heating load.

Equipment may be oversized provided one or more of the following conditions are met:

It can be demonstrated to the satisfaction of the enforcing agency that oversizing will not increase building source energy use; or

1. Oversizing is the result of standby equipment that will operate only when the primary equipment is not operating. Controls must be provided that prevent the standby equipment from operating simultaneously with the primary equipment; or

2. Multiple units of the same equipment type are used, each having a capacity less than the design load, but in combination having a capacity greater than the design load. Controls must be provided to sequence or otherwise optimally control the operation of each unit

based on load.

Item #1 above is the condition on this project. It just took some research to dig this out of the code (actually, the handbook for the code).

 

[north star]

* * * *

Wayne,

Just out of curiosity, ...couldn't they have added the software &

associated controls to the existing system to make it compliant,

rather than adding a whole other system "WITH" programmable

thermostats?

FWIW, our AHJ municipal building has the programmable

thermos that reduce energy consumption after normal business

hours & on weekends / holidays.

& & & &

 

[GCtony]

See this quite often. Most modern office building using vav systems have an override that allows a tenant that is working "off-hours" to turn the building system on when the main system is off. The probelm with this is you could have a tenant that occupies 5% of the building on a sunday but he's turning on the equipment for the whole building just to condition his space. Years ago, one of my owners had an energy audit done on his buildings, They then started asking tenants that have strange hours to install their own suplimental system since it's much cheaper using that than kicking on the building system.