[riverman]

Need a little help.

Working on a job where the original white box spec called for a 15 ton heating/cooling RTU for a single story office space. Unit is installed (no ducting), now a tenant is found and their specs call for a "constant volume system with electric heat duct heaters in ducts that serve exterior zones"

Pretty straight forward office area with large open interior area with EXTERIOR zones as conference rooms, private offices and lunch room. They are asking for (6) electric duct heaters to serve each individual exterior area.

Wondering how a system like this works? Where does the MAIN t-stat get located for the RTU? I would gather that the main t-stat is located in the interior zone and that it functions like any single RTU/t-stat would (call for heat, call for cool, entire system gets hot or cold air).

Now, I assume that the electric duct heaters are for providing heat to those exterior zones when the interior zone (where t-stat is) is calling for cooling when exterior needs heating. Also I would assume that those electric duct heaters could provide additional heat (booster coil) when the RTU is in the heating mode?

Do these electric duct heaters have room sensors to control their operation?

Lastly, if I am on the right track, what is the proper way of sizing the electric duct heaters?

Thanks for the help!

 

[TitleLess]

Typical small lab installation, not very efficient.

Constant volume constant discharge air from the air handler, then reheat each zone. Yours is only heating the exterior, and the inner zones will probably get cold,,, thi system will have problems, it is not a good application for an office building unless you are reheating the inner zones also,,,, you could devise some sophisticated control logic to reset the dis air temp, but you are probably stuck with a unit that has very little modulation to work with unless it is in economizer mode. The logic could cover up some things here, but it will never be a good system.Why don't they have variable volume in the inner areas ? Constant volume with reheat on the perimeter is excellent, but you can't do it to well without reheat in the inner parts of the building.

The sizing question is straight forward, the engineer should have speced it for you, generally they are sized in 3 stages and a ten degree rise per stage.

Yes they have room sensors, the electric heat in your scenario ( assuming no good BAS system) will have the sensor mounted in the space and it will trim the supply air to that zone, to maintain the setpoint of that sensor.

 

[ralphtheplumber]

What Eddy said, plus a couple things.

You're going to need reheat on all the zones, not just the exterior. These systems are usually set up to just blast 55º air during occupied hours.. The rtu will have a discharge air controller, and the zone stats will kick the reheat on and off. It's the cheapest and easiest way to get one unit to heat and cool at the same time, but it's not very efficient.

Orion makes a controller for this, if you need one.

 

[ralphtheplumber]

Riverman, are you from Peoria by any chance?

 

[drrtu]

Review your local codes, CV with terminal reheat will not meet 90.1 or the other energy codes like IECC. The other guys are correct, install a D.A. control with reset, and then reheat all the zones. Designs like this run really stabile but are energy pigs I have seen a few examples that do not use reheat on the big all interior zone. CV with reheat was a common design practice in the late 60s and early 70s.

 

[just_opinion]

Reheat is still legal if used in hospital/surgury that constantly exhausting air (without going too much red tape)

If you are in the warmer climate area like me (SE Texas) the you can get away with reheating in exterior zones only.

But it requires complicate DDC control to make everyone happy.

I have several doctors building like yours and I have to use DDC control and many hours of programming to meet everyone crietria. (trust me, if a zone is 0.5 degrees off, they call)

I use Inet2000 as my DDC controls. In your case all you need is a 7716 board, a 4DO-4AO add-on board, VFD module, dumd damper and actuator. AND many hours of programming. You are in business

If you go cheap, then you will have all kind of headage before they fire you (why do think we inherite these building and use DDC controls)

Good luck man

 

[riverman]

I do not like the idea either, just what the Architect mentioned.

I will more than likely just propose a Vari-Trac VAV system with electric heat in exterior zones.

 

[al o]

keep it simple, its a small system, as long as there's not big solar gain on the exterior offices put the main stat in the interior set for auto change over fan must run constant when occupied. install reheat stats in each office & size reheats to provide 40' temp rise if you have high solar gains install a main stat that allows for temprature averaging usually 4 sensors are required