Recommendations on OAT Control

[prorefco]

I am in the process of updating the controls on 9 rooftop heatpumps. I am installing networked DDC controllers in each unit. Now I am addressing the auxiliary heaters and have only one digital output available on each controller for the auxiliary heater, yet I have three stages of auxiliary heat in each unit. My choices as I see it are: 1 install an expansion board on each unit, 2 utilize outdoor air thermostats to stage in the extra heaters as load increases. 3 any ideas??

I would like to use klixon type OAT's but can't seem to find any with the correct temperatures. I was figuring to have one stage available all the time, second stage available when the OAT drops to about 35ºF and the third available when the OAT reaches about 25ºF.

Of course I could use generic adjustable gas filled bulb-type OAT controls used on most residential heatpumps but would need two per unit. This gets a little expensive yet cheaper than an expansion board. If anyone knows any part numbers of klixon t-stats that open on rise and set at these temperatures, please let me know...

Thanks in advance,

Tom

[viceman]

do you have any analog outputs left ? if you do kele sells a o-10 or 4-20 to digital output transducer. ramp a pid up and stage the heaters.

[crab master]

Agree w/viceman.

Or a Johnson A350P, enabled by the DDC heating call, with the sensor mounted in the supply ductwork sending a 0-10VDC signal to a kele digital output transducer. There is likely a way to do this with all A350 series items.

http://www.kele.com/olcat/TC11/a350_w351_P352_R353.pdf

http://www.kele.com/olcat/OT8/ucs-421e.pdf

[prorefco]

Thanks for the suggestions. I thought about the A-350 system staged based on OD temp but this particular job requires something less expensive... The Kele also looks good but again, most likely more dollars than I can squeeze out of this one... ]

What I have is two heating stages available on the new controller. The unit has 1st stage heat pump and three electric resistance stages, 5KW each. I am thinking of just using outdoor t-stats to enable the auxiliary stages with two ganged on one OD t-stat, one stage connected to the heat pump stage and one to the second stage...

Thanks again,
Tom

[duke of earl]

Or you can keep it cheap and simple and wire all the stages in parallel with the heat signal and put sussessive delay on make relays on stage 2 and 3. Bring the stages on with a timed function instead of outside air temp. just a suggestion.

[duke of earl]

Oh and don't forget to bring all the stages on in the defrost mode!

[mechdorn]

Don't get mixed up with the additional controls, purchase the expansion board, or tie all the stages together. 5 KW is a minimal amount of heat, for a RTU. How often is the electric heat required? Usually if the electric heat is required the space temp is already 3-4 deg from the setpoint. At this point, a 5 KW heater isn't much help. Let it rip with full heat.

[keepyoucool2003]

Or you can keep it cheap and simple and wire all the stages in parallel with the heat signal and put sussessive delay on make relays on stage 2 and 3. Bring the stages on with a timed function instead of outside air temp. just a suggestion.

This was my thinking, if they are worried about energy consumption, slightly long delays after the first stage. This is better than OA lockout, if they need heat, they need heat. You don't want to get the call someday when the building is cold in the morning because the heat pump plus 1 stage can't bring it up since it's not cold enough outside to bring the next stage on.

[duke of earl]

OA lockout is not the best way to do this. If you are using Optimal Start properly in the DDC than the system will make setpoint by the start of the occupied period. Never lock out the heat via outside air temp.

[prorefco]

Thanks once again for all the opinions. After reviewing your replies and surveying this building's occupancy, loads and needs, I have decided to wire the first electric heat stage on the first heating stage (with the heat pump) and the second & third stage electric heat to the second heating stage on the DDC. The DDC software allows me to set whatever stage-up time delay I want, so no external timers are necessary. Further, I will utilize the optimal start feature in the DDC. I MIGHT also use a OAT control to shift second stage (two electric stages) up to first stage DDC during extreme cold weather to help eliminate droop, but haven't decided that just yet.

One more question I have though, is the optimal start setting. I am using the CV-C controllers from Wattmaster Autozone controls. This allows a setting between 1.0 and 5.0. I assume this adjusts the aggressiveness of the optimal start, but their literature doesn't really detail this function, and I haven't attended any of the factory courses as of yet. Do any of you know how to calculate this setting?

I installed this control system in my own home (and garage) three years ago for a learning platform and it's a very nice system for the price. While it's not as flexible as the big boy systems (JCI, Honeywell, Carrier, etc.) because the software is canned, for the money it has a lot of nice features (not to mention the software is FREE)....

Anyway, if anyone knows about the optimal start settings, let's hear it...


Thanks again,

Tom